Chemical Kinetics — Practice

Question 1

A first-order reaction has k = 1.0e-3 s^-1. Find t_half and the time for 75% completion.

Accepted Answer

t_half = 0.693/1e-3 = 693 s. 75% completion needs two half-lives = 1386 s.

Question 2

The half-life of a reaction is inversely proportional to the initial concentration. What is the order?

Accepted Answer

Second order, since t_half is proportional to 1/[A]0 for a second-order reaction.

Question 3

A zero-order reaction has k = 0.04 mol/L/s with initial concentration 1 M. Find t_half and time for 90% completion.

Accepted Answer

t_half = [A]0/(2k) = 1/0.08 = 12.5 s. For 90%, [A] = 0.1 M, t = ([A]0 - [A])/k = 0.9/0.04 = 22.5 s.

Question 4

k = 2.0e-3 s^-1 at 300 K and 8.0e-3 s^-1 at 400 K. Find Ea.

Accepted Answer

log(4) = (Ea/2.303 x 8.314)(1/300 - 1/400) = (Ea/19.147)(8.33e-4). 0.6021 = Ea x 4.35e-5, so Ea ~ 13.8 kJ/mol.

Question 5

Doubling [A] doubles the rate; doubling [B] quadruples it. Write the rate law and overall order.

Accepted Answer

Rate = k[A][B]^2; overall order = 1 + 2 = 3.

Practice — Chemical Kinetics