Enthalpy

• Enthalpy change, H = difference between the enthalpy of the products and the enthalpy of the reactants, change H = Hp + Hr
• Enthalpy, H = heat content that is stored in a chemical system
• Law of conservation of energy = states that energy cannot be created or destroyed
• Exothermic change, - = energy transferred from the system to the surroundings
• endothermic change, + = energy transferred from the surroundings to the system
• Activation energy, Ea = minimum energy required for a reaction to take place
• Standard conditions:

  • standard pressure - 101KPa
  • Standard temperature - 298K
  • standard concentration - 1 mol dm-3
• Standard state = physical states of a substance under standard conditions
• Standard enthalpy change of reaction = enthalpy change that Accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions
• Standard enthalpy change of formation = enthalpy change when one mole of a compound is formed from its elements under standard conditions
• Standard enthalpy change of combustion = enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions
• Standard enthalpy change of neutralisation = enthalpy change when one mole of water is produced from a reaction of a base and an acid
• Exothermic - heat out of the system:

  • any energy loss by the chemical system is balanced by the same energy gain by the surroundings
  • enthalpy change is negative
  • temperature of the surroundings increase as they gain energy
• Endothermic - heat into the system:

  • any energy gain by the chemical system is balanced by the same energy loss by the surroundings
  • enthalpy change is positive
  • temperature of the surroundings decrease as they lose energy
• q=mc/\T

  q = heat energy, J

  m = mass of surroundings, g

  c = specific heat capacity, J g-1 K-1

  /\T = change in temperature, °C
• Average bond enthalpy = energy required to break one mole of a specified type of bond in a gaseous molecule
• Average bond enthalpy:

  • energy is always required to break bonds
  • bond enthalpy are always endothermic
  • always have a positive enthalpy value
• Bond breaking:

  • endothermic
  • energy is required
  • enthalpy change is positive
• Forming bonds:

  • endothermic
  • energy is released
  • enthalpy change is negative
• Limitation of average bond enthalpy:

  • actual energy involved in breaking or making bonds would be slightly different
  • average bond enthalpy is calculated using bond enthalpies’ from different chemical environment, not the actual bond enthalpy of the bond
• Kelvin scale of temperature (K) = starts at absolute 0k and is equivalent to -273°C
• Mass of the surroundings, m/g = have to identify the materials that are changing temperature
• Hess’ law = if a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route