# What is made at the copper cathode when electrolyzing water?

I just electrolyzed water with $$\ce{NaHSO4 -> HSO4- <=> H+ + SO4^2-}$$ with copper electrodes.

At the anode this reaction happened:

$$\ce{Cu -> Cu^2+ + 2e-} \hspace{26 mm} E_0 = -0.34V$$

$$\ce{Cu^2+ + 6H2O -> [Cu(H2O)6]^2+}$$

Can anyone confirm this last reaction? There was a blue-greenish substance falling to the bottom.

At the cathode this reaction happened:

$$\ce{2H+ + 2e- -> H2} \hspace{27 mm} E_0 = 0V$$

$$\ce{2H2O + 2e- -> H2 + 2OH-} \hspace{5 mm} E_0 = -0.83V$$

$$\ce{OH- + H+ -> H2O}$$

However, additionally, the cathode ($$\ce{Cu}$$) turned black. What reaction could have happened there? I electrolyzed at $$12V$$ using a car battery charger.

## 1 Answer

Look at your redox potentials for the half reactions. 12V is tremendous overkill. At the cathode (reducing) you will make hydrogen to which copper is inert. Could you reduce sulfate to sulfide or sulfur and have that react with copper? CuS is black.

At the anode (oxidizing) you will oxidize water to oxygen - but one atom at a time. It will be very reactive to the copper surface. CuO is black. You will also oxidize copper to Cu(II). It coordinates four waters square planar. The apical waters are distant and weakly bound given the Jahn–Teller effect,

One possibility is to add a series resistor to lower the working voltage.

• I'm thinking there will be so little $\ce{SO4^2-}$ around the cathode that there shouldn't be any $\ce{CuS}$. – Friend of Kim Feb 16 '14 at 16:47
• Because the redox potential for $\ce{Cu -> Cu^2+}$ is lower than that of water, shouldn't it be the only reaction happening? – Friend of Kim Feb 16 '14 at 16:53
• The copper half-reaction is 0.34 V. You are putting in 12 V. How many volts oxidizing is the fluorine half-reaction? How many volts reducing is potassium metal? – Uncle Al Feb 16 '14 at 20:37
• Are you saying that the sulfate could be reduced to sulfide even though there is a lower potential needed to make hydrogen gas? At school we learned that this would never happen, even though I think that this may be an unlikely statement. – Friend of Kim Feb 17 '14 at 20:58
• The standard reduction potential of potassium metal is -2.93 volts; you've got 12 volts. "Never happen" is a relative statement. If it smells of hydrogen sulfide, it's happening. I wouldn't bet a grade on footnotes. One day you will be at a lab bench. Reality plays a dead hand - but its all trump. – Uncle Al Feb 18 '14 at 4:38