practical 2

• describe a method to collect data to determine enthalpy change

  1. weigh out approximately 4g of anhydrous copper sulfate and record precise mass (weighing by difference)
  2. add 25cm3 of deionised water to a polystyrene cup and record the temperature at t=0
  3. start the timer and record the temperature every minute, continuously stirring
  4. add the anhydrous copper sulfate at minute 4 rapidly and continue to stir, do not record the temperature at minute 4
  5. continue to stir and record the temperature from minutes 5-15
• describe how you would determine the enthalpy change from the data collected

  1. plot a graph of time (x) against temperature (y), drawing seperate lines of best fit for minute 0-3 and 5-15, extrapolating both lines to minute 4
  2. use the extraplation to calculate the temperature change at minute 4, which theoretically should have occured on addition of the solid
• how may this reaction method be improved?

  • use an electronic thermometer
  • use a flame colorimeter as flame is enclosed, spiral chimney is made of copper, fuel burns in pure oxygen
• what are some errors with this method?

  • heat loss to surroundings
  • reaction may be incomplete or slow
  • specific heat capacity and density of water and solutions may be different
  • some water may evaporate
  • if using a fuel, incomplete combustion may occur
• why must the reaction mixture be continuously stirred?

  ensures the reaction mixture is homogenously mixed to encourage even temperature change
• why must the solid be added rapidly?

  to ensure instantaneous reaction
• why is the temperature not recorded at t=4?

  it is impossible to record the immediate temperature change so must be extrapolated to find the instantaneous change in temperature
• how could you decrease the percentage uncertainty of using the same thermometer?

  increase the change in temperature by increasing the mass of the solid or concentration of acid/alkali if two solutions used