Question

Which of the following statements is true?

Fast reactions have small rate constants.

Slow reactions have large rate constants.

A rate equation contains concentration terms for all reactants involved in a one-step mechanism.

A rate equation contains concentration terms for all the reactants involved in a multi-step reaction.

Answer 1

Ans:

1. Fast reactions have small rate constants – FALSE
2. Slow reactions have large rate constants – FALSE
3. A rate equation contains concentration terms for all reactants involved in a one-step mechanism –TRUE
4. A rate equation contains concentration terms for all the reactants involved in a multi-step reaction –FALSE

Explanation for 1 and 2;

• For a reaction aA + bB --> Products, rate equation is given by

r = k [A]^x [B]^y

where, r = rate of the reaction,

k= rate constant of the reaction at some temperature T

x, y = powers of the reactant concentration which may or may not be equal to

stoichiometric coefficients.

• From the above equation it can be seen that rate and rate constants are directly proportional to each other. So, a faster reaction will have larger rate constant.

Explanation for 3 and 4;

• An elementary reaction (one-step mechanism) is a reaction where all reactant species react in one step to form the products. So all the reacting species appear in elementary reaction and the rate equation will be derived from this.

• Whereas, a complex reaction (multi-step mechanism) is a multi step reaction. Of all the reactions, the slowest step will determine the rate/rate equation. Since the reaction happens in multi step, all the reactants need not necessarily present in the rate determining step of the reaction.