To determine a reactivity series for a range of metals using a voltmeter Background: You have performed displacement reactions and have concluded that some metals are more reactive than others. You have also learned that these displacement reactions involve the oxidation of a reactive metal and reduction of the ions of the least reactive metal. Since oxidation and reduction in these cases involves the transfer of electrons and the movement of electrons constitutes an electrical current, it should be possible to use electrical methods to determine numerically if a metal is more reactive than another. We need to determine the energy driving the reaction and the term associated with electrical energy is the volt. We will, therefore, set up some simple cells and connect the metals via a voltmeter to measure the electrical potential (an energy related term measured in volts). It is predicted that the order of the voltages measured will match the observations from displacement reactions. Apparatus Stopclock (to measure seconds). Voltmeter (high resistance to minimise current flow). 2 plug to croc leads (to connect electrodes to the voltmeter). 100cm3 beaker. Approximately 100cm3 of 0.5M Na2SO4 solution. (Other solutions could be used, preferably NOT including ions of the metals being tested or chloride ions.) Diagram Electron flow circuit: V Voltmeter Leads Electrodes Stopclock Ion flow circuit: Electrodes Electrolyte Optional extras: Magnetic stirrer / glass rod Thermometer Method – Preliminary Work: After following these instructions, you may think of ways to improve your experiment in order to obtain either better results or a better understanding of the experiment. Feel free to develop the experiment as you see fit. o Abrade the electrodes’ surfaces using a fine paper. Take care not to leave fingerprints or other marks on the surface. o Connect cleaned copper and zinc electrodes to the voltmeter. o Make sure the voltmeter is switched on before placing the two electrodes in the electrolyte at the same time. o Notice the behaviour of the voltmeter reading over a minute or so. Also note the sign of the voltage. o What happens if the electrodes are swapped round? Results: Metal on +ve terminal (Red) Measurement Time (sec) Voltage Measured (V) Copper Metal on -ve terminal (Black) Zinc More reactive metal 10 15 20 30 Zinc Metal on -ve terminal (Black) Copper More reactive metal 10 15 20 30 60 Observations Metal on +ve terminal (Red) Measurement Time (sec) Voltage Measured (V) 60 Observations You will notice at least two important features about the voltage reading… 1. The values “jump about” a lot, and 2. The values decrease, quickly at first but then more gradually. This poses a problem of how to take an accurate measurement. In fact it will be impossible to get measurements which give the same value each time the experiment is repeated (except by chance). There are many variables which are affecting your results so we have to be very careful with our method so as not to disturb the system more than necessary and to make our measurements as consistent as possible. Always start with freshly prepared, clean electrodes. Keep electrodes as far apart as possible. Possible further preliminary investigations: You have seen that the measurements decrease quickly. How can you make readings from separate tests compatible with each other? (Note: Do not expect them to be the same – what, in your opinion constitutes consistent results for this experiment?) Does stirring have any effect? The process is temperature dependent. What reasonable precautions should you take to make sure that measurements are as consistent as possible? What if you connect two electrodes of the same metal? Investigation: You are to select pairs of metal electrodes and determine a series for the metals. Compare this series with that of the reactivity series obtained from displacement reactions. In your evaluation, comment on difficulties experienced in taking measurements and how you tackled them. How accurate do you think your measurements are, and with this in mind, how precisely should they be recorded? Try to justify any assumptions made.