q = mc ΔT
Hence, q = 4.18 x 28.1 x 50.0
= 5872.9 J
Moles of aq copper sulphate used in the experiment:
Volume x concentration = 50.0 x 0.500 = 0.0250 moles
1000 1000
Moles of Zinc power used in the experiment:
Mass = 4.761 = 0.07283
mr 65.37 =0.0728(3s.f)
In the method given, I was asked to weigh out the mass of the Zinc powder between 4.5g and 5g. This shows that the reagent Zinc is in excess as the mass of Zinc does not effect the outcome of the results.
The limiting reagent for this experiment is copper sulphate solution. Hence, I will be using the moles of copper sulphate to work out the rest of the calculation in the experiment.
I will then use the values that I have worked out from the previous calculation to work out that molar enthalpy change for the reaction:
ΔH = -q = -5872.9 = -234.9
Moles x 1000 0.0250 x 1000 = -235(3s.f)
Note: This is assuming that the experiment is under standard condititions.
During this experiment, I must also consider the errors into account as they could affect the overall results. The apparatus that I used for this experiment is the most likely cause of error for example, transferring the Zinc powder from the weighing boat to the polystyrene cup is an error as there would be Zinc powder leftover on the weighing boat but in this investigation, it would not be the case as Zinc powder is in excess so the error would not take into account.
First of all, I will work out the maximum percentage error in using each piece of the apparatus. This is to ensure that I have include all the possible errors in the apparatus and so that I can comment on whether if the apparatus contributes in affecting the overall results.
The maximum percentage error in apparatus:
Error in balance = ±0.001g = 0.001 X 100
4.761 = 0.02100 %( 4s.f)
Error in the thermometer = ±0.2 = 0.2 X 100
28.1 = 0.7117 %( 4s.f)
Error in 50ml burette pipette= ±0.05 = 0.05 X100
50.0 = 0.1%
Overall maximum percentage error= 0.8327%
= 0.833% (3s.f)
Hence, after working out the maximum percentage error, I can use the figure to estimate the degree of uncertainty in my enthalpy change. The enthalpy change is -235 in 3s.f. I have decided to use 3s.f as an appropriate degree of accuracy so the maximum percentage error 0.8327% will be 0.833% in s.f. In my opinion, the maximum percentage error is small so therefore I don't think it will effect my enthalpy change to an extent as the smaller the number of the maximum percentage error, then the more accurate my enthalpy change will be. I also thought of including the error of the stopwatch but it wouldn't affect the maximum percentage error much so therefore I decided to exclude it from my calculation as the error is tiny and can be discarded.
Conclusion
The reaction between Zinc and Copper Sulphate solution is an exothermic reaction meaning that the reaction releases heat to its surrounding. Therefore, theoretically, the temperature of the solution would increase when the reaction is taking place. This is proven in my results table as when I added the Zinc powder to Copper Sulphate solution at the fourth minute, the temperature dramatically increase from 19.2°C to 40.2°C. The reaction is spontaneous as the temperature increase very rapidly and then gradually slowing down in the later time interval.
From my graph, I have drawn the best fit line for the data that I plotted onto the graph to show a trend that the temperature gradually decrease after a set amount of time. The best fit line proves the accuracy of my results as the best fit line crosses most of the data that I plotted which shows that the data that I collected is reasonably reliable. The best fit line also helps me to identify the anomalies of my values as I can clearly distinguish the anomalies from my values by using the best fit line.
However, I must also take into account of the two anomalies which I circled on the graph and these points do not respond to the trend of the values. This may be due to the possible errors that could have taken place during the experiment.
For example, at the fifth minute after that Zinc powder has been added, the temperature that I collected (40.2°C) is an anomaly as it does not fit into the best fit line. This may be due to the fact that I haven't stirred the solution properly with the glass rod so the heat may not spread through some parts of the solution so by recording the solution with the thermometer, I may receive an unreliable value so therefore I must take into account that it is important to stir the solution vigorously to ensure that the heat is spread out throughout the solution and to ensure that the precipitate has completely dissolved and no sludge remained.
In order to see how well the performance of my results are, I will need to use the theoretical value of the enthalpy change and compare it with the value of my enthalpy change so that I can see how far my value is from the correct value. The theoretical values that I used are from the chemistry data book; JG Stark, H G Wallace, 1982, Chemistry data book, page 57. The values that are used for Copper aq and Zinc aq are:
Δhf° (Cu2+ (aq)) = +64.4
Δhf° (Zn2+ (aq)) = -152.4
By using the Hess Law, I will work out the theoretical enthalpy change by using these values from the data book.
Δh = -(+64.4) + (-152.4)
= -216.8 Kjmol¯¹
I will then compare my value with the theoretical value of the enthalpy change. To do this, i will work out the percentage difference between my experimental value and the theoretical value. This is to see how big the percentage difference between my value and the theoretical value as the smaller the percentage, the more accurate my value will be to the theoretical value.
So first, I will work out the difference between my experimental value and the theoretical value.
-216.8-(-234.916) = 18.116 Kjmol¯¹
Then, I will use this value to find the percentage difference between my experimental value and the theoretical value.
% difference: difference X 100 18.116
theoretical value = -216.8 = -8.35608%
= -8.36% (3s.f)
