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- by Manogya Dahal almost 2 years ago

For chemical systems, the value of ΔS was not always possible to determine. As a result, a new thermodynamic function was needed to predict the direction of spontaneity. J. Gibbs introduced a new function known as Gibbs' free energy. Denoted by the letter G, it is mathematically defined by

G = H - TS

Where,

H= Enthalpy of the system

T= Absolute temperature (Kelvin scale)

S= Entropy of a system

Let us consider an isothermal change at temperature T from initial state 1 to final state 2.

Then for the initial state,

G_{1}= H_{1} - TS_{1}

And, for final state,

G_{2}=H_{2}-TS_{2}

The change of free energy ΔG is given by

ΔG =G_{2}-G_{1}

=(H_{2}-H_{1})-T(S2-S_{1})

Thus,

__ΔG=ΔH-TΔS__

This equation is also known as **Gibbs Helmholtz Equation**.

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