Equilibrium Constant
K = [products]^coefficients / [reactants]^coefficients
The equilibrium constant K expresses the ratio of product to reactant concentrations at equilibrium. Large K means products favored; small K means reactants favored.
Variables
Ratio of concentrations at equilibrium
Equilibrium concentrations of products
Equilibrium concentrations of reactants
Exponents from balanced equation
Example Calculation
Scenario
For N2 + 3H2 ⇌ 2NH3, at equilibrium [N2] = 0.10 M, [H2] = 0.30 M, [NH3] = 0.20 M. Calculate K.
Given Data
Calculation
K = [NH3]^2 / ([N2][H2]^3) = (0.20)^2 / ((0.10)(0.30)^3) = 0.040 / 0.0027
Result
K = 15
Interpretation
K > 1 indicates products are favored at equilibrium. At these conditions, a significant amount of ammonia forms.
When to Use This Formula
- ✓Writing equilibrium expressions from balanced equations
- ✓Calculating K from equilibrium concentrations
- ✓Using K to find unknown equilibrium concentrations
- ✓Comparing Q to K to predict reaction direction
Common Mistakes
- ✗Including solids or pure liquids in K expression
- ✗Getting products and reactants reversed
- ✗Forgetting to raise concentrations to coefficient powers
- ✗Confusing K with Q (reaction quotient)
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Common questions about this formula
K is the equilibrium constant at equilibrium. Q is the reaction quotient at any point. If Q < K, reaction goes forward; if Q > K, reaction goes backward; if Q = K, system is at equilibrium.
Pure solids and liquids have constant concentrations (activity = 1). Their amounts don't affect equilibrium position, only the surface area affects rate of reaching equilibrium.