91Ó°ÊÓ

Enthalpy reflects the capacity to do a non-mechanical work and the capacity to release heat. It is denoted by H and its unit is joules.

Entropy indicates the degree of randomness of a system and is abbreviated as S. Its unit is J . K^-1.

Free energy indicates the part of the total energy of a system which is available for useful work. A change in the free energy of a process is ΔG.

$\mathrm{Δ}G=\mathrm{Δ}H−T\mathrm{Δ}S$

For a spontaneous press process ΔG < zero and such processes are said to be exergonic. Non spontaneous process has a positive ΔG value and are said to be endergonic.

According to the question ΔH = 15 kJ and ΔS = 50 J · K^−1

a) at 10°C

$\begin{array}{l}\mathrm{Δ}G=\mathrm{Δ}H−T\mathrm{Δ}S\\ \mathrm{Δ}G=15−(10×50)\\ \mathrm{Δ}G=−485\end{array}$

As ΔG is - 485 (negative), the reaction will be spontaneous.

b) at 80°C

$\begin{array}{l}\mathrm{Δ}G=\mathrm{Δ}H−T\mathrm{Δ}S\\ \mathrm{Δ}G=15−(80×50)\\ \mathrm{Δ}G=−3985\end{array}$

As ΔG is -3985 (negative), the reaction will be spontaneous.

Thus, the reaction will be spontaneous at 10°C and 80°C.