9.1 Enthalpy changes

• Enthalpy is a measure of the heat energy stored in a chemical system
• Enthalpy change is the difference between the enthalpy of reactants and products
• Enthalpy change for an exothermic reaction is negative
• Exothermic reaction = chemicals lose energy to surroundings
• Endothermic reaction = chemicals gain energy from the surroundings
• Enthalpy change of an endothermic reaction is positive
• Standard enthalpy changes are measured using standard conditions

  • Pressure of 101kPa
  • Temperature of 298 K
• All substances in enthalpy must be in their standard states
• Standard enthalpy change of reaction 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
• Standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
• Standard enthalpy change of formation of an element in its standard state is 0
• Standard enthalpy change of combustion is the enthalpy change when one mole of a compound is completely burnt in oxygen under standard conditions
• Standard enthalpy change of neutralisation is the energy change that accompanies the reaction of an acid by a base to form one mole of water under standard conditions
• Bond breaking requires energy
• Bond forming releases energy
• Enthalpy change = total energy to break bonds - total energy of bonds made
• Bond enthalpy is the average energy required to break 1 mole of covalent bonds in gaseous molecules