Energy Transfer During Exothermic and Endothermic Reactions

SPEC 4.5.1.1 • REQUIRED PRACTICAL 4 • PAPER 1

Energy Transfer: Exo & Endothermic

The two definitions, the four examiner alerts, and the RP4 method that scores top marks every time.

Energy Is Conserved

Every chemical reaction either releases energy to or absorbs energy from its surroundings. Energy itself is never created or destroyed — only transferred.

Two key terms

System — the reactants and products of the reaction.

Surroundings — everything outside the system: the solution, the cup, the air, the thermometer.

"If a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the amount transferred." — AQA Specification 4.5.1.1

Why this matters

When we measure temperature in a polystyrene cup during RP4, we're measuring the temperature of the surroundings — not the reaction itself. The cup minimises further energy escape, so the reading more accurately reflects energy released by the system.

Exothermic Reactions

Definition · Mark scheme

An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases. Energy moves OUT of the system. ΔT is positive.

Examples (spec)

Combustion (burning fuels)

Neutralisation (acid + alkali)

Many oxidation reactions

Most displacement reactions (e.g. Zn + CuSO₄)

Everyday uses (spec)

Hand warmers — sodium ethanoate solution crystallises, releasing heat

Self-heating cans — calcium oxide + water → Ca(OH)₂, exothermic

Examiner Alert

"Exothermic loses matter" — WRONG. Exothermic reactions release energy, not mass. Mass is always conserved (Lesson 3.01).

Endothermic Reactions

Definition · Mark scheme

An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases. Energy moves INTO the system. ΔT is negative.

Examples (spec)

Thermal decomposition (e.g. CaCO₃ → CaO + CO₂)

Citric acid + sodium hydrogencarbonate (sherbet)

Everyday uses (spec)

Sports injury packs — instant cold packs cool damaged tissue

Often based on ammonium nitrate dissolving in water — endothermic

Memorise this

EXO = EXIT (energy exits the system → warms surroundings). ENDO = energy enters the system → cools surroundings. The temperature change is always reported for the surroundings.

Quick Classification Check

For each scenario, decide: exothermic or endothermic? Use only the temperature data — that's the spec criterion.

ScenarioType & reason

HCl is added to NaOH; thermometer rises from 21 °C to 27 °C.Exothermic — surroundings warmed

Citric acid + sodium hydrogencarbonate; mixture cools from 22 °C to 14 °C.Endothermic — surroundings cooled

Magnesium ribbon ignites in air, giving off bright white light and heat.Exothermic — combustion is always exothermic

Calcium carbonate is heated strongly to decompose into calcium oxide.Endothermic — thermal decomposition needs energy in

Classification rule

If the temperature of the surroundings rises, the reaction is exothermic. If it falls, it's endothermic. Don't confuse "needs heat to start" with endothermic — combustion needs a spark to ignite, but it's exothermic once going.

RP4 — Apparatus

RP4 investigates how varying one reactant (mass, volume, or concentration) affects the highest temperature reached. Each piece of apparatus serves a purpose.

ApparatusReason

Polystyrene cupInsulator — minimises heat loss to surroundings

Lid (cardboard)Reduces heat loss to the air above

ThermometerMeasures initial and highest temperatures

Glass beakerSupports cup; prevents tipping

Measuring cylinderFixed volume of liquid (controls heat capacity)

BalanceMeasures mass of solid reactant added

RP4 — The Method

Adapted from the AQA-published procedure used in P1 Jun 2024 Q04 (zinc + copper sulfate solution):

Measure a fixed volume of one reactant (e.g. 25 cm³ of acid) into the polystyrene cup using a measuring cylinder.

Record the initial temperature with the thermometer.

Add a known mass (or volume) of the other reactant — this is your independent variable.

Stir the mixture with the thermometer to mix evenly.

Record the highest temperature reached.

Calculate ΔT = T_highest − T_initial.

Repeat with different masses (or volumes/concentrations) of the second reactant.

Repeat the whole investigation and find a mean ΔT for each value.

Critical step

Step 5 — "highest temperature reached" — is the most-missed mark in RP4. Students who write "measure the temperature" lose this mark. We unpack why on the next slide.

Alert 1 — The "Highest" Word

Examiner Alert

The exothermic reaction's temperature peaks then falls — heat escapes to the surroundings. So a single later reading is too low. The mark scheme requires "measure the highest temperature reached".

Source: P1 Jun 2022 Q02.1 mark scheme — indicative content includes "measure the highest temperature reached" verbatim.

Loses the mark

"Measure the temperature"

"Measure the temperature after adding the reactant"

"Measure the final temperature"

Wins the mark

"Measure the highest temperature reached"

As soon as the reaction finishes, heat escapes. The peak is the only reading that reflects the reaction's actual energy release.

RP4 — Variables

RP4 questions test variables every series. The mark scheme demands each control variable carries a reason.

Independent

Mass of solid (or volume / concentration of one reactant)

Dependent

Highest temperature reached (or ΔT)

Controls × 4

Each with a reason — see table

Control variableReason

Same volume of liquid each runDifferent volumes have different heat capacities → ΔT would differ for the same energy

Same starting temperatureConsistent baseline — so ΔT comparison is fair

Same insulating container (cup + lid)Same rate of heat loss between runs

Same concentration of fixed reactantOnly the IV is changing — isolates the variable being tested

The mark for control variables

"Same volume" is half a mark; "same volume so the heat capacity of the mixture is constant" is the full mark. The reason is what examiners credit.

Alert 2 — Polystyrene vs Glass

Examiner Alert

Two phrasing errors lose marks every year:

❌ "Polystyrene is an insulator and glass is a conductor." Both insulate and conduct to some extent.

❌ "No energy is lost from the polystyrene cup." Some energy always escapes — polystyrene reduces the rate but doesn't eliminate loss.

Source: P1 Jun 2024 Q04.3 mark scheme.

Loses marks

"Polystyrene is an insulator, glass is a conductor"

"No energy is lost in polystyrene"

"Polystyrene keeps all the heat in"

Wins marks

"Polystyrene is a better (thermal) insulator"

"Less energy is transferred to the surroundings"

The principle

Comparative wording — "better insulator", "less energy lost", "more accurate" — is what the mark scheme rewards. Absolute statements ("no energy lost", "is an insulator") fail because they're scientifically false.

Alert 3 — Never Heat Externally

Examiner Alert

Some students added "heat the mixture with a Bunsen burner" to their RP4 method. This invalidates the entire experiment — RP4 measures energy released by the reaction itself. External heat from a Bunsen contaminates the reading: you can't tell how much of the temperature rise was from the reaction and how much was from the flame.

Source: P1 Jun 2022 Q02.1 examiner report.

RP4 is unique among the practicals

In RP1 (soluble salts) you heat to crystallise. In RP6 (chromatography) you don't heat. In RP4, the only heat source allowed is the chemical reaction itself. The cup, the thermometer, and the reaction together form a closed measurement system. Adding a Bunsen breaks it.

Mental rule

If a method involves measuring the temperature change caused by a reaction, never add external heat. The chemistry is the heat source.

Reading RP4 Graphs

For the Jun 2024 zinc + copper sulfate experiment, results plot highest temperature against mass of zinc added:

The two-phase pattern

Below 0.8 g of zinc: highest temperature rises with mass added — more reaction, more energy released. Above 0.8 g: temperature plateaus — copper sulfate is now the limiting reactant, and adding more zinc cannot release more heat.

Alert 4 — Explain the Plateau

Examiner Alert

Most students described the trend ("temperature rises then plateaus") but failed to explain the plateau. The plateau happens because one reactant becomes the limiting reactant — adding more of the variable reactant beyond that point cannot produce more reaction.

Source: P1 Jun 2024 Q04.2 mark scheme — full 4 marks require: "until 0.8 g zinc is added", "(so) there is no additional reaction", "(because) zinc is in excess", "(or) copper sulfate is used up".

The 4-mark answer structure

1

State the trend with reasoning: "Temperature rises because the reaction is exothermic " (or "energy is transferred to the surroundings").

2

Identify the turning point with a value from the graph: " until 0.8 g of zinc is added ".

3

State the consequence above the turning point: " no additional reaction ".

4

Explain WHY: "zinc is now in excess " OR "copper sulfate is used up " (limiting reactant).

Heat acceptable for energy

The mark scheme accepts "heat" as an alternative to "energy" throughout 4.5.1.1. Both score equally.

Common Errors — RP4

What students writeCorrect version

"Measure the temperature""Measure the highest temperature reached"

"Polystyrene is an insulator, glass is a conductor""Polystyrene is a better insulator than glass"

"No energy is lost"" Less energy transferred to surroundings"

"Reaction stops" (for plateau)"Reactant X is in excess / reactant Y is used up"

"Heat the mixture with a Bunsen"No external heating — never in RP4

"Exothermic loses matter""Exothermic releases energy to the surroundings"

Pattern

Three categories of error: imprecise wording (missing "highest"), absolute claims (no energy lost), and missing reasoning (plateau described not explained). The mark scheme rewards specificity, comparison, and explanation.

Practice — Mean Calculation

A student repeats the experiment four times with the same mass of zinc. The highest temperatures recorded were:

37.6 °C 37.2 °C 37.8 °C 37.4 °C

Calculate the mean highest temperature.3 MARKS

Adapted from P1 Jun 2024 Q04.5

MP1

Sum the four values:

37.6 + 37.2 + 37.8 + 37.4 = 150.0

MP2

Divide by number of repeats:

150.0 ÷ 4

MP3

Final answer:

Mean = 37.5 °C

📝

Mark scheme notes: Tolerance ± 0.3 °C. The alternative working "150/4" earns the second mark. The °C unit is bracketed in the MS — include it as good practice but the third mark is for the value 37.5. Quote to one decimal place to match the data.

Practice — Plateau Explanation

A student investigated the energy change of the reaction between zinc and copper sulfate solution. Figure 3 (slide 12) shows highest temperature against mass of zinc added. Below 0.8 g, the line rises; above 0.8 g, it plateaus.

Explain the results shown in Figure 3. Do not refer to anomalous points. Use data from Figure 3.4 MARKS

Source: P1 Jun 2024 Q04.2 (verbatim)

MP1

The temperature rises because the reaction is exothermic (or "energy is transferred to the surroundings").

MP2

The temperature rises until 0.8 g of zinc is added.

MP3

Above 0.8 g, there is no additional reaction (so no further temperature rise).

MP4

Because zinc is now in excess (or copper sulfate is used up — limiting reactant).

📝

Structure: trend → "exothermic" → turning-point value → limiting-reactant explanation. "Heat" is acceptable for "energy".

Practice — Polystyrene Cup

Explain why using a polystyrene cup gives more accurate results than using a glass beaker.2 MARKS

Source: P1 Jun 2024 Q04.3 (verbatim)

MP1

Polystyrene is a better (thermal) insulator (than glass).

MP2

Less energy is transferred to the surroundings (or "less energy is lost").

⚠️

Mark loss check: "Polystyrene is an insulator, glass is a conductor" scores 0 — both insulate to some degree. "No energy is lost" scores 0 — some energy always escapes. The comparative wording ("better", "less") is what the mark scheme rewards.

Checkpoint — Lesson 5.01

You should now be able to:

Definitions & classification

Define exothermic (energy to surroundings, T rises) and endothermic (energy from surroundings, T falls) — with spec examples

Classify a reaction as exo or endothermic from temperature data alone

State that mass is conserved — exothermic releases energy , not matter

RP4 method & variables

Describe the polystyrene-cup setup with labelled apparatus

Always include " highest temperature reached "

State IV, DV, and at least three controls — each with a "so that…" reason

Never heat a RP4 mixture externally

Examiner-proof answers

Compare polystyrene and glass comparatively ("better insulator", "less energy lost")

Explain a graph plateau using limiting-reactant language

Use mark-scheme phrasing verbatim where you know it

Next up

Lesson 5.02 — Reaction Profiles. We turn the energy change into a graph and add activation energy. The same exothermic/endothermic distinction comes back, but now drawn precisely with arrows the mark scheme will accept.