AP Chemistry Unit 7: Mechanics of Equilibrium and Calculations

Equilibrium Constant (K)

Describes the ratio of product concentrations to reactant concentrations at equilibrium.

$K
ightarrow 1$

Indicates comparable amounts of reactants and products are present at equilibrium.

$K
gtr 1$

Indicates products are favored and the reaction goes to completion.

$K
less 1$

Indicates reactants are favored and the reaction barely proceeds.

$K_c$

Equilibrium constant using molar concentrations.

$K_p$

Equilibrium constant using partial pressures.

Relationship between $Kc$ and $Kp$

$Kp = Kc(RT)^{ riangle n}$, where $ riangle n$ is the change in moles of gases.

Reversing the Reaction Rule

$K{new} = \frac{1}{K1}$ when a reaction is reversed.

Multiplying Coefficients Rule

$K{new} = (K1)^n$ when stoichiometric coefficients are multiplied by $n$.

Adding Reactions for Equilibrium Constant

$K{total} = K1 \times K_2$ for reactions that are sums of two or more steps.

Reaction Quotient (Q)

Calculated like $K$ but using initial concentrations to predict direction of shift.

$Q < K$

Indicates the reaction will shift right, towards products.

$Q > K$

Indicates the reaction will shift left, towards reactants.

$Q = K$

Indicates the system is at equilibrium.

R.I.C.E. Table Method

A method to organize initial concentrations, changes, and equilibrium concentrations.

$I$ in R.I.C.E.

Represents initial concentrations in the table.

$C$ in R.I.C.E.

Represents changes to reach equilibrium in the table.

$E$ in R.I.C.E.

Represents equilibrium concentrations in the table.

Small x Approximation

Assuming $x$ is negligible when $K$ is very small, simplifying calculations.

Particle Diagrams

Graphical representations showing relative amounts of reactants and products.

Common Mistake: Solids and Liquids

Do not include pure solids or liquids in $K$ or $Q$ expressions.

Confusion with Initial vs. Equilibrium

Only equilibrium values should be used in the $K$ expression.

Forgetting Coefficients in Powers

Students often forget to apply stoichiometric coefficients in $K$ expressions.

Rate vs. Equilibrium

A large $K$ indicates reaction direction, not rate; rates depend on activation energy.

Unit Mismatch Error

Avoid mixing concentration units with partial pressure without conversion.

Change in Moles of Gases ($\Delta n$)

The difference between moles of gaseous products and moles of gaseous reactants.