The concentration of copper sulphate solution is 0.5 mol/litres. One mole of copper sulphate solution can be calculated as shown below:
CuSO4
1 mole = (1 x 64) + (1 x 32) + (4 x 16)
= 64 + 32 + 64
= 160g
The concentration of copper sulphate solution is therefore 0.16 mol/litres because 160 grams of copper sulphate is dissolved in 1000 cm3 of water. Similarly, 0.5 mol/litres can be worked out by calculating half a mole of copper sulphate, i.e. 160 2 = 80g. So, in copper sulphate 80g are dissolved in 500 cm3.
In my experiment, I think it will be best to use powdered zinc, as opposed to pieces of zinc. It will be easier and quicker to weigh out 80g of powdered zinc if it is in powdered form, than if it is in pieces that have to be cut up. During the experiment the rate of reaction will be much quicker if I use powdered zinc because it will have a much larger surface area than pieces of zinc and so, would dissolve faster.
I have decided that the mass of zinc should increase by 0.2g each time it is repeated. I will use the following masses: 0.5g; 0.7g; 0.9g; 1.1g; 1.3g; 1.5g and 1.7g. When I carry out the experiment with each of these masses, there should be no unreacted copper sulphate solution left.
I have decided to use 40 cm3 of copper sulphate solution. Enough solution must be used so that the bulb of the thermometer will be completely covered by it throughout the experiment.
The number of moles of copper sulphate solution is worked out using the following equation:
No. of moles = volume x concentration
1000
= 40 _ x 0.5
1000
= 0.02 moles
By finding the number of moles in the copper sulphate solution, I can calculate the exact amount of zinc which will react with it:
The ratio of CuSO4 : Zn
is 1 : 1
0.02 : 0.02
This means that one mole of copper sulphate molecules will react with one mole of zinc atoms, and so, 0.02 moles of copper sulphate molecules will react exactly with 0.02 moles of zinc atoms.
The mass of zinc atoms that will be needed can be calculated using the following equation:
No. of moles = _ Mass of zinc ___
1 Relative Molecular Mass (RAM)
No. of moles x RAM = 1 x Mass of zinc
0.02 x 65 = 1 x Mass of zinc
1.3g = Mass of zinc
This means that 1.3g of zinc powder will react exactly with 0.5 moles of copper sulphate solution.
If a larger mass of zinc was used during the experiment, there will be no reaction – there is no more copper sulphate solution to react with, and so, no more heat energy is given out.
If a smaller mass than 1.3g of zinc is added, there will be some heat energy given out, but the copper sulphate solution will not reach the optimum temperature until 1.3g of zinc is added.
The amount of zinc added to the copper sulphate will be related to the amount of heat energy given out. After 1.3g of zinc is in excess, and the copper sulphate is used up, I predict that the temperature will remain the same. This prediction can be displayed in the graph below:
Temperature
Rises (0C)
0
0 1 1.3 2
Amount of Zinc
Added (g)
The observations I expect to make when the grey zinc powder is added to the blue copper sulphate solution are:
- Fizzing and bubbles;
- Heat given off;
- The solution eventually turning colourless.
I will need to measure the zinc powder with electronic weighing scales that are accurate to two decimal places. I will measure the volume of copper sulphate using a pipette and I will measure the temperature of it with a thermometer.
The controlled variables are those that I will need to keep constant throughout my experiment. These will be both the volume and the concentration of the copper sulphate solution.
The dependant variables will be measuring the temperature of the copper sulphate solution and the independent variable will be the different masses of zinc powder used.
Experimental Procedure
Aim
In this experiment, I will be investigating how temperature is affected when different masses of zinc are added to copper sulphate.
Apparatus
- 7 polystyrene cups and 1 lid to hold the solution and insulate it when the reaction is taking place:
-
A pipette, so 25cm3 of copper sulphate solution can be accurately measured:
- 7 beakers to hold the different masses of zinc powder:
- Electronic weighing scales, accurate to 2 decimal places so the zinc can be weighed out as accurate as possible.
- Electronic thermometer, accurate to 1 decimal place to record the temperatures of the copper sulphate solution before the reaction, and the zinc sulphate solution after the reaction has taken place.
Safety
As always, the normal laboratory rules will apply during this experiment, for example, blazers and scarves to be kept outside, long hair tied back and the workbenches being kept clear. Because both copper sulphate solution and zinc powder are irritant, it is important to wear safety goggles throughout the experiment. It is also important to avoid getting the zinc powder under your nails or on your hands, so it is a good idea to wash your hands well after the experiment.
Fair Test
In order to ensure that there will be a fair test, it is necessary to keep some aspects of the experiment constant. The same size and material of the container holding the solution should be used, i.e. 7 identical polystyrene cups. The same volume and concentration of copper sulphate solution should be used and zinc powder should be used throughout the experiment. When the temperature of the zinc sulphate solution is taken, I will expect some copper to form at the end of the thermometer. In order to receive an accurate reading as possible, I should remove this as best I can. I will also be repeating the whole experiment again so my results can be as reliable as possible.
Method
- Collect all the apparatus required for the experiment, that is, 7 polystyrene cups and 1 lid, 7 small beakers, a pipette, electronic weighing scales and an electronic thermometer.
-
Measure 40cm3 of copper sulphate solution into each of the 7 polystyrene cups using the pipette. Measure and record the temperature of the solution.
- Weigh out the range of masses of zinc powder selected using the electronic weighing scales, e.g. 0.5g; 0.7g; 0.9g; 1.1g; 1.3g; 1.5g and 1.7g. First weigh the beaker used to hold the zinc and add the mass of zinc required on to this weight. For example, if the beaker weighed 50.4g and 0.5g of zinc is required, then enough zinc should be added until the weight reaches 50.9g.
- Add the 0.5g of zinc powder, to the first cup of copper sulphate solution and put on the lid. Place the sensor of the thermometer into the cup and stir so the zinc powder will dissolve faster. Record the highest temperature that the solution reaches.
- Be sure to remove any copper that has formed on the tip of the sensor, as this may be the cause of any results being inaccurate.
- Record the temperature of the copper sulphate solution before and after the reaction has taken place and repeat Step No. 4 with each mass of zinc powder.
- Repeat this experiment again so the results will be accurate.
I will record my results in the following tables:
Temperatures of Copper and Zinc Sulphate Solution Before and After Reaction
Average Temperature Difference of Copper and Zinc Sulphate Solution
After I have recorded my results, I will plot a graph showing what the relationship is between the mass of zinc powder and the average difference in temperature.
Results
Temperatures of Copper and Zinc Sulphate Solution Before and After Reaction for First Experiment
Temperatures of Copper and Zinc Sulphate Solution Before and After Reaction for Second Experiment
Average Temperatures of Copper and Zinc Sulphate Solution Before and After Reaction
Interpretation of Results
By looking at my graph, I can see that there is a relationship between the mass of zinc and the temperature rise – as the mass increased, the temperature of the copper sulphate solution increased. This can be confirmed by looking at the results obtained:
When 1g of zinc powder is added, the temperature is 9.8g;
When 1.2g is added, the temperature is 10.35g;
When 1.4g is added, the temperature reaches 10.8g.
Before I conducted my experiment, I predicted that as more zinc was added, the temperature would rise. For this part, my predictions were correct, but the temperature didn’t level off when 1.3g of zinc was added as I had expected, but rose slightly instead. I expected this to be the case because at this point, the zinc is in excess and all the copper sulphate solution has been reacted. Because the reaction has finished taking place, there should not have been anymore heat given out, and so the temperature should have remained the same when larger masses of zinc were added. When I was carrying out my experiment, I noticed how some zinc powder remained in the bottom of the beakers each time. For this reason, my masses of zinc were not entirely accurate – only 1g, instead of 1.3g could have been used, for example. This would explain why the temperature continued to rise when larger masses were being added.
The mass of zinc that I didn’t include in my line of best fit on my graph was 0.5g. The temperature difference I calculated for this mass was 6.7°C. Although this is lower than the next temperature of 6.85°C for 0.7g, there is not a large enough difference between the two for them to make sense. From my graph I can see that ideally the temperature difference for 0.5g should have been 3.25°C. I may also have measured the volume of copper sulphate solution for this cup, or the weight of zinc powder incorrectly, which would also explain why this is an anomalous result.
By looking at the results of my experiment, I can confirm what I had said at the beginning of this investigation.
I know that it is a redox reaction because the following equation justifies what my results show:
Reduction
Cu+2(aq) + Zn(s) → Zn+2(aq) + Cu(s)
Oxidation
Cu+2 has been reduced because it has gained two electrons and the Zn has been oxidised because it has lost two electrons. I know this is correct because when I was carrying out my experiment, copper solid formed on the tip of the sensor on the thermometer after the zinc powder was added.
In this experiment, zinc replaced copper sulphate because it is more reactive than it is. This displacement reaction can be shown in the following equation:
Zinc + copper sulphate → zinc sulphate + copper
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Grey Blue Colourless Red
Metal Solution Solution Solid
Looking at the Reactivity Series of Metals can reinforce this. By seeing that copper is further down the series than zinc, we know that it is less reactive, and so will be displaced by the zinc.
The reaction between zinc and copper sulphate is exothermic because overall more energy is given out (i.e. heat energy). This is due to more bonds being formed than broken, as illustrated below:
Cu — Cu
Zn — Zn
atoms atoms
This is an endothermic reaction because energy is put in so the bonds can be broken.
The bonds that are formed are:
Zn — SO4
Cu — Cu
atoms atoms
This is an exothermic reaction because energy is released and so, bonds are formed.
There are more bonds formed than broken so again, the reaction between zinc and copper sulphate is exothermic.
The powdered zinc increased the rate of reaction because it would have had a larger surface area than pieces of zinc with the same mass. This means that more of the zinc is in contact with the copper sulphate solution and so it will dissolve faster and react faster.
The temperature of the copper sulphate solution rose because of the reaction occurring with the zinc powder. As they reacted together, energy was released and heat was given off.
Following the results I believe I have produced enough evidence to support the statement that in the reaction between zinc powder and copper sulphate solution, more heat is given out when more zinc is added and that it is an exothermic reaction.
Questions
“What was the temperature change for 1.3g of zinc?”
In my first experiments, I recorded the temperature change to be from 22.0°C to 32.4°C, a difference of 10.4°C. For my second experiments, I recorded the copper sulphate solution to be 24.4°C and the zinc sulphate solution 35.2°C – a temperature change of 10.8°C. From these figures I calculated that the average temperature change for 1.3g of zinc was 10.6°C.
“If it takes 4.2J to raise the temperature of water by 1°C, how much heat energy was required to raise the temperature of the reaction mixture to the new temperature?”
Assuming 1g = 1cm³ and so 40g is = 40cm³, the equation to calculate the heat energy is:
Heat Energy = Mass of water x Joules x Temperature change
= Mass of CuSO4 x 4.2 (J) x 10.6 (°C)
= 40 cm³ of CuSO4 x 44.52
= 1780.8 J
This means that the amount of heat energy that is given out when 1.3g of zinc is used is 1780.8 joules.
“Calculate the heat change of the reaction per mole of zinc.”
1 mole of zinc = 65g
No. of moles = Mass
RAM
= 1.3
65
= 0.02 moles of zinc
0.02 moles of zinc = 1780.8 J
x 50 x 50
1 mole of zinc = 89 040 J/Mole
Evaluation
Throughout the experiment, there was always zinc left in the bottom of the beakers and some remained undissolved in the bottom of the polystyrene cup. This made the results inaccurate because not all of the zinc was used each time. A layer of oxide may also have formed on the zinc I was using. This extra oxide would make the zinc powder heavier, so the exact amount would not have been weighed out, 1g may have been used instead of 1.3g for example. I also found it a little difficult to measure the copper sulphate solution accurately with the pipette.
I would improve the experiment by measuring the zinc powder onto some paper, instead of in a beaker. The chance of calculating a mistake would be reduced and there would be no possibility of any significant amount of zinc remaining on the paper, as it did in the beaker.
Investigating the Factors Affecting the Temperature Change Between Zinc and Copper Sulphate.
Nicola Donnelly 12D
Miss Donnelly
