What is the enthalpy of combustion of a substance?

The enthalpy of combustion of a substance is defined as the amount of heat released when one mole of that substance is burned completely in oxygen. This process typically involves the reaction of the substance with oxygen to produce carbon dioxide and water, if it is a hydrocarbon, for example. The enthalpy of combustion is an important value in thermodynamics as it gives insights into the energy efficiency of fuels and materials and is a key factor in calculating the energy released in various chemical reactions.

In contrast, the alternative definitions given do not pertain to combustion. The first describes the process of evaporation, which involves a phase change from liquid to gas and is associated with a different energy change, known as enthalpy of vaporization. The third choice refers to the reaction with water, which is not related to combustion; instead, it may involve hydration or hydrolysis processes depending on the nature of the substance. The fourth choice involves freezing, which is a phase change from liquid to solid and is associated with the enthalpy of fusion, not combustion. Therefore, the correct identification of the enthalpy of combustion aligns specifically with the complete burning of a substance in the presence of oxygen.