The dependent variable that I will be measuring will be the temperature change of the reaction. To do this I will have to measure the initial and final temperature of the reaction.
Preliminary experiments
I conducted preliminary experiments using 1g of zinc, in order to predict, with greater accuracy, the outcome of my experiments. It showed that the experiment was going to be exothermic, and that the temperature given off increased as the experiment progressed. I also decided that I would only time the experiments for a maximum of ninety seconds each, as this was ample time for the results to be recorded onto a graph.
Apparatus
To do the experiment I will need the following equipment:
Powdered zinc – In powdered form as this increases the surface area and gives better results
Copper sulphate solution (0.5M)
Measuring cylinder – To make sure the volume of copper sulphate is controlled
Timer – To time the 5-second intervals
Weighting scales – To accurately measure the amount of zinc
Polythene cups – polythene, as this is a heat insulating material. Using a different cup for each section of the experiment so that each one is fair. Also using plastic lids to stop any unwanted substances from interfering with the experiment and to stop heat from being easily lost.
Thermometer – To measure temperature changes from start to intervals to the final temperature.
Method
During this experiment I will be measuring the temperature given off as I expect the reaction to be exothermic.
I will use 25ml of copper sulphate solution and 0.8125 grams of zinc in each experiment. I manage to work this out using Avogadro's theory of moles. First, I had to work out how many moles there are in 25ml of copper sulphate at 0.5M. We need to convert the ‘ml’ units into ‘dm’ units. I therefore divided the 25 into 1000. I timed this answer by how strong the concentration of the copper sulphate was, 0.5M. Consequently, we have the sum (25/1000) x 0.5= 0.0125 moles of copper sulphate. We then need to times this number by the atomic weight of zinc, which is 65. 0.0125x65 = 0.8125 grams. This is the amount of zinc needed to disassociate copper in 25ml of copper sulphate solution at 0.5M.
It is important to note that the starting temperature of each experiment will be 25oC. I will record the temperature change in five-second intervals from zero to seventy. I have to make sure that the zinc used is definitely powdered zinc as any strips or lumps of zinc would alter the rate of reaction due the surface area.
I will record my results in a table and then convert the data onto a graph. I will do this by plotting temperature against time. By drawing a line of best fit we can interpret the rates of reactions by calculating the gradient. I will repeat my experiment to ensure accurate and fair results.
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Measure out 25cm3 of 0.5M copper sulphate solution using the measuring cylinder
- Pour the copper sulphate into the polystyrene cup
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Put the thermometer in and wait until the temperature reaches 25oC
- Using the weighing scales, measure out 0.41, 0.51, 0.61, 0.71, 0.81, 0.91, 1.01, 1.11 grams of zinc.
- Add the zinc, simultaneously start the timer and put the plastic lid on immediately.
- Stir the solution using the thermometer as a rod.
- Record the temperatures at five-second intervals.
- Repeat experiment for accurate results.
Safety
There is a possible hazard when carrying out this experiment as the copper sulphate solution or the zinc sulphate solution could be an irritant to the skin therefore contact with the skin should try to be avoided. Goggles must also be worn to protect any harmful substances from entering your eyes. If contact is made with the skin during the experiment you must wash the affected area thoroughly.
Prediction
I predict that this experiment should be a displacement reaction. Therefore the copper in copper sulphate should be displaced by the zinc to form zinc sulphate and copper. This should occur as zinc is higher in the reactivity series than copper.
The reactivity series is as follows:
Most Reactive
Least Reactive
As I am varying the amounts of zinc, I can predict that the greater the amounts of zinc the faster the rate of reaction. This is because there is more reactant for the copper sulphate to react with. Therefore the temperature should rise in direct proportion the mass of zinc. Heat should be given off so I can predict that the experiment will be exothermic.
Conclusion
From my experiment I can conclude that it was an exothermic reaction, which shows that my prediction of this was correct. The graphs show that as the reaction time increases, the more the reactants give off heat energy.
The lines of best fit highlight an error in my prediction as I predicted that the greater the amounts of zinc the faster the rate of reaction. From the gradients of the lines of best fit we can see that this is untrue. In both experiments the 1.11g of zinc does not have the fastest rate of reaction and the temperature does not rise as high as 1.01, 0.91 or 0.81 grams of zinc.
I also predicted that the mass of zinc would rise in direct proportion to the temperature. We can see that this prediction is not true as then you would expect the temperature to almost double from 0.41 to 0.81 and from 0.51 to 1.01, which does not happen.
Evaluation
In general my experiment has been a success as it has produced fairly accurate results. The reason for the experiment not following the exactly correct pattern could be:
Not all the zinc reacted with the copper.
Some heat might have escaped from the open cup.
There could have been errors in the reading.
The stirring could not have been thorough and consistent.
These results can be further improved by:
Using a more insulating lid to prevent heat loss.
Using more accurate and sensitive scales.
Using a pipette or burette instead of a measuring cylinder will increase the accuracy of the measuring of the copper sulphate.
I could improve my results to my experiment if:
More repetitions of the experiment were done. This would provide more results and add to the evidence for the experiment and also increase accuracy.
The experiment was tried with different metals, ones that are at different levels in the reactivity series. This would be done to see how the use of different reactions change the temperature and energy changes and if the theory still works with different metals. Also, using salts with a metal lower in the reactivity series than the base can show whether distance in the reactivity series affects the energy change.
Different concentrations of the copper sulphate were used. This can also show the difference between the temperature and energy changes.
Data logging could be used so that there is now chance in human error in the experiment.
Results
Repeats in bold
