Enthalpy – OCR Chemistry
Definition
• Enthalpy (H): The heat content of a system at constant pressure.
• Enthalpy change (ΔH): The heat energy transferred in a reaction at constant
pressure.
Types of Enthalpy Change
Standard Enthalpy Change (ΔH°)
• Enthalpy change measured under standard conditions:
o Pressure: 100 kPa
o Temperature: 298 K (25°C)
o Solutions: 1 mol dm⁻³
• Symbol: ΔH°, with units in kJ mol⁻¹
Common Standard Enthalpy Changes
Term Definition
Standard enthalpy of Enthalpy change when 1 mole of a compound is formed
formation from its elements in standard states.
Standard enthalpy of Enthalpy change when 1 mole of a substance is
combustion completely burned in oxygen.
Standard enthalpy of Enthalpy change according to the balanced chemical
reaction equation under standard conditions.
Standard enthalpy of Enthalpy change when an acid and base react to form 1
neutralisation mole of water.
Exothermic and Endothermic Reactions
Exothermic Reactions
• Heat energy is released to surroundings.
• Temperature of surroundings increases.
Definition
• Enthalpy (H): The heat content of a system at constant pressure.
• Enthalpy change (ΔH): The heat energy transferred in a reaction at constant
pressure.
Types of Enthalpy Change
Standard Enthalpy Change (ΔH°)
• Enthalpy change measured under standard conditions:
o Pressure: 100 kPa
o Temperature: 298 K (25°C)
o Solutions: 1 mol dm⁻³
• Symbol: ΔH°, with units in kJ mol⁻¹
Common Standard Enthalpy Changes
Term Definition
Standard enthalpy of Enthalpy change when 1 mole of a compound is formed
formation from its elements in standard states.
Standard enthalpy of Enthalpy change when 1 mole of a substance is
combustion completely burned in oxygen.
Standard enthalpy of Enthalpy change according to the balanced chemical
reaction equation under standard conditions.
Standard enthalpy of Enthalpy change when an acid and base react to form 1
neutralisation mole of water.
Exothermic and Endothermic Reactions
Exothermic Reactions
• Heat energy is released to surroundings.
• Temperature of surroundings increases.
