Chemical Equilibrium Cheat Sheet

A printable reference covering equilibrium constants, reaction quotient, Le Chatelier's principle, $K_p$, and ICE tables for grades 11-12.

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Chemical equilibrium describes reversible reactions where forward and reverse reaction rates become equal. This cheat sheet helps students connect equilibrium arrows, concentration changes, pressure changes, and temperature changes to measurable quantities. It is useful for solving problems involving $K_c$ , $K_p$ , $Q$ , and ICE tables. Students need these tools to predict whether a reaction favors products or reactants at equilibrium. The core idea is that equilibrium is dynamic, not stopped. For a reaction such as $\mathrm{N_2(g)+3H_2(g) \rightleftharpoons 2NH_3(g)}$ , the equilibrium constant compares product and reactant concentrations using stoichiometric exponents. The reaction quotient $Q$ uses the same form as $K$ but describes any moment before equilibrium is reached. Le Chatelier's principle predicts how a system shifts when concentration, pressure, volume, or temperature changes.

Key Facts

For $a\mathrm{A}+b\mathrm{B} \rightleftharpoons c\mathrm{C}+d\mathrm{D}$ , the concentration equilibrium constant is $K_c=\frac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b}$ .
Pure solids and pure liquids are omitted from equilibrium expressions, so $\mathrm{CaCO_3(s)}$ and $\mathrm{CaO(s)}$ do not appear in $K_c$ .
The reaction quotient is $Q_c=\frac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b}$ using current concentrations rather than equilibrium concentrations.
If $Q<K$ , the reaction shifts forward toward products, and if $Q>K$ , the reaction shifts backward toward reactants.
For gases, $K_p=K_c(RT)^{\Delta n}$ , where $\Delta n$ equals moles of gaseous products minus moles of gaseous reactants.
Changing concentration, pressure, or volume shifts the equilibrium position but does not change $K$ at constant temperature.
Only temperature changes alter the value of $K$ , with added heat favoring the endothermic direction and removed heat favoring the exothermic direction.
In an ICE table, equilibrium concentration is calculated as $[\text{equilibrium}]=[\text{initial}]+[\text{change}]$ using stoichiometric ratios from the balanced equation.

Vocabulary

Dynamic equilibrium: A state in which the forward and reverse reactions continue at equal rates, so macroscopic concentrations remain constant.
Equilibrium constant: The value $K$ that relates product and reactant amounts at equilibrium for a specific reaction at a specific temperature.
Reaction quotient: The value $Q$ that uses current concentrations or pressures to predict which direction a reaction will shift.
Le Chatelier's principle: The rule that a system at equilibrium shifts to reduce the effect of an imposed change.
ICE table: A problem-solving table that organizes initial, change, and equilibrium amounts for reacting substances.
Heterogeneous equilibrium: An equilibrium involving substances in more than one phase, such as gases with solids or liquids.

Common Mistakes to Avoid

Including solids and liquids in $K_c$ expressions is wrong because pure solids and pure liquids have constant activity and are omitted.
Using initial concentrations in $K_c$ is wrong because $K_c$ must be calculated only from equilibrium concentrations.
Forgetting stoichiometric exponents is wrong because coefficients become powers in expressions such as $K_c=\frac{[\mathrm{NH_3}]^2}{[\mathrm{N_2}][\mathrm{H_2}]^3}$ .
Saying a catalyst changes $K$ is wrong because a catalyst speeds up both forward and reverse reactions equally and does not change equilibrium position.
Assuming pressure changes affect every equilibrium is wrong because pressure and volume changes mainly matter when gases are present and $\Delta n_{\text{gas}}\ne 0$ .

Practice Questions

1 For $\mathrm{N_2(g)+3H_2(g) \rightleftharpoons 2NH_3(g)}$ , write the expression for $K_c$ .
2 At equilibrium for $\mathrm{H_2(g)+I_2(g) \rightleftharpoons 2HI(g)}$ , $[\mathrm{H_2}]=0.20\,\mathrm{M}$ , $[\mathrm{I_2}]=0.20\,\mathrm{M}$ , and $[\mathrm{HI}]=1.60\,\mathrm{M}$ . Calculate $K_c$ .
3 For $\mathrm{PCl_5(g) \rightleftharpoons PCl_3(g)+Cl_2(g)}$ , calculate $Q_c$ when $[\mathrm{PCl_5}]=0.50\,\mathrm{M}$ , $[\mathrm{PCl_3}]=0.20\,\mathrm{M}$ , and $[\mathrm{Cl_2}]=0.10\,\mathrm{M}$ .
4 For an exothermic equilibrium reaction, explain how increasing temperature affects the equilibrium position and the value of $K$ .