Intermediate Topic 464 of 1102 Kinetics & Equilibrium
Equilibrium Constant
The ratio of product to reactant concentrations raised to stoichiometric powers at equilibrium.
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In-depth explanation of Equilibrium Constant
The equilibrium constant K expresses the position of a reversible reaction at equilibrium. For a general reaction aA + bB <-> cC + dD it is written as K = [C]^c[D]^d / ([A]^a[B]^b), with each concentration raised to its stoichiometric coefficient. The value depends only on temperature and not on the initial amounts of reactants or on the presence of a catalyst, so it is a characteristic property of the reaction under given conditions. Pure solids and pure liquids are omitted from the expression because their activity is taken as one. A large K means that products are favoured at equilibrium, while a small K means that reactants dominate.
Going deeper: Equilibrium Constant
Key ideas: Write K with products over reactants, powers equal to coefficients, using concentrations for solutes (Kc) or pressures for gases (Kp). Substances in K are in equilibrium, not initial values. K is dimensionless in rigorous thermodynamic treatment but is often quoted with units. Always state the balanced equation and temperature with the number.
Common mistakes and connections: Typical errors include inserting solids or pure liquids, using initial concentrations, and inverting the ratio. Students also forget that K changes if the equation is rewritten. Another misconception is that a catalyst or a change in concentration changes K; only temperature does. K links to free energy through delta G standard = -RT ln K and to rate constants through K = kf/kr.
Visualization of Equilibrium Constant
A large K, like 10, means products dominate the equilibrium mixture; K depends only on temperature, not on starting amounts or catalyst. The interactive lesson shows this as a live animation or chart.
Worked example and use case
For N2 + 3H2 <-> 2NH3 the expression is K = [NH3]^2 / ([N2][H2]^3).
Frequently asked questions about Equilibrium Constant
What is Equilibrium Constant?
The ratio of product to reactant concentrations raised to stoichiometric powers at equilibrium.
Is Equilibrium Constant a beginner, intermediate or advanced topic?
Equilibrium Constant is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Equilibrium Constant?
Study these earlier topics in Kinetics & Equilibrium first: Quantum Yield, Reversible Reactions, Dynamic Equilibrium.
What comes after Equilibrium Constant?
The next topics in Kinetics & Equilibrium are Kc and Kp and Reaction Quotient.
Can I practise Equilibrium Constant with quizzes?
Yes. The interactive lesson includes a self-check question on Equilibrium Constant, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Equilibrium Constant fits in Kinetics & Equilibrium
Equilibrium Constant is topic 45 of 91 in Kinetics & Equilibrium.