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Chemical Kinetics

Are diamonds forever? Are diamonds forever? Diamond is precious among the nine gemstones. We often hear the expression ‘Diamonds Are Forever’ (also a spy film in James Bond series). Is this true? No. Diamond is one of the naturally occurring allotropes of carbon, the other common allotrope being graphite. According to thermodynamics graphite is the most stable allotrope of carbon and diamond can be converted into graphite spontaneously after billions of years! But the reactivity is very slow due to high activation energy and hence diamond is often said to be forever. Thermodynamics explains the stability of diamond but fails to explain its reactivity.

Learning Objectives

After completing the topic, the student will be able to:

  • Define the average and instantaneous rate of a reaction and classify the types of reactions based on their rate.
  • Express the rate of reaction in terms of change concentration of either reactant or product and relate the change in concentration of products and reactants using stoichiometric coefficients.
  • Give the rate law/ rate expression and appreciate the significance of rate constant.
  • Distinguish between elementary reaction and complex reaction and define order and molecularity of the reaction.
  • Discuss the factors effecting rate of reaction.
  • Derive integrated rate equations and explain the significance of half‐life (t1/2).
  • Solve problems based on integrated rate equations and determine the order of reaction using different methods.
  • Discuss various theories that describe the reaction rates and mechanisms and predict the effect of temperature on rate of reaction.
  • Describe potential energy diagrams for exothermic, endothermic, catalyzed and non‐catalyzed reactions.
  • List various types of catalysts and give the details of catalytic action.
Total energy change during a chemical reaction Total energy change during a chemical reaction In the formation of water from hydrogen and oxygen gases, the total energy remains constant through out the process or reaction.
Chemical Kinetics

Chemistry by its nature is concerned with a chemical reaction. In any chemical reaction there is breakage of existing bonds and formation of new bonds. Chemical Kinetics deals with at what extent the bonds are breaking or making. In simple words Chemical Kinetics deals with the rate of reaction and the conditions that affect the rate of reaction. The word Kinetics is derived from the Greek word “kinesis ” which means movement.

Before going to the details of the topic, we should ask ourselves why there is a need to study chemical kinetics? Or what is the necessity of chemical kinetics? According to thermodynamics, if a reaction has to be taken place there must be a decrease in free energy (Gibbs free energy, ΔG). If ΔG is negative (−ve) the reaction will take place without any external force (spontaneous). If ΔG is positive (+ve) it won't take place without external force (non‐spontaneous).

Thermodynamics and chemical kinetics

Let us consider the formation of water (H2O):

2H2(g) + O2 (g) 2H2O (l)     ΔG = −221 KJ/mole

When we mix H2 gas and O2 gas in a container at room temperature and at atmospheric pressure we observe only a little bit of water or no water is formed.

Consider the decomposition of N2O4:

Since ΔG is +ve for the above reaction, according to thermodynamics it is non spontaneous but it occurs rapidly. By observing the above two reactions, formation of water is spontaneous but it won't occur rapidly where as decomposition of N2O4 is non‐spontaneous but it occurs rapidly.

In summary we can say that thermodynamics can predict the feasibility(either spontaneous or non‐spontaneous) of the reaction but, it can't predict the rate of reaction. This is the major drawback of thermodynamics. But chemical kinetics overcomes this drawback. So, chemical kinetics is a complementary treat of thermodynamics, which deals with the rate of reaction and conditions that affect the rate of reaction.

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