Question

One mole of $NaCl\left(s\right)$ on melting absorbed $32.76kJ$ of heat and its entropy is increased by $30J{K}^{-1}$. What is the melting point of sodium chloride?

Answer 1

Gibb's free energy ΔG is defined as heat absorbed ΔH and entropy ΔS are related to each other by following equation.

$△G=△H-T\times △S$

During melting of the substance,temperature of the substance remains constant because the reaction is in equilibrium.

Therefore, during melting,$△G=0$

$0=△H-T\times △S$

$△H=T\times △S$

Here T is the melting temperature also known as melting point temperature.

We have been given ΔH = 30.5 kJ. Let us convert this to J

$\Delta H=30500J$

$\Delta S=28.8J/K$

Calculating these values.

$30500J=T\times 28.8J/K$

$\frac{30500J}{28.8J/K}$

$T=1059$

The melting point of NaCl is 1059K-273=Celsius.