The E^θ (M²⁺/M) value of a metal depends on the energy changes involved in the following:

1. Sublimation: The energy required for converting one mole of an atom from the solid state to the gaseous state.

M(s) → M(g) Δ_sH (Sublimation energy)

2. Ionization: The energy required to take out electrons from one mole of atoms in the gaseous state.

M(g) → M²⁺(g)  Δ_iH (Ionization energy)

3. Hydration: The energy released when one mole of ions are hydrated.

M²⁺ → M²⁺ Δ_hydH(Hydration energy)

Now, copper has a high energy of atomization and low hydration energy. Hence, the E^θ (M²⁺/M) value for copper is positive.