Hess's Law

Hess's Law MDCAT MCQs with answers

7 past paper MCQs on Hess's Law, 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.

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1.The Born-Haber cycle is used to determine the lattice energy of ionic compounds. It is an application of: (MDCAT 2022)

  1. Henry's law
  2. Le Chatelier's principle
  3. Hess's law
  4. the common ion effect
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Correct answer: (c) Hess's law

The cycle adds the enthalpies of a set of steps to reach the same final state, which is Hess's law.

2.The heat of formation of CO and CO2 are -26.4 Kcal and -94.0 Kcal respectively. The heat of combustion of carbon monoxide according to Hess's Law will be: (SIBA MDCAT 2025)

  1. +26.4Kcal
  2. -67.6Kcal
  3. +94.0Kcal
  4. -120.4Kcal
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Correct answer: (b) -67.6Kcal

Burning carbon monoxide is the formation of carbon dioxide less the formation of the monoxide, $-94.0-(-26.4)$, or $-67.6\,\mathrm{kcal}$.

3.ΔH can be measured indirectly by applying (KMU MDCAT 2024)

  1. Avogadro's law
  2. Faraday's law
  3. Gas's law
  4. Hess's law
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Correct answer: (d) Hess's law

Hess’s law lets an enthalpy change be got from any convenient route between the same two states.

4.DeltaH° for sublimation of one mole of iodine: H2(g) + I2(g) → 2HI(g) DH = 51.8kJ/mol H2(g) + I2(s) → 2HI(g) DH = -10.5kJ/mol (NUMS MDCAT 2024)

  1. 41.3 kJ/mol
  2. 62.3 kJ/mol
  3. 53.5 kJ/mol
  4. 36.5 kJ/mol
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Correct answer: (b) 62.3 kJ/mol

Subtracting the second equation from the first leaves the sublimation, $51.8-(-10.5)$, that is $62.3\,\mathrm{kJ\,mol^{-1}}$.

5.Q.88 Born-Haber cycle is used to determine the Lattice energy of ionic compounds. It is the application of (UHS MDCAT 2023)

  1. Henry's law
  2. Le - Chatleir's Principle
  3. Hess's law
  4. Common ion effect
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Correct answer: (c) Hess's law

The cycle adds enthalpies round a closed path, which is Hess’s law.

6.Which of the following law Is an application of first law of thermodynamics? (PMC MDCAT 2022)

  1. Hess's law
  2. Dalton's law
  3. Moseley's Law
  4. Avogadro's law
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Correct answer: (a) Hess's law

Hess’s law follows from the conservation of energy applied to enthalpy.

7.Standard enthalpy of combustion of graphite at 25 oC is –393.51 kJ mol-1 and that of diamond is –395.41 kJ mol-1. The enthalpy change for graphite is: (UHS MDCAT 2015)

  1. – 1.91
  2. +2.1
  3. –2.1
  4. +1.91
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Correct answer: (d) +1.91

Subtracting the two combustion enthalpies leaves $+1.91\,\mathrm{kJ\,mol^{-1}}$ for graphite turning to diamond.

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