Question

What is the physical significance of free energy?

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Answer 1

Q. What is the physical significance of free energy?

Answer :

According to the first law of thermodynamics, we have

q = ΔE + $W_{expansion} + W_{non - expansion}$

at constant pressure

$W_{expansion}$ = PΔV

Thus, q = ΔE + PΔV + $W_{non - expansion}$

⇒ Δn + $W_{non - expansion}$ ................(i)

For a reversible process at constant temperature T, we have

ΔS = $\frac{q_{rev}}{T}$

$q_{rev}$ = T ΔS .................(ii)

From (i) and (ii), we get

TΔS = OH + $W_{non - expansion}$ .....................(iii)

ΔH - TΔS = $-W_{non - expansion}$

For a process occursing at constant T and P, we have

ΔG = ΔH - TΔS ...............(iv)

From (iii) and (iv), we have

$[ΔG]_{TP}$ = $-W_{non - expansion}$

Thus, decrease in free energy is a measure of useful or net work obtain during a reversible process at constant temperature and pressure.

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