Practical 2: Measurement of an enthalpy change

• We should use a polystyrene cup with a lid to reduce heat loss

  We need to use a cooling curve to get an accurate value of the change in temperature
• Cooling curve

  >Record the temperature of the zinc sulphate every minute up to 3 mins to ensure the temp is constant

  >At the 4th min add the Mg powder to the solution and don't record the temp and then continue to record the temp every minute up till 10 mins

  >Plot a graph of temp vs time

  > Draw 2 lines of best fit and extrapolate to the 4th minute

  >Then work out the change in temperature reached using the graph
• Determine an enthalpy change which cannot be measured directly

  • Add Xg of solid into a weighing boat and record the mass of the solid and weighing boat.
  • Record the mass of weighing boat and work out the exact amount used.
  • Add 100cm3 of distilled water using a measuring cylinder to a polystyrene cup
• Determine an enthalpy change which cannot be measured directly

  • Record the temperature of the water every minute up to 3 minute
  • Add the solid to the polystyrene cup , do not record the temperature at the 4th minute.
  • Continue to stir the mixture and record the temperature every minute up to 15 minutes
  • Plot a cooling curve - temp x axis and time y axis
  • extrapolate both line to point of mixing and work out change in Temperature
• Why can the enthalpy change not be directly measured?

  We cannot add the exact amount of water to turn CuSO4 into CuSO4.5H2O as some will dissolve
• Explain why the experimental value is different to the value in a data book

  Heat loss to surrounding, CuSO4 will react with water vapout and become hydrated some of the solid will clump togther