# 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.

• 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