Topic 6

• enthalpy change

  heat energy change measured at constant pressure
• Standard conditions for enthalpy change:

  • specified temperature - normally 298K
  • 100 kPa
  • 1 mole of specified substance
  • all reactants and products in their standard state
  • units are always kJ/mol
• exothermic

  where heat energy is transferred from the system to the surroundings, and has a negative sign
• endothermic

  where heat energy is transferred from the surroundings to the system, and has a positive sign
• standard enthalpy change of formation

  enthalpy change when 1 mole of substance is formed from its elements in their standard state at 100 kPa and a specified temperature
• The standard enthalpy change of formation of an element is always 0.
• standard enthalpy change of combustion

  enthalpy change when 1 mole of a substance burns completely in excess oxygen under standard conditions of 100 kPa and a specified temperature
• The standard enthalpy change of combustion is always exothermic, so always has a negative sign.
• standard enthalpy change of neutralisation

  enthalpy change when an acid and an alkali react under standard conditions of 100 kPa and a specified temperature to form 1 mole of water
• The standard enthalpy change of neutralisation is always exothermic, so always has a negative sign. It's ionic equation is H+ + OH- -> H2O.
• standard enthalpy change of atomisation

  enthalpy change for the formation of one mole of gaseous atoms from the element in standard state
• The standard enthalpy change of atomisation is always exothermic, so always has a negative sign.
• energy transferred (J) = mass (g) x specific heat capacity (J g-1 C-1) x temperature change (C)
• Once energy transferred is calculated, divide by 1000 to convert from J to kJ, then divide by moles.