Chemical equilibria
Study H2 Chemistry dynamic equilibrium, predict shifts with Le Chatelier’s principle and solve Kc, Kp and equilibrium-composition problems.
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Before you begin
Many reactions do not go to completion: they reach a dynamic equilibrium in which the forward and reverse reactions continue at the same rate. This topic explains what equilibrium is, predicts how it responds to changes with Le Chatelier’s principle, describes the equilibrium mixture with Kc and Kₚ, calculates equilibrium compositions and applies the ideas to the Haber process.
Be comfortable with: solution concentration, Dalton’s law and partial pressures, enthalpy changes and energy profiles, and rearranging simple equations.
Learning goals
- Dynamic Equilibrium and Le Chatelier
- Equilibrium Constants (Kc and Kp)
- Equilibrium Composition Calculations
- Haber Process (Case Study)
Syllabus statements covered
- explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium
- state Le Chatelier’s Principle and apply it to deduce qualitatively (from appropriate information) the effects of changes in concentration, pressure or temperature, on a system at equilibrium
- deduce whether changes in concentration, pressure or temperature or the presence of a catalyst affect the value of the equilibrium constant for a reaction
- deduce expressions for equilibrium constants in terms of concentrations, Kc, and partial pressures, Kp [treatment of the relationship between Kp and Kc is not required]
- calculate the values of equilibrium constants in terms of concentrations or partial pressures from appropriate data
- calculate the quantities present at equilibrium, given appropriate data (such calculations will not require the solving of quadratic equations)
- describe and explain the conditions used in the Haber process, as an example of the importance of an understanding of chemical equilibrium in the chemical industry
Lessons
Work through them in order.
- Dynamic equilibriumExplain equal forward and reverse rates and why equilibrium concentrations can be different.
- Le Chatelier’s principlePredict a controlled equilibrium response and distinguish composition changes from changes in K.
- Equilibrium constants and KcWrite Kc expressions, derive their units and interpret the equilibrium ratio.
- Partial pressures and KpFind gas partial pressures from mixture composition and use them to write and calculate Kp.
- Equilibrium composition calculationsUse ICE tables to find equilibrium compositions, Kc and Kp from starting amounts and one equilibrium value.
- The Haber processWeigh yield against rate and cost to explain the conditions used in the Haber process.
Practise and check
Or choose
Topic reference
Quick reference
| Question | First move | What to show |
|---|---|---|
| Effect of a concentration change | decide which reaction uses up the added substance or replaces the removed one | direction of shift and the effect on the named species; K unchanged |
| Effect of a pressure change | count the moles of gas on each side | higher pressure favours fewer moles of gas; no shift if equal |
| Effect of a temperature change | find the endothermic direction from Δ H | higher temperature favours the endothermic direction; K changes |
| Effect of a catalyst | both reactions speed up equally | equilibrium reached sooner; position and K unchanged |
| Write Kc or Kₚ | products over reactants, coefficients as powers | leave out pure solids and liquids; work out the units |
| Find partial pressures | mole fraction × total pressure | count every gas in nₜₒₜₐₗ; partial pressures add to the total |
| Find an equilibrium composition | ICE table with changes in the ratio of the coefficients | substitute the equilibrium row only; check the answer in K |
Core knowledge to remember
- Reversible reaction: a reaction that can go in both directions under the same conditions.
- Dynamic equilibrium: the rates of the forward and reverse reactions are equal, so the concentrations stay constant; both reactions continue. It needs a closed system.
- Le Chatelier’s principle: if a change is made to a system in dynamic equilibrium, the position of equilibrium moves to oppose the change.
- Equilibrium constant: for aA + bB < = > cC + dD, Kc = [C]^c[D]^d/[A]^a[B]^b using equilibrium concentrations, and Kₚ is written in the same way using equilibrium partial pressures.
- Partial pressure: the pressure a gas would exert if it alone occupied the whole volume; pX = xXPₜₒₜₐₗ.
- For a given equation, K depends only on temperature. Concentration, pressure and catalysts do not change it.
- The Haber process runs at about 200 atm and 450 °C with an iron catalyst: a compromise between yield, rate and cost.
Common mistakes
- Saying the reactions stop at equilibrium, or that the concentrations of reactants and products are equal.
- Counting solids, liquids or aqueous species when predicting the effect of pressure.
- Saying that a catalyst increases the yield, or that changing a concentration changes K.
- Taking the powers in K from formula subscripts instead of equation coefficients.
- Substituting initial values instead of equilibrium values into Kc or Kₚ.
- Using the total pressure in place of each partial pressure in Kₚ.
- Leaving out the units of Kc or Kₚ.