ChemistryDynamic Equilibrium and Le Chatelier's Principle
Atlas stands at a laboratory bench beside a sealed flask of reddish-brown nitrogen dioxide gas, adjusting a pressure gauge with one hand while watching the color shift before her eyes — she grins and jots a note as the gas lightens, equilibrium responding in real time.
- Explain what dynamic equilibrium means in terms of forward and reverse reaction rates.
- Identify how changes in concentration, temperature, and pressure shift a system at equilibrium.
- Predict the direction of equilibrium shift when a stress is applied, using Le Chatelier's Principle.
- Compare the equilibrium positions of exothermic and endothermic reactions when temperature changes.
- Interpret a written equilibrium expression to determine which side of the reaction is favored under given conditions.
Key terms
- Dynamic equilibrium
- A state where the forward and reverse reaction rates are equal so concentrations stay constant.
- Le Chatelier's Principle
- A system at equilibrium responds to a stress by shifting to partially relieve that stress.
- Equilibrium constant K
- The ratio of product to reactant concentrations at equilibrium, each raised to its coefficient.
- Reaction quotient Q
- The same concentration ratio as K but evaluated at any moment, not only at equilibrium.
- Catalyst
- A substance that speeds both forward and reverse reactions equally without shifting the equilibrium position.
What dynamic equilibrium really means
At equilibrium a reaction has not stopped; both the forward and reverse processes continue at full speed, but their rates have become equal so no net change is visible. In the NO2 and N2O4 flask, brown NO2 molecules combine into colorless N2O4 exactly as fast as N2O4 molecules break apart, leaving the color steady. This is why equilibrium is called dynamic rather than static. The equilibrium constant K captures the resulting balance as a fixed ratio of product to reactant concentrations at a given temperature, and any disturbance triggers a measurable shift back toward that ratio.
Comparing Q to K to predict direction
The reaction quotient Q uses the same expression as K but is calculated from the concentrations present at any instant. Comparing Q to K tells you which way a reaction must move. When Q is less than K there is too little product, so the system shifts right toward products. When Q is greater than K there is too much product, so the system shifts left toward reactants. When Q equals K the system is already at equilibrium and does not shift. This quantitative test complements Le Chatelier reasoning and removes guesswork when concentrations are known.
Worked examples
Predict the shift for N2(g) + 3H2(g) reversible 2NH3(g) when total pressure is increased.
- Count moles of gas on each side: left has 1 + 3 = 4, right has 2.
- Increasing pressure favors the side with fewer moles of gas.
- The right side has fewer moles, so the system relieves the stress by shifting that way.
Answer: Equilibrium shifts right, toward more ammonia.
For A(g) reversible B(g) with K = 0.04, a mixture has [A] = 0.1 and [B] = 0.5 mol/L. Which way does it shift?
- Write the quotient: Q = [B]/[A] = 0.5/0.1 = 5.
- Compare Q to K: 5 is greater than 0.04, so Q > K.
- When Q exceeds K there is excess product, so the system shifts toward reactant A.
Answer: The reaction shifts left to produce more A until Q equals K.
Activity
For each stress applied to the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat, drag the label to show whether equilibrium shifts LEFT, shifts RIGHT, or does NOT SHIFT.
Practice
For the exothermic reaction 2SO2 + O2 reversible 2SO3, predict the direction of shift when the temperature is raised.
Given K = 10 for X reversible Y and a mixture with [X] = 2.0 and [Y] = 5.0, compute Q and state the direction of shift.
Common mistakes to avoid
- A catalyst shifts equilibrium toward productsA catalyst speeds forward and reverse reactions equally, so it only reaches equilibrium faster without changing its position or K.
- Temperature changes never affect KTemperature is the one common factor that does change K, raising it for endothermic and lowering it for exothermic reactions.
Check your understanding
The reaction CO(g) + 3H₂(g) ⇌ CH₄(g) + H₂O(g) is at equilibrium. The volume of the container is suddenly decreased, increasing the pressure. What happens to the equilibrium position?
A student claims that adding a catalyst to a reaction at equilibrium will cause more product to form because the forward reaction speeds up. Is this claim correct?
For the endothermic reaction N₂(g) + O₂(g) ⇌ 2NO(g), what effect does increasing the temperature have on the equilibrium constant, K?
At a given temperature, the equilibrium constant for A(g) ⇌ B(g) is K = 0.04. A student prepares a mixture with [A] = 0.1 mol/L and [B] = 0.5 mol/L. Which statement correctly describes this mixture?
Recap
Dynamic equilibrium means equal forward and reverse rates, not a stopped reaction. Le Chatelier's Principle predicts that concentration, pressure, and temperature stresses cause a shift that partially relieves them, while a catalyst only speeds attainment of equilibrium. Comparing the reaction quotient Q to the constant K gives a precise way to predict the shift direction.
Reflect
Why is it valuable for an industrial chemist to know how a reaction responds to pressure and temperature stress?