Quiz | Chemical Kinetics: Mechanisms

1

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

The energy of reaction

The Arrhenius energy

The activation energy

The catalyst energy

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?

A plot of ln k as a function of T is linear

A plot of k as a function of T is linear

A plot of k as a function of 1/T is linear

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

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?

- 15 kJ

+ 15 kJ

+ 40 kJ

+ 95 kJ

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?

k [NO2][F]

k [NO2][F2]

k [NO2]2[F2]

k [NO2]2[F2][F]

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?

k [NOCl][Cl]

k' [NO][Cl]2

k'' [NOCl]2/[NO]

All of the above

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?

It decreases ΔHrxn for the reaction

It provides a new path for the reaction

It lowers the activation energy

It is not consumed in the reaction

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