Properties of the Equilibrium Constant
Introduced| English | Meaning |
|---|---|
| equilibrium constant/ˌiːkwɪˈlɪbrɪəm ˈkɒnstənt/ | A value relating the equilibrium activities, concentrations or partial pressures of products and reactants at a specified temperature. |
Rules for reshaping K
- Flip a reaction and its constant flips too.
- Double it, and the constant gets squared.
- Combine reactions, and their constants multiply.
- A few tidy rules let you build any K from known ones.
Reverse the reaction
- Reverse a reaction and the equilibrium constant 平衡常数 becomes its reciprocal, $1/K$.
- What was product-favoured becomes reactant-favoured.
- The balance simply flips.
Reversing a reaction inverts K; halving the coefficients takes its square root
A reaction has $K = 5$. What is $K$ for the reverse reaction? (as a decimal)
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Reversing gives $1/K = 1/5 = 0.2$.
Reversing a reaction replaces $K$ with its ____.
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The reverse reaction's constant is $1/K$.
Multiply the coefficients
- Multiply a reaction by $n$ and $K$ becomes $K^n$.
- Double it gives $K^2$; halve it gives $\sqrt{K}$.
- The power matches the scaling factor.
A reaction has $K = 3$. If you double every coefficient, the new $K$ is...?
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Doubling gives $K^2 = 3^2 = 9$.
If you halve all the coefficients of a reaction, its $K$ becomes...
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Scaling by $\tfrac{1}{2}$ raises $K$ to the power $\tfrac{1}{2}$.
Add reactions
- Add two reactions and multiply their $K$ values.
- $K_{total} = K_1 \times K_2$.
- This mirrors Hess's law, but with multiplication instead of addition.
When two reactions are added, their equilibrium constants are...
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$K_{total} = K_1 \times K_2$ when reactions add.
A reaction has $K = 4$. What is $K$ for the reverse reaction?
- Reversing gives the reciprocal.
- $K_{reverse} = 1/4 = 0.25$.
How K changes when you rewrite
Rewriting an equation changes K in a predictable way.
Pure solids and liquids are included in the $K$ expression.
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Only gases and aqueous species appear in $K$.
The three operations are different: reversing inverts $K$ ($1/K$), scaling by $n$ raises $K$ to the $n$, and adding reactions multiplies their $K$ values. Do not mix them up. And pure solids and liquids never appear in $K$.
Reshaping a reaction reshapes its equilibrium constant: reversing gives $1/K$, multiplying the equation by $n$ gives $K^n$, and adding reactions multiplies their $K$ values. It parallels Hess's law, except $K$ multiplies where $\Delta H$ adds.