Chemical Kinetics

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Chemical Kinetics

Chemical Kinetics

The word kinetics deriv from the Greek word ‘kinesis‘ meaning ‘movement.’ Chemical kinetics is the branch of chemistry which deals with the study of reaction rates, factors affecting the rate of reactions and the mechanism by which the reactions proceed. In other words,
Chemical kinetic is the study of chemical reactions with respect to reaction rates, effect of various variables, rearrangement of atoms and formation of intermediates

Slow and Fast Reactions

Some reactions such as ionic reactions occur instantaneously and thus, are called fast reactions, e.g., precipitation of silver chloride occurs instantaneously by mixing aqueous solutions of silver nitrate (AgNO3) and Sodium chloride (NaCl)

AgNO3 + NaCl → AgCl↓ + NaNO3

On the other hand, some reactions take few days, months or years for their completion. Such reaction is called slow reactions, e.g., rusting of iron in the presence of air and moisture. Also, there are some reactions like an inversion of cane sugar, hydrolysis of starch, etc. which proceed with moderate speed. Generally, under the chemical kinetics, chemical reactions with an average speed are studied.

Some Important Terms related to Reactions

Bond Energy

The amount of energy required to break one mole of the bond of a particular type between two atoms in the gaseous state is called bond energy. It is expressed in kJ mol-1. Larger the bond dissociation energy, stronger will be the bond in the molecule. Energy is required to break a bond, i.e., bond breaking is an endothermic process and energy is released when a bond is formed, i.e., the bond formation is an exothermic process.

Heat of Reaction

The heat of reaction is the quantity of heat evolved or absorbed in a reaction.

H2 (g) +Br (l) → 2HBr (g) + 72.8 kJ mol-1

Heat of formation

The energy released or absorbed for the formation of one mole of a compound from its constituent element is called heat of formation

C (s) 2H2 (g) → CH4 (g) 74.81 kJ mol-1

Heat of combustion

The heat energy evolved during the combustion of one mole of a substance in the presence of the excess of oxygen is called heat of combustion.

C6H12O6 (g) +6O2 (g) → 6CO2 (g) + 6H2O (l) + 2802.0 kJ mol-1

Rate of Reaction

The speed of a reaction or rate of a reaction can be defined as the change in concentration of a reactant or product in unit time. To be more specific, it can be expressed concerning
  1. the rate of decrease in concentration of any one of the reactant or
  2. a rate of increase in the concentration of any one of the products.

Unit of Rate of Reaction

Unit of rate is concentration time-1, e.g., if the concentration is in mol L-1 and time is in Second then the unit will be mol L-1s-1. However, in gaseous reactions, when the concentration of gases is expressed regarding their partial pressures, the unit of rate of reaction will be atm s-1.

Factors influencing Rate of Reaction

Rate of reaction depends upon several experimental conditions which are described below
  1. Effect of Concentration of Reactants Rate of a chemical reaction at a given temperature may depend on the concentration of one or more reactants and products. In general, the rate of reaction increases with increase in the concentration of the reactants, because the number of collisions between the molecules increases with increases in concentration.
  2. Temperature Generally, rate of reaction increases with increase in temperature and vice-versa. This is because, at high temperature, molecules possess high kinetic energy and hence, the high velocity which increases the chance of combination of molecules.
  3. Nature of Reactants Rate of a reaction is also affected by the quality of reactants. e.g., sodium and potassium react vigorously with ordinary water temperature, but iron responds only with steam.
  4. A surface area of Reactants  Larger the surface area of reactants, higher is the rate of reaction because more sites are available for the reaction. The surface area of a solid can be increased by converting it into powdered form. e.g., the reaction of zinc dust with sulphuric acid takes place rapidly than the response of zinc piece with sulphuric acid.
  5. Presence of Light Rate of some chemical reactions increases in the presence of light (radiations), e.g., oxidation of chloroform takes place in the presence of light
  6. Effect of Presence of a Catalyst Rate of reaction increases in the presence of a catalyst. A catalyst is specific in nature, and it increases the rate of a reaction by providing an alternative path of lower activation energy to the reactants.

Activation Energy

Before involving in a chemical reaction, the reactant molecules absorb some extra energy and come together to form an activated complex. This activated compound or complex is unstable because its potential energy is very high. Thus, it decomposes into products. Therefore activation energy is the additional energy which the reacting molecules must acquire to form an activated complex. Lower the value of activation energy faster will be a reaction.

Electrolysis

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Electrolysis
The word ‘electrolysis’ is derived from two words ‘electro’ meaning electrical energy and ‘lysis‘ meaning dissociation (breakdown). The process of decomposition of a molten substance or its aqueous solution by passing an electric current is called electrolysis. In other words, electrolysis is a process in which electrical energy is used to bring about a non-spontaneous chemical reaction. William Nicholson showed the electrolysis of water.

Components Required for Electrolysis

The three components required for the process of electrolysis are

Electrolyte

The compound which can conduct electricity in liquified state or in an aqueous state is termed as the electrolyte. e.g., acid, base, salt, etc. On the basis of dissociation, electrolytes are of two types: strong electrolyte and weak electrolyte. Strong electrolytes dissociate completely while weak electrolytes dissociate only partially, NaCl, KCl, CaCl2, MgSO4 are the examples of strong electrolytes while CH3COOH is an example of weak electrolyte. Non-electrolytes are bad conductor of heat because they do not dissociate into their ions when dissolved in water. e.g., urea, glucose, sugar, etc.

Electric Current

The flow of electrons in a conductor is termed as electric current. This result in the transfer of ions to the respective terminal.

Electrode

A solid electric conductor through which an electric current enters or leaves electrolytes is termed as an electrode. It is used to make electrical contact with some part of circuit

Arrhenius Theory of Electrolytic Dissociation or Decomposition

This theory was proposed by Sweden chemist Arrhenius in 1894 in order to explain the behavior of electrolytes in aqueous solutions. Main postulates of this theory are as follows
  1. When an electrolyte is dissolved in water, it dissociates into its ions i.e., cations (positive ions) and anions (negative ions) and this phenomenon are called ionization.
  2. In ordinary conditions, weak electrolytes dissociate in solution to a small extent and the solution of these electrolytes contains ions which are in equilibrium with unionized molecules. Such an equilibrium is called ionic equilibrium.
  3. Electrolysis takes place only at electrodes
  4. The conductivity of the solution depends upon the number of ions present in the solution.

Faraday’s Laws of Electrolysis

Michael Faraday performed various experiments on the phenomenon of electrolysis and their results were published in 1833-34. On the basis of these experiments, Faraday gave the following two laws called the Faraday’s laws of electrolysis.

 First Law

The amount of chemical reaction which occurs at an electrode during electrolysis by current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).

m =ZQ = Zit

where, Z = electrochemical equivalent, m =substance deposited in gram i= current in ampere, t = time in second, Q = charge in coulomb When, i = 1A and t=1s then m=Z i.e,. The electrochemical equivalent is defined as the mass of substance deposited when 1-ampere electricity is passed for 1 second (or 1-coulomb charge is passed) through a solution.

Second Law

A number of different substances liberated at the electrodes by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights (Atomic mass of metal ÷ Number of electrons required to reduce the cation). If W1, W2, W3 are the deposited amounts of the substances and E1, E2, E3 are their respective chemical equivalnet weights then

W1/W2 = E1/E2 and W1/W2=E2/E3 or Z∝ E

Thus, the electrochemical equivalent of a substance is directly proportional to the chemical equivalent of the substance
  • Faraday If an electric current due to 96487 coulomb charge is passed for 1 second through an electrolyte, it will deposit one equivalent weight of the substance. This amount of electric current is called 1 Faraday
1 Faraday = 96500 C mo-1
  • It is basically the charge on 1 mole of electron, its exact value is 96487.

Products of Electrolysis

Products of electrolysis depend upon the nature of material being electrolysed and the type os electrodes being used. e.g., if we use molten NaCl, the products of electrolysis are sodium metal and chlorine gas. During the electrolysis of an aqueous sodium chloride solution, the products are NaOH, Cl2 and H2. Electrolysis of an aqueous solution of copper sulphate using copper elecctrodes produces copper at cathode. Products of electrolysis also depend on the different oxidizing and reducing species present in the electrolytic cell and their standard electrode potentials.
  • A potential difference develops between the electrode and the electrolyte, this is called electrode potential
  • When the concentration of all the species involved in a half cell is unity, then the electrode potential is known as standard electrode potential.

Application of Electrolysis

In Electrorefining of Metals Pure form of copper, silver gold are obtained by electrorefining process in which anode is made up of impure metal and a thin strip of pure metal acts as cathode. A salt solution of the metal is generally used as electrolyte. The copper obtained by this process in 99.9% pure. In Electroplating Objects Electroplating is the process of electrolysis in which the desired metal is deposited on the another material to provide shiny appearance and prevent it from corrosion and scratch. e.g., chromium plating is done on many object such as car parts, etc., Jewellery makers electroplate gold on silver or copper or nickel ornaments by placing these metals in a solution having a salt of gold and by passing an electric current. Tin cans used for storing food are made by electroplating tin onto iron. In Electrotyping It is used in printing industries for making blocks, graphics, etc. e.g., in the large printing press, a thin layer of copper is coated on the printing paper by using copper voltmeter and replacing cathode by printing paper because on copper coated papers, excellent prints appear. In Electrometallurgy Electrometallurgy is the process of extraction of certain metals like calcium, aluminum, magnesium, etc from respective compounds. In manufacturing of Compounds  Chemical compounds like chloroform, ethane acetylene, drugs are manufactured by the process of electrolysis. In the evaluation of equivalent weight of Metals The equivalent weight of the metals is calculated by using the formula w ∝ E. In Electrolytic Capacitor Here, aluminum metal is used for making both the electrodes. The mixture of boric acid, glycerine, and aqueous ammonia is used as the electrolyte. On passing electric current, a layer of aluminum hydroxide deposits on the anode which acts as dielectric for the electrodes  

Ideal Gas Law

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Mass (m), volume (V), pressure (p), and temperature (T) is of a gas are the measureable properties. The laws which inter-relate these properties are called gas laws.

Perfect Gas or Ideal Gas

The gas whose molecules are point masses (mass without volume) and do not attract each other, is called ideal or perfect gas. It is a hypothetical concept which can not exist in reality. The gases such as hydrogen, oxygen or helium which can not be liquefied, are called parmanent gases. Properties of ideal Gas are as follows
  1. It strictly obeys Boyle’s law, Charle law and the law of pressure under all consitions of temperature and pressure.
  2. Its pressure coefficient and the volume coefficient are exactly equal to each other.
  3. A perfect gas can not be converted into liquid or solid state, because a force of attraction is necessary between the molecules in case of liquid or solid state.

Ideal Gas law or Equation

The tree laws (Boyle’s law, Charles law and Avogardo’s law) can be combined together in a single equationn which is known as ideal gas equation.

Gas law

  1. Boyle’s Law (pressure-voulme relationship) According to this law, at constant temperature, pressure of a fixed amount of gas varies inversely with its volume, i.e.,

p∝1/V(at constant T) or pV = k (constant) or p1V1=p2V2

at constant temperature, pressure of the gas is directly proportional to the density of a fixed mass of the gas.

i.e.,    p ∝ d

2. Charles’ Law (Temperature-Volume relationship) According to this law, at constant pressure, the volume of a fixed mass of a gas is directly proportional to its absolute temperature i.e., decreases with a decrease in temperature.

V∝T (at constant p) or V1/T1 = V2/T2

The lowest hypotheticcal or imaginary temperature at which gases are supposed to occupy zero volume, is called absolute zero. 3. Gay Lussac’s Law (Pressure-Temperature relationship) According to this law, at constant volume, pressure of a fixed amount of a gas varies directly with the temperature, i.e.,

p ∝ T or p/T= constant or p1/T1=p2/T2

4. Avogadro’s Law (Volume-Amount relationship) According to this law, equal volumes of all the gases under the same conditions of temperature and pressure contain the equal number of molecules, i.e.,

V ∝ n (at constant T and p)

where, n=number of molecules at STP, gram molecular mass or 1 mole of gas occupies volue of 22.4 L. Number of molecules in one mole of a gas has been determined to be 6.022 x 1023 This number is known as Avogadro’s constant 5. Combined Gas Law This is the relationship for the simultaneous variation of the variables. If temperature, volume and pressure of a fixed amount of gas vary from T1, V1 and p1 to T2, V2 and p2 then we can write

pV/T=nR or p1V1/T1=p1V2/T2

6. Dalton’s Law of Partial Pressure It states that the total pressure exerted by gaseous mixture of two or more non-reacting gases is equal to the sum of the partial pressure of each individual component in a gas mixture, i.e.,

ptotal= p1+p2+p3…..pn (at constant T, V)

where, p1, p2, p3 … are the partial pressures of individual gases. 7. Graham’s Law of Diffusion According to this law, at constant temperature and pressure, the rate of diffusion (r) of a gas is inversely proportional to the square root of its density (d) i.e.,

Graham's Law of Diffusion

(Diffusion is the process of spontaneous mixing of different gases and the volume of a gas diffused per unit time, is called rate of diffusion) This law is applicable 
  1. In the production of marsh gas
  2. In the separation of gaseous mixture
  3. In the determination of vapour densities of gases
  4. In the Separation of Isotopes

Indeal Gas Equation

At constant T and n;

V ∝ 1/p (Boyle’s Law)

At constant p and n;

V ∝ T (Charles’ Law)

At constant p and T;

V ∝ n (Avogadro’s law)

V ∝ nT/p or V = RxnT/p

where, R is proportionality constant, On rearranging the above equation, we obtain

pV=nRT (ideal gas equation)

R=pV/nT

R is called universal gas constant and has vol. 8.314 J mol-1 K-1 or 0.0821 L atm mol-1 K-1 Ideal gas equation is a relation between four variables and it describes the state of any gas, therefore, it is also called equation of state.

Colloidal Solution

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A colloidal solution is a heterogeneous system which is made up of two phases; dispersed phase (as solute) and a dispersion medium (as solvent). The substance distributed as the colloidal particles is called the dispersed phase and the second phase in which the colloidal particles are scattered is called the dispersion medium. Size of colloidal particles is in between 1 nm to 1000 nm. Milk, face creams, sponge, rubber, pumice, blood, gems, etc are the examples of colloids. When one constituent particle of a solution is scattered around the another, then it is called dispersion.

Classification of Colloids

  1. On the basis of physical state of dispersed phase and dispersion medium, colloids are classified into following types
    Dispersed Phase Dispersion Medium Type Examples
    Solid Solid Solid Sol Coloured Gemstone, Milky glass
    Solid Liquid Sol Milk of Magnesia, Mud, paints, cell fluids
    Solid Gas Aerosol Smoke, automobile exhaust
    Liquid Solid Gel Jelly, cheese, butter
    Liquid Liquid Emulsion Milk, face cream, Hair cream
    Liquid Gas Aerosol Fog, Clouds, Mist, insecticide sprays
    Gas Solid Solid sol Foam, rubber, sponge, pumice stone
    Gas Liquid Foam Shaving cream, froth, whipped cream
  2. On the basis of nature of interaction between dispersed phase and dispersion medium, colloidal sols are divided into two categories
    Lyophilic Colloids Lyophobic Colloids
    These are solvent loving colloids These are solvent hating colloids
    These are directly formed by mixing substances like gum, gelatin, starch, rubber, etc with a suitable solvent. These sols can be prepared only by special methods
    These are quite stable sols These are not stable.
    These are also called reversible sol These sols are also called irreversible sols.
    e.g., Sol of starch e.g., Gold Sol
  3. On the basis of the type of particles of the dispersed phase, colloids are classified into three categories.
    1. Multimolecular Colloids. In this type of colloids, colloidal particles are aggregates of a large number of atoms or smaller molecules. e.g., gold sol, sulfur sol, etc.
    2. Macromolecular Colloids. Macromolecules in suitable solvents form solutions in which the size of macromolecules may be in colloidal range. These colloids are quite stable and resemble true solutions in many respects, e.g., naturally occurring macromolecules starch, cellulose, proteins and enzymes; and those of man-made macromolecules polythene, nylon, polystyrene, synthetic rubber, etc.
    3. Associated Colloids (Micelles) The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called Critical Micelle Concentration (CMC).
These substances behave as normal strong electrolytes at low concentration e.g., soap solution in water at particular temperature and at particular concentration
  • Cleansing action of soap and detergents is due to the emulsification and micelle formation

Properties of Colloidal Solutions

  1. It is a heterogeneous permanent system and can not be filtered by ordinary filter papers. The dispersed particles can not be seen through naked eye but can be distinctly seen through ultra microscope.
  2. When light passes through a sol, its path becomes visible due to scattering of light by colloidal particles. It is called Tyndall effect
  3. The continuous zig–zag motion of colloidal particles is called Brownian Movement (first observed by British botanist, Robert Brown). This is independent of the nature of colloid but depends on the size of the particles and viscosity of the solution. Smaller the size, lesser is the viscosity and faster is the motion. Brownian movement is a stirring effect, so it is responsible for the stability of soil. Suspension and true solution do not show Brownian movement.
  4. Colloidal particles always carry an electric charge. e.g., Haemoglobin (blood) is positively charged sol while sols of starch, gelatin, charcoal are negatively charged sols.
  5. Colloidal solutions impart color due to scattering of light. The color of the colloidal solution depends on the wavelength of light scattered by dispersed particles.
  6. The process of precipitation of colloid on adding a small quantity of electrolyte is called coagulation. During this process, the particles of electrolyte carrying a charge opposite to that present on the colloidal particles, neutralize them, so they get precipitated. Alum or ferric chloride is applied on minor acts, they stop bleeding by coagulating the blood.

Emulsions and Suspension

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These are liquid-liquid colloidal systems. If a mixture of two immiscible or partially miscible liquids is shaken, an emulsion is obtained. These are generally stabilized by adding certain substances like protein, gum, soap, alcohol, etc called the emulsifying agents or emulsifiers.

Types of Emulsions

Emulsions are of two types; oil dispersed in water (o/w type) and water dispersed in oil (w/o type). Milk and vanishing cream are the examples of o/w type emulsions. In the milk, liquid fat is dispersed in water. Butter and cream are the examples of w/o type emulsions.

Properties of Emulsions

  1. It is a dispersion of finely divided droplets into another liquid.
  2. Emulsions also show Brownian movement and Tyndall effect.
  3. These can be broken down into constituent liquids by heating, freezing centrifuging, etc.

Suspension

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended through the bulk of the medium. Chalk water, the pulluted water of the river, smoke in atmospheric air, muddy water, etc are the examples of suspension.

Properties of a Suspension

  1. It is heterogeneous mixture
  2. The particles of a suspension can be seen by naked eye. Their size is of the order of 10-5 cm or more.
  3. The particles of a suspension scatter a beam of light passing through it and make its path visible.
  4. The solute particles settle down when a suspension is left undisturbed, i.e., suspension is unstable. They can be separated from the mixture by the process of filteration. When the particles settle down, the suspension breaks and it does not scatter light anymore.

Solutions

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Solutions
In our daily life, we rarely come across pure substances. Most of these are mixtures containing two or more substances. These mixtures are also called solutions. Depending upon their composition (particles size of the component), these are divided into true, solutions, suspension and colloids. Their utility or importance in life depends on their composition.

Solution or True Solution

A solution is a homogenous mixture of two or more substances in which at a constant temperature the relative amounts of components can change up to a certain definite limit. Lemonade, soda water, etc are the examples of solutions in our daily life.

Components of a Solution

A binary solution (a mixture of two substances) has two components; solvent and solute
  1. Solvent. The component of the solution that is present in the largest quantity, is known as the solvent. The solvent which has greater dielectric constant is a better solvent. The dielectric constant of water is large (80), so it is called universal solvent. Solvents are used in making perfumes, drugs, in the processing of various foodstuff and in beverages. These are also used in dry cleaning.
  2. Solute. One or more components present in the solution other than the solvent, are called solutes. In general, in a binary solution amount of solute is smaller than solvent.
e.g., a solution of iodine in alcohol (solvent) known as ‘tincture of iodine’, has iodine (solid) as a solute. Aerated drinks like soda water contain CO2 as solute and water as a solvent.

Properties of a Solution

  1. A solution is a homogenous mixture (a mixture of uniform composition).
  2. The particles of a solution are smaller than 1 nm (10-9 m) in diameter. So, they can not be seen by naked eye.
  3. Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution. In other words, they do not exhibit Tyndal effect.
  4. The components of a solution (i.e., solute and solvent) diffuse into each other in such a way that they can not be distinguished.
  5. The solute particles cannot be separated from the mixture by the process of filtration because the size of solute particles is very tiny. The solute particles do not settle down when left undisturbed, i.e., a true solution is stable, permanent and transparent.

Types of Solutions

  1. Depending upon the amount of solute in a given solvent, the solution can be classified into following types.
    1. Unsaturated Solution A solution in which more solute can be dissolved without increasing temperature is called unsaturated solution.
    2. Saturated solution A solution in which no solute can be dissolved further at a given temperature, is called saturated solution.
    3. Supersaturated Solution When a saturated solution is heated, its capacity to hold more solute increases and it is called supersaturated solution. The supersaturated solution contains an excess amount of dissolved solute in it which is beyond the capacity of the solution at a given temperature. If a small crystal of solute is added to it, the excess solute immediately crystallizes out.
  2. On the basis of states of solute and solvent, the solution may be of the following types
Types of Solution Solute Solvent Common Examples
Gaseous Solutions Gas Gas Mixture of gases, air
Liquid Gas Chloroform mixed with N gas, solution of gas in water
Solid Gas Campohor in nitrogen gas, iodine in air
Liquid Solution Gas Liquid Oxygen dissolved in water, CO2 dissolved in water
Liquid Liquid Ethanol dissolved in water, bromine dissolved in carbon disulfide, H2SO4 in water
Solid Liquid Glucose (sugar) dissolved in water, I2 in CCl4, lead in mercury
Solid Solutions Gas Solid Solution of hydrogen in palladium
Liquid Solid Amalgam of mercury with sodium
Solid Solid Copper dissolved in gold

Aqueous and Non-aqueous Solutions

When the solute is dissolved in water, it is known as an aqueous solution, e.g., ethanol in water. When the solute is dissolved in a solvent other than water, it is known as a non-aqueous solution. e.g., iodine in alcohol (tincture of iodine)

Acidic and Basic Solutions

Acidic solutions have more H+ ions than that of OH- ions while basic solutions have more OH- ions than that of H+ ions.

Neutral Solutions

They have equal concentration of H+ ions (hydrogen ions) and OH- ions (hydroxyl ions) Concentration of Solution It is defined as the amount of solute present in a given amount (mass or volume) of solution or solvent. Solutions with relatively low concentration are called dilute solutions, while those with relatively high concentration are called concentrated solution. Various ways of Expressing the Concentration of a Solution
  1. Mass percentage of a component = Mass of the component/total mass of solution x100
  2. Volume percentage of a component = Volume of Component/Total volume of Solution x 100
  3. Mole fraction of a Component = Number of moles of the component/ Total number of moles of all the components
  4. Pars per million (ppm) = Number of parts of the component/ Total number of parts of all components of solution x 10^6
  5. Molarity (M) = Moles of solute/Volume of solutions in liter
  6. Molality (m) = Mole of solute/ Mass of solvent in kg

Solubility

The maximum amount of a solute that can be dissolved in a given amount of solvent (generally 100g) at a given temperature and pressure, is known as its solubility at that temperature. If at a given temperature w g of solute is dissolved in W g of solvent (water) then.

Solubility of solute in solvent = wx100/W

Factors affecting solubility of a Solute in a Solvent

  1. Nature of Solute and Solvent (like dissolves like) Polar solute like sodium chloride dissolves in a polar solvent like water. Similarly, non-polar solute like cholesterol, bromine, etc. dissolves in a non-polar solvent like benzene, carbon tetrachloride (CCl4), etc.
  2. Effect of Temperature Usually solubility of a solute increases with increase in temperature of solution i.e., their dissolution process is endothermic (proceeds with the substances such as calcium nitrate, calcium oxide, sodium sulphate, calcium hydroxide and calcium citrate decreases with increase in temperature i.e., their dissolution process is exothermic (involves evolution of energy or heat). The solubility of a gas in a liquid decrease with increase in temperature.
  3. Effect of pressure Pressure has no effect on the solubility of solids in a liquid. But solubility of gases in liquids increases with increase in pressure.
  4. Size of Substance Solubility decreases as the molecular mass of substance increases.

What is Biodiversity? Why is biodiversity important?

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What is Biodiversity?

Biodiversity is termed as the variability among living organisms from all sources, including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are the part, and this includes diversity within species, between species and that of ecosystems. It also refers to the interrelatedness of genes, species, and ecosystem and in turn, interaction with the environment. Biodiversity found on Earth today consists of many million of distinct biological species, the product of four billion years of evolution. The Word “Biodiversity” is thought to have first been coined as a contraction of term “biological diversity” in 1985 and then popularized by a number of authors. Conservation and sustainable use of biodiversity is fundamental to ecologically sustainable development. Biodiversity is part of our daily lives and livelihood, and constitutes resources upon which families, communities, nations and future generations depend.

Why is Biodiversity Important?

The diverse biological entities, along with natural resources from air, land, and water, provide the basis for life and also the sustainability and development of societies. Holistically, biodiversity facilitates the process of renewal of biomass, soil, water and air, free recycling and purification in nature, and also ensures co-existence and cultural balance of hosts of living organisms together with their friends and foes, such as scavengers, predators, pests, and pathogens. Biodiversity is also important to maintain the ‘web of life’. The building blocks of plants, animals, and human are identical and are made of the four elements – carbon, oxygen, and hydrogen. There are many other services provided by Biodiversity listed below
  • Protection of water resources
  • Soil formation and protection
  • Nutrient storage and recycling
  • Pollution breakdown and absorption
  • Biodiversity provides the wood products, ornamental plants, medical resources, Future resources.
  • They provide diversity in genes which helps to develop better breed of animals, high yielding varieties of crops and other benefits
  • Higher biodiversity in the tropical region is helping tourism industries to grow more
  • Biodiversity having some cultural values as well
So, biodiversities having infinite benefits for humans, it’s our responsibility to conserve biodiversity, that our future generation gets the same benefits as we have. We have to give same earth to our future generation as we get from our ancestors.

Effects of Chemical Reactions in Our Daily Life

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Effects of Chemical Reactions in Our Daily Life

Fermentation of fruits, digestion of food inside the stomach of the human body, respiration, burning of fuel, corrosion and rancidity are some examples of reaction in our daily life.

Corrosion

It is the process of oxidative deterioration of a metal surface by the action of substances present in the environment to form unwanted corrosion products. In other words, it is the process of formation of oxide or other salts on the surface of a metal when it is exposed to the atmosphere. In this process, the metal surface which is in direct contact with air and moisture gets oxidized and forms a mixture of oxide and hydroxides. The process is continued until the metal is not destroyed completely. e.g., conversion of iron into rust [Fe2O3.xH2O], tarnishing of silver (due to the formation of Ag2S), development of green coating of Cu(OH)2.CuCO3 (basic copper carbonate) on copper and bronze. It is basically an electrochemical process. Corrosion of iron is called rusting. It is accelerated by the presence of impurities, H+ electrolytes such as NaCl, gases such as CO2, CO2, NO, NO2 etc. It is prevented by the following methods
  • By electroplating
  • By surface coating (coating of surface with oil, grease, paint, and varnish)
  • By alloying
  • By galvanization of iron (process of deposition of a thin layer of zinc over iron surface)
  • By anodizing (generally done by using aluminum metal as anode because it becomes passive due to the formation of its oxide layer over its surface)
  • Food cans are coated with tin, not with Zn because Zn is more reactive than iron and hence readily converts into toxic substances.
  • Formation of a layer of aluminium oxide over aluminium surface protects the metal from further corrosion.
  • Platinum gold, silver are the metals that do not undergo corrosion and hence, are called noble metals.

Fermentation

Louis Pasteur discovered fermentation in 1857. In this process, complex organic compounds are decomposed by micro-organisms such as yeast bacteria into simpler organic compounds. It is an exothermic process, CO2 gas (H2 and CH4 in small amount) is evolved in this process and the apperance of gas seems like boiling the fermentative solution of the substance. Examples of fermentation are
  1. Conversion of milk into curd through lactobacilli
  2. Preparation of wine and vinegar from sugarcane juice or preparation of ethyl alcohol from glucose by using yeast.
  3. In the baking industry for making bread, pastries, and cakes.

Rancidity

When oils and fats or foods containing oils and fats are exposed to air or oxygen, they get oxidized due to which the food becomes stale and its color and smell changes. This process is called rancidity. It is prevented by the following methods
  • Antioxidants like BHT (Butylated hydroxytoluene), N2 (dinitrogen) are added to foods containing fats and oils.
  • The food is kept in airtight containers in the refrigerator or deep freezers

Chemical Reaction

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Chemical Reaction

The process in which substances (reactants) react to form new compounds (products), is known as a chemical reaction. This process involves the breaking of old bonds and formation of new bonds. If bond energies of reactants are greater than the bond energies of products, the reaction occurs with the evolution of energy in the form of heat. However, in an opposite condition, absorption of energy takes place.

Properties of a Chemical Reaction

A Chemical reaction can be observed with the help of any of the following observations.
  1. Change in state
  2. Change in color
  3. Evolution of a gas
  4. Change in temperature
  5. Formation of Precipitate

Chemical Equation

The short representation of a chemical reaction with the help of symbols of elements or formula of compounds is called chemical equation.
  1. The substances or compounds which take part in a reaction are called reactants. These are written on the left hand side (LHS) with a plus sign (+) in between them.
  2. The substances or compounds formed in the course of reaction are called products. These are written on the right hand side (RHS) with a plush sign (+) in between them.
  3. The arrow head (→) point towards the products which shows the direction of reaction. e.g., zinc reacts with sulphuric acid to form zinc sulphate and hydrogen gas.

Zinc + Sulphuric Acid → Zinc Sulphate + Hydrogen

Rules of Writing a balanced Chemical Reaction Equation

i) The number of atoms of reactants should be equal to the number of atoms of products. (According to the law of conservation of mass).

Fe + H2O → Fe3O4 + H2

As per rule, the above equation is incorrect and can be correctly written as

3Fe + 4H2O → Fe3O4 + 4H2

ii) The physical States of reactants and products should be mentioned along with their chemical formula in parenthesis. The above equation can be written in accordance with to rule ii)

3Fe (s) + 4H2O (g) → Fe3O4 (s) + 4H2 (g)

Types of Chemical Reactions

Different types of chemical reactions with examples of chemical reactions Combination Reaction A reaction in which a single new product is formed from two or more reactants, is called a combination reaction. Such reactions may occur in between the element or compounds. For example, formation of slaked lime by the reaction of calcium oxide with water

CaO (s) + H2O (l)→ Ca(OH)2 (aq)

Other examples of combination reactions are
  1. Burning of coal C (s) + O2 (g) → CO2 (g)
  2. Formation fo water from H2(g) and O2 (g)

2H2 (g) + O2 (g) → 2H2O (l)

Decomposition Reaction

A chemical reaction in which a single reactant (compound) breaks down to give simpler products, is called a decomposition reaction. The decomposition reactions require energy in the form of heat, light or electricity. Therefore, decomposition reactions are of three types

Thermal Decomposition

When a decomposition is carried out by heating, it is called thermal decomposition. For example, decomposition of calcium carbonate to calcium oxide and carbon dioxide upon heating

CaCO3 (s) → CaO (s) + CO2 (g)

Another example of thermal decomposition is the decomposition of lead nitrate to lead oxide, nitrogen dioxide (brown fumes) and oxygen.

2Pb(NO3)2 → 2PbO (s) + 4 NO2 (g) + O2

Photolysis

When a decomposition reaction is brought about by sunlight, it is called photolysis For example,    2AgCl (s) → 2 Ag (s) + Cl2 (g)
  • The above reaction is used in black and white photography since silver chloride or silver bromide turns grey in sunlight.
  • When metal salts are heated, their ions emit various colors of light
  • A decomposition reaction is the reverse of the combination reaction.
  • Decomposition reaction of calcium carbonate is used in various industries. e.g., in the manufacturing of cement

Electrolysis

When a decomposition reaction is brought about by electricity, it is called electrolysis

2 H2O (l) → 2H2 + O2

Displacement Reaction

A reaction in which more reactive element displaces less reactive element from its compound present in the dissolved state, is called a displacement reaction. For example, when an iron nail is suspended in an aqueous solution of copper sulfate for 20 minutes, it becomes brownish and the blue color of the solution is slightly faded. This indicates that iron has displaced copper from copper sulfate solution.

Fe (s) + CUSO4 (aq) → FeSO4 (aq) + Cu (s)

Zinc and lead are more reactive elements than copper, so they displace Cu from the aqueous solutions of its compounds.

Double Displacement Reaction

A chemical reaction in which there is an exchange of ions between the reactants to give new substances is called a double displacement reaction.

Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s)↓ + 2 NaCl (aq)

In the above reaction, precipitates are formed. So, this reaction is also known as precipitation reaction.

Neutralisation Reaction

Acids and bases neutralize each other to form corresponding salts and water. This reaction is called neutralization reaction. If acid and base both are strong, 57.1 kJ heat is in released during the process.

HCl + NaOH → NaCl + H2O

Isomerisation or Rearrangement Reaction

A chemical reaction in which the atoms of the molecule of a compound undergo rearrangement is called an isomerization or rearrangement reaction. It is generally seen in case of organic compounds. For Example, isomerization of ammonium cyanate into urea.

NH4CNO → 2NH3

Reversible and Irreversible Reaction

A chemical reaction which proceeds in both the directions is called a reversible reaction. For example, the formation of ammonia from nitrogen and hydrogen by Haber’s process.

N2 + H2 ↔ 2NH3

A chemical reaction which proceeds only in one direction is called irreversible reaction.

2NaOH + H2SO4

Hydrolysis Reaction

It is the reaction between salts of a weak acid or a weak base with water. Due to high dielectric constant, water has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some iconic compounds are hydrolyzed in water.

CH3COONa + H2O → CH3COOH + NaOH

Photochemical Reaction

These chemical reactions take place in the presence of sunlight.

6CO2 + 12H2O → C6H12O6 + 6H2O +6O2

The rate of a photochemical reaction is affected by the intensity of light. The photosensitizer is a substance which brings about a reaction without undergoing any chemical change itself. In the process of photosynthesis, chlorophyll acts as a photosensitizer.

Exothermic and Endothermic Reactions

Reactions occurring with the evolution of energy are called exothermic reactions, e.g., respiration, decomposition, burning of natural gas, etc whereas reactions for the occurrence of which energy is absorbed, are called endothermic reactions. e.g., digestion

A+B → C + ΔE (exothermic)

A+B → C – ΔE (endothermic)

Oxidation and Reduction

Oxidation It is defined as a chemical reaction in which a substance gains oxygen or any other electronegative element or loses hydrogen or electrons and shows an increase in oxidation number.

2Cu + O2 → 2CuO (Copper is oxidized to CuO)

CuO + H2 → Cu + H2O (Hydrogen is oxidized to H2O)

Reduction It is defined as a chemical reaction in which a substance gains hydrogen or electropositive element or electrons or loses oxygen or electronegative element and shows decrease in oxidation number Oxidation Reduction Reaction Oxidising Agent and Reducing Agent The acceptor of electrons is an oxidizing agent (oxidant). The donor of electrons is reducing agent (reductant). In short, a substance which is oxidized or oxidation number of which is increased acts as a reducing agent while a substance which is reduced or oxidation number of which is decreased acts as an oxidizing agent.

Chemical Bond, Ionic Bond, Covalent Bond, Hydrogen Bond

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Chemical Bond, Ionic Bond, Covalent Bond, Hydrogen Bond
The attractive force which holds various constituents (atoms, ions, etc) together in different chemical species to maintain a mutual atomic and a definite but specific geometrical shape, is called a chemical bond. During the formation of the chemical bond, energy is released, i.e., it is an exothermic process and occurs with the decrease in free energy of the system.

Valency

The combining power (or capacity of mutual combination) of an element is called its valency. Valency of noble gases, previously called inert gases [i.e., Helium (He), Neon (Ne), Argon (ar), Krypton (Kr), Xenon (Xe) and Radon (Rn( is zero. They generally do not take part in chemical bonding because they have completely filled or very stable outermost shell, ie., 8 electrons (octet) in their outermost shel (except He atom, which has 2 electrons in its outermost shell). In other words, noble gases have no tendency to lose, gain or share electrons and hence, they exist in free or monoatomic form. Valency of an element is equal to the number of electrons lost or gained during the formation of a compound. It generally depends upon the number of valence electrons. If an atom has 1, 2, 3, or 4 valence electrons, then its valency = number of valence electrons If an atom has 5, 6, 7 or 8 valence electrons, then its valency = 8- number of valence electrons. e.g, in Na (2, 8, 1) there is one valence electron, so it loses the valence electron to achieve octet in valence shell and forms Na+(2, 8).

Electronic theory of Chemical Bonding (Octet Rule)

Kossel and Lewis in 1916 developed a theory of chemical combination between atoms known as electronic theory of chemical bonding which was based on inertness of noble gases. According to this theory, “atoms combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet or stable configuration just like noble gases in their valence shells”. This is known as octet rule.

Types of Chemical Bonds

Bonding is broadly divided into two classes i) Chemical Bonding (Ionic, covalent and coordinate bond0 ii) Physical Bonding (hydrogen and Van der Waals bonding)

Ionic Bond or Electrovalent Bond

The bond formed, as a result of the electrostatic attraction between the positive and negative ions, is termed as electrovalent bond. The transfer of electrons takes place in such a way so that each combining element achieves complete octet (or duplet in case of H and He). The compound formed by the transfer of electrons from a metal to a non-metal atom is known as ionic or electrovalent compound. e.g., the formation of sodium chloride from sodium and chlorine.
  1. The sodium atom has one electron in its valence shell so it loses its one electron to attain octet in its outer shell and forms Na+ ion.
  2. Similarly, Cl atom has 7 electrons in its valence shell so it gains one electron from sodium to attain octet in its outer shell and forms Cl- ion.
  3. The Na+ and Cl- ions bind together by electrostatic forces of attraction
Other examples of ionic compounds are MgCl2, Cao, NH4Cl, NaOH, etc.
  • The electrovalency is equal to the number of unit charge (s) present on the ion(s). Thus, in the above example electrovalency of Na and Cl is 1.

Characteristics of Ionic or Electrovalent Compounds

  1. Ionic compound are generally crystalline solids, i.e., have definite shape and are somewhat hard and brittle because of the long distance orders of their constituent particles.
  2. They have high melting and boiling points because of strong electrostatic forces of attraction.
  3. These are non-volatile and have high density.
  4. These are soluble in polar solvent like water because of high dielectric constant of water and insoluble in non-polar solvents like benzene, acetone, kerosene, petrol, etc. because these are covalent in nature.
  5. They can conduct electricity in their aqueous state and in the molten state due to the presence of mobile ions i.e, in these states they behave like an electrolyte. However, these are unable to conduct electricity in solid state because ions are held together by strong electrostatic forces of attraction i.e., ions are not free.
  6. Ionic compounds undergo very fast reactions in solution.

Covalent Bond

The bond formed due to sharing of electrons between two similar atoms or different atoms having almost similar electronegativity, is called the covalent bond. The sharing occurs in such a way that the formed molecules achieve a permanent electronic structure just like noble gases. The compounds that are the result of sharing of electrons are called covalent compounds. For example, the formation of Cl2, molecule. Each Cl atom has one electron short for the octet configuration. Both Cl atom attains the outer shell octet by sharing of a pair of electrons. Each Cl atom contributes one electron to the shared pair.

Single, Double and Triple Covalent Bonds

When two atoms share one electron pair, the covalent bond between them is called a single covalent bond. When Two atoms share two pairs of electrons, the covalent bond between them is called a double covalent bond
Single, Double and Triple Covalent Bonds
Single, Double and Triple Covalent Bonds
Source: quora
When the combining atoms share three pairs of electrons, the covalent bond between them is called a triple covalent bond

What is Covalency?

In a covalent compound, covalency of an atom is the number of electrons which are supplied by that atom for sharing. e.g., in H2 molecules, covalency of each hydrogen is 1, in N2 molecules, covalency of each nitrogen atom is 3 and in CH4 covalency of C is 4 and of H is 1.

Characteristics of Covalent Compounds

  1. In general, covalent compounds are gases or liquids. Some of the compounds are soft solids with a few exceptions like a diamond, silica (SiO2), carborundum (SiC), etc.
  2. They have relatively low melting and boiling points because of weaker intermolecular forces of attraction.
  3. These are insoluble in water but soluble in non-polar solvents like benzene, carbon tetrachloride, etc. However some of them like sugar, alcohol are miscible with water due to hydron bonding.
  4. Covalent compounds are bad conductors of electricity in molten as well as in solid state. However, graphite can conduct electricity as it possesses free electrons.
  5. The covalent bond is rigid and directional, therefore covalent molecules is rigid and directional, therefore, covalent molecules have specific shapes.
  6. The reactions of the covalent compound are slow and complex.

Types of Covalent Bond

On the basis of polarity, the covalent bond can be of two types.
  1. Non-polar Covalent Bond: When a covalent bond is formed between two similar atoms, e.g., H2, O2, Cl2, N2, or F2, the shared pair of electrons is equally attracted by two atoms and situated exactly between two identical nuclei. The bond so formed is called the non-polar covalent bond.
  2. Polar Covalent Bond: In case of a heteronuclear molecule like HF, the shared pair of electrons between two atoms gets displaced more towards fluorine, since electronegativity of F is greater than H. The resultant bond is polar covalent bond.
Polar, Non polar covalent Bond

Coordinate Bond or Coordinate Covalent Bond or Dative Bond

It is a special type of covalent bond in which both the shared electrons are donated by only one of the atoms. The atom supplying the elctrons pair is called donar atom and that receiving the electrons pair is called acceptor atom. This bond is represented by placing an arrow pointing towards the acceptor. e.g., Formation of hydronium ion (H3O-).

Characteristics of Compounds having coordinate BOnds

  1. These compounds exist in all the three states i.e., solid, liquid and gas under ordinary conditions.
  2. The melting and boiling points of these compounds are higher than that of covalent compounds are higher than that of ionic compounds.
  3. Like covalent compounds, these are poor conductors of electricity.
  4. They are sparingly soluble in water but readily soluble in organic solvents.
  5. Like covalent compounds the reactions of these compounds are slow

Hydrogen Bond

The attractive forces between a hydrogen atom (which is already covalently to a strongly electronegative atom i.e., N, O or F) and an electronegative atom is known as a hydrogen bond. Thus, it is formed between the hydrogen atom of one molecule and an electronegative atom of another molecule.
Hydrogen Bond
Hydrogen Bond
Source: wiki
These bonds are present in inorganic molecules like water as well as inorganic molecules like DNA.

Van der Waals’ Forces

These are the sum of attractive or repulsive interaction between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules. These are relatively weak as compared to normal chemical bond and can be of the following three types.
  1. Forces between two permanent dipoles (Keesom forces)
  2. Forces between a permanent dipole and a corresponding induced dipole (Debye forces).
  3. Forces between two instantaneously induced dipoles (London or dispersion of forces). London forces dominate the interaction of non-polar molecules.
 
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