The E^⊖(/M) value for copper is positive (+0.34V). What is possible reason for this? (Hint: consider its high Δ_aH^⊖ and low Δ_{hyd}H^⊖)
The positive E^⊖ value for copper (+0.34V) is because the high energy required to transform Cu(s) to Cu²⁺(aq) is not balanced by its hydration enthalpy.
Copper has a high atomization enthalpy (Δ_aH^⊖ = 339 kJ mol⁻¹) and significant ionization enthalpies. The hydration enthalpy (Δ_{hyd}H^⊖ = -2121 kJ mol⁻¹), though negative, is not sufficiently large to compensate for the total energy input required for atomization and ionization processes.
This unique behaviour explains why copper cannot liberate H₂ from acids and only reacts with oxidizing acids like nitric acid and hot concentrated sulphuric acid.
Explanation
The textbook explicitly states that 'The high energy to transform Cu(s) to Cu²⁺(aq) is not balanced by its hydration enthalpy.' Table 4.4 shows Cu has Δ_aH^0 = 339 kJ mol⁻¹ (atomization) and Δ_{hyd} = -2121 kJ mol⁻¹ (hydration). The hint guides students to consider both these parameters. The positive value indicates that Cu²⁺ ions are relatively stable and Cu metal is less reactive, which is why it cannot displace H₂ from non-oxidizing acids.