Enthalpy Changes

Understanding Enthalpy Changes

  • Enthalpy change refers to the heat energy change in a chemical reaction at constant pressure.
  • Symbolised by ΔH, positive for endothermic reactions (energy absorbed) and negative for exothermic reactions (energy released).
  • Endothermic reactions are ones where the surroundings get colder, like photosynthesis or the thermal decomposition of calcium carbonate.
  • Exothermic reactions are ones where the surroundings heat up, such as combustion reactions or the oxidation of glucose in the body.

Measurement of Enthalpy Changes

  • Enthalpy changes can be measured using a calorimeter, a device that measures the heat absorbed or released during a reaction.
  • Calorimetry involves performing the reaction in an insulated container and measuring the temperature change.
  • Calculating enthalpy change requires knowledge of the mass of substance, specific heat capacity, and change in temperature.

Hess’s Law

  • Hess’s Law states that the total enthalpy change for a reaction is the same, no matter what route is taken, provided the initial and final conditions are the same.
  • Assists in calculating enthalpy changes for hard-to-measure reactions by measuring easier related reactions.

Standard Conditions

  • Standard enthalpy change of formation (ΔHf°) is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.
  • Standard enthalpy change of combustion (ΔHc°) is the enthalpy change when one mole of substance is completely burned in oxygen under standard conditions.
  • Standard conditions are a pressure of 100kPa and a stated temperature, usually 25°C.

Enthalpy Change in Industry

  • Understanding enthalpy changes is vital for control and safety in industry, particularly in chemical manufacturing.
  • Design of industrial processes considers enthalpy changes to optimise energy efficiency and ensure safety.
  • Exothermic reactions may be utilised for energy production, while endothermic reactions may be used for cooling processes.