IB Chemistry SLTopic 5 — The Extent of Chemical ChangePaper 1 & 2Core idea~12 min read
Features of Dynamic Equilibrium
Most reactions you meet early on run in one direction until something runs out. Reversible reactions do not. They settle into a state where the chemistry never actually stops — it just stops looking like anything is happening.
📚 What you need to know
A reversible reaction can run forwards and backwards, shown by two half arrows: ⇌
Equilibrium can only be reached in a closed system, where nothing enters or leaves.
At equilibrium the rate of the forward reaction equals the rate of the reverse reaction.
Both reactions are still running. That is what dynamic means.
Concentrations of reactants and products are constant — not equal.
Macroscopic properties such as colour, pressure and density stay constant.
The same equilibrium is reached whether you start from the reactants or from the products.
Reactions that go both ways
Some reactions go to completion: the reactants are used up, and that is the end of it. In a reversible reaction the products can react together to re-form the original reactants, so the reaction runs in both directions at once.
The classic school example is copper(II) sulfate. Heat the blue hydrated crystals and they turn white as water is driven off; add water back to the white powder and the blue returns.
A reversible reaction
CuSO4·5H2O(s) ⇌ CuSO4(s) + 5H2O(l)
Read the two half arrows as a single statement, not two separate reactions. The top arrow is the forward reaction, left to right as written. The bottom arrow is the reverse. Which one you call “forward” is decided entirely by how the equation is written down.
What “dynamic” actually means
Put the reactants in a sealed container and the forward reaction starts fast, because the reactant concentrations are at their highest. There is no product yet, so the reverse reaction has nothing to work with and its rate is zero.
As the reaction proceeds, reactants are used up, so the forward rate falls. Products build up, so the reverse rate rises. Sooner or later the two rates meet.
Equilibrium is not the moment the reaction dies. It is the moment the two opposing rates match, and then keep matching.
Think of a busy shop with a door in and a door out. Once people are entering at the same rate as they are leaving, the number inside stays the same all afternoon — even though nobody is standing still. Constant totals, constant movement. That is dynamic equilibrium.
Constant is not the same as equal
This is the sentence that costs marks. At equilibrium the concentration of each substance stops changing, so we call it constant. That tells you nothing at all about whether the concentrations are the same as each other — and usually they are nowhere near.
A reaction that barely gets going sits at equilibrium with mostly reactants. A reaction that almost finishes sits at equilibrium with mostly products. Both are perfectly good equilibria; both have constant concentrations. Where the balance point falls is a separate question, and it is the one the equilibrium constant answers.
Because the concentrations are constant, so is everything that depends on them: colour, density, and, for gases in a fixed volume, pressure. These are the macroscopic properties, and a constant value for any of them is your experimental evidence that equilibrium has been reached.
The same destination from either end
Here is the neat part. If you start with only the reactants, or only the products, and the conditions and total amounts are the same, you end up in exactly the same place.
The dashed lines are the equilibrium concentrations. Notice they are constant but not equal — there is five times as much B as A in both flasks.
Why the system has to be closed
A reverse reaction needs the products to still be there. Let one of them wander off and it can never react back, so the forward reaction just keeps going until the reactants are gone.
Same reaction, same temperature, opposite outcome. In the open container every molecule of carbon dioxide that leaves is one that can never react back.
A reaction that takes place entirely in solution can reach equilibrium in an open beaker, because nothing has anywhere to go. The moment a gas is involved, you need a stopper.
Physical changes do it too
Equilibrium is not only a chemical idea. Seal some ethanol in a bottle and liquid evaporates while vapour condenses. Once those two rates match, the amount of liquid stops falling, and the vapour above it reaches a fixed pressure.
A physical equilibrium
C2H5OH(l) ⇌ C2H5OH(g)
Leave the bottle open and the same thing happens as with the carbonate: vapour drifts away, condensation can never catch up, and eventually the bottle is dry.
WORKED EXAMPLE
A sealed flask contains a mixture of colourless N2O4 and brown NO2 at equilibrium. The brown colour has been the same shade for an hour. A student concludes that the reaction has stopped. Explain why the student is wrong, and state what is actually true.
What the constant colour tells youColour depends on the concentration of NO₂, so a constant colour means a constant concentration.What it does not tell youConstant does not mean stopped. Molecules are still reacting in both directions.the two rates are equal, not zeroN₂O₄ is still breaking apart and NO₂ is still joining up, at exactly the same rate, so the amounts never change.
WORKED EXAMPLE
Two identical sealed flasks are held at the same temperature. Flask 1 contains 0.60 mol H2 and 0.60 mol I2. Flask 2 contains 1.20 mol HI. Predict how the final mixtures compare, and justify your answer. H2(g) + I2(g) ⇌ 2HI(g)
Step 1 — check the atomsFlask 1 holds 1.20 mol of H atoms and 1.20 mol of I atoms. So does flask 2.Step 2 — same conditions, same starting materialFlask 1 approaches equilibrium from the reactant side, flask 2 from the product side.the two final mixtures are identicalThe route does not matter. Same closed system, same temperature and same atoms means the same equilibrium position.
WORKED EXAMPLE
State two pieces of experimental evidence that a reaction mixture has reached equilibrium, and one observation that would prove it has not.
Evidence for equilibriuma constant coloura constant pressure at fixed volume and temperatureAny macroscopic property that depends on concentration will do — colour, pressure, density, pH.Evidence againstA colour that is still deepening, or a pressure still climbing, means concentrations are still changing, so the rates are not yet equal.
💡 Exam tip
Write the two conditions in full: the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations remain constant.
Always add “in a closed system”. It is often worth a mark on its own.
Use the word constant, never equal, when describing concentrations.
Never say the reaction has stopped, finished or is balanced in amount. Say the rates are equal.
If asked how you would know experimentally, name a macroscopic property and say it stops changing.
⚠️ Common mix-up
“The concentrations are equal.” They are constant. This single word loses marks every year.
“The reaction has stopped.” Nothing has stopped; the two directions have levelled up.
Assuming equilibrium means 50 : 50. The mixture can be almost all reactant or almost all product.
Forgetting the closed system, then being surprised that an open flask of a gas-producing reaction goes to completion.
Thinking the forward reaction happens first and then the reverse starts. The reverse begins the instant the first product molecule exists.
Up next: The Equilibrium Law — you now know that reactions settle somewhere. The next step is writing down exactly where, in a form you can put a number to.
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