energetics

• This is a revision video for AQA chemistry
• The PowerPoints used in this video can be purchased for revision purposes
• Enthalpy


  The heat change in a reaction at constant pressure, given the symbol ΔH
• Exothermic reaction


  • Combustion of fuels
• Bond making


  Exothermic process, releases energy, ΔH is negative
• Calorimetry experiment

  1. Measure mass of fuel

  2. Measure temperature rise of water

  3. Calculate energy transferred using Q = m x c x ΔT

  4. Calculate moles of fuel used

  5. Enthalpy change = Energy transferred / Moles of fuel
• This is an exothermic reaction, it's a fuel producing heat energy
• ΔHny

  • 364.5 kJ per mole
• Calculating enthalpy change of neutralization

  1. Q = mc∆T

  2. Moles of HCl = concentration x volume

  3. ΔH = -Q/n
• Hess's Law


  The total enthalpy change of a reaction is independent of the route taken
• Hess cycle


  1. Formation cycle

  2. Combustion cycle
• Going against the arrow in a Hess cycle

  Change the sign of the enthalpy value
• Going with the arrow in a Hess cycle

  Keep the sign of the enthalpy value the same
• Enthalpy of combustion of methanol is -732 kJ/mol
• Enthalpy of formation of pentane is -177 kJ/mol