Reversible reactions and dynamic equilibrium
Many reactions are reversible: the products can react to form the reactants again, shown by the sign ⇌. In a closed system the forward and reverse reactions go on at the same rate once dynamic equilibrium is reached. The reactions have not stopped, but the concentrations of all species stay constant.
The equilibrium constant
- equilibrium constant; depends only on temperature
A large K_c (much more than 1) means the equilibrium lies to the right: mostly products. A small K_c means it lies to the left: mostly reactants. K_c changes only with temperature.
At equilibrium a mixture contains 0.40 mol/dm³ of N₂O₄ and 0.10 mol/dm³ of NO₂ in the reaction N₂O₄ ⇌ 2NO₂. Find K_c.
- Write the expression.
- Substitute.
Answer: K_c = 0.025 mol/dm³, so the equilibrium lies on the left.
For A + B ⇌ C, enter the equilibrium concentrations. See K_c = [C] ÷ ([A][B]). Then try a different mixture and compare the reaction quotient Q with K_c.
Le Chatelier's principle
- Le Chatelier's principle
- If a change is made to a system at equilibrium, the position of equilibrium shifts to oppose the change.
| Change | Shift | Effect on K_c |
|---|---|---|
| Increase the concentration of a reactant | Towards the products | None |
| Remove a product | Towards the products | None |
| Increase the pressure (gases) | Towards the side with fewer moles of gas | None |
| Increase the temperature | In the endothermic direction | Changes |
| Add a catalyst | No shift; equilibrium is reached faster | None |
Industrial applications: the Haber process
| Condition | Effect on yield | Chosen value and reason |
|---|---|---|
| Pressure | High pressure favours ammonia (4 moles of gas become 2) | About 200 atmospheres: a compromise between yield and the cost of strong equipment |
| Temperature | Low temperature favours ammonia (the reaction is exothermic) but is slow | About 450 °C: a compromise between yield and rate |
| Catalyst | None on yield; much faster | Iron |
| Removing ammonia | Shifts equilibrium right | Ammonia is liquefied and removed; unreacted gases are recycled |
Key terms
- Dynamic equilibrium
- A state in which the forward and reverse reactions proceed at equal rates.
- Equilibrium constant
- K_c, the ratio of product to reactant concentrations at equilibrium.
- Le Chatelier's principle
- A system at equilibrium shifts to oppose any change made to it.
- Reaction quotient
- Q, the same expression as K_c but with the concentrations at any moment.
Summary
- At dynamic equilibrium the rates are equal and concentrations are constant.
- K_c = [products]^coefficients ÷ [reactants]^coefficients; it depends only on temperature.
- Le Chatelier's principle predicts the shift for changes in concentration, pressure and temperature.
- Haber process: 450 °C, 200 atm, iron catalyst: a compromise between yield and rate.