Equilibrium constant
For aA + bB ⇌ cC + dD, the equilibrium constant is K = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ). Pure solids and liquids are left out. K is large if products are favoured and small if reactants are favoured. It changes only with temperature.
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Analogy
A scoreboard that always lands on the same ratio: no matter how the game starts, the final ratio is fixed.
Example
N₂ + 3H₂ ⇌ 2NH₃Kc = [NH₃]² / ([N₂][H₂]³)K > 1: mostly productsK < 1: mostly reactants
In practice
The Haber process for making ammonia is designed around K for N₂ + 3H₂ ⇌ 2NH₃.
Practice
Flashcards
- Which species are left out of K?Pure solids and pure liquids.
- What is the only thing that changes K?Temperature.
Interview questions
- For H₂ + I₂ ⇌ 2HI with K = 64, find equilibrium concentrations starting from 1.0 M H₂ and 1.0 M I₂.
- Set up an ICE table
- (2x)²/(1−x)² = 64
- Take square root: 2x/(1−x) = 8
- x = 0.80, so [HI] = 1.6 M and [H₂] = 0.20 M
Questions people ask
Why are solids and pure liquids left out of K?
Their concentration does not change in a meaningful way, because it is fixed by their density. So they would be a constant that just folds into K. Only gases and dissolved species, whose concentrations really change, appear in the expression. Water as a solvent is also left out.
What happens to K if I reverse the equation or multiply it?
If you reverse the reaction, the new K is 1/K. If you multiply all coefficients by n, the new K is K raised to the power n. If you add two reactions, you multiply their K values. These rules are used to build K for complex reactions.