In a reversible reaction, the forward and reverse reaction rates become equal, and the system reaches chemical equilibrium. The relationship between concentrations at this point is expressed by the equilibrium constant.
Basic Knowledge
For the reaction aA + bB ⇌ cC + dD, the equilibrium constant is: Kc = [C]c[D]d / ([A]a[B]b). For gas-phase reactions, the partial-pressure form Kp = (PCc·PDd) / (PAa·PBb) is also used, with the relationship Kp=Kc(RT)Δn.
The equilibrium constant K depends only on temperature and does not change with concentration, pressure, or catalysts. When K≫1, products predominate; when K≪1, reactants predominate; and when K≒1, the equilibrium is intermediate.
Pure liquids and pure solids are not included in the equilibrium-constant expression because their activities are taken as 1 (for example, in CaCO3(s)⇌CaO(s)+CO2(g), only K=PCO2).
Chemical equilibrium（Dynamic equilibrium in which the forward and reverse rates are equal）
Equilibrium constant Kc（Concentration form; temperature-dependent）
Kp（Partial-pressure form; Kp=Kc(RT)Δn）
Pure solids and pure liquids（Omitted because their activities are 1）
ΔG°=-RTln K（Free energy and the equilibrium constant）
Reaction quotient Q（Concentration ratio at any point; the forward reaction proceeds when Q
Further Exploration (Principles and Applications)
The equilibrium constant is related to the Gibbs free energy of thermodynamics by ΔG° = -RT ln K. When K=1, ΔG°=0; when K>1, the reaction is exothermic(ΔG°<0); when K<1, the reverse reaction is favored.
Equilibrium calculations form the basis for designing optimum conditions for industrial reactions such as ammonia synthesis, the water-gas equilibrium(CO+H2O⇌CO2+H2), and esterification equilibrium. Comparing the reaction quotient Q with K shows whether the forward or reverse reaction proceeds(Q
- Dynamic equilibrium=equal forward and reverse rates
- K=depends only on temperature
- K≫1: products predominate
- Pure solids and pure liquids are omitted
- Kp=Kc(RT)Δn
- ΔG°=-RT ln K
- Forward reaction when Q
Points to Note (Common Confusions and Frequent Mistakes)
① The equilibrium constant changes only with temperature(it is unchanged by concentration, pressure, or catalysts). ② Pure solids and pure liquids are not included in the expression for K. ③ K is not dimensionless and has units such as [mol/L]Δn depending on the reaction order, although it is sometimes treated as dimensionless for convenience. ④ Use consistent pressure units when calculating Kp.
Practice
- Write the expression for the equilibrium constant Kc of N2+3H2⇌2NH3.
- Write the expression for the equilibrium constant Kp of CaCO3(s)⇌CaO(s)+CO2(g).
- If Q=0.5 at the beginning of a reaction and the equilibrium constant is K=2.0, in which direction will the reaction proceed?
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