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CH1012
REDOX REACTIONS III
Necessarily, oxidation (a substance loses electrons) and reduction (a substance gains  electrons) occur simultaneously and hence the title "redox" reaction.
                  2 electrons transferred
Zn  +  Cu2+  +   SO42-  ®  Zn2+  +  Cu +  SO42-
    SO42- are spectator ions - no change LHS/RHS.
Half-reactions   - conceptual rxns used to divide up the overall rxn into 2 halves which show the gain and loss of electrons:
Cu2+  +   2e-  ®  Cu          reduction
Zn                 ® Zn2+  +  2e-               oxidation
          “lossox”    loss of e-  =  oxidation