The equilibrium constant Kc
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All 12 Flashcards — The equilibrium constant Kc
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Question
What is the equilibrium constant K_{c}?
Answer
The **fixed ratio** of product to reactant concentrations at equilibrium, at a given temperature — it shows **how far** a reaction goes.
Question
How do you write the K_{c} expression?
Answer
**Products over reactants**, each concentration **raised to the power of its balancing coefficient**; use [ ] for equilibrium concentration in mol dm⁻³.
Question
Write K_{c} for N_{2}(g) + 3H_{2}(g) ⇌ 2NH_{3}(g).
Answer
$K_{c} = \dfrac{[\text{NH}_{3}]^{2}}{[\text{N}_{2}][\text{H}_{2}]^{3}}$ — the 2 and 3 become powers.
Question
What does a large K_{c} (>> 1) mean?
Answer
**Products are favoured** — the equilibrium lies to the **right** (mostly products).
Question
What does a small K_{c} (<< 1) mean?
Answer
**Reactants are favoured** — the equilibrium lies to the **left** (mostly reactants).
Question
What is K_{c} for the reverse reaction?
Answer
The **reciprocal**: K_{reverse} = **1 / K_{forward}**.
Question
Which is the only change that alters K_{c}?
Answer
A change in **temperature** — concentration, pressure and a catalyst all leave K_{c} unchanged.
Question
Endothermic forward reaction: what happens to K_{c} as T rises?
Answer
K_{c} **increases** (the position shifts towards products).
Question
Exothermic forward reaction: what happens to K_{c} as T rises?
Answer
K_{c} **decreases** (the position shifts towards reactants).
Question
First step in calculating K_{c} from amounts in a flask?
Answer
Convert each **amount (mol)** to a **concentration (mol dm⁻³)** using **c = n/V**, then substitute.
Question
Which species are left out of a K_{c} expression?
Answer
Pure **solids** and pure **liquids** — only gases and dissolved (aqueous) species appear.
Question
Does a large K_{c} mean the reaction is fast?
Answer
**No** — K_{c} describes the **extent** (how far), not the **rate** (how fast).
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Full study notes for The equilibrium constant Kc
Topic 5.3 hub
How far? The extent of chemical change
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