Online Test — Chemical Kinetics
20 Questions • 15 min • Chapter MCQ
15:00
Question 1 of 20
The rate of a reaction is generally expressed in units of:
$\text{mol L}^{-1}$
$\text{mol L}^{-1}\text{s}^{-1}$
$\text{s}^{-1}$
$\text{L mol}^{-1}$
Explanation: Rate is change of concentration per unit time, so its unit is $\text{mol L}^{-1}\text{s}^{-1}$.
Question 2 of 20
The order of a reaction is:
always equal to molecularity
the sum of the powers of concentration terms in the rate law
always a whole number
equal to the number of reactants
Explanation: Order is the experimentally found sum of the concentration exponents in the rate law.
Question 3 of 20
The unit $\text{s}^{-1}$ for the rate constant corresponds to a reaction of order:
zero
first
second
third
Explanation: First-order $k$ has units $(\text{mol L}^{-1})^{1-1}\text{s}^{-1} = \text{s}^{-1}$.
Question 4 of 20
Molecularity of a reaction can never be:
one
two
three
zero
Explanation: Molecularity counts colliding species in an elementary step and is a positive integer; it cannot be zero.
Question 5 of 20
For a zero-order reaction the integrated rate law is:
$[R] = [R]_0 - kt$
$\ln[R] = \ln[R]_0 - kt$
$\frac{1}{[R]} = \frac{1}{[R]_0} + kt$
$[R] = [R]_0 e^{-kt}$
Explanation: A zero-order reaction obeys $[R] = [R]_0 - kt$.
Question 6 of 20
For a first-order reaction $k$ equals:
$\frac{[R]_0 - [R]}{t}$
$\frac{2.303}{t}\log\frac{[R]_0}{[R]}$
$\frac{0.693}{[R]_0}$
$\frac{[R]_0}{2t}$
Explanation: The integrated first-order law gives $k = \frac{2.303}{t}\log\frac{[R]_0}{[R]}$.
Question 7 of 20
The half-life of a first-order reaction is:
$\frac{[R]_0}{2k}$
$\frac{0.693}{k}$
$\frac{1}{k[R]_0}$
$0.693\,k$
Explanation: For first order $t_{1/2} = \frac{0.693}{k}$, independent of initial concentration.
Question 8 of 20
For a first-order reaction $t_{1/2}$ is:
proportional to $[R]_0$
independent of $[R]_0$
inversely proportional to $[R]_0$
proportional to $[R]_0^2$
Explanation: No concentration term appears in $t_{1/2} = \frac{0.693}{k}$.
Question 9 of 20
A plot of $\log[R]$ against $t$ being a straight line indicates a reaction of order:
zero
first
second
fractional
Explanation: First-order kinetics gives $\log[R] = \log[R]_0 - \frac{k}{2.303}t$, a straight line.
Question 10 of 20
The acid hydrolysis of an ester in dilute aqueous solution is:
true first order
true second order behaving as pseudo-first-order
zero order
third order
Explanation: It is second order, but with water in excess it behaves as pseudo-first-order.
Question 11 of 20
In the Arrhenius equation $k = Ae^{-E_a/RT}$, $A$ is the:
activation energy
frequency (pre-exponential) factor
rate of reaction
temperature coefficient
Explanation: $A$ is the frequency or pre-exponential factor.
Question 12 of 20
The slope of a $\log k$ versus $\frac{1}{T}$ plot is:
$-\frac{E_a}{R}$
$-\frac{E_a}{2.303R}$
$\frac{E_a}{2.303R}$
$\log A$
Explanation: From $\log k = \log A - \frac{E_a}{2.303R}\cdot\frac{1}{T}$ the slope is $-\frac{E_a}{2.303R}$.
Question 13 of 20
A catalyst increases the rate of a reaction by:
raising the temperature
increasing $\Delta H$
lowering the activation energy
increasing the concentration of reactants
Explanation: A catalyst provides a path of lower $E_a$, increasing $k$.
Question 14 of 20
A catalyst does NOT change the:
rate of reaction
activation energy
enthalpy change $\Delta H$
reaction pathway
Explanation: $\Delta H$, the equilibrium position and reactant/product energies are unchanged by a catalyst.
Question 15 of 20
For $\text{Rate} = k[A]^2[B]$, the overall order is:
1
2
3
4
Explanation: Overall order $= 2 + 1 = 3$.
Question 16 of 20
The temperature coefficient of a reaction is usually about:
$0.5$ to $1$
$2$ to $3$
$10$ to $20$
$100$
Explanation: The ratio $\frac{k_{T+10}}{k_T}$ is generally between $2$ and $3$.
Question 17 of 20
The rate constant of a second-order reaction has units:
$\text{s}^{-1}$
$\text{mol L}^{-1}\text{s}^{-1}$
$\text{L mol}^{-1}\text{s}^{-1}$
dimensionless
Explanation: For $n = 2$, $k$ has units $(\text{mol L}^{-1})^{-1}\text{s}^{-1} = \text{L mol}^{-1}\text{s}^{-1}$.
Question 18 of 20
After two half-lives of a first-order reaction, the fraction of reactant left is:
$\frac{1}{2}$
$\frac{1}{4}$
$\frac{1}{8}$
$\frac{1}{16}$
Explanation: Each half-life halves the amount, so after two it is $\left(\frac{1}{2}\right)^2 = \frac{1}{4}$.
Question 19 of 20
For an elementary reaction, the order is equal to:
zero
the molecularity
twice the molecularity
the number of products
Explanation: In a single elementary step, order equals molecularity.
Question 20 of 20
The decomposition of $NH_3$ on a hot platinum surface is:
first order
second order
zero order
third order
Explanation: The surface is saturated, so the rate is independent of $[NH_3]$ — a zero-order reaction.