Hess's law and average bond enthalpies

Overview

Conservation of energy and Hess's law, enthalpy cycles from combustion and formation data, average bond enthalpies, and worked calculations for ammonia, hydrogen and hydrogen chloride.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   State the principle of conservation of energy.

   Model answer

   Energy cannot be created or destroyed, only transferred from one form to another. The total energy of a reaction and its surroundings stays constant.

2. 2

   State Hess's law.

   Model answer

   Hess's law states that the enthalpy change for a reaction is independent of the route taken. The change depends only on the initial and final states.

3. 3

   How does Hess's law follow from the conservation of energy?

   Model answer

   Two routes between the same reactants and products must involve the same total energy change. A different total would create or destroy energy on a round trip.

4. 4

   Why are enthalpy cycles needed for reactions such as C(s) + ½O₂(g) → CO(g)?

   Model answer

   The enthalpy change cannot be measured directly because some carbon dioxide always forms as well. An enthalpy cycle calculates it from combustion data instead.

5. 5

   Use the enthalpies of combustion of carbon (−393.5 kJ mol⁻¹) and carbon monoxide (−283.0 kJ mol⁻¹) to find ΔH for C(s) + ½O₂(g) → CO(g).

   Model answer

   The enthalpy change is −110.5 kJ mol⁻¹. The working is −393.5 − (−283.0) = −110.5 kJ mol⁻¹.

6. 6

   Calculate ΔH for CaCO₃(s) → CaO(s) + CO₂(g), given enthalpies of formation of −1207 kJ mol⁻¹ for CaCO₃, −635 kJ mol⁻¹ for CaO and −394 kJ mol⁻¹ for CO₂.

   Model answer

   The enthalpy change is +178 kJ mol⁻¹. The working is (−635 + −394) − (−1207) = +178 kJ mol⁻¹.

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