UnofficialAP ChemistryUnit 7 of 9

AP Chemistry Unit 7: Equilibrium

Equilibrium constants, Q versus K and Le Chatelier's principle.

Unit overview

Many reactions do not go to completion; they reach dynamic equilibrium, where forward and reverse rates are equal and concentrations stop changing. The equilibrium constant $K$ compares products to reactants, each raised to its coefficient, and omits pure solids and liquids.

The reaction quotient $Q$ has the same form as $K$ but uses current concentrations. If $Q < K$ the reaction shifts toward products; if $Q > K$ it shifts toward reactants. Le Chatelier's principle predicts how a system responds to stress: adding a reactant, removing a product, changing pressure for gases or changing temperature. Only temperature changes the value of $K$ . Solubility equilibria use $K_{sp}$ .

Key topics

Equilibrium constant: $K_c$ from concentrations and $K_p$ from partial pressures.
Reaction quotient: Compare $Q$ with $K$ to predict the direction of shift.
ICE tables: Initial, change and equilibrium rows to find unknown concentrations.
Le Chatelier's principle: Predicting shifts from concentration, pressure and temperature changes.

Worked example

For $A \rightleftharpoons 2B$ , at equilibrium $[A] = 0.5$ M and $[B] = 0.2$ M. Find $K_c$ .

$K_c = \dfrac{[B]^2}{[A]}$ .
$K_c = \dfrac{0.2^2}{0.5} = \dfrac{0.04}{0.5}$ .
$K_c = 0.08$ .

Answer: $K_c = 0.08$

Common mistakes

Saying concentrations of reactants and products are equal at equilibrium; their rates are equal.
Including solids or pure liquids in the expression for $K$ .
Claiming a catalyst shifts equilibrium; it only speeds up reaching it.

Exam tips

Justify a shift by comparing $Q$ and $K$ , not only by quoting Le Chatelier.
When $K$ is very small, the small- $x$ approximation simplifies ICE algebra; check it afterwards.

Chemistry Core Formulas (6 flashcards)

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