Rate Constant

Rate Constant MDCAT MCQs with answers

12 past paper MCQs on Rate Constant, 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.The effect of temperature on the rate of a reaction is given by: (MDCAT 2020)

  1. Henderson's equation
  2. the general gas equation
  3. the Arrhenius equation
  4. the van der Waals equation
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Correct answer: (c) the Arrhenius equation

$k=Ae^{-E_a/RT}$ is the Arrhenius equation.

2.Which of the following is the CORRECT Arrhenius equation? (MDCAT 2024)

  1. $k=Ae^{-E_a/QT}$
  2. $k=Ae^{-E_a/RT}$
  3. $k=Ae^{-E_a/ST}$
  4. $k=Ae^{-E_a/uT}$
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Correct answer: (b) $k=Ae^{-E_a/RT}$

$k=Ae^{-E_a/RT}$, with $R$ the gas constant and $E_a$ the activation energy.

3.The specific rate constant (k) of a reaction is related to the concentration of reactants: (KMU MDCAT 2025)

  1. Directly
  2. Inversely
  3. Exponentially
  4. Independently
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Correct answer: (d) Independently

The rate constant depends on temperature and on the catalyst, never on how much reactant is present.

4.K is called specific rate constant because it is: (SIBA MDCAT 2025)

  1. Rate per unit concentration
  2. Temperature independent
  3. Depends on concentration
  4. Unitless
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Correct answer: (a) Rate per unit concentration

It is the rate when every reactant is at unit concentration, that is the rate per unit concentration.

5.For a first order reaction A → B, the rate constant is 0.0458 s⁻¹. Calculate rate of the reaction if the concentration of reactant is 0.35mol dm⁻³. (KMU MDCAT 2024)

  1. 0.012 mol dm⁻³ s⁻¹
  2. 0.014 mol dm⁻³ s⁻¹
  3. 0.016 mol dm⁻³ s⁻¹
  4. 0.018 mol dm⁻³ s⁻¹
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Correct answer: (c) 0.016 mol dm⁻³ s⁻¹

Rate is $k[A]=0.0458\times0.35$, which is $0.016\,\mathrm{mol\,dm^{-3}\,s^{-1}}$.

6.The unit of rate constants is the same as the rate of reaction in which of the following order of reaction? (SZABMU MDCAT 2023)

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

For a zero order reaction the rate equals $k$, so the two have the same unit.

7.Arrhenius equation is an equation of: (PMC MDCAT 2022)

  1. Curve
  2. Parabola
  3. Straight line
  4. Hyperbola
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Correct answer: (c) Straight line

Taking logarithms turns it into $\ln k=\ln A-E_a/RT$, a straight line against $1/T$.

8.What Is the unit of slope obtained from Arrhenius equation? (PMC MDCAT 2022)

  1. K
  2. J/mol K
  3. mol/K
  4. K-
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Correct answer: (a) K

The slope is $-E_a/R$, and joules per mole over joules per mole per kelvin leaves kelvin.

9.Rate constant is denoted by? (PMC Practice 2021)

  1. K
  2. Kr
  3. Kv
  4. Ks
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Correct answer: (a) K

The rate constant is written $k$.

10.The unit of the rate constant is the same as that of the rate of reaction in: (PMC MDCAT 2020)

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

For a zero order reaction the rate equals $k$.

11.Unit of K in first order Reaction is: (UHS MDCAT 2017)

  1. s-1
  2. moles dm-3 s-1
  3. moles dm-3
  4. mol-1 dm3’
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Correct answer: (a) s-1

For a first order reaction the rate constant has the unit of reciprocal seconds.

12.The rate constant ‘k’ is 0.693 min-1. The half-life for the 1st order reaction will be: (UHS MDCAT 2016)

  1. 1 min
  2. 2 min
  3. 0.693 min
  4. 4 min
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Correct answer: (a) 1 min

$t_{1/2}=0.693/k=0.693/0.693$, that is one minute.

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