RP 2: Measuring Enthalpy Change

• What is Hess' Law?

  The principle that the total enthalpy change of a reaction is independent of the pathway between the initial and final states.
• How do you measure enthalpy change of a reaction experimentally?

  • q = mcΔt
  • m = mass of solution that changes temperature (usually water).
  • c = specific heat capacity (usually of water).
  • Δt = temperature change (C or K).
  • q = heat energy taken in / released in J.
  • divide q by number of moles of limiting reactant.
  • + or - to show exo/endo.
• Why may theoretical/experimental values for enthalpy change vary?

  1. Heat loss to apparatus and surroundings.
  2. Incomplete combustion.
  3. Non-standard conditions.
  4. Evaporation of alcohol or water.
• How do you prevent heat loss?

  • Insulate beaker with a polystyrene cup and lid.
  • Avoid large temperature differences between the surroundings and the calorimeter.
  • Use a bomb calorimeter.
• How can accuracy be improved (other than heat loss)?

  • Read thermometer at eye level to eliminate parallax errors.
  • Stir solution so temp is easily distributed.
  • Use a digital thermometer for more accurate readings.
  • Use greater concentrations and masses.