Why this unit matters
Most real chemical systems are dynamic, and equilibrium tools let you predict how they respond to disturbances.
What you will learn
- Write K expressions from balanced equations and interpret magnitude of K.
- Use Q versus K to determine reaction shift direction.
- Apply Le Chatelier reasoning to concentration, pressure, and temperature changes.
Understand the core ideas
Equilibrium describes dynamic balance, not a stopped reaction. In a closed reversible system at equilibrium, forward and reverse rates are equal, so measurable concentrations stay constant even while microscopic conversion continues in both directions. The equilibrium constant K captures the product-to-reactant ratio at equilibrium for a specific temperature, using exponents from stoichiometric coefficients. Large K values indicate product-favored equilibrium composition, while small K values indicate reactant-favored composition, but neither value implies complete conversion. AP Chemistry emphasizes separating composition information from rate information, because a system can reach equilibrium slowly or quickly depending on kinetics while still sharing the same K at that temperature. You should also distinguish homogeneous and heterogeneous systems when writing expressions, since pure solids and liquids have activity approximated as 1 and are not included in K forms. Temperature must be held fixed when comparing K values across conditions. Concentration units should be consistent when evaluating Q and K comparisons.
Comparing Q to K predicts spontaneous shift direction before re-equilibration. If Q < K, the system has too little product relative to equilibrium and shifts forward; if Q > K, it shifts in reverse. Le Chatelier reasoning then evaluates imposed disturbances such as concentration changes, volume or pressure changes in gas systems, and temperature changes that alter K itself. Some interventions do not change equilibrium composition under certain constraints, for example adding inert gas at constant volume in an ideal-gas mixture does not alter reacting partial pressures. Catalysts also do not change K, even though they can speed approach to equilibrium. Strong AP responses reference the balanced equation, write the correct Q or K expression, and justify shift direction with ratio logic rather than memorized phrases. This avoids common errors when coefficient exponents or omission rules are applied inconsistently. It also clarifies why shift direction is a consequence of ratio imbalance.
Key terms
- equilibrium constant
- Constant K relating equilibrium activities or concentrations of products and reactants at a fixed temperature.
- reaction quotient
- Quantity Q calculated like K but from current, not necessarily equilibrium, composition.
- Le Chatelier principle
- Rule that a disturbed equilibrium shifts in the direction that partially counteracts the disturbance.
- dynamic equilibrium
- State in which forward and reverse reaction rates are equal while microscopic reaction continues.
Use Q versus K to predict shift direction
Reaction: N2(g) + 3H2(g) <-> 2NH3(g). At a given temperature, let Kc = 0.50. Suppose current concentrations are [N2]=0.40 M, [H2]=0.20 M, [NH3]=0.10 M.
- 1) Write Qc expression: Qc = [NH.
- 2) Substitute values: Qc .
- 3) Compute denominator: 0.40 x 0.008 = 0.0032, and numerator is 0.01.
- 4) Compute Qc and compare: Qc = 0.01/0.0032 = 3.125, which is greater than Kc = 0.50, so shift is toward reactants.
A common misconception
Claim: A catalyst changes the equilibrium constant and moves equilibrium toward products.
Correction: A catalyst changes rates of forward and reverse reactions similarly, so it helps equilibrium be reached faster but does not change K or the final equilibrium composition.
Lessons in this unit
- Dynamic equilibrium modelDistinguish static appearance from equal forward and reverse rates.
- Equilibrium expressionsConstruct Kc or Kp correctly from stoichiometric coefficients.
- Q versus K reasoningPredict spontaneous shift direction before equilibrium is reached.
- Le Chatelier applicationsPredict and justify shifts due to external changes.
Study task
Unit checkpoint
For N2(g) + 3H2(g) <-> 2NH3(g), what is Kc?
Kc = [NH.