A reversible reaction is one where the products can react to form the reactants again. In a closed container, such a reaction can reach dynamic equilibrium, where the forward and reverse reactions carry on at the same rate and the amounts of each substance stay constant.
This module sits after rates of reaction and before acids, bases and salts. It is also where industrial chemistry starts to make sense, because manufacturers must balance how much product they get against how fast they get it.
What do you need before you start?
- Writing and balancing symbol equations, including state symbols.
- The collision idea for rate: more frequent, more energetic collisions mean a faster reaction.
- Exothermic and endothermic reactions: which direction releases energy and which takes it in.
- Reading a simple table of amounts or percentages.
One orienting example
Dinitrogen tetroxide is a colourless gas. It forms nitrogen dioxide, a brown gas, in a reversible reaction:
N₂O₄(g) ⇌ 2NO₂(g) (forward reaction endothermic)
Sealed in a tube at room temperature, the gas is pale brown. Warm the sealed tube and it darkens. Cool it and it lightens again.
Read that in three steps. First, the double arrow says both directions happen.
Second, the colour stops changing at a fixed temperature, so the mixture is at equilibrium, yet both reactions are still going. Third, warming favours the endothermic direction, so more brown NO₂ forms.
Every lesson in this module is a longer version of those three steps. This is a teacher demonstration described for understanding only; any practical work belongs in a supervised school laboratory.
In what order should you study the lessons?
- Represent a reversible reaction: learn the ⇌ symbol and write balanced reversible equations, because every later lesson starts from an equation.
- Explain a dynamic state: learn what “equilibrium” really says, and why “the reaction has stopped” is wrong.
- Predict a shift under a stated change: use concentration, pressure and temperature, one change at a time.
- Separate rate change from equilibrium position: stop mixing up “faster” with “further”.
- Interpret a yield-rate compromise: describe why industrial conditions are a compromise.
Then test yourself with the mixed practice set. The mole and equation-ratio tutor and the equation balance reasoning trainer help with the equation side.
Common traps
- Writing “the reaction stops” at equilibrium. It does not stop; the rates are equal.
- Saying a catalyst moves the equilibrium. It speeds up both directions equally.
- Applying a pressure rule to a reaction with the same number of gas particles on each side.
- Forgetting that the energy sign tells you which way a temperature change pushes.
- Using one word, “faster”, for two ideas: how quickly equilibrium is reached and how far it goes.
How to use the practice set
Attempt every question on paper first. When you get one wrong, name the type of slip before reading the route list: equation, meaning of equilibrium, direction of shift, rate versus position, or compromise wording. The practice page routes each type back to its lesson.
If you would like a teacher to watch that reasoning live, look at our online one-to-one Chemistry tuition.