Equilibrium Constant (Kc/Kp) & Reaction Quotient (Q) Checker
Compare K and Q to determine whether a reaction will shift toward products or reactants. Explore Kc vs Kp and learn about chemical equilibrium.
Check K vs Q for Your Reaction
Enter an equilibrium constant (Kc or Kp), add your reactants and products with concentrations or partial pressures, and we'll compute Q to show whether the system will shift toward products or reactants.
Q < K → Shifts Toward Products
Forward reaction is favored
Q > K → Shifts Toward Reactants
Reverse reaction is favored
Q ≈ K → At Equilibrium
No net change in concentrations
Note: Pure solids and liquids are omitted from K and Q expressions (activity ≈ 1).
Understanding Equilibrium Constants and Reaction Quotients
Equilibrium Constant (K)
The equilibrium constant K describes the ratio of product concentrations to reactant concentrations at equilibrium. A large K means products are favored; a small K means reactants are favored.
For: aA + bB ⇌ cC + dD
Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ
Reaction Quotient (Q)
The reaction quotient Q is calculated the same way as K, but using current (non-equilibrium) concentrations. Comparing Q to K tells us which direction the reaction will shift.
- Q < K: Shifts toward products →
- Q > K: Shifts toward reactants ←
- Q = K: At equilibrium
Kc vs Kp
Kc uses molar concentrations (mol/L) for aqueous and gas-phase species. Kp uses partial pressures (atm) and only applies to gaseous species.
Kp = Kc × (RT)^Δn
Δn = moles of gas products − moles of gas reactants
R = 0.08206 L·atm·K⁻¹·mol⁻¹
Solids & Liquids
Pure solids (s) and pure liquids (l) are omitted from equilibrium expressions because their concentrations are essentially constant. Their "activity" is defined as 1.
Example:
CaCO₃(s) ⇌ CaO(s) + CO₂(g)
Kp = P(CO₂)
Key Concepts to Remember
- •K depends only on temperature, not on concentrations. Changing concentrations changes Q, not K.
- •A large K means products are favored at equilibrium—it does NOT mean the reaction is fast.
- •The Kc ↔ Kp relationship only applies to gas-phase reactions under ideal gas assumptions.
- •When Q = K, the forward and reverse reaction rates are equal and no net change occurs.
This tool is for educational purposes only. It uses idealized models that assume ideal behavior (activities ≈ concentrations or partial pressures). Real systems may deviate due to non-ideal interactions.
Frequently Asked Questions
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