Question

The free energy change is ______ kJ mol^-1 when 1 mole of NaCl is dissolved in water at 25°C. Lattice energy of NaCl = 777.8 kJ mol^-1; ΔS for dissolution = 0.043 kJ mol^-1; and hydration energy of NaCl = - 774.1 kJ mol^-1.

Options

• 9.114

• 1.08

• 5.203

• 2.09

Answer 1

The free energy change is 9.114 kJ mol^-1 when 1 mole of NaCl is dissolved in water at 25°C. Lattice energy of NaCl = 777.8 kJ mol^-1; ΔS for dissolution = 0.043 kJ mol^-1; and hydration energy of NaCl = - 774.1 kJ mol^-1.

Explanation:

`Delta"H"_"disso" = Delta"H"_"Lattice" + Delta"H"_"hydr"`

`= 777.8 + (- 774.1)` kJ mol^-1 

= 3.7 kJ mol^-1

Given `Delta"S"_"disso"` = 0.043 kJ mol^-1 

from `Delta "G" = Delta "H" - "T" Delta "S"`

= 3.7 - (25 + 273) · 0.043

= 3.7 - 12.514

= - 9.114 kJ/mol