Reversible Reactions, Energy Changes and Reversible Reactions, and Equilibrium

SPEC 4.6.2.1 – 4.6.2.3 • PAPER 2

Reversible Reactions & Dynamic Equilibrium

The ⇌ symbol, energy conservation across both directions, and the most-misunderstood concept on Paper 2 — what "equilibrium" actually means.

What You'll Learn

Three sub-spec points, each with its own examiner trap. Get the wording right and the marks follow — the concept itself is straightforward.

By the end of this lesson

Recognise reversible reactions and the ⇌ symbol — and how changing conditions can reverse them (spec 4.6.2.1).

Apply the energy rule: if forward is exothermic, reverse is endothermic — by the same amount(spec 4.6.2.2).

Define dynamic equilibrium using rate language — and avoid the three errors examiners flag every year (spec 4.6.2.3).

Why this matters

Examiner reports flag this as one of the highest-error topics on Paper 2. Three specific student misconceptions appear across every year. This lesson targets each one with the verbatim mark-scheme phrase that earns the mark.

The ⇌ Symbol — Reactions That Go Both Ways

In a normal reaction, the arrow → points one way: reactants become products, full stop.

In a reversible reaction, products can react together to re-form the reactants. Two arrows are drawn, one above the other, to show this — written as a single double-headed symbol:

A + B ⇌ C + D

Definition

Reversible reaction: a reaction in which the products can react together to re-form the original reactants. Shown by the ⇌ symbol.

Spec point 4.6.2.1

The direction of a reversible reaction can be changed by changing the conditions (e.g. heating it one way, cooling or adding a substance the other).

Classic Demo — Copper Sulfate

The standard classroom example of a reversible reaction. Same chemicals; the colour flips depending on which direction the reaction is going.

How the demo runs

Heat blue crystals in a test tube → water vapour drives off → white powder remains. Add water back to the white powder → instantly turns blue again. Same chemicals, opposite directions, set by the conditions.

Energy: Same Magnitude, Opposite Direction

Energy is conserved across a reversible reaction. If one direction releases energy, the reverse direction must absorb the same amount.

Spec point 4.6.2.2 (verbatim)

If the forward reaction is exothermic, the reverse reaction is endothermic. The same amount of energy is transferred in each case.

Same magnitude. Opposite direction of energy flow. No energy is created or destroyed.

Worked example: copper sulfate

Forward (heating blue crystals): endothermic — energy taken in to drive off water.

Reverse (adding water to white powder): exothermic — same amount of energy released as a temperature rise.

Mark-scheme phrasing

"Reverse reaction is endothermic by the same amount." Don't just say "the reverse is endothermic" — examiners want the magnitude rule made explicit.

Worked Example — Energy Calculation

P2 Jun 2022 Q02.4 · Spec 4.6.2.2 3 MARKS

When 238 g of hydrated cobalt chloride is heated until the mass does not change, 88.1 kJ of energy is taken in.

A student heats 2.00 g of hydrated cobalt chloride until the mass does not change. Calculate the energy taken in, to 3 significant figures.

M1

Set up the proportion. Energy taken in scales linearly with mass — same compound, same reaction, just less of it. energy = (2.00 / 238) × 88.1

M2

Calculate.= 0.7403... kJ

M3

Round to 3 s.f. as the question requires. = 0.740 kJ ✓

SPEC

The principle: energy taken in (forward, endothermic) for 2.00 g = energy released when 2.00 g of the white powder is rehydrated (reverse, exothermic). Same magnitude — that's spec 4.6.2.2 in calculation form.

Practice — Identify the Reaction

P2 Jun 2022 Q02.5 · Spec 4.6.2.2 1 MARK

Energy is taken in from the surroundings when hydrated cobalt chloride is heated.

What type of reaction takes place when hydrated cobalt chloride is heated?

MS

Primary mark-scheme answer: endothermic (reaction) ✓ — the headline answer the MS expects.

ALT

Allowed alternatives (any one earns the mark):

reversible (reaction) — accepted because the reverse direction can be triggered by adding water

(thermal) decomposition (reaction) — accepted because heating breaks one substance into simpler ones

TIP

Why "endothermic" is the headline: the question stem gives you "energy is taken in from the surroundings" — that's the textbook signal for endothermic. Answer that first; the alternatives also score but only one mark is on offer.

Closed Systems — The Setup for Equilibrium

Equilibrium can only be reached if nothing is escaping or entering the apparatus. This is what the spec calls a closed system.

Definition (verbatim spec)

Closed system: apparatus which prevents the escape of reactants and products. The reaction is sealed off from the surroundings.

Why this matters

If a gas escapes, the products keep being removed — the reverse reaction can't get going, so equilibrium is never reached. Sealed apparatus traps everything in, allowing the forward and reverse reactions to balance.

Exam wording

If a question describes a sealed flask or stoppered apparatus, it's signalling "closed system" — equilibrium is on the cards. An open beaker is not a closed system.

Dynamic Equilibrium — Three Conditions

When a reversible reaction runs in a closed system, it eventually reaches a state called dynamic equilibrium. Three conditions must all be true:

Closed System

Nothing escapes; nothing enters.

Same Rate

Forward & reverse occur at exactly the same rate.

Constant Amounts

Concentrations stop changing — net change is zero.

Definition (memorise — verbatim spec)

Dynamic equilibrium: the state in a closed system where the forward and reverse reactions occur at exactly the same rate, so the amounts of reactants and products remain constant.

"Dynamic" is the key word

"Dynamic" = both reactions are still happening. As fast as products form, they convert back to reactants. The reaction has not stopped — it's just running in both directions at matched speeds.

What Equilibrium Looks Like — Rates Over Time

Plot the rate of the forward and reverse reactions against time, and the picture of dynamic equilibrium becomes clear.

Reading the graph

At the start, only the forward reaction can run — there are no products yet. As products build up, the reverse reaction speeds up. The forward slows down as reactants run low. When the two curves meet, both reactions continue at the same rate — that's equilibrium.

Examiner Alert #1 — "Stopped" Is Wrong

EXAMINER ALERT — P2 Jun 2023 Q09.8

Students wrote "the reaction has stopped" to describe equilibrium — and lost both marks. The reactions have not stopped. They continue, in both directions, at exactly the same rate. The mark scheme requires the word rate.

✗ Loses Marks

"At equilibrium the reaction has stopped because the amounts aren't changing."

Two errors in one sentence: claims reactions stop, no mention of rate.

✓ Full Marks

"The forward and reverse reactions are taking place at exactly the same rate."

Both reactions ongoing; explicit reference to rate. Verbatim mark-scheme answer.

Memorise this phrase

"forward and reverse reactions take place at exactly the same rate"

Examiner Alert #2 — Constant ≠ Equal

Once a reaction reaches equilibrium, the amounts of reactants and products are constant — they stop changing. That does not mean the amounts are equal.

COMMON MISCONCEPTION

Most equilibria sit far over to one side. There may be a lot more product than reactant (or vice versa). The two amounts stay the same over time — but they are usually very different from each other.

✗ Wrong

"At equilibrium the amounts of reactants and products are equal."

✓ Right

"At equilibrium the amounts of reactants and products are constant (not necessarily equal)."

Quick test

If you wrote "equal", reread your sentence and check it makes physical sense. Most reactions don't sit at 50/50 — equilibrium just means the ratio has settled and stopped shifting.

Worked Example — Mark-Scheme Wording

P2 Jun 2023 Q09.8 · Spec 4.6.2.3 2 MARKS

Hydrochloric acid is added to an aqueous solution of copper compound A:

copper compound A + hydrochloric acid ⇌ copper compound B + water

The concentrations of the substances in this reaction do not change at dynamic equilibrium. Explain why.

2 mk

Full-mark answer (verbatim from MS):"the forward and reverse reactions are taking place at exactly the same rate"

1 mk

For 1 mark only: "the reactions are taking place at exactly the same rate" — without specifying that one is forward and one is reverse.

WHY

Why two marks if you say it fully: the mark scheme rewards (a) both directions running — i.e. naming forward and reverse — and (b) at the same rate. Drop either half and you drop a mark.

NOTE

The mark scheme explicitly says "ignore references to closed systems" — that's already given by the question, so don't waste line space restating it.

Practice — Equilibrium MCQ

P2 Jun 2024 Q03.2 · Spec 4.6.2.3 1 MARK

Water molecules break down into hydrogen ions and hydroxide ions:

H₂O ⇌ H⁺ + OH⁻

Which sentence describes this reaction at equilibrium? Tick one box.

▢ Water molecules break down at a higher rate than they reform.

▢ Water molecules break down and reform at the same rate.

▢ Water molecules break down at a lower rate than they reform.

MS

Answer: "Water molecules break down and reform at the same rate." ✓

WHY

The spec definition says forward rate = reverse rate at dynamic equilibrium. The middle option restates that exact rule using the words from the equation.

TRAP

The other two options describe a system that is not yet at equilibrium — the rates are unequal, so the amounts are still changing. That's a system on its way to equilibrium, not at equilibrium.

Practice — Identify Reaction Types

P2 Jun 2024 Q03.1 · Spec 4.6.2.1 2 MARKS

Hydrogen gas reacts with oxygen gas to produce water. Water is decomposed into hydrogen gas and oxygen gas using electricity.

Which two words describe the reaction between hydrogen gas and oxygen gas? Tick two boxes.

▢ Alloying

▢ Combustion

▢ Corrosion

▢ Endothermic

▢ Reversible

MS

Answers: Combustion ✓ AND Reversible ✓ (1 mark each)

A

Why combustion: hydrogen burning in oxygen to produce water is the textbook combustion reaction (and exothermic).

B

Why reversible: the reverse direction is given in the question — water can be decomposed back into hydrogen and oxygen using electricity (electrolysis). Both directions are achievable, so ⇌ applies.

TRAP

Endothermic is wrong for the forward direction: combustion is exothermic. The reverse (electrolysis) is endothermic, but the question asks about the forward reaction only.

PEE Worked Example — 3-Mark Description

Typical exam question · Spec 4.6.2.3 3 MARKS

Describe what is happening at dynamic equilibrium in a closed system.

P

Point — what's happening: the forward and reverse reactions are both still occurring; the reaction has not stopped.

E

Explain — the rate rule: the rate of the forward reaction is exactly the same as the rate of the reverse reaction. As fast as products form, they convert back to reactants.

E

Effect — the consequence: the net change is zero, so the amounts (concentrations) of reactants and products remain constant — though not necessarily equal.

MK

Likely mark allocation (typical 3-mark equilibrium description): 1 mk for "both reactions continue", 1 mk for "exactly the same rate", 1 mk for "constant amounts". Real past papers may bundle these differently — but if you hit all three points, you secure the marks regardless.

Common Errors — At-A-Glance

Run your answer past this table before moving on. If anything you wrote matches the left-hand column, fix it.

What students write ✗What earns marks ✓Why it matters

"Reaction has stopped at equilibrium""Forward and reverse reactions continue at exactly the same rate "Equilibrium is dynamic — both reactions still occurring

"Amounts are equal""Amounts are constant (not necessarily equal)"Constant ≠ equal; most equilibria favour one side

No mention of rate at allAlways include "rate" in the definitionMark scheme requires the rate language

"Reverse is endothermic" (no quantity)"Reverse is endothermic by the same amount "Energy magnitude rule explicitly required (4.6.2.2)

Using a single arrow (→) for a reversible reactionAlways use the double-headed symbol ⇌ for reversible reactionsWrong symbol = wrong reaction type stated

Summary

Spec 4.6.2.1 — Reversible Reactions

Symbol: ⇌ (not →) for reactions that can run in both directions

The direction is set by the conditions — change the conditions and the reaction can run the other way

Classic demo: hydrated CuSO₄ ⇌ anhydrous CuSO₄ + water (blue ⇌ white)

Spec 4.6.2.2 — Energy in Both Directions

If forward is exothermic, reverse is endothermic — same amount of energy transferred in each case

Energy is conserved across the reversible reaction; same magnitude, opposite direction

Calculation type: scale energy in proportion to mass (P2 Jun 2022 Q02.4)

Spec 4.6.2.3 — Dynamic Equilibrium

Three conditions: closed system + forward and reverse at exactly the same rate + constant amounts

"Dynamic" because both reactions continue — the reaction has not stopped

Constant ≠ equal. Amounts stop changing; they don't become equal to each other

The Phrase to Memorise

"forward and reverse reactions take place at exactly the same rate;

amounts remain constant"

Ready for 6.06 [Higher Tier]

Next up: Le Chatelier's Principle — how concentration changes shift the equilibrium position. You'll need everything from this lesson plus the new direction-shift logic.