Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- A reversible process can go forward (reactants → products) and in reverse (products → reactants). It is written with ⇌.
- Physical examples: evaporation and condensation; a gas dissolving in and escaping from a liquid; a salt dissolving and precipitating.
- Chemical examples: proton transfer in acid–base reactions and electron transfer in redox reactions.
- In a closed system at constant temperature, a reversible process reaches equilibrium.
- Equilibrium is dynamic: both processes continue, at equal rates, so there is no net change.
Key relationships
| Idea | What it means | How you see it |
|---|---|---|
| Equilibrium condition | forward rate = reverse rate | rate–time lines meet and stay together |
| Constant composition | each concentration or partial pressure stops changing | concentration–time or pressure–time lines go flat |
| All species present | the reaction does not go to completion | no line falls to zero |
| Stoichiometry still holds | changes are in the ratio of the coefficients | e.g. for A ⇌ 2B, B rises twice as much as A falls |
Assumptions behind the picture
- The system is closed: no matter escapes or enters. An open beaker that loses a gas never settles.
- The temperature is constant. A change of temperature moves the system to a new equilibrium (Topic 7.9 onwards).
- Every reaction you meet in this unit is treated as reversible, even if the reverse is very slow.
Mistakes to avoid
- Constant ≠ equal. Equilibrium concentrations are usually different from each other.
- Crossing ≠ equilibrium. Two concentration lines can cross while both are still changing.
- Not stopped. The reaction continues both ways; only the net change is zero.
- Rates, not rate constants, are equal at equilibrium.
- Reverse rate starts at zero when only reactants are present, then grows as product builds up.
- Use the coefficients when you read a graph: the changes must match the balanced equation.
Quick self-check
- A sealed jar of water at constant temperature has a steady water level. Which two rates are equal? (The rate of evaporation and the rate of condensation.)
- For A ⇌ 2B starting with 1.00 atm of A, P_A levels off at 0.70 atm. What is P_B at equilibrium? (0.60 atm.)
- At equilibrium, is the forward rate zero? (No. It equals the reverse rate, and both are greater than zero.)
Next: practice questions.