Enthalpy change of the reaction between zinc and copper sulphate solution
John XXX
June 4, 2021
1 Aim
To determine the enthalpy change for a metal displacement reaction by adding a known mass of zinc powder to
excess aqueous copper(II) sulphate solution and recording the rise in temperature
2 Apparatus required
2.1
–
–
–
–
–
–
–
2.2
per group
100 ml beaker
25 ml beaker
50 ml measuring cylinder
Plastic cup ×3
0-110 °C thermometer
Stop watch
250 cm3 conical flask
General
– Electronic balance capable of weighing to at least 0.1g
3 Chemical required
– 1.0 mol/dm3 aqueous copper(II) sulphate solution
– 2.0g zinc powder
4 Notes
Safety Hazard: The neutrilisation of zinc and copper sulphate is exothermic, which will release a lot of heat and
the aqueous solution in the plastic cup may be splashed out, which will cause safety hazard.
Environmental care: The zinc powder can not be poured to the soil directly because the zinc will harm the
environment.
5 Results
Volume of aqueous copper(II) sulphate solution = 40.0 ± 0.5cm3
Mass of zinc powder used
= 2.0 ± 0.1g
Time/min
0.00
1.00
2.00
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
Temperature ±0.5 °C
25.0
25.0
25.0
25.0
39.0
54.0
60.0
59.5
59.0
58.0
57.5
57.0
56.0
55.0
Table 1: Table of results
Figure 1: Graph of results
Relevant Qualitative Observations
Before the zinc powder adding to the copper sulphate solution, copper sulphate solution is a transparent blue liquid.
The reaction between Copper sulphate and Zinc is an exothermic reaction which can react immediately and release
a large amount of heat energy. During the reaction, the thermometer will demonstrate the rapid rise in temperature.
When the reaction finished, the temperature of the solution will gradually decrease to room temperature. After
the reaction, the colour of the solution becomes greyish instead of blue, and the copper in the form of red powder
precipitate can be found at the bottom of the beaker.
6
Data Analysis
a) Equation for the reaction: Zn(s) + CuSO4(aq) −−→ ZnSO4(aq) + Cu(s)
Mass of CuSO4 :
= 40.0ml × 1.0g/ml = 40.0g
Mass of Zn:
= 2.0g
b) Initial Temperature (T0 )= 25.0 °C
Extrapolation (T1 )
= 62.0 °C
∆Temperature
= 37.0 °C
c) Total energy released Q = mc∆T = 40.0 × 4.18 × 37.0 = 6186.4J
Amount of Zn
Amount of CuSO4
Enthalpy change:
2.0g
= 0.031mol
65.38g/mol
40.0g
=
= 0.251mol (in excess)
63.55g/mol + 32.07g/mol + 16g/mol × 4
Q −6186.4J
∆H = =
= −199.56kJ/mol ≈−200kJ/mol
n 0.031mol
=
0.5
×1 = 1.25%
40.0
0.1
Uncertainty of mass of Zn =
= 5%
2.0
1.0
Uncertainty of temperature =
= 2.7%
37.0
d) Uncertainty of mass of H2 O =
Total uncertainty of heat energy released: = 1.25% + 2.7% + 5% = 8.95%
Absolute uncertainty of energy released: = 8.95% × 6186.4 = 553.68J = 0.554kJ ≈ 0.6kJ
Total energy released:
= 6186.4 ± 0.6kJ
Absolute uncertainty of ∆H
= 8.95% × 199.56 = 17.86kJ/mol ≈ 20kJ/mol
Enthalpy change
= −200 ± 20kJ/mol
Uncertainties - neglecting uncertainty in molar mass, as an order of magnitude less than volumetric method
Range
uncertainties, and taking the uncertainty of repeated values as being ± √ .
2 n
Quantity
Volume of copper sulphate solution prepared
∆Temperature
Mass of zinc powder prepared
Volume of measuring cylinder
Thermometer
Total energy released
Enthalpy change (∆H)
Value
40.0
37.0
2.0
50.0
110.0
6.1864
-200
Absolute uncertainty
0.5
1.0
0.1
0.5
0.5
0.6
20
Percentage uncertainty
1.25%
2.7%
5%
1%
0.45%
9.7%
10.0%
Table 2: The uncertainties in the experiment
7
7.1
Evaluation
The review of systematic errors
There is various possible systematic errors in the experiment that may affect the final accuracy of the value we
gained:
1. Heat loss: The heat loss during the reaction is the major issue that affecting the final precision and accuracy of
calculation of enthalpy change. Although the heat loss to the surrounding is already being considered as we
used plastic cup which is a poor conductor of heat to be the reaction container, heat loss is still unavoidable;
2. Impurity in Zinc: The zinc powder is not prepared by our students and we do not know the purity of the zinc
powder. If the zinc power that we take has some impurity in it, the amount of copper sulphate solution reacted
with it will be influenced as well, further creating bigger uncertainty of the enthalpy change;
3. Time interval: The interval of reading of time is every 30 seconds, that interval may not be small enough
to gain sufficient data because the increasing in temperature during the reaction is fast and 30 seconds a big
interval. Also, we need more data to plot a more reliable;
4. Not shake the thermometer during the reaction: Because the reaction is only occur in the sites where zinc
powder and copper sulphate meets each other, that’s why the temperature in the solution will varies among
different point. Thus, it is crusial to stir the solution with thermometer which act as a glass rod to ensure all
the zinc powder reacted with copper sulphate during the same period of time and recorded temperature will be
more accurate;
5. The solution may be splashed out: Because the reaction is exothermic and we are stirring the solution during
the reaction, it is readily easy to splashed out the solution incautiously. The loss of solution may affect the
temperature we recorded. It is also dangerous that if the hot liquid splashed into eyes;
6. The intervals of apparatus: Due to the apparatus in school is limited, so the intervals of those equipment may
not be small enough for highly required accuracy.
7.2
The review of random errors
1. Weighing and Reading: In weighing the solids of zinc powder and reading the volume of solution as well as
the reading of thermometer, the random errors always affect the final reading of the value and further affect
the accuracy of the values. For example, in reading the volume of the solution, the parallax may mislead the
students. The bird eye’s view gives rise to the reading above the actual volume whereas the bottom view causes
the reading less than the actual volume;
2. Repeating the experiments over time: In order to reduce the random errors, the experiment should be repeated
over time.
7.3
The improvement for the errors
1. Use better insulator to minimise the heat loss, for example, set two plastic cups and trap a lay of air between
two cups. Thin layer of air is a extremely good insulator of heat;
2. Use lid to both prevent the heat loss and the spray of acid;
3. To avoid the parallax, the eye levels of observers should keep the same as the label on the measuring cylinder
and thermometer. Also, the measurement should be taken from the lowest point of meniscus;
4. To prepare the zinc powder with minimised impurity;
5. Be ware do not splash the solution, stir the solution steadily to ensure the zinc powder is reacting with copper
sulphate;
6. Use apparatus with small intervals which greatly reduced uncertainty;
7. Use digital thermometer which have more accurate reading of temperature.