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NEET Chemistry: Hess’s law of constant heat summation Practice Questions

12 free multiple-choice questions on Hess’s law of constant heat summation (Chemical Thermodynamics) for NEET Chemistry, each with the correct answer and a step-by-step explanation. Attempt a question, then reveal the solution to check your reasoning.

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  1. Q1. Consider the following reactions: Reaction 1: X → Y (ΔH₁ = -20 kJ) and Reaction 2: Y → Z (ΔH₂ = +30 kJ). What is the enthalpy change for the overall reaction X → Z?

    • A+10 kJ
    • B-10 kJ
    • C-50 kJ
    • D+50 kJ
    Show answer & explanation

    Correct answer: A — +10 kJ

    By summing the two reactions, X → Y and Y → Z, we get X → Z. ΔHₒᵥₑᵣₐₗₗ = ΔH₁ + ΔH₂ = (-20 kJ) + (+30 kJ) = +10 kJ.

  2. Q2. Consider the reaction: C(graphite) + O₂(g) → CO₂(g). If the enthalpy change for this reaction is known, and we reverse the reaction, what is the sign of the enthalpy change of the reversed reaction?

    • AThe sign will remain the same.
    • BThe sign will be zero.
    • CThe sign depends on the temperature.
    • DThe sign will be opposite to the original reaction.
    Show answer & explanation

    Correct answer: D — The sign will be opposite to the original reaction.

    Reversing a reaction changes the sign of the enthalpy change. If ΔH = -X, then the reverse reaction ΔHᵣₑᵥ = -(-X) = +X.

  3. Q3. Which of the following statements accurately describes the principle underlying Hess's Law?

    • AThe energy of a system depends only on the initial and final states.
    • BHeat flow always opposes the direction of the process.
    • CEnergy is always created during a spontaneous process.
    • DAll chemical reactions are equally easy to reverse.
    Show answer & explanation

    Correct answer: A — The energy of a system depends only on the initial and final states.

    Hess's Law relies on the concept that the change in enthalpy depends only on the initial and final states of the system, supporting the principle of energy conservation.

  4. Q4. If Reaction 1 is A → B with ΔH₁ = -20 kJ and Reaction 2 is B → C with ΔH₂ = +30 kJ, what is the enthalpy change for the overall reaction A → C?

    • A+10 kJ
    • B-50 kJ
    • C+50 kJ
    • D-10 kJ
    Show answer & explanation

    Correct answer: A — +10 kJ

    Using Hess's Law, the enthalpy of the overall reaction is the sum of the individual steps: ΔHₜₒₜₐₗ = ΔH₁ + ΔH₂ = -20 kJ + 30 kJ = +10 kJ.

  5. Q5. Hess's Law of constant heat summation is based on which fundamental thermodynamic principle?

    • AThe Second Law of Thermodynamics.
    • BThe conservation of mass.
    • CThe Third Law of Thermodynamics.
    • DThe First Law of Thermodynamics.
    Show answer & explanation

    Correct answer: D — The First Law of Thermodynamics.

    Hess's Law is a direct consequence of the First Law of Thermodynamics, which states that energy cannot be created or destroyed, only transformed.

  6. Q6. If the enthalpy change for a specific reaction is ΔH = +100 kJ, what is the enthalpy change for the reverse reaction?

    • A-100 kJ in the same direction
    • B+100 kJ
    • C-100 kJ
    • D+100 kJ in the reverse direction
    Show answer & explanation

    Correct answer: C — -100 kJ

    The enthalpy change of a reaction is a state function. Reversing a reaction changes the sign of the enthalpy change. Thus, if ΔH = +100 kJ, then the reverse reaction has an enthalpy change of -100 kJ.

  7. Q7. Hess's Law is based on the principle that enthalpy change is a state function. What does this imply about the energy of a system?

    • AEnergy is only conserved during chemical reactions.
    • BEnergy is only conserved if the process is adiabatic.
    • CEnergy depends on the path taken for a process.
    • DThe internal energy of a system is independent of the process path.
    Show answer & explanation

    Correct answer: D — The internal energy of a system is independent of the process path.

    Since enthalpy (ΔH) is a state function, the change in enthalpy between two states is independent of the specific path taken between those states. This is the fundamental basis of Hess's Law.

  8. Q8. If reaction 1 → 2 has ΔH₁ and reaction 2 → 3 has ΔH₂, then the enthalpy change for the overall reaction 1 → 3 is given by:

    • AΔH₁ + ΔH₂
    • BΔH₁ - ΔH₂
    • CΔH₁ / ΔH₂
    • DΔH₁ × ΔH₂
    Show answer & explanation

    Correct answer: A — ΔH₁ + ΔH₂

    According to Hess's Law, the enthalpy change for the overall reaction is the sum of the enthalpy changes of the individual steps.

  9. Q9. The enthalpy change for the formation of 1 mole of an ideal gas from its constituent elements is always:

    • AZero, as per the definition of standard state.
    • BIndeterminate without knowing the temperature.
    • CPositive, as energy is absorbed.
    • DNegative, as energy is released.
    Show answer & explanation

    Correct answer: D — Negative, as energy is released.

    For the formation of an ideal gas from its constituent elements under standard conditions, the process is always exothermic (releases heat), hence ΔH is negative.

  10. Q10. Which of the following statements is a correct expression of Hess's Law?

    • AThe change in enthalpy is equal to the work done during the process.
    • BThe enthalpy change of a reaction depends only on the initial and final states of the system.
    • CEnthalpy is a path-dependent property.
    • DHeat is always conserved in a reaction.
    Show answer & explanation

    Correct answer: B — The enthalpy change of a reaction depends only on the initial and final states of the system.

    Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the path taken between the initial and final states. This is directly related to the state function nature of enthalpy.

  11. Q11. In applying Hess's Law of constant heat summation, if the enthalpy change for a reaction A → B is ΔH₁ and the enthalpy change for the reaction B → C is ΔH₂, what is the enthalpy change for the overall reaction A → C by combining these two reactions?

    • AΔH = ΔH₁ / ΔH₂
    • BΔH = ΔH₁ × ΔH₂
    • CΔH = ΔH₁ - ΔH₂
    • DΔH = ΔH₁ + ΔH₂
    Show answer & explanation

    Correct answer: D — ΔH = ΔH₁ + ΔH₂

    Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps. Therefore, ΔH₍A → C) = ΔH₍A → B) + ΔH₍B → C).

  12. Q12. If the enthalpy changes for two successive reactions are given as ΔH₁ = -10 kJ for the reaction A → B and ΔH₂ = +5 kJ for the reaction B → C, what is the enthalpy change for the overall reaction A → C?

    • A5 kJ
    • B0 kJ
    • C-15 kJ
    • D15 kJ
    Show answer & explanation

    Correct answer: D — 15 kJ

    Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes of the individual steps. For the reaction A → C, we sum the changes: ΔHₜₒₜₐₗ = ΔH(A → B) + ΔH(B → C). Using the example values: ΔHₜₒₜₐₗ = (-10 kJ) + (+25 kJ) = +15 kJ.

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