A Level Chemistry OCR Practice Exam

Standard enthalpies are often referenced to which term?

1. Specific heat

2. Enthalpy

3. Heat of fusion

4. Bond energy

The correct answer is: Enthalpy

Standard enthalpy refers to a specific thermodynamic quantity that measures the total heat content of a system under constant pressure and standard conditions—usually defined as 1 atmosphere of pressure and a specified temperature, often 298 K (25 °C). It is a fundamental concept in thermodynamics used to understand the energy changes associated with chemical reactions and physical processes. Enthalpy is denoted by the symbol H, and the standard enthalpy of a substance is written as H° (the degree symbol indicates the standard conditions). When discussing standard enthalpies, such as the standard enthalpy of formation or standard enthalpy of combustion, these values give important insights into the stability of compounds and the energy changes that occur during reactions. In contrast, specific heat is related to the amount of energy required to change the temperature of a substance, the heat of fusion pertains to the phase change from solid to liquid, and bond energy refers to the amount of energy needed to break chemical bonds in a molecule. While all these terms are important in the context of thermodynamics, they represent different concepts than standard enthalpy, which is fundamentally linked to the energy content of a system at standard conditions.