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
• Enthalpy change


  • Measured in kJ/mol
  • Occurs under standard conditions of 100 kPa pressure and 298 K temperature
• Endothermic reactions


  Reactions that absorb energy from the surroundings, ΔH is positive
• Endothermic reaction


  • Decomposition of calcium carbonate
• Exothermic reactions


  Reactions that release energy to the surroundings, ΔH is negative
• Exothermic reaction


  • Combustion of fuels
• Bond breaking


  Endothermic process, requires energy input, ΔH is positive
• Bond making


  Exothermic process, energy is released, ΔH is negative
• Mean bond enthalpy


  Average energy required to break or form a particular type of bond
• Calculating enthalpy change using mean bond enthalpies

  1. Total energy to break bonds

  2. Total energy released when bonds are formed

  3. Enthalpy change = Energy to break bonds - Energy released when bonds are formed
• Calorimetry


  Experimental method to determine the enthalpy change of a reaction
• Calorimetry experiment


  1. Measure mass of fuel

  2. Measure temperature rise of water

  3. Calculate energy transferred using Q = mcΔT

  4. Calculate enthalpy change per mole of fuel
• There is a difference between energy (in J) and enthalpy (in kJ/mol)
• Calorimetry experiment with solution

  1. Add acid first and measure temperature

  2. Add alkali and stir, measure temperature change

  3. Acid-alkali reactions often show a reasonable temperature change
• ΔHny


  Energy change divided by moles minus 14212
• This is an exothermic reaction, it's a fuel producing heat energy
• ΔHny


  • 364.5 kJ per mole
• Calorimetry experiment of a solution

  1. Add acid

  2. Measure temperature

  3. Add alkali

  4. Measure temperature change
• Mass


  Mass of acid + mass of alkali (assuming density = 1 g/cm³)
• ΔS = -5016 J/mol·K (exothermic reaction)
• Hess's Law


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


  • Formation cycle
  • Combustion cycle
• Using Hess Cycle


  1. Draw cycle with reactants, products, and elements in standard states

  2. Substitute data into cycle

  3. Calculate ΔH or ΔS using cycle