The Reaction Quotient and 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. |
| reaction quotient/rɪˈækʃn ˈkwəʊʃənt/ | An expression with the same form as an equilibrium constant but evaluated using the current composition, which need not be at equilibrium. |
A number that predicts direction
- At balance, a special ratio of products to reactants stays fixed.
- Compute that same ratio at any moment and compare.
- The comparison tells you which way the reaction will go.
- Two symbols, one powerful tool.
The equilibrium constant K
- The equilibrium constant 平衡常数 $K$ is the fixed ratio at equilibrium:
- Each concentration is raised to its coefficient.
Compare Q with K to tell which way a reaction will shift
In the expression for $K$, each concentration is raised to its...
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Concentrations are raised to their balanced coefficients.
The reaction quotient Q
- The reaction quotient 反应商 $Q$ has the same form, but at any moment.
- It measures where the reaction currently stands.
- At equilibrium, $Q = K$.
At equilibrium, $Q$ equals $K$.
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When $Q = K$, the system is at equilibrium.
The reaction quotient $Q$ is like $K$ but measured at the current ____.
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$Q$ uses the same expression at any instant, not only at equilibrium.
Predicting the direction
- If $Q < K$, there are too few products, so it shifts forward.
- If $Q > K$, there are too many products, so it shifts backward.
- If $Q = K$, it is already at equilibrium.
What goes into K?
Sort each species by whether it appears in the equilibrium expression.
If $Q < K$, the reaction shifts...
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Too few products means it moves forward to make more.
If $Q > K$, the reaction shifts...
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Too many products means it moves backward.
A reaction has $K = 10$ but currently $Q = 2$. Which way does it go?
- $Q < K$, so there are too few products.
- The reaction shifts forward to make more.
Pure solids and liquids must be included in the $K$ expression.
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False. Their activities are constant, so pure solids and liquids are left out; only gases and aqueous species appear in $K$.
$Q$ and $K$ use the same expression — $Q$ is just "$K$ right now," before equilibrium. Include only gases and aqueous species; pure solids and liquids are left out. And compare: $Q < K$ goes forward, $Q > K$ goes backward.
The equilibrium constant $K$ is the products-over-reactants ratio (each to its coefficient) at equilibrium. The reaction quotient $Q$ is the same ratio at any moment. Comparing them predicts direction: $Q < K$ shifts forward, $Q > K$ backward, $Q = K$ is equilibrium.