Exothermic and Endothermic Reactions

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.1 — Exothermic and endothermic reactions defined by heat given out or taken in
• 3.2 — Simple calorimetry experiments for combustion, displacement, dissolving and neutralisation
• 3.8 — Practical: investigate temperature changes accompanying dissolving, neutralisation, displacement and combustion (required practical)

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Every chemical reaction involves a transfer of energy, and the direction of that transfer is what this page classifies. An EXOTHERMIC reaction GIVES OUT heat energy to the surroundings. An ENDOTHERMIC reaction TAKES IN heat energy from the surroundings.

The way you tell them apart in a laboratory is to put a thermometer in the mixture and watch, and here is the point that catches almost everybody. In an EXOTHERMIC reaction the temperature GOES UP. That sounds obvious, and it becomes confusing the moment you ask what is being measured, because the thermometer is not measuring the reaction — it is measuring the SURROUNDINGS.

Think of it like a radiator warming a room

A radiator gives out heat, and the way you know is that the ROOM gets warmer. Nobody measures the radiator's own energy content; you measure what happens to everything around it. Chemistry is exactly this, and the confusion arises because we say 'the reaction gives out heat' and then measure the water it is sitting in. Exothermic means energy leaves the reacting chemicals and arrives in the surroundings — so the surroundings, and the thermometer in them, get hotter. Endothermic means energy is pulled the other way, out of the surroundings and into the chemicals, so the surroundings cool and the thermometer falls.

Statement 3.2 — the four kinds of experiment, and which way each goes

1. COMBUSTION — burning a fuel. Strongly EXOTHERMIC, always. The temperature of water heated by the flame rises.
2. NEUTRALISATION — an acid with an alkali. EXOTHERMIC. Mixing the two solutions in a polystyrene cup raises the temperature by several degrees.
3. DISPLACEMENT — a more reactive metal displacing a less reactive one, such as zinc into copper sulfate. EXOTHERMIC.
4. DISSOLVING — this one goes EITHER WAY. Some salts dissolve exothermically and warm the water; others dissolve endothermically and cool it noticeably.
5. So three of the four are reliably exothermic, and dissolving is the interesting case that makes the classification worth having at all.

Ammonium nitrate dissolving in water is the standard endothermic example, and it cools the water enough to be startling — this is how instant cold packs for sports injuries work. Anhydrous copper(II) sulfate dissolving, by contrast, warms the water measurably.

The required practical is these four experiments done with a thermometer, and the apparatus is chosen with one enemy in mind: HEAT LOSS TO THE ROOM. If the heat produced escapes before it is measured, every temperature rise comes out too small.

Statement 3.8 — the apparatus and why each part is there

1. A POLYSTYRENE CUP rather than a glass beaker, because polystyrene is a poor conductor and reduces heat loss through the walls.
2. A LID, because a great deal of heat escapes from an open surface by evaporation and convection.
3. The cup stood in a BEAKER, which adds a layer of trapped air as further insulation.
4. A thermometer reading to 0.5 °C or better, since the changes being measured may be only a few degrees.
5. STIR before taking the final reading, so the temperature is even throughout rather than measured at one hot spot.
6. Record the temperature BEFORE mixing and the highest or lowest reached afterwards, and take the difference.

Combustion is the awkward one to do well, and it is worth knowing why. Burning a fuel under a container of water loses an enormous proportion of the heat to the air around the flame and to the container itself, so measured values for fuels are typically far below the true figure. The other three experiments happen inside the cup, which is why they can be insulated properly.

Reading a set of results and classifying each

A student records: (a) acid + alkali, 21.0 °C to 27.5 °C; (b) ammonium nitrate dissolving, 20.8 °C to 14.2 °C; (c) zinc into copper sulfate, 20.5 °C to 31.0 °C. Classify each and give the temperature change.

1. (a) The temperature ROSE by 6.5 °C, so heat was given out to the surroundings: EXOTHERMIC. This is neutralisation, which always is.
2. (b) The temperature FELL by 6.6 °C, so heat was taken in from the surroundings: ENDOTHERMIC. Dissolving can go either way, and ammonium nitrate is the standard endothermic case.
3. (c) The temperature ROSE by 10.5 °C, so EXOTHERMIC. This is displacement, which is reliably exothermic.
4. Note that (b) and (c) have similar-sized changes in opposite directions, so the SIGN of the change is the classification and the size is a separate question.
5. The size is what the next lesson turns into an energy in joules.

Answer: (a) exothermic, +6.5 °C. (b) endothermic, −6.6 °C. (c) exothermic, +10.5 °C.

One last thing worth naming now because the next two lessons depend on it. The energy has not been created or destroyed in any of these — it has MOVED, between the chemicals and their surroundings. What the thermometer measures is that movement, and the whole of this topic is about accounting for where it went.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 3.1 — the two definitions

Exothermic

A reaction that GIVES OUT heat energy to the surroundings. The temperature of the surroundings RISES.

Endothermic

A reaction that TAKES IN heat energy from the surroundings. The temperature of the surroundings FALLS.

Surroundings

Everything other than the reacting chemicals — the water, the container and the air. This is what the thermometer measures.

Calorimetry

Measuring the heat energy transferred in a reaction, usually by recording a temperature change in a known mass of water.

Statement 3.2 — the four reaction types

Required practical 3.8 — measuring temperature changes

1. Stand a polystyrene cup inside a glass beaker, which provides an extra layer of insulation.
2. Measure a fixed volume of the first solution, such as 25 cm³, into the cup using a measuring cylinder.
3. Record its starting temperature with a thermometer reading to at least 0.5 °C.
4. Add the second reagent, put a lid on the cup and stir gently with the thermometer.
5. Record the highest temperature reached for an exothermic reaction, or the lowest for an endothermic one.
6. Calculate the temperature change by subtracting the starting temperature from the final one.
7. Repeat for each of the four reaction types — dissolving, neutralisation, displacement and combustion — and compare.

Variables

Independent (changed) — The reaction being investigated
Dependent (measured) — The change in temperature of the solution

Sources of error

Reducing heat loss — and why each measure matters

• POLYSTYRENE CUP — a poor conductor, so little heat escapes through the walls
• A LID — stops heat escaping by evaporation and convection from the open surface
• Stand the cup in a BEAKER — traps a layer of air as extra insulation
• STIR before the final reading, so the temperature is even and not measured at a hot spot
• COMBUSTION is the hardest to insulate, since the flame is outside the container — measured values for fuels are always well below the true ones

Model answer [3 marks]

A student mixes an acid and an alkali and the temperature rises from 20.5 °C to 27.0 °C. State the temperature change and classify the reaction, explaining your answer. [3]

The temperature change is 27.0 − 20.5 = 6.5 °C, an increase. The reaction is exothermic, because heat energy has been given out by the reacting chemicals to the surroundings, and it is that energy arriving in the solution which has raised its temperature. Neutralisation reactions are always exothermic.

Model answer [3 marks]

Explain why a polystyrene cup with a lid is used rather than an open glass beaker. [3]

Polystyrene is a poor conductor of heat, so much less energy escapes through the walls of the cup than would escape through glass. The lid prevents heat being lost from the open surface by evaporation and convection. Reducing heat loss in this way means that more of the energy released stays in the solution, so the temperature change measured is closer to the true value rather than being too small.

Not this: In an exothermic reaction the temperature falls, because the reaction has lost energy.

This: The thermometer measures the SURROUNDINGS, not the chemicals. Energy leaving the chemicals arrives in the surroundings, so the surroundings — and the thermometer — get HOTTER. Exothermic always means a temperature rise.

Mark-losing trap. EXOTHERMIC = temperature RISES. The thermometer is in the surroundings, not in the chemicals.

Mark-losing trap. DISSOLVING can be either way. Combustion, neutralisation and displacement are always exothermic.

Mark-losing trap. Heat loss makes every measured rise TOO SMALL — insulate, lid, and stir before reading.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — In an exothermic reaction, what happens to the temperature of the surroundings?
2. Grade 7 · Classify [2 marks] — A solid is added to water and the temperature falls from 21.0 °C to 15.5 °C. Classify the process and give the temperature change.
3. Grade 8 · Explain [3 marks] — Explain why a temperature rise measured in an open glass beaker is smaller than the true value.
4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of how to measure the temperature change when an acid neutralises an alkali.
5. 9+ · Analyse [6 marks] — A student measures the energy released by burning ethanol and gets a value about a third of the accepted figure. They conclude their ethanol was impure. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who proved heat is a form of energy and measured the exchange rate, and the one who built the first calorimeter, out of ice. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

James Prescott Joule — the one who proved heat is a form of energy and measured the exchange rate

Every measurement on this page assumes heat is a quantity of energy that can be counted, and that was Joule's achievement against considerable resistance. Heat was widely thought to be a fluid called caloric, and Joule showed instead that a fixed amount of mechanical work always produces a fixed amount of heat — measuring, by stirring water with falling weights, temperature rises of a fraction of a degree. He is the right person to ask about measuring small changes accurately, since the whole practical on this page turns on doing exactly that.

• “How do you measure a temperature rise of a fraction of a degree?”
• “What was heat thought to be before you?”
• “How did stirring water with falling weights prove anything?”
• “Why did people find your results so hard to believe?”
• “What did you do about heat escaping from your apparatus?”

Antoine Lavoisier — the one who built the first calorimeter, out of ice

Lavoisier built the first true calorimeter with Laplace, and its design is worth knowing because it sidesteps the thermometer entirely. A reaction was placed in a chamber packed with ice; the heat released melted some of the ice, and the meltwater was collected and weighed. The mass of water gave the heat directly, with no temperature measurement at all. He used it to show that a guinea pig and a burning candle produce heat and carbon dioxide in the same proportion — the first evidence that respiration is a form of combustion.

• “How did you measure heat without relying on a thermometer?”
• “How does melting ice tell you how much energy was released?”
• “What did the guinea pig experiment show?”
• “Why did you think respiration was a kind of burning?”
• “What was hardest about keeping the heat from escaping?”

Carry on through the course

• Previous lesson: Tests for Ions
• Next lesson: Calorimetry Calculations
• Calorimetry Calculations — Turning a temperature rise into an energy in joules, and then into a value per mole.
• Energy Diagrams and Bond Energies — Why breaking bonds costs energy, why making them repays it, and why the balance decides everything.
• Rates of Reaction — Four things that make a reaction faster, and two ways of measuring whether they did.
• Collision Theory — One explanation that accounts for every factor on the previous page.
• Catalysts — A substance that speeds a reaction up, takes part in it, and is there unchanged at the end.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification