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.