The steps! separate the reaction into "half-reactions" Cu → Cu2+                             NO3- → NO2 balance oxygen (use water) and hydrogen (use H+ for acidic solutions) Cu → Cu2+                           2 H+ + NO3- → NO2 + H2O balance charges, using electrons Cu → Cu2+  + 2 e-             e- + 2 H+ + NO3- → NO2 + H2O multiply half-reactions as needed to make electrons the same in both Cu → Cu2+  + 2 e-             2(e- + 2 H+ + NO3- → NO2 + H2O) finally, combine the half-reactions once more, simplifying where possible Cu + 4 H+ + 2 NO3- → Cu2+ + 2 NO2 + 2 H2O