Question

Calcualte the enthalpy change when infinitely dilute solution of `CaCI_(2)` and `Na_(2)CO_(3)` are mixed. `Delta_(f)H^(Theta)` for `Ca^(2+)(aq), CO_(3)^(2-)(aq)`, and `CaCO_(3)(s)` are `-129.80, -161.65`, and `-288.50 kcal mol^(-1)` respectively.

Answer 1

Infinitely dilute solutions are completely ionised.
`{:(Caunderset(darr).^(2+)CI_(2)^(2-) +Na_(2)^(2+)CO_(3)^(2-)),(CaCO_(3)darr+2NaCI):}`
`{:(Caunderset(darr).^(2+)+2CI^(Theta)+2Na^(darr)+CO_(3)^(2-)),(Ca^(2+)+CO_(3)^(2-)+2Na^(o+)+CO_(3)^(2-)):}`
`Ca^(2+) +CO_(3)^(2-) rarr CaCO_(3)(s)`
`-DeltaH^(Theta) = Delta_(f)H^(Theta)CaCO_(3) -[Delta_(f)H^(Theta)ca^(2+)+Delta_(f)H^(Theta)CO_(3)^(2-)]`
`= - 288.5 -(-129.80-161.65)`
`DeltaH^(Theta) = 2.95 kcal`