Chemical Kinetics

Defination:

Chemical kinetics is the study and discussion of chemical reactions with respect to reaction rates, effect of various variables, re-arrangement of atoms, formation of intermediates etc. There are many topics to be discussed, and each of these topics is a tool for the study of chemical reactions. By the way, the study of motion is called kinetics, from Greek kinesis, meaning movement.

At the macroscopic level, we are interested in amounts reacted, formed, and the rates of their formation. At the molecular or microscopic level, the following considerations must also be made in the discusion of chemical reaction mechanism.

• Molecules or atoms of reactants must collide with each other in chemical reactions.
• The molecules must have sufficient energy (discussed in terms of activation energy) to initiate the reaction.
• In some cases, the orientation of the molecules during the collision must also be considered.

Reaction Rates

Chemical reaction rates are the rates of change in concentrations or amounts of either reactants or products. For changes in amounts, the units can be one of mol/s, g/s, lb/s, kg/day etc. For changes in concentrations, the units can be one of mol/(L s), g/(L s), %/s etc.

With respect to reaction rates, we may deal with average rates, instantaneous rates, or initial rates depending on the experimental conditions.
Thermodynamics and kinetics are two factors that affect reaction rates. The study of energy gained or released in chemical reactions is called thermodynamics, and such energy data are called thermodynamic data. However, thermodynamic data have no direct correlation with reaction rates, for which the kinetic factor is perhaps more important. For example, at room temperature (a wide range of temperatures), thermodynamic data indicates that diamond shall convert to graphite, but in reality, the conversion rate is so slow that most people think that diamond is forever.

Factors Influence Reaction Rates

Many factors influence rates of chemical reactions, and these are summarized below. Much more extensive discussion will be given in other pages.

1. Nature of Reactants
   Acid-base reactions, formation of salts, and exchange of ions are fast reactions. Reactions in which large molecules are formed or break apart are usually slow. Reactions breaking strong covalent bonds are also slow.
2. Temperature
   Usually, the higher the temperature, the faster the reaction. The temperature effect is discussed in terms of activation energy.
3. Concentration Effect
   The dependences of reaction rates on concentrations are called rate laws. Rate laws are expressions of rates in terms of concentrations of reactants. Keep in mind that rate laws can be in differential forms or integrated forms. They are called differential rate laws and integrated rate laws. The following is a brief summary of topics regarding rate laws.
• rate laws: differential and integrated rate laws.
• Integrated rate laws: First Order Reactions
  Second Order Reactions

Rate laws apply to homogeneous reactions in which all reactants and products are in one phase (solution).

4. Heterogeneous reactions: reactants are present in more than one phase
   For heterogeneous reactions, the rates are affected by surface areas.
5. Catalysts: substances used to facilitate reactions
   By the nature of the term, catalysts play important roles in chemical reactions.

Reaction Mechanisms

The detailed explanation at the molecular level how a reaction proceeds is called reaction mechanism. The explanation is given in some elementary steps. Devising reaction mechanisms requires a broad understanding of properties of reactants and products, and this is a skill for matured chemists. However, first year chemistry students are often given a mechanism, and be asked to derive the rate law from the proposed mechanism. The steady-state approximations are a technique for deriving a rate law from the proposed mechanism.