Determining an Enthalpy Change of Reaction

Determining an Enthalpy Change of Reaction

Aim: to determine the enthalpy change for the displacement reaction:

By adding an excess of zinc powder it a measured amount of aqueous copper (II) sulphate and measuring the temperature change over a period of time, you then calculate the enthalpy change for the reaction.

Formula

This will give us heat which is opposite to enthalpy.

Diagram

00.00.00

25.5 °C

s

Stopwatch

Temperature Display

Zinc powder

Lid Temperature probe

Data Collection

Time (s)

Temperature (°C) ± 0.1 °C Trial 1

Trial 2

Trial 3

0

22.8

22.4

22.5

30

22.8

22.4

22.6

60

22.8

22.4

22.5

90

22.8

22.5

22.5

120

22.8

22.5

22.5

150

39.8

40.8

51.2

180

55.3

67.9

69.8

210

65.5

70.2

69.5

240

69.4

69.0

67.9

270

68.2

67.3

66.5

300

66.6

65.9

65.1

330

65.3

64.5

63.7

360

63.9

63.2

62.5

390

62.7

61.9

61.3

420

61.5

60.7

60.0

450

60.4

59.5

58.9

480

59.2

58.5

57.8

510

58.2

57.4

56.8

540

57.4

56.3

56.0

570

56.4

55.3

55.1

600

55.6

54.4

54.4

630

54.9

53.5

53.4

660

54.2

52.7

52.7

690

53.6

51.9

52.0

720

53.0

51.1

51.3

This data was then represented in 3 graphs, 1 for each trial. The graphs showed the change in temperature over the time span. See attached hand drawn graphs.

Qualitative Observations

Before reaction takes place During reaction

After reaction takes place

The copper sulphate, mostly made up of water, is a clear bright blue. As the zinc is added to the copper sulphate, the solution begins to heat up until it reaches a maximum at which point it begins to cool down After the reaction is complete we are left with the products, reddish copper and white zinc sulphate.

Data Processing

Graphs On the graphs the red line represents the slope at the time of cooling or the rate of cooling, and the green line indicates where the reaction began. The point at which the two lines meet is the theoretical maximum temperature for that trial. (see graphs). This is the max temperature value we will use for our calculations.

Trial Max theoretical Temperature found from graph (°C)

1

2

3

74.0

74.0

72.8

As we have two of the same values for our temperature (74.0) the last value may be an outlier but there is no way to be sure of this with only three trials so we will need to include it in our calculations.

Calcualtions

average max temp = 73.6°C

Here we are finding the average of our maximum temperatures, to have one final, more accurate value for our enthalpy change formula. we In the formula need the change in temperature therefore we must calculate the average minimum temperature of the three trials. This is just the lowest temperature recorded for each trial.

average min temp = 22.6°C

= 51°C

To find you subtract the average minimum temperature from the average maximum temperature.

Then we use the heat equation to find Q, assuming that the copper sulphate is made up completely of water and has a c value of 4.18 KJKg-1

We used 25ml of copper sulphate which equals 25g = 0.025kg

We want to find the heat change per mole so we divide by the number of moles

This is now the heat produce per mole of copper sulphate reacted.

Finding the Uncertainty

We take the uncertainty of the instrument and divide it by the smallest measurement taken to give us the biggest uncertainty then we add up the 2 (pipette and temperature) and times by 100 to give us the percentage uncertainty.

Final answer Q=

± 0.44KJmol-1

But because we are looking for enthalpy change we just have to switch the sign as enthalpy change is opposite to heat change.

Final value for Enthalpy change

Conclusion

When we compare our value of approximately -213KJmol-1 it is very close to the accepted value of -217KJmol-1. However it is not within the uncertainty. When I did the percentage yield formula I found that I was less than 2% off (only 1.76%). There was bound to be a difference as experimental conditions are never perfect. Also I did wonder if because we used the specific heat capacity of water and not copper sulphate if it made a difference.

Evaluation

Error

Explanation

Improvement To improve this I would proposed using 2 or maybe 3 calorimeters and placing them inside each other and then taping them together airtight. This would add insulation but also create airspace, which would insulate well. Then I would cut out a polystyrene top with a much smaller hole so that the temperature probe can fit more snuggly. This should minimize heat loss significantly and hopefully give a more accurate value for ∆H.

Heat loss

This was our major point of error and probably the main reason why our value was below the literature value. The calorimeter was polystyrene but the top would have let out a lot of heat to the surroundings that would not have shown in our data as they would not have warmed the temperature probe

C value of water or copper sulphate

I felt that by using the specific heat capacity of water for our calculations we decreased the accuracy of our result, as we did not know the specific heat capacity of copper sulphate. There is no way to know if this made our value smaller or larger.

Stirring the zinc so it reacts fully

If this experiment were re done I would recommend When the zinc is added it larger quantities of both often gets stuck on the sides copper sulphate and zinc to and does not all react. It is make sure that they can be in excess but it may still mixed properly although have affected our results you will need to make sure you are careful not to spill them over the side.

The SHC of copper sulphate is 4.184KJKG-1K-1. Obviously this does not seem like it would make a big difference but I recommend that it was used in future calculations