Equilibrium Constant
K compares product to reactant concentrations at equilibrium.
About this topic
The equilibrium constant K is the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of its coefficient. For aA + bB ⇌ cC + dD, K = [C]^c[D]^d / ([A]^a[B]^b). Pure solids and liquids are left out. A large K (much greater than 1) means products are favoured; a small K (much less than 1) means reactants are favoured.
K depends only on temperature for a given reaction. The reaction quotient Q has the same form but uses current concentrations: if Q is less than K, the reaction shifts toward products; if Q is greater, it shifts toward reactants; if equal, it is at equilibrium. For gases, Kp uses partial pressures. Reversing a reaction gives 1/K, and doubling the coefficients squares K.
Worked example: for H2 + I2 ⇌ 2HI at equilibrium, [H2] = 0.10, [I2] = 0.10 and [HI] = 0.80 mol/L, so K = 0.80 squared / (0.10 x 0.10) = 64. Equilibrium calculations with ICE tables (initial, change, equilibrium) find unknown concentrations. K for the Haber process falls as temperature rises, showing it is exothermic.
Students often include solids, forget the powers, or put reactants on top. The value of K says nothing about how fast equilibrium is reached.
Key ideas
- K = [products]/[reactants]
- Large K favors products
- Depends only on temperature
Quick check
Use cases: where Equilibrium Constant helps
- Homework: review the key idea behind Equilibrium Constant before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Equilibrium Constant without notes.
- Visual learning: play with the interactive lab to see how Equilibrium Constant changes when you adjust its inputs.
- Teaching a friend: explain Equilibrium Constant in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is Equilibrium Constant?
K compares product to reactant concentrations at equilibrium.
What will I learn in Equilibrium Constant?
K = [products]/[reactants]. Large K favors products. Depends only on temperature.
Is Equilibrium Constant part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Equilibrium Constant lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Equilibrium Constant?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
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