When electrolysing a potassium nitrate solution, there are four ions present: 1. H+ From water (H- H+ + OH- ) 2. OH- From water (H2O H+ + OH- ) 3. K+ From Potassium Nitrate 4. OH3- From Potassium Nitrate H+ and K+ compete for electrons at the cathode. K+ is the more stable of the two, so therefore it is H+ that reacts: 2H+ (aq) + 2e- –> H2(g) OH- and NO3- compete to lose electrons at the anode. NO3- is more stable, therefore OH- will react. 0H- (aq) –> 0H- (aq) + e- Uncharged OH is unstable so it reacts: 40H- (aq) –> 2 H2O(1) + O2 (g).
When electrons are released by OH- ions when they react are pushed round the circuit by the power supply into the gaps left by the electrons reacting with the H+ Ions: Therefore the rate of electrons flowing round the circuit is dictated by how fast the reactions are occurring. This means that current can be used as a measure of the rate of reaction Variables There are several variables I could change, these are: Voltage. Concentration of Potassium Nitrate. Temp of Potassium Nitrate. Temperature of water. The size of the electrodes I will only change one variable at a time.
I have decided to experiment with two variables if I have time. I will experiment with changing the voltage, as this should have an effect on the rate of reaction because the voltage is the force pushing the electrons round the circuit. The greater the voltage, the more electrons that are pushed round the circuit, and the more electrons that are available. This will in turn create a greater rate of reaction because pushing the electrons round the circuit with more pressure, so the are more electrons available to react. This in turn increases the current.
I will also experiment with the concentration of potassium nitrate, this should have an effect on the ROR because with less potassium nitrate in the solution, the electrons are less likely to cause a reaction, so the rate of reaction will be slower, this is basically the collision theory. Preliminary work Our preliminary work was designed to help us choose a range for our variables, and discover any problems that could occur in our future experiments. We set up our equipment like so: I tested a range of voltages and concentrations, here are my results: Volts (V) Current (amps) 2 0 (approx).
3 We tested with concentration, and found these results. NOTE: each test was done with 100ml of KNO3 and a voltage of 12. Concentration of KNO3 (aq) Current (amps) From my preliminary work I worked out a sensible range for my variables: Concentration of Potassium Nitrate: 0%, 25%, 50%, 75%, 100% – these are spread far enough apart to give distinguishable results, however are close enough to show any trends that may occur. Voltage (V): 2, 4, 6, 8, 10, 12 – this gives me a good range to work with. I also found a few things, which could impair my results if I am not extra careful:
I will have to be especially careful whilst measuring out potassium nitrate. I will have to be especially careful when taking readings, as we are not using digital ammeters. Safety Issues I will follow the below precautions, to ensure my investigation goes as smoothly and safely as possible: I will wear safety goggles as potassium nitrate is harmful. I will be careful not to spill any potassium nitrate on my body or clothing. Prediction I can make this prediction, based on my theory above (see theory above), that the higher the voltage (V) used, the greater the reaction rate.
The greater the voltage, the more electrons that are pushed round the circuit, and the more electrons that are available. This will in turn create a greater rate of reaction, and the current should increase. For my second variable- potassium nitrate concentration, I can make this prediction based on my theory above. This variable should have an effect on the ROR because with less potassium nitrate in the solution, the electrons are less likely to cause a reaction, so the rate of reaction will be slower, and this is basically the collision theory.