Enthalpy Change, Endothermic Reactions, and Exothermic Reactions

Enthalpy Change, Endothermic Reactions, and Exothermic Reactions

In chemistry, the term 'enthalpy' refers to the total heat content of a system. Enthalpy change, specifically, denotes the heat exchange during chemical reactions, where pressure remains constant. This concept plays a crucial role in understanding the behavior of various types of reactions, including endothermic and exothermic ones.

Enthalpy Change

Enthalpy change is calculated by determining the heat of a process when pressure is held constant. The capital letter H represents 'enthalpy,' while the symbol Δ (delta) indicates a change in the quantity. Therefore, ΔH denotes the change in enthalpy during a chemical reaction.

For example, when two moles of hydrogen react with one mole of oxygen to create two moles of water, the characteristic enthalpy change is 570 kilojoules (kJ). In a thermochemical equation, the enthalpy change is typically placed after a balanced chemical equation and on the same line.

Endothermic Reactions

An endothermic reaction occurs when a system absorbs heat from the surroundings. This type of reaction is characterized by an increase in temperature and a positive enthalpy change, indicating that the heat released during the reaction is less than the heat absorbed. Examples of endothermic reactions include the dissociation of water, where two moles of water molecules are broken down into three moles of atoms, requiring additional energy. Another example is the decomposition of ammonium nitrate (Nh4NO3) to nitrous oxide, which absorbs more energy than it releases.

Exothermic Reactions

Conversely, an exothermic reaction involves a decrease in temperature and a negative enthalpy change. In this scenario, the system releases heat into the surroundings, causing a drop in temperature. An example of an exothermic reaction is the combustion of methane (CH4) and ethylene (C2H4). These reactions release energy, lowering the temperature of the system and raising it in the surroundings.

It is essential to note that the enthalpy change is not dependent on the environmental conditions, like ambient pressure or temperature, but rather on the characteristics of the chemical species themselves.