Enthalpy

• Enthalpy (H)


  The heat content of a system at constant pressure
• System


  • An isolated part of the universe
• Change in enthalpy (ΔH)


  The heat added to a system at constant pressure
• Types of enthalpy changes

  • Exothermic
  • Endothermic
• Breaking bonds requires energy, so bond breaking is endothermic
• Energy is given out when bonds are made, so bond making is exothermic
• Bond enthalpy


  The energy required to break a bond
• Average bond enthalpy is less accurate than specific bond enthalpy calculations
• Standard enthalpy change of combustion

  The enthalpy change when 1 mole of a substance is completely burned in excess oxygen under standard conditions
• Standard enthalpy change of formation

  The enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions
• Elements have a standard enthalpy of formation of 0 kJ/mol
• Hess's law states that the enthalpy change of a reaction is independent of the route of the reaction
• The law of conservation of energy states that energy cannot be created or destroyed, only changed in form
• Calculating enthalpy changes from experimental data

  1. Measure mass (m) and temperature change (ΔT)

  2. Use q = mcΔT to calculate heat energy (q)

  3. Divide q by the amount of substance (n) to get the enthalpy change (ΔH)
• Factors affecting accuracy of enthalpy change measurements

  • Accurate thermometer
  • Powdered solids for faster reaction
  • Accurate measurement of solution volume and mass
  • Continuous stirring for even heat distribution
  • Regular temperature recording
  • Minimising heat loss from calorimeter
• Measuring enthalpy changes for combustion reactions

  1. Use a bomb calorimeter

  2. Measure temperature change of water

  3. Control volume of water

  4. Ensure complete combustion in pure oxygen

  5. Minimise heat loss from calorimeter