Chemical Equilibrium and Le Chatelier Quantitative Cheat Sheet

A printable reference covering equilibrium constants, reaction quotients, ICE tables, Le Chatelier shifts, and temperature effects for grades 11-12.

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Chemical equilibrium describes reversible reactions where forward and reverse reaction rates are equal, so concentrations stay constant. This cheat sheet helps students connect equilibrium expressions, numerical calculations, and Le Chatelier predictions. It is useful for solving $K_c$ , $K_p$ , $Q$ , and ICE table problems quickly and accurately. Students in grades 11-12 need these tools to explain both reaction direction and equilibrium composition. The core idea is that $K$ describes the ratio of products to reactants at equilibrium, while $Q$ describes the same ratio before equilibrium is reached. Comparing $Q$ to $K$ predicts whether a system shifts toward products or reactants. ICE tables organize initial amounts, changes, and equilibrium amounts for quantitative problems. Le Chatelier's principle explains how concentration, pressure, volume, and temperature changes affect equilibrium position.

Key Facts

For $aA + bB \rightleftharpoons cC + dD$ , the concentration equilibrium constant is $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ .
Pure solids and pure liquids are omitted from $K_c$ and $Q_c$ because their activities are treated as constant.
The reaction quotient is $Q_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ using current concentrations, not necessarily equilibrium concentrations.
If $Q < K$ , the reaction shifts right toward products; if $Q > K$ , it shifts left toward reactants; if $Q = K$ , the system is at equilibrium.
For gases, $K_p = K_c(RT)^{\Delta n}$ , where $\Delta n = \text{moles of gaseous products} - \text{moles of gaseous reactants}$ .
In an ICE table, equilibrium concentration equals initial concentration plus change, so $[\text{equilibrium}] = [\text{initial}] + [\text{change}]$ .
Increasing temperature favors the endothermic direction, while decreasing temperature favors the exothermic direction.
A catalyst speeds up both forward and reverse reactions equally, so it changes the time to reach equilibrium but does not change $K$ .

Vocabulary

Chemical equilibrium: A dynamic state in which the forward and reverse reaction rates are equal and concentrations remain constant.
Equilibrium constant: The value $K$ that compares product and reactant amounts at equilibrium for a specific reaction temperature.
Reaction quotient: The value $Q$ that compares product and reactant amounts at any moment before or during equilibrium.
ICE table: A setup that organizes initial concentration, change in concentration, and equilibrium concentration for each species.
Le Chatelier's principle: A rule stating that an equilibrium system shifts to reduce the effect of an imposed change.
Dynamic equilibrium: An equilibrium condition where reactions continue in both directions even though macroscopic concentrations do not change.

Common Mistakes to Avoid

Including solids or liquids in the equilibrium expression is wrong because pure solids and pure liquids have constant activity and are omitted from $K$ .
Using initial concentrations in $K_c$ is wrong because $K_c$ must be calculated using equilibrium concentrations only.
Reversing the reaction without changing $K$ is wrong because the reverse reaction has $K_{\text{reverse}} = \frac{1}{K_{\text{forward}}}$ .
Ignoring coefficients in the equilibrium expression is wrong because coefficients become exponents, such as $[C]^c$ and $[A]^a$ .
Saying a catalyst shifts equilibrium is wrong because a catalyst lowers activation energy for both directions and does not change $K$ or equilibrium position.

Practice Questions

1 For $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ , write the expression for $K_c$ .
2 For $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$ , calculate $Q_c$ when $[H_2] = 0.20\,\text{M}$ , $[I_2] = 0.10\,\text{M}$ , and $[HI] = 0.50\,\text{M}$ .
3 For $A(g) \rightleftharpoons 2B(g)$ , $K_c = 4.0$ and initially $[A] = 1.00\,\text{M}$ and $[B] = 0\,\text{M}$ . Set up the ICE table expression needed to solve for equilibrium concentrations.
4 For an exothermic equilibrium reaction, explain why increasing temperature shifts the equilibrium toward reactants instead of products.

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