A Level Chemistry OCR Practice Exam

What does enthalpy (H) represent in a chemical system?

1. The total mass of reactants in a reaction

2. The heat content stored in the system

3. The kinetic energy of the particles

4. The change in temperature during a reaction

The correct answer is: The heat content stored in the system

Enthalpy, represented by the symbol H, is a measure of the total heat content within a chemical system at constant pressure. It encompasses not only the internal energy of the system but also accounts for the energy required to make room for it by displacing its surroundings (which corresponds to the pressure-volume work). When examining reactions, the change in enthalpy (ΔH) is particularly important, as it indicates whether a reaction absorbs heat (endothermic) or releases heat (exothermic). The other options do not accurately capture the essence of enthalpy. The total mass of reactants pertains to the conservation of mass but does not relate to the energy content of the system. Kinetic energy refers solely to the energy of motion of particles, while enthalpy includes potential energy components as well. The change in temperature during a reaction is associated with thermal changes but does not directly equate to the overall heat content stored in the system, as en thalpy takes into account both temperature and other factors.