Reversible and irreversible reactions; chemical equilibrium
A reversible reaction runs both ways. In a closed vessel it reaches equilibrium, where forward and backward rates are equal and concentrations stop changing.
Reversible vs irreversible
| Feature | Reversible | Irreversible |
|---|---|---|
| Direction | Both forward and backward | Forward only |
| Symbol | ⇌ | → |
| Completion | Never complete in a closed vessel; reaches equilibrium | Goes to completion |
| Usually seen in | A closed vessel | An open vessel, or when a product escapes or is used up |
| Examples | N₂(g) + 3H₂(g) ⇌ 2NH₃(g); H₂(g) + I₂(g) ⇌ 2HI(g); PCl₅ ⇌ PCl₃ + Cl₂ | Burning of paper or wood; 2Mg + O₂ → 2MgO; AgNO₃ + NaCl → AgCl + NaNO₃ |
How equilibrium is reached
After this point the amounts stay constant, though both reactions carry on.
Characteristics of chemical equilibrium
- It is reached only in a closed system, where nothing escapes.
- It is dynamic: forward and backward reactions continue at the same rate.
- The concentrations of reactants and products stay constant, but need not be equal.
- It can be reached from either side, starting with reactants or with products.
- A catalyst helps reach equilibrium faster but does not change it.
- A similar balance in a physical change, such as water evaporating and condensing in a closed bottle, is called physical equilibrium.
Same reaction, open or closed
Heat limestone in an open dish: the CO₂ escapes, so CaO cannot react back. The change goes to completion and behaves as irreversible. Heat it in a closed steel vessel: the CO₂ stays inside, so CaO and CO₂ react back to form CaCO₃. The reaction becomes reversible and reaches equilibrium.
In a closed soda bottle, CO₂ in the gas space and CO₂ dissolved in the drink are at equilibrium; opening the cap upsets it. Cooking an egg or burning firewood are irreversible changes: you cannot undo them.
1 marks: “The symbol used to show a reversible reaction is ______.” ⇌ (double half-arrows).
2 marks: “What is chemical equilibrium? Why is it called dynamic?” The state in which the forward and backward rates of a reversible reaction are equal, so concentrations stay constant. It is dynamic because both reactions keep going.
4 marks: “Give four differences between reversible and irreversible reactions, with one example of each.” Direction, symbol, completion, type of vessel; examples N₂ + 3H₂ ⇌ 2NH₃ and burning of paper.
Equilibrium is a tie, not a stop: both reactions keep running at equal speed.