The Magnitude of the Equilibrium Constant, K
Specification Reference R2.3.3
Quick Notes:
- The equilibrium constant (K) indicates how far a reaction proceeds at equilibrium (which direction is favoured).
- K is temperature dependent.
- What K values tell us:
- K >> 1 : reaction goes nearly to completion (products favoured)
- K > 1 : more products than reactants at equilibrium
- K = 1 : reactants and products are present in roughly equal amounts
- K < 1 : more reactants than products at equilibrium
- K << 1 : very little reaction occurs (reactants strongly favoured)
- For the reverse reaction, K = 1/K1
Full Notes:
What Does the Value of K Mean?
The equilibrium constant (K) tells us the relative amounts of products and reactants present at equilibrium.
- K > 1: Products are favoured. The reaction lies to the right.
- K < 1: Reactants are favoured. The reaction lies to the left.
- K = 1: Position of equilibrium lies in the middle.
Extremes of K values:
- K >> 1 (e.g. K = 105): The reaction is nearly complete. Products dominate.
- K << 1 (e.g. K = 10−5): Very little product is formed. Reactants dominate.
Reversing a Reaction: What Happens to K?
If you reverse a chemical equation, the new K value becomes the inverse of the original one (at the same temperature).
Example Reversing K
If A ⇌ B has K = 4
then
B ⇌ A has K = 1/4.
This means that if the forward reaction strongly favours products, the reverse reaction will strongly favour reactants.
Summary Table: Interpreting K
| K value | Equilibrium position | Favoured species |
|---|---|---|
| K >> 1 | Far to the right | Products |
| K > 1 | Right of centre | Products |
| K = 1 | Centre | Neither strongly favoured |
| K < 1 | Left of centre | Reactants |
| K << 1 | Far to the left | Reactants |
Summary
- K shows whether products or reactants are favoured at equilibrium.
- K >> 1 means the reaction is almost complete with products dominating.
- K << 1 means very little product forms and reactants dominate.
- Reversing a reaction gives K = 1/K.
- The value of K is temperature dependent.
Linked Course Question
Reactivity 3.1 — Linked Course Question
How does the value of K for the dissociation of an acid convey information about its strength?
The larger the value of Ka (the acid dissociation constant), the more the acid dissociates into ions — meaning it is a stronger acid. A small Ka means little dissociation, so the acid is weak.
In simple terms:
- Large Ka → strong acid (more H+ released)
- Small Ka → weak acid (less H+ released)