Order of Reaction

Order of Reaction MDCAT MCQs with answers

18 past paper MCQs on Order of Reaction, from the Reaction Kinetics unit of MDCAT Chemistry. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

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1.Consider the hypothetical equation $aA+bB\to cC+dD$. Which of the following represents the correct rate equation? (MDCAT 2024)

  1. Rate $=k[A][B]$
  2. Rate $=k[A]^a[B]^b$
  3. Rate $=k[A]^a$
  4. Rate $=k[B]^b$
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Correct answer: (b) Rate $=k[A]^a[B]^b$

For an elementary step the order in each reactant is its coefficient, so the rate law carries both concentrations raised to those powers.

2.If a reaction rate does not change with concentration, then it is: (MDCAT 2024)

  1. third order
  2. second order
  3. first order
  4. zero order
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Correct answer: (d) zero order

A rate independent of concentration is $\text{rate}=k[A]^0$, which is zero order.

3.A reaction is first order with respect to A and third order with respect to B, the rate equation is: (NUMS MDCAT 2025)

  1. Rate = K[A][B]2
  2. Rate = K[A]2[B]
  3. Rate = K[A][B]3
  4. Rate = K[A]
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Correct answer: (c) Rate = K[A][B]3

The rate law carries each concentration to the power of its own order.

4.For a reaction A → Product, if concentration of A is doubled, product increases 4 times. The order of reaction will be: (NUMS MDCAT 2025)

  1. Zero
  2. 1st
  3. 2nd
  4. 3rd
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Correct answer: (c) 2nd

The rate goes up by $2^2$ when the concentration doubles, so the order is two.

5.The order of chemical reaction is defined as: (SZABMU MDCAT 2025)

  1. The number of reactants involved in balanced equation
  2. The number of products formed in balanced equation
  3. The power to which the concentration of a reactant is raised in the rate equation
  4. The rate constant of the reaction
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Correct answer: (c) The power to which the concentration of a reactant is raised in the rate equation

The order in a reactant is the power to which its concentration is raised in the rate equation.

6.A reaction is first order with respect to A and second order with respect to B, the rate equation is (KMU MDCAT 2024)

  1. Rate = k [A]
  2. Rate = k [A][B]
  3. Rate = k [A]²[B]
  4. Rate = k [A][B]²
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Correct answer: (d) Rate = k [A][B]²

Each concentration is raised to the power of its own order.

7.6CO2 + 6H2O → sunlight → C6H12O6 + 6O2. Predict the order of reaction: (NUMS MDCAT 2024)

  1. 2nd
  2. 1st
  3. Zero
  4. 3rd
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Correct answer: (c) Zero

A photochemical reaction goes at a rate set by the light, not by the concentrations, so it is zero order.

8.Which mechanism is consistent with rate law Rate = K[NO]2[H2]? (NUMS MDCAT 2024)

  1. 2NO + H2 → N2 + H2O2 (slow), H2O2 + H2 → 2H2O (fast)
  2. NO + H2O → N2 + H2O2 (slow), H2O2 + H2 → 2H2O (fast)
  3. 2NO + 2H2 → N2O + H2 (slow), H2O + H2 → H2O2 (fast)
  4. NO + NO → N2 + O2 (slow), O2 + H2 → H2O (fast)
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Correct answer: (a) 2NO + H2 → N2 + H2O2 (slow), H2O2 + H2 → 2H2O (fast)

The rate law follows the slow step, and only the first scheme has two molecules of nitric oxide and one of hydrogen in it.

9.Q.86 What is the overall order of this rate equation? Rate =k[H3][NO2]2 (UHS MDCAT 2023)

  1. 1
  2. 2
  3. 3
  4. 4
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Correct answer: (c) 3

The overall order is the sum of the powers, $1+2$, that is three.

10.If a reaction is first order with respectd to a reactant then the rate will be__________if the concentration of reactant is doubled: (SZABMU MDCAT 2023)

  1. Doubled
  2. Halved
  3. One fourth
  4. Quadrupled
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Correct answer: (a) Doubled

First order means the rate is proportional to that concentration.

11.When nitric oxide reacts with ozone, the order of reaction will be: (NUMS MDCAT 2023)

  1. 2nd
  2. 3rd
  3. 1st
  4. Zero
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Correct answer: (a) 2nd

The reaction is first order in each reactant, so second order overall.

12.The number of reactant molecules whose concentrations affect the rate of reaction is called: (PMC MDCAT 2022)

  1. Rate law
  2. Molecularity
  3. Order of reaction
  4. Rate constant
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Correct answer: (b) Molecularity

The number of molecules taking part in the rate determining step is the molecularity.

13.The reaction in which rate depends only on one reactant concentrations, although more than one reactants are present in the reaction mixture is known as __________ (PMC MDCAT 2022)

  1. 1st order reaction
  2. 2nd order reaction
  3. Pseudo first order reaction
  4. Third order reaction
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Correct answer: (c) Pseudo first order reaction

With one reactant in great excess its concentration hardly changes, so the reaction behaves as first order.

14.The rate law of a particular reaction is Rate = k[NO][O3]. The order of this reaction with respect to ozone is: (PMC MDCAT 2022)

  1. 1st
  2. 2nd
  3. 3rd
  4. Zero
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Correct answer: (a) 1st

Ozone appears to the first power, so the order in it is one.

15.The order of reaction for which half-life is inversely proportional to the concentration of reaction is? (PMC Practice 2021)

  1. Second Order
  2. Zero Order
  3. First Order
  4. Third Order
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Correct answer: (a) Second Order

For a second order reaction $t_{1/2}=1/k[A]_0$.

16.For which of the following order of the reaction, rate of reaction is inversely proportional to the concentration reaction? (PMC MDCAT 2020)

  1. 1st order reaction
  2. 2nd order reaction
  3. Negative order of reaction
  4. Zero order of reaction
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Correct answer: (c) Negative order of reaction

A rate falling as the concentration rises means the order in that reactant is negative.

17.If concentration time graph of a reactant indicates a constant half life, then the order reaction with respect to that reactant is: (UHS MDCAT 2018)

  1. Zero order
  2. Half order
  3. Second order
  4. First order
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Correct answer: (d) First order

Only for a first order reaction is the half life independent of the concentration.

18.Rate of first order reaction depends on: (UHS MDCAT 2017)

  1. Concentration of one reactant
  2. Concentration of two reactants
  3. Concentration of three reactants
  4. Independent of the initial concentration
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Correct answer: (a) Concentration of one reactant

A first order reaction has its rate proportional to a single concentration.

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