Question

The following questions are case based questions. Read the passage carefully and answer the questions that follow.

The rate of a chemical reaction is expressed either in terms of decrease in the concentration of reactants or increase in the concentration of a product per unit time. The rate of the reaction depends upon the nature of reactants, concentration of reactants, temperature, presence of catalyst, surface area of the reactants and presence of light. The rate of reactions is directly related to the concentration of reactant. Rate law states that the rate of reaction depends upon the concentration terms on which the rate of reaction actually depends, as observed experimentally. The sum of powers of the concentration of the reactants in the Rate law expression is called the order of reaction, while the number of reacting species taking part in an elementary reaction which must collide simultaneously, in order to bring about a chemical reaction is called molecularity of the reaction.

1. Answer the following questions:  [2]
   1. What is a rate determining step?
   2. Define complex reaction.
2. What is the effect of temperature on the rate constant of a reaction?  [1]
                                                       OR
   Why is molecularity applicable only for elementary reactions whereas order is applicable for elementary as well as complex reactions?  [1]
3. The conversion of molecule X to Y follows second order kinetics. If concentration of X is increased threе times, how will it affect the rate of formation of Y?  [1]

Answer 1

(a)

1. The slowest stage in a chemical reaction that controls how quickly the reaction as a whole proceeds is known as the rate determining step.
2. The order is not equal to the molecularity of the reaction when the entire reaction takes place in several steps rather than just one. complicated molecules or intermediates may be formed as part of a complicated reaction's mechanism, which consists of multiple elementary steps.

(b) The rate of the reaction and the rate constant both rise with temperature.

                                                         OR

A few simple reactions happen before a complex reaction happens. It’s possible for each elementary process to have a different number of molecules. This means that the molecularity of each step could be different. So, it is not possible to figure out the molecularity of the complex process as a whole. There is no relationship between the number of steps in a complex reaction and its order. The order is set by the step that happens the fastest.

(c) For the process X → Y

For the second order, 

Rate = k[X]², and X = a mole/Liter

If you triple the amount of X in a solution, you get [X] = 3a mol/L

Rate = k (3a)² = 9 ka²