Required Practical 2

• how do you measure enthalpy change?

  1. weigh between 3.9-4.10g of anhydrous copper (II) sulphate in a dry, stoppered weighing boat. keep the stock of solids in a closed container during weighing. the precise mass should be recorded

  2. construct a suitable table of results to allow you to record the temperature at minute intervals for 15 minutes

  3. using a measuring cylinder, place 25cm3 of distilled water into a polystyrene cup and record its initial temperature (t=0). start the timer and then continue to record the temperature each minute, for three minutes

  4. at the fourth minute, add the powdered anhydrous copper (II) sulphate to the water in the polystyrene cup but do not record the temperature. at the fifth minute continue the temperature readings at minute intervals, up to fifteen minutes. stir the solution in the polystyrene cup as this is done

  5. plot a graph of temperature (on the y-axis) against time. draw two separate best fit lines; one which joins the points before the addition and one which joins the points after the addition - extrapolate both lines to the fourth minute

  6. use your graph to determine the temperature change at the fourth minute, which theoretically should have occurred immediately on addition of the solid
• what are some possible errors with this practical?

  - heat transfer to or from surroundings (usually heat loss)

  - the method assumes all solutions have the heat capacity of water

  - neglecting the specific heat capacity of the calorimeter, any heat absorbed by the apparatus is ignored

  - reaction or dissolving may be incomplete or slow

  - density of solution is taken to be the same as water

  - some of the water could have evapourated

  - room temperature could have changed

  - incomplete combustion if using a fuel
• how do you improve accuracy of this practical?

  - an electronic temperature sensor and data logging software could be used to plot the graph accurately

  - a flame calorimeter could be used to improve accuracy:

  -> spiral chimney is made of copper

  -> flame is enclosed

  -> fuel burns in pure oxygen rather than air so complete combustion
• why are polystyrene beakers used?

  good insulators and have a high specific heat capacity
• How can you reduce the uncertainty in the mass measurement?

  - Use a larger mass

  - Use a balance with a greater resolution
• What is percentage uncertainty and how do you calculate it?

  Percentage uncertainty in a measurement =

  100 x absolute uncertainty/calculated value
• How can you prevent heat loss to surroundings/apparatus?

  - insulate the beaker baby placing it in a polystyrene cup with a lid

  - avoid large temperature differences between surroundings and calorimeter

  - use a bomb calorimeter