OCR - A-Level - Chemistry - Enthalpy Changes

OCR - A-Level - Chemistry - Enthalpy Changes

1. Enthalpy is the amount of thermal energy stored by chemicals in a system.

   • Represented as H
     • ΔH, +ve means it is endothermic
       • +ve because gaining energy
       • In endothermic reactions, products have more energy than reactants. Energy taken in from the surroundings.
     • ΔH, -ve means it is exothermic
       • -ve because losing energy
       • In exothermic reactions, products have less energy than reactants. Energy given out to the surroundings.
     • With a catalyst, activation energy needed is reduced.
       • Exothermic
       • Endothermic

2. A system is the reactants and products.

3. Standard conditions

   • 100kPa
   • 298K (25 degrees Celsius)
   • 1.0M concentration
   • Standard state is the most stable physical state of a substance under standard conditions
     • Magnesium is Mg(s)
     • Hydrogen is H2(g)
     • Water is H2O(l)

4. Enthalpy change of reaction

   • Balanced chemical equation

5. Enthalpy change of formation

   • Only 1 mole of the product

6. Enthalpy change of combustion

   • Only 1 mole of the fuel substance

7. Enthalpy change of neutralisation

   • Only 1 mole of water

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