Notes On K,Q,G Relationship And Le Chatelier'S Principle - CBSE Class 11 Chemistry
The change in the Gibbs free energy of a chemical reaction can be defined as the difference in the Gibbs free energy of the products and that of the reactants. This can be represented as ΔG = G(products) - G(reactants) When ∆G =0, (that means G(reactants) = G (products) At this stage, the system is said to have achieved equilibrium. ∆G > 0, the process is non - spontaneous The change in the Gibbs energy for a reaction in which all the reactants and products are in the standard state is known as standard Gibbs energy change (∆G°) The relationship between Gibbs energy changeand standard Gibbs energy changeis given as    ∆G = ∆G° + RTlnQ where, R is universal gas constant T is absolute temperature Q is reaction quotient For a reaction A + B C + D '.' Q = $\frac{\text{[C][D]}}{\text{[A]}\text{[B]}}$ ΔG = ΔG° + RTln$\frac{\text{[C][D]}}{\text{[A]}\text{[B]}}$ At equilibrium, when ΔG = 0 and Q = Kc, the equation ΔG = ΔG° + RTln$\frac{\text{[C][D]}}{\text{[A]}\text{[B]}}$ Becomes ΔG = ΔG° + RTlnKc = 0 ΔG° = - RTlnKc lnKc = $\frac{\text{-}∆\text{G}⁰}{\text{RT}}$$\frac{\text{}}{\text{}\text{}}$ Taking antilog on both sides, K = e  - ∆ G ⁰ RT   If ΔG° < 0, then $\frac{\text{-}∆\text{G}⁰}{\text{RT}}$$\frac{\text{}}{\text{}}$ becomes positive and e - ∆ G ⁰/ RT >1 If k > 1 It is a spontaneous reaction. If ΔG° > 0, then $\frac{\text{-}∆\text{G}⁰}{\text{RT}}$$\frac{\text{}}{\text{}}$ becomes positive and e - ∆ G ⁰/ RT <1 If k < 1 It is a non-spontaneous reaction. According to Le Chatelier's principle: "If a system at equilibrium is subjected to a change in the temperature, pressure or concentration of the reactants or the products that govern the equilibrium, then the equilibrium position shifts in the direction in which this change is reduced or nullified."

#### Summary

The change in the Gibbs free energy of a chemical reaction can be defined as the difference in the Gibbs free energy of the products and that of the reactants. This can be represented as ΔG = G(products) - G(reactants) When ∆G =0, (that means G(reactants) = G (products) At this stage, the system is said to have achieved equilibrium. ∆G > 0, the process is non - spontaneous The change in the Gibbs energy for a reaction in which all the reactants and products are in the standard state is known as standard Gibbs energy change (∆G°) The relationship between Gibbs energy changeand standard Gibbs energy changeis given as    ∆G = ∆G° + RTlnQ where, R is universal gas constant T is absolute temperature Q is reaction quotient For a reaction A + B C + D '.' Q = $\frac{\text{[C][D]}}{\text{[A]}\text{[B]}}$ ΔG = ΔG° + RTln$\frac{\text{[C][D]}}{\text{[A]}\text{[B]}}$ At equilibrium, when ΔG = 0 and Q = Kc, the equation ΔG = ΔG° + RTln$\frac{\text{[C][D]}}{\text{[A]}\text{[B]}}$ Becomes ΔG = ΔG° + RTlnKc = 0 ΔG° = - RTlnKc lnKc = $\frac{\text{-}∆\text{G}⁰}{\text{RT}}$$\frac{\text{}}{\text{}\text{}}$ Taking antilog on both sides, K = e  - ∆ G ⁰ RT   If ΔG° < 0, then $\frac{\text{-}∆\text{G}⁰}{\text{RT}}$$\frac{\text{}}{\text{}}$ becomes positive and e - ∆ G ⁰/ RT >1 If k > 1 It is a spontaneous reaction. If ΔG° > 0, then $\frac{\text{-}∆\text{G}⁰}{\text{RT}}$$\frac{\text{}}{\text{}}$ becomes positive and e - ∆ G ⁰/ RT <1 If k < 1 It is a non-spontaneous reaction. According to Le Chatelier's principle: "If a system at equilibrium is subjected to a change in the temperature, pressure or concentration of the reactants or the products that govern the equilibrium, then the equilibrium position shifts in the direction in which this change is reduced or nullified."

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