Energetics

• Enthalpy


  The change in energy when a chemical reaction occurs, measured at constant pressure
• Enthalpy change


  Denoted by the symbol ΔH, measured in kJ/mol, under standard conditions of 100 kPa and 298 K
• Endothermic reactions


  • Absorb energy from the surroundings, feel colder
  • Reactants have lower energy than products
  • ΔH is positive
• Exothermic reactions


  • Release energy to the surroundings, feel warmer
  • Reactants have higher energy than products
  • ΔH is negative
• Bond breaking and bond making

  1. Breaking bonds is endothermic, ΔH positive

  2. Making bonds is exothermic, ΔH negative
• Mean bond enthalpy


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

  Total energy to break bonds - Total energy released when forming bonds = ΔH
• Calorimetry


  A method to determine the enthalpy change of a reaction by measuring the temperature change of a solution
• Calorimetry experiment


  1. Measure mass of fuel burned and temperature change of water

  2. Use Q = m x c x ΔT to calculate energy transferred

  3. Divide energy by moles of fuel to get enthalpy change
• Calorimetry can be used to determine the enthalpy change of combustion of fuels
• Calorimetry can also be used to determine the enthalpy change of acid-base reactions
• 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


  Measure temperature change when mixing acid and alkali in a polystyrene cup
• Calorimetry experiment


  1. Add acid

  2. Measure initial temperature

  3. Add alkali

  4. Measure final temperature
• Acid-alkali reactions are used for calorimetry experiments because they show a reasonable temperature change
• Q = mcΔT


  Energy change = mass x specific heat capacity x temperature change
• Assume density of liquids is 1 g/cm³ so mass = volume
• Calculating enthalpy of neutralization

  1. Find moles of HCl

  2. Energy change / moles of HCl = enthalpy change
• Hess's Law


  Total enthalpy change of a reaction is independent of the route taken
• Hess Cycles


  • Formation cycle
  • Combustion cycle
• Using Hess Cycles


  1. Draw cycle with reactants, products and elements

  2. Substitute data into cycle

  3. Calculate enthalpy change by going with/against arrows
• Enthalpy changes are exothermic if negative, endothermic if positive
• Sum of enthalpy changes around a Hess Cycle should equal zero