Question

What is enthalpy of formation? How does it differ from the enthalpy of combustion?

Short answer

Answer: The main differences between enthalpy of formation and enthalpy of combustion are: 1. Enthalpy of formation refers to the energy change during the formation of a compound, while enthalpy of combustion deals with the energy change during combustion. 2. Enthalpy of formation involves the formation of a compound from its constituent elements, whereas enthalpy of combustion deals with the complete oxidation (burning) of a compound with oxygen. 3. Enthalpy of formation can have either positive or negative values, while enthalpy of combustion is always negative since combustion is an exothermic process. 4. The standard enthalpy of formation of any pure element in its standard state is defined to be zero, whereas enthalpy of combustion values are non-zero.

Step-by-step solution

Definition of Enthalpy of Formation

Enthalpy of formation is the change in enthalpy (ΔH) that occurs during the formation of one mole of compound from its constituent elements in their standard states. In other words, it refers to the amount of energy released or absorbed during the formation process.

Definition of Enthalpy of Combustion

Enthalpy of combustion refers to the change in enthalpy (ΔH) that occurs during the complete combustion of one mole of a substance with oxygen, resulting in the formation of products like water and carbon dioxide. It is usually expressed in the form of heat energy released during the process.

Key Differences Between the Two

There are a few differences between enthalpy of formation and enthalpy of combustion, which are listed below: 1. Enthalpy of formation refers to the energy change during the formation of a compound, while enthalpy of combustion deals with the energy change during combustion. 2. Enthalpy of formation involves the formation of a compound from its constituent elements, whereas enthalpy of combustion deals with the complete oxidation (burning) of a compound with oxygen. 3. Enthalpy of formation can have either positive or negative values, depending on whether the reaction is endothermic (absorbs energy) or exothermic (releases energy). On the other hand, enthalpy of combustion is always negative since combustion is always an exothermic process. 4. The standard enthalpy of formation of any pure element in its standard state is defined to be zero, while for enthalpy of combustion, the values are non-zero. By understanding these key differences, students can visualize and interpret the concepts of enthalpy of formation and enthalpy of combustion.