RP2 - measurement of an enthalpy change

Question 1

why is it impossible to measure the enthalpy change of a reaction directly?

Answer 1

this is because it is not possible when adding water to anhydrous copper sulphate to prevent it forming a solution

Question 2

why did we not just use the highest temperature recorded to calculate change in temperature?

Answer 2

because this is not the maximum change in temperature for the reaction, this would have occurred just after the addition of the solid
measuring the temperature at this time would have led to an inaccurate result as at this point all the solution would have been at the same temperature

Question 3

what would explain differences in data book values and the ones you obtained and explain how this difference could be minimised?

Answer 3

heat loss is the biggest source of error and could be minimised by putting a lid on the calorimeter

Question 4

outline a method for determining an enthalpy change which cannot be measured directly

Answer 4

1. weigh out between 3.90g and 4.10g of anhydrous sulphate in a dry stoppered weighing bottle, keeping the stock of solid in a closed container during weighing. Take care to avoid skin contact. The precise mass should be recorded.
2. Construct a suitable table of allow you to record temperature at minute intervals up to 15 minutes
3. Using a measuring cylinder place 25cm3 of deionised water into a polystyrene cup and record its temperature at the beginning, start the timer and then record the temperature again every minute stirring the liquid continuously
4. at the 4th minute add the powdered anhydrous copper sulfate rapidly to the water and continue to stir but do not record the temperature. At the 5th minute and for every minute up to 15 minutes stir and record the temperature
5. Plot a graph of temperature against time. Draw 2 separate best fit lines, extrapolating both lines to the 4th minute
6. Use your graph to determine the temperature change at the 4th minute
7. weigh out between 6.20g and 6.30g of hydrated copper sulfate in a dry stoppered weighing bottle. The precise mass should be recorded.
8. Construct a suitable table of results to allow you to record temperature at minute intervals up to 15 minutes
9. Using a measuring cylinder place 23cm3 of deionised water into a polystyrene cup
10. Repeat the procedure
11. Plot the graph similar to the first experiment and determine temperature change