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.

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