Spectator Ions

Whilst the answer:

Sn_(s) | SnSO_4(aq) || CuSO_4(aq) | Cu_(s)

is totally correct it is more usual to write

Sn_(s) | Sn²⁺_(aq) || Cu²⁺_(aq) | Cu_(s).

If we consider the CuSO_4(aq) | Cu_(s) half-cell then the reaction can be written as:

Cu²⁺_(aq) + SO₄^2-_(aq) + 2e = Cu_(s) + SO₄^2-_(aq),

but as the sulfate ions are unaltered the equation may be simplified to:

Cu²⁺_(aq) + 2e = Cu_(s) .

Similarly the CuSO_4(aq) | Cu_(s) half cell can be simplified to Cu²⁺ | Cu_(s). The sulfate ions are sometimes referred to as "spectator ions."