# Quiz | Chemical Kinetics: Mechanisms

1

How do we call the minimum amount of energy needed to start a reaction?

The activation energy Ea is the energy that must be provided to compounds to result in a chemical reaction

2

What is the relationship between the rate constant k and the temperature as predicted by the Arrhenius equation?

Arrhenius equation:  ln K = - Ea/RT + ln A

ln k is proportional to 1/T ⇒ A plot of ln k as a function of 1/T is linear

3

The activation energy in the forward direction of an elementary step is 40 kJ. The activation energy in the reverse direction is 55 kJ. What is ΔHrxn for this step?

ΔHrxn = Ea forward – Ea reverse = 40 – 55 = - 15 kJ

4

Suppose the reaction: 2 NO2 + F2 → 2 NO2F occurs by the following mechanism:
(1) NO2 + F2 → NO2F + F (slow)
(2) NO2 + F → NO2F (fast)
What is the rate law of this reaction?

The step which is much slower than any of the other steps controls the overall reaction rate
⇒ step (1) is the rate-determining step

Elementary reactions have reaction orders equal to the stoichiometric coefficients for each reactant
⇒ rate law of (1) = k[NO2][F2]

5

Suppose the reaction: 2 NOCl $⇌$ 2 NO + Cl2 occurs by the following mechanism:
(1) NOCl $⇌$ NO + Cl (fast, equilibrium)
(2) NOCl + Cl → NO + Cl2 (slow)
What is the rate law of this reaction?

Elementary reactions have reaction orders equal to the stoichiometric coefficients for each reactant
⇒ rate law of (1) = k1 [NOCl];    rate law of (-1) = k-1 [NO][Cl];    rate law of (2) = k [NOCl][Cl]

For reversible reactions, rate of forward reaction = rate of reverse reaction at equilibrium
⇒ k1 [NOCl] = k-1 [NO][Cl]

The step which is much slower than any of the other steps controls the overall reaction rate
⇒ step (2) is the rate-determining step

The rate law of this reaction is: k [NOCl][Cl] = k' [NO][Cl]2 = k'' [NOCl]2/[NO]

6

Which of the following statements concerning a catalyst is incorrect?

A catalyst lowers the activation energy barrier of the reaction by providing a new path for the reaction without being consumed in the reaction