Reversible reactions and equilibrium
Reversible reactions
- A reversible reaction can go both ways — the products can react to reform the reactants. Symbol: $\rightleftharpoons$.
- Example (a test for water): blue hydrated copper(II) sulfate heated → white anhydrous copper(II) sulfate; adding water turns it blue again.
Practice
Adding water to white anhydrous copper(II) sulfate turns it:
Anhydrous copper(II) sulfate turns blue with water — used as a test for water.
Equilibrium
- In a closed system, equilibrium is reached when:
- the forward rate = the reverse rate, and
- the concentrations stop changing.
Practice
A reversible reaction is at equilibrium when:
At equilibrium the forward and reverse reactions occur at equal rates, so concentrations stay constant.
Moving the equilibrium
- Temperature: heating shifts it the endothermic way; cooling the exothermic way.
- Pressure (gases): more pressure shifts it to the side with fewer gas molecules.
- Concentration: adding more of a substance shifts it to the other side (to use it up).
- A catalyst does not move the position — it only reaches equilibrium faster.
Practice
A catalyst changes the position of equilibrium.
A catalyst only helps reach equilibrium faster; it does not move the position.
You've got it
Key idea
- reversible ($\rightleftharpoons$): hydrated ⇌ anhydrous copper sulfate is a test for water
- equilibrium: forward rate = reverse rate, concentrations constant (closed system)
- shift it with temperature, pressure, concentration; a catalyst doesn't shift it