9.6.3 HSC Question Feedback

2002 aii) – To reverse a galvanic cell and make it an electrolysis cell we have to reverse both the anode and cathode reactions. If we apply the exact opposite voltage produced by the galvanic cell all we do is stop the galvanic cell reaction. To reverse the cell we have to apply GREATER than the galvanic cell potential.

2003 bii) Any time you describe an experiment to measure the rate of electrolysis you MUST specify HOW you measured the rate – eg In our experiment we measured the mass change of the anode as an indication of the rate of reaction.

2006 bi) For the risk assessment and HOW it INFLUENCED our PROCEDURE– we used slightly toxic CuSO₄ – this may have limited our concentration to lower (safer) concentrations. We also used a power pack that limited the voltage to 12V to minimise the risk of electrocution but this limited our experiment to less than 12 V

2006 bii) When providing a conclusion from YOUR OBSERVATIONS you have to state the observations – eg the electrolysis cell that had the higher voltage had a greater mass loss at the anode in a fixed time. Thus it was concluded that higher voltages increase rates of electrolysis.

2007 ai) Graphite rods are used because they are INERT (meaning that they wont react in the redox reactions) CONDUCTORS (this allows them to transport re electrons required for electrolysis) – a lot of people forgot to mention that they are conductors. GRAPHITE IS NOT A METAL!

2008 ciii) It did say “Identify” the factors that affect rates of electrolysis but which of these two answers (that take basically the same time) is going to score more highly?:

·        Voltage, electrode spacing, electrode size and electrolyte concentration all affect electrolysis rates

OR

·        ­ voltage, ¯ electrode spacing, ­ electrode size and ­ electrolyte concentration all increase electrolysis rates.

2009 d) Usually w =e would use an Ag cathode so the Ag+ conc would stay constant in solution. This reaction gives a cell potential of 0V. However, i) if you think about you will see that a spontaneous metal displacement reaction would happen between the Ni and AgNO3. thus we do need to provide a -ve potential to the Ni to prevent it dissolving. ii) A potential of 0V actually means it is in equilibrium, the Ag will ppt and dissolve at both electrodes not plate the Ni. To actually get a silver plate we need to ensure that the reduction (deposition) of Ag only happens at the spoon (cathode) and oxidation (dissolution) of silver only happens at the silver anode - ie a voltage with the -ve terminal at the spoon is needed.

2010 bii) The cathode could be identified by the hydrogen gas or hydroxide ions  BUT HOW could you test for them – ie test with pop test or use a phenolphthalein indicator…

2012 di) Validity means measuring what you intended ie the effect of ONE variable (IV) on the dependent variable. As soon as you change multiple variables you can no longer validly say which changed variable affected the DV. Many people neglected to mention that in a valid test only one variable is changed.

2020 dii) Not the greatest Q for us. All through the Q student were drawing on the reactions at the electrodes until they get to the Q where it specifically states to draw the expected results on your diagrams and everyone stops drawing them on. You should have include 3 diagrams (control and two different variables changed – one in each of the other two pictures). You should have specified the constants and drawn on th results – ie bubbles and Cu deposition showing MORE (or less) depending on what variable you were testing.

Lastly – in their diagram they had a graphite and copper electrode. You should have used the sae chemicals in your example ie the Cu as the cathode th graphite as the anode.