The hypothesis is based on the following explanation. The enthalpy change of a reaction is the heat evolved when a reaction takes place. All chemical bonds hold heat energy within them. Enthalpy change of reaction is the difference between the heat held in the bonds of reactants, and the heat held in the bonds of the products, of a reaction. If the bonds in the products are stronger than the bonds in the reactants then heat will be given off by the reaction as a strong bond is more stable and holds less energy within it. This is called an exothermic reaction and would be considered to have a negative enthalpy change of reaction because heat energy has been given up into the surroundings. Likewise a reaction in which the product bonds are weaker than the reactant bonds will take up heat from the surroundings and will be considered to have a positive enthalpy change as heat energy has been gained. This is called an endothermic reaction. My hypothesis is based on the understanding that the more reactive metals will create more stable compounds on reaction, meaning that less energy will be held in their strong and stable bonds.
I have chosen to test my hypothesis using the reaction between copper sulphate solution and metals of various reactivity. I have chosen this reaction because it is simple and safe, and because based on the standard electrode potential for the reduction of copper ions to copper metal it should be spontaneous with a good number of common metals.
For example: The reaction between Lithium metal and copper sulphate. The half cell equations are:
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Cu2+ + 2e- Cu = + 0.34 v
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2 Li 2 Li+ + 2e- = + 6.06 v
- Thus the reaction will be spontaneous with a positive S.E.P of 6.4 v
I can then measure the heat given off by this reaction to calculate the enthalpy change of the reaction. I calculate the enthalpy change using:
Δ H = m c ( Tinitial – Tfinal) in joules, where c = specific heat capacity water, m = mass of copper sulphate, and T = temperature.
The controlled variables in the investigation are:
- The quantity of copper sulphate used. (25 cubic centimetres)
- The quantity of metal used. (In excess in all cases)
The independent variable is the choice of metal in the experiment. This changes from experiment to experiment. The dependant variable is the aspect which will change because of the changing metal: It is ΔH in all cases in this investigation.
The uncontrolled variables are aspects over which I have no effective control. These are things like the ambient temperature in the cup which may change with external temperatures. I am minimising this by taking a new initial temperature reading for every experiment. The surface area of the metal is an uncontrolled variable in that it affects the rate of reaction. Rate of reaction has no impact on the enthalpy change but the temperature change will be harder to measure the slower the reaction, as there will be a greater period in which heat can escape from the reaction vessel where temperature is measured. This can be mitigated by using metal filings wherever possible.
Apparatus: Thermometer
- Insulated Styrofoam cup
- Plastic lid with hole for above
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0 – 100 oC thermometer Cup
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samples of common metals
- Copper sulphate solution
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Stopwatch
Method:
The reaction will be conducted in the insulated Styrofoam cup to minimise the heat loss to the surroundings. The lid should also minimise the heat lost to air circulation. The thermometer will be introduced through a small hole in the lid and the initial and final temperatures of the reaction measured in situ thus. 25cm3 of copper sulphate will be measure into the cup and then 0.5 (g) ( an excess) of the metal will be added and the stopwatch started. A temperature reading will be taken every fifteen seconds until the temperature of the reaction has dropped for three readings in succession. This means that the reaction has completed, passing its peak temperature. Heat leaving the cup is responsible for the fall in temperature after this point. It is important to take further readings after the peak temperature has passed as these allow the true peak temperature to be extrapolated accounting for the heat lost to surroundings during the experiment. A graph of time on the x axis and temperature on the y axis will be plotted for each reaction. I will draw a best fit line through the points with a positive gradient and another through the points with negative gradients. Where these two line cross will be the true peak temperature and thus enthalpy change of the reaction. This is because heat loss error will mean that the temperature measured will not take account of all the heat that has been generated by the reaction.
Temp (oC)
Time (s)
To provide sufficient data for analyses I have chosen five metals with which to conduct the reaction. I have chosen them on the basis that I expect them to react with copper ions on the basis of their standard electrode potentials and that I expect them to cover a reasonable range of the reactivity series, so that their should be significant differences in the enthalpy change values for each of them. Therefore the metals for which I shall measure the enthalpy change of reaction with copper sulphate are:
- Lithium
- Magnesium
- Zinc
- Nickel
- Lead
