Exothermic & Endothermic Processes

Understanding Enthalpy Changes

In chemistry, energy changes occur during reactions. These changes are measured as enthalpy changes (94H). Depending on whether energy is released or absorbed, reactions are classified as exothermic or endothermic.

Exothermic Reactions

• Energy Release: Exothermic reactions release energy to the surroundings, usually in the form of heat.
• Negative 94H: The enthalpy change (94H) is negative because the products have less energy than the reactants.
• Examples: Combustion of fuels, respiration.

Endothermic Reactions

• Energy Absorption: Endothermic reactions absorb energy from the surroundings.
• Positive 94H: The enthalpy change (94H) is positive because the products have more energy than the reactants.
• Examples: Photosynthesis, melting of ice.

Measuring Enthalpy Changes

Enthalpy changes can be measured for different processes, such as combustion, neutralisation, and solution.

1. Enthalpy of Combustion (94H_c)

• Definition: The enthalpy change when one mole of a substance is completely burned in oxygen.
• Example:

  Combustion of methane: $$\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l)$$

2. Enthalpy of Neutralisation (94H_n)

• Definition: The enthalpy change when an acid and a base react to form one mole of water.
• Example:

  Neutralisation of hydrochloric acid and sodium hydroxide: $$\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$$

3. Enthalpy of Solution (94H_sol)

• Definition: The enthalpy change when one mole of a solute dissolves in a solvent to form a solution.
• Example:

  Dissolution of ammonium nitrate: $$\text{NH}_4\text{NO}_3(s) \rightarrow \text{NH}_4^+(aq) + \text{NO}_3^-(aq)$$