A Level Chemistry OCR Practice Exam

What is the (standard) enthalpy change of combustion (∆Hcө)?

1. The change when one mole of a substance reacts with oxygen

2. The enthalpy change during an endothermic reaction

3. The change of heat in a reversible reaction

4. The enthalpy change for condensation of water

The correct answer is: The change when one mole of a substance reacts with oxygen

The standard enthalpy change of combustion (∆Hcө) refers to the energy change that occurs when one mole of a substance reacts completely with oxygen under standard conditions (typically 1 bar pressure and a specified temperature, usually 298 K). This definition is fundamental in thermodynamics and is crucial for calculating the energy released in combustion reactions, which are exothermic in nature. In this context, answer A accurately captures the concept by specifying the reaction with oxygen, highlighting that the measurement is per mole of the substance in question. It is also important to note that this process involves the breaking of chemical bonds in the substance and the formation of new bonds in the products (usually carbon dioxide and water), resulting in the release of energy. The other options do not align with the definition of enthalpy change of combustion. They either describe different types of reactions or energy changes, rather than focusing specifically on combustion. This reinforces the correctness of the first option as the precise definition of what the standard enthalpy change of combustion entails.