The Equilibrium Constant (K) in chemistry is a key concept that defines the ratio of product concentrations to reactant concentrations at equilibrium for a reversible reaction. The equilibrium constant provides a quantitative measure of how far a reaction proceeds and helps chemists predict the reaction’s direction under specific conditions.

Equilibrium Constant (K)

ContentsIntroductionDefinition & ExpressionTypes (Kc, Kp)Relation: Kp & KcSignificance of KReaction Quotient (Q)Worked ExampleMCQsQuizFAQsReferences

Introduction

The Equilibrium Constant (K) is a dimensionless (or unit-specific) value that quantifies the ratio of products to reactants at equilibrium for a reversible chemical reaction. It tells you how far the reaction proceeds under given conditions and helps predict the direction the reaction will move when disturbed.

Definition and Expression

For a general reversible reaction:

aA + bB ⇌ cC + dD

The equilibrium constant K (in concentration form, Kc) is defined as:

K = ([C]^c × [D]^d) / ([A]^a × [B]^b)

Here [X] denotes the molar concentration of species X at equilibrium and a,b,c,d are stoichiometric coefficients.

Types of Equilibrium Constants

• K_c: Equilibrium constant in terms of molar concentrations (M).
• K_p: Equilibrium constant using partial pressures for gaseous species (atm or bar).
• K_x / Other: Mole-fraction-based forms or activity-based constants are used in specialized contexts.

Relationship Between K_c and K_p

For gaseous reactions:

Kp = Kc (RT)^{Δn}

Where:

• R = universal gas constant (0.082057 L·atm·mol⁻¹·K⁻¹),
• T = temperature in Kelvin,
• Δn = (moles gaseous products) − (moles gaseous reactants).

Significance of the Equilibrium Constant

• Magnitude of K indicates position of equilibrium:
  • K ≫ 1 — products favored (reaction proceeds near completion).
  • K ≪ 1 — reactants favored (little product formed).
  • K ≈ 1 — comparable amounts of reactants and products at equilibrium.
• Predicts direction: Comparing reaction quotient Q with K shows whether the system will move forward or backward.
• Depends on temperature: K changes with temperature; heating/cooling shifts K depending on reaction enthalpy.

Reaction Quotient (Q)

Q uses the same mathematical form as K but with initial (or any non-equilibrium) concentrations/pressures. Compare Q with K:

• Q < K → reaction proceeds forward (makes more products).
• Q > K → reaction proceeds backward (makes more reactants).
• Q = K → system is at equilibrium.

Example Calculation

Consider the Haber-type reaction (gases):

N2(g) + 3H2(g) ⇌ 2NH3(g)

If at equilibrium:

• [N2] = 0.50 M
• [H2] = 1.50 M
• [NH3] = 0.75 M

Then

Kc = [NH3]^2 / ([N2] × [H2]^3) = (0.75)^2 / (0.50 × (1.50)^3) = 0.5625 / 1.6875 = 0.333

MCQs: Quick check

1. Which expression correctly gives K for aA + bB ⇌ cC + dD?
   • a) ([A]^a [B]^b)/([C]^c [D]^d)
   • b) ([C]^c [D]^d)/([A]^a [B]^b) ✅
2. The relationship between K_p and K_c contains which factor?
   • a) (RT)^Δn ✅
   • b) (PV)^n
3. If Q < K, the reaction will:
   • a) move backward
   • b) move forward ✅

Short Quiz

1. What is the practical meaning of a very large K value?
2. How do you get K_p from K_c for gas reactions?
3. Given initial concentrations, how would you use Q to tell the reaction direction?

Answers (brief):

1. Products are strongly favored (reaction proceeds nearly to completion).
2. Use Kp = Kc (RT)^{Δn} with Δn = (products gas moles − reactants gas moles).
3. Compute Q using initial values: if Q < K → forward, if Q > K → backward, if Q = K → equilibrium.

Frequently Asked Questions

Q: Does K have units?

A: Formally K can be unitless when activities are used; when concentrations are used you may see units, but many textbooks present K as dimensionless by dividing by standard states.

Q: Why does K change with temperature?

A: K depends on the reaction’s enthalpy. Changing temperature shifts the equilibrium and thus the ratio of products to reactants at equilibrium (Le Chatelier & van ‘t Hoff relation).

Q: Can catalysts change K?

A: No — catalysts speed up the attainment of equilibrium by increasing both forward and reverse rates, but they do not change the equilibrium constant K.

• Wikipedia — Chemical equilibrium (overview)

Summary

The equilibrium constant K is a compact, quantitative description of where a reversible chemical reaction lies at equilibrium. It helps chemists predict reaction behavior, choose conditions to favor products or reactants, and interpret how temperature or pressure changes will alter a system.