Laws of Thermodynamics

Laws of Thermodynamics MDCAT MCQs with answers

8 past paper MCQs on Laws of Thermodynamics, from the Thermochemistry and Energetics of Chemical Reactions unit of MDCAT Chemistry. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

Read the notes first Practise the unit as a quiz

1.Calculate the work done when 1 mole of an ideal gas expands from 15 dm³ to 20 dm³ against a constant external pressure of 2 atmospheres. (KMU MDCAT 2024)

  1. -10 atm.dm³
  2. -5 atm.dm³
  3. 5 atm.dm³
  4. 10 atm.dm³
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Correct answer: (a) -10 atm.dm³

$w=-P\Delta V=-2\times5$, which is $-10\,\mathrm{atm\,dm^3}$; the sign says the gas did the work.

2.Expansion takes place when gas evolves during reaction between carbohydrates and dilute HCl. Predict energy change and work done: (NUMS MDCAT 2024)

  1. w is positive, q is negative
  2. w is negative, q is positive
  3. q is positive, no work done
  4. w is positive, q is positive
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Correct answer: (b) w is negative, q is positive

The gas pushes back the surroundings, so the system does work and the sign of $w$ is negative while heat is absorbed.

3.The correct equation for the first law of thermodynamics is: (SZABMU MDCAT 2023)

  1. ∆E = q + w
  2. ∆E = w - q
  3. ∆E = ∆q - p∆v
  4. ∆E = ∆q + p∆v
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Correct answer: (a) ∆E = q + w

The first law is $\Delta E=q+w$: heat put in and work done on the system.

4.Which one of the following is the correct demonstration of first law of thermodynamics? (PMC MDCAT 2022)

  1. △H= qw
  2. △E=q – P△V
  3. △H=q+w
  4. △E = q+w
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Correct answer: (d) △E = q+w

The first law is $\Delta E=q+w$.

5.With different number of moles of reactants and products, the volume of system? (PMC Practice 2021)

  1. Remains unchanged
  2. Changes
  3. Decrease
  4. Increase
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Correct answer: (b) Changes

A change in the number of gas molecules changes the volume, and the system then does or absorbs work.

6.Heat exchanged between the system and surrounding at constant volume is shown by the relation? (PMC Practice 2021)

  1. ΔH= qv+ w
  2. ΔE=(q)v
  3. ΔH=(q)v
  4. ΔE=(q)v + PV
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Correct answer: (b) ΔE=(q)v

At constant volume no work is done, so the heat exchanged is the change in internal energy.

7.At constant volume the heat supplied is equal to: (PMC Practice 2021)

  1. Work done
  2. Internal energy change
  3. Enthalpy
  4. Entropy
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Correct answer: (b) Internal energy change

With no work done, the first law leaves the heat supplied equal to the rise in internal energy.

8.Which of the following condition is constant in Bomb calorimeter? (PMC Practice 2021)

  1. Pressure
  2. Volume
  3. Temperature
  4. All of these
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Correct answer: (b) Volume

A bomb calorimeter is a sealed steel vessel, so the volume cannot change.

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