Thanks for the reply billuminati, but isn't the oxidation reaction Cu(s) -> Cu2+(aq) + 2e- as the anode is copper, and copper is the stronger reductant?
Or is this because at SLC the electrochemical series doesn't hold true?
Edit: I observed gas at the cathode too, so was there gas produced here as well?
Normally you would have Cu oxidised at the anode because you need to replenish Cu2+ concentration in the electrolyte which gets depleted at the cathode to produce solid Cu. My year 12 EPI was on electrolysis of CuSO4 (which is chemically very similar to Cu(NO3)2) with a Cu anode and a graphite inert cathode and the theoretical equations were:
Anode: Cu(s) —> Cu2+(aq) + 2e-
Cathode: Cu2+(aq) + 2e- —> Cu(s)
Your experimental design is very similar to mine. However, you’ve stated that a gas was produced and the oxidation of H2O is the only way to rationalise it. I think you’re looking at a case of non-SLC conditions that may have caused some side reactions. If your anode mass decreased, then oxidation of Cu has definitely occurred and it’s probably best to list the theoretical equations (but also mention the H2O oxidation reaction to give O2)