Reversible reactions and equilibria

Most reactions you have met so far go one way only: reactants turn into products and that is the end of it. But some reactions are reversible — the products can react together to re-form the reactants. The reaction can go forwards AND backwards.

We show a reversible reaction with a special double arrow, ⇌, instead of the normal single arrow →:

• The forward reaction is left → right.
• The backward reaction is right → left.

Demo 1 — hydrated ⇌ anhydrous copper(II) sulfate. This is the demo examiners love, because the colour change is so clear:

$\text{CuSO}_4{\cdot}5\text{H}_2\text{O} \rightleftharpoons \text{CuSO}_4 + 5\text{H}_2\text{O}$

• Forward (heat it): blue hydrated copper(II) sulfate loses its water and turns into white anhydrous copper(II) sulfate. Water vapour is driven off.
• Backward (add water): dripping water onto the white powder turns it blue again — and the reaction releases heat.

This colour change is also used as a test for water: white anhydrous copper(II) sulfate turns blue when water is added.

Demo 2 — ammonium chloride. Heating white solid ammonium chloride breaks it down; the gases re-combine on the cooler part of the tube to re-form the white solid:

$\text{NH}_4\text{Cl} \rightleftharpoons \text{NH}_3 + \text{HCl}$

• Forward (heat): white solid ammonium chloride → ammonia gas + hydrogen chloride gas (both colourless).
• Backward (cool): the two gases react again and re-form white ammonium chloride solid higher up the test tube.

If you carry out a reversible reaction in a closed system — a sealed container that nothing can escape from — something special happens. The reaction reaches a state called dynamic equilibrium.

How it builds up. Imagine sealing the reactants in a flask:

• At the start, only the forward reaction can happen (there are no products yet to go backwards).
• As products build up, the backward reaction starts and gets faster.
• Meanwhile the forward reaction slows down as the reactants get used up.
• Eventually the two rates become EQUAL → this is dynamic equilibrium.

Why it is called "dynamic". "Dynamic" means moving / still happening. At equilibrium the reaction has NOT stopped — both the forward and backward reactions are still going on. They are just happening at exactly the same rate, so for every particle of product made, one is turned back into reactant. There is no net change.

Why a closed system is needed. If the container were open, products (such as a gas) could escape. The backward reaction could then never get going, so the forward and backward rates would never become equal — equilibrium would never be reached. A closed system keeps everything inside so both directions can balance.

A helpful picture — running on a treadmill. You run hard (your legs keep moving) but you stay in the same place. At dynamic equilibrium the reactions keep going hard, but the overall composition stays the same.

This is the idea students find hardest, and the one examiners test most: at equilibrium the concentrations of the reactants and products stay CONSTANT — even though both reactions are still happening.

How can both be true at once?

• The forward reaction is making product (and using up reactant).
• The backward reaction is making reactant (and using up product).
• Because the two rates are equal, product is made as fast as it is broken down.
• So the amount of each substance does not change — concentrations stay constant.

It is not that nothing is happening. It is that the two opposite changes cancel out, so there is no NET change.

Worked picture (numbers). Imagine that, every second:

• 1000 product molecules are formed by the forward reaction, and
• 1000 product molecules are broken down by the backward reaction.

The net change is zero — the concentration of product stays exactly the same, second after second. Individual molecules are constantly being converted back and forth, but the populations are stable. That is dynamic equilibrium.

Important: "concentrations constant" does NOT mean the amounts of reactant and product are equal to each other. They are usually different amounts — they just each stay the same over time.

Changing the conditions. The position of equilibrium (how much product vs reactant there is) can be shifted by changing the temperature, pressure or concentration. For example, removing a product makes the system make more of it. (At Double Award you only need to know that these changes can shift the position — the detailed rules are a higher-up topic.)

This topic is mostly describe and explain marks. The marks are won by using the exact mark-scheme phrases, so learn them word for word.

Defining dynamic equilibrium (a very common 2–4 mark question). A full answer needs:

1. the forward and backward reactions occur at the same rate (equal rates);
2. the concentrations of reactants and products stay constant;
3. it happens in a closed system;
4. (for the top mark) the reactions are still happening — equilibrium is dynamic, not stopped.

Describing the demos. For copper(II) sulfate, always give the colours and the direction: blue → white on heating (water driven off); white → blue on adding water. For ammonium chloride: white solid → two colourless gases on heating; gases → white solid on cooling.

The trap to avoid. Never say the reaction "stops" at equilibrium, and never say the amounts of reactant and product become "equal". Both lose marks. Say the rates are equal and the concentrations are constant.