Enthalpy changes

• Enthalpy H is a measure of the heat energy in a chemical system.
• A chemical system refers to the atoms, molecules, or ions making up the chemicals.
• Enthalpy change is the enthalpy of products minus the enthalpy of reactants.

  ΔH= H(products) - H(reactants)
• The law of conservation of energy states that energy cannot be created or destroyed.
• When a chemical reaction involving an enthalpy change, heat energy is transferred between the system and surroundings.

  • the system is the chemicals- reactants and products
  • the surroundings are the apparatus, the laboratory and everything that is not in the chemical system
  • the universe is everything and includes both system and surroundings
• Exothermic- heat out of the system

  The conservation of energy means that energy is transferred from the system to the surroundings

  • the chemical system releases heat energy to the surroundings
  • any energy loss by the chemical system is balanced by the same energy gain by the surroundings
  • ΔH is negative
  • the temperature of the surroundings increases as they gain energy 
• Endothermic- heat into the system

  the conservation of energy means that energy is transferred from the surroundings to the system

  • the chemical system takes in heat energy from the surroundings
  • any energy gain by the chemical system is balanced by the same energy loss by the surroundings
  • ΔH is positive
  • the temperature of the surroundings decreases as they lose energy
• The energy input required to break bonds acts as an energy barrier to the reaction, known as the activation energy. This is the minimum energy required for a reaction to take place.
• Standard conditions:

  • standard pressure is 100 kPa
  • standard temperature 298 k 25℃
  • standard concentration is 1 mol dm-3
  • standard state is the physical state of a substance under standard conditions
• The standard enthalpy change of a reaction ΔrH is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard States
• The standard enthalpy change of formation ΔfH is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
• The standard enthalpy change of combustion ΔcH is the enthalpy change that takes place when one mole of substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
• The standard enthalpy change of neutralisation ΔneutH is the standard entity change that accompanies the reaction of an acid by a base to form one mole of water, under standard conditions, with all reactants and products in their standard states