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3.1.1Physical Chemistry: Part 1

Exothermic and Endothermic — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions

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📚 Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Hub / Topical Revision
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Part 1 of 8Key ideas

Key ideas
ΔH = total energy to break bonds (reactants) − total energy released making bonds (products), in kJ/mol.
If ΔH 0 it is endothermic.
Activation energy is measured from the reactants up to the top of the energy hump.
On an exothermic pathway diagram the products are below the reactants; on an endothermic one they are above.
Breaking bonds always absorbs energy; making bonds always releases energy.
Energy is conserved
Energy is conserved energy given out by the reaction = energy gained by the surroundings.
Temperature change (°C) = final temperature − initial temperature.
A catalyst lowers the activation energy but does not change ΔH.
Further examples
Further examples combustion, oxidation and neutralisation are exothermic; electrolysis, thermal decomposition and the first stages of photosynthesis are endothermic.
Uses
Uses hand warmers and self-heating cans rely on exothermic reactions; sports cold packs rely on an endothermic one. Endothermic reactions are less common than exothermic ones.
Differs between reactions
Activation energy differs between reactions; a reaction with a higher activation energy needs more energy to start.

Part 2 of 8Learning objectives

1
State that exothermic reactions transfer thermal energy to the surroundings and endothermic reactions take it in.
2
Interpret and draw reaction pathway diagrams showing ΔH and activation energy.
3
Explain enthalpy change in terms of bond breaking and bond making.
4
Calculate ΔH from bond energies (Supplement).

Part 3 of 8Essential knowledge

Essential knowledge
An exothermic reaction transfers thermal energy to the surroundings, raising their temperature (e.g. combustion, neutralisation, respiration).
An endothermic reaction takes in thermal energy from the surroundings, lowering their temperature (e.g. thermal decomposition, photosynthesis).
The transfer of thermal energy during a reaction is the enthalpy change, ΔH.
Reaction pathway diagrams show reactants, products, ΔH (arrow from reactant level to product level) and activation energy (arrow from reactants to the peak).
Supplement
ΔH is negative for exothermic reactions and positive for endothermic reactions.
Supplement
Supplement activation energy Ea is the minimum energy that colliding particles must have in order to react.
Supplement
Supplement bond breaking is endothermic and bond making is exothermic; the overall ΔH depends on which is larger.
Supplement
ΔH = sum of bond energies broken − sum of bond energies formed; count every bond (e.g. CH4 has four C–H bonds, O=O is a double bond).
Temperature changes can be measured in simple calorimetry using an insulated polystyrene cup and thermometer.
Uses
Uses exothermic reactions in hand warmers and self-heating cans; endothermic reactions in instant cold packs.
Supplement
Bond energies can be rearranged to find an unknown value. For N2 + 3H2 → 2NH3, ΔH = −92 kJ/mol, N≡N = 945, H–H = 436: bonds broken = 945 + 3(436) = 2253; so 2253 − 6(N–H) = −92, giving N–H = 2345 ÷ 6 ≈ 391 kJ/mol.

Part 4 of 8Process & sequence

    • Measure a known volume of one reactant (e.g. 25 cm3 acid) into a polystyrene cup (insulator) in a beaker.
    • Record the starting temperature with a thermometer.
    • Add the second reactant (e.g. alkali, metal powder or a solid) and stir.
    • Record the highest (or lowest) temperature reached.
    • Calculate temperature change = final − initial; a rise means exothermic, a fall means endothermic.
    • Repeat and average; use a lid to reduce thermal energy loss.

    Reading an exothermic reaction pathway diagramwrite these steps in this order; each step is a marking point.

    1. Energy on the vertical axis, progress of reaction on the horizontal axis.
    2. Reactants shown at a higher energy level on the left.
    3. Curve rises to a peak; the height from reactants to peak is the activation energy.
    4. Curve falls to products at a lower energy level on the right.
    5. Downward arrow from reactant level to product level is ΔH (negative).

Part 5 of 8Worked examples

−184 kJ/mol
Calculate ΔH for H2 + Cl2 → 2HCl. Bond energies (kJ/mol): H–H 436, Cl–Cl 242, H–Cl 431.
  • Bonds broken = 436 + 242 = 678 kJ. Bonds made = 2 × 431 = 862 kJ. ΔH = 678 − 862 = −184 kJ/mol (exothermic).
−818 kJ/mol
Calculate ΔH for CH4 + 2O2 → CO2 + 2H2O. Bond energies (kJ/mol): C–H 413, O=O 498, C=O 805, O–H 464.
  • Broken
    • 4(413) + 2(498) = 1652 + 996 = 2648 kJ.
    • Made: 2(805) + 4(464) = 1610 + 1856 = 3466 kJ.
    • ΔH = 2648 − 3466 = −818 kJ/mol.
+6.5 °C
Temperature of acid rises from 19.5 °C to 26.0 °C after adding alkali. Classify the reaction.
  • Change = 26.0 − 19.5 = +6.5 °C; surroundings got hotter, so the reaction is exothermic (ΔH negative).

Part 6 of 8Summary

    • Exothermic: thermal energy transferred to surroundings, temperature rises, ΔH negative.
    • Endothermic: thermal energy taken in from surroundings, temperature falls, ΔH positive.
    • Bond breaking is endothermic; bond making is exothermic.
    • Activation energy is the minimum energy for colliding particles to react.
    • ΔH = bonds broken − bonds made.

Part 7 of 8Keep going

  1. Exothermic & Endothermic Reactions IGCSE Chemistry 0620: what to do next

    These Exothermic & Endothermic Reactions IGCSE Chemistry 0620 notes covered the key ideas, worked examples and exam technique for this topic. Check your Exothermic & Endothermic Reactions IGCSE Chemistry 0620 understanding with the practice questions below, then reread any Exothermic & Endothermic Reactions IGCSE Chemistry 0620 point you were unsure about before moving on.

    Exothermic & Endothermic Reactions IGCSE Chemistry 0620: revision checklist

    • Revise the main Exothermic & Endothermic Reactions IGCSE Chemistry 0620 definitions until you can write them from memory.
    • Practise Exothermic & Endothermic Reactions IGCSE Chemistry 0620 exam questions under timed conditions.
    • Link each Exothermic & Endothermic Reactions IGCSE Chemistry 0620 idea to a diagram or a worked example.
    • List the common Exothermic & Endothermic Reactions IGCSE Chemistry 0620 mistakes and how to avoid them.

Part 8 of 8Compare: Compare

Exothermic vs endothermic
FeatureExothermicEndothermic
energy transferto surroundingsfrom surroundings
temperature of surroundingsincreasesdecreases
sign of ΔHnegative (−)positive (+)
products' energy vs reactants'lowerhigher
bond energiesenergy released making bonds > energy absorbed breaking bondsenergy absorbed breaking bonds > energy released making bonds
examplescombustion, neutralisation, respiration, metal + acidthermal decomposition, photosynthesis, dissolving ammonium nitrate, citric acid + sodium hydrogencarbonate

Key terms

Command terms

Command terms
State / Name
Give a short answer only; no explanation is needed (1 mark = 1 fact).
Define
Give the precise meaning. Learn the syllabus definitions word-for-word: every keyword in the definition is a marking point.
Describe
Say what happens / what the data show, with key features and figures. Do not give reasons.
Explain
Give reasonswhat happens AND why/how, linked with “because”, “so”, “therefore”.
Suggest
Apply knowledge to an unfamiliar situation; any sensible, scientifically-based answer can score.
Compare
Give similarities AND differences, each as a direct comparison (“X is … whereas Y is …”).
Outline
Set out the main points briefly.
Calculate
Work out from given facts; show working, give the unit and the requested significant figures.
Identify
Name/select from given information.
Predict
Give an expected result without a full justification.
Evaluate
Judge the importance or quality of the evidence/method, with reasons.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Exothermic reaction · Endothermic reaction · Enthalpy change (ΔH) · Activation energy (Ea) · Reaction pathway diagram · Bond breaking · Bond making · Bond energy · Surroundings · Combustion · Thermal decomposition · Temperature change
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Use the exact scientific term and learn syllabus definitions word-for-word.
2
Write “rate of … increases”, not “… increases”.
3
Say “concentration gradient”, never just “gradient”; name particles precisely (atoms, molecules, ions) — never “it”.
4
Give units with every numerical answer and use the significant figures asked for; show all working.
5
Compare = a similarity AND a difference; describe = what happens; explain = why it happens.
6
Repeats let you identify anomalies and calculate a mean — they do not make an experiment “valid”.

Words to avoid

1
exothermic reactions give out heat so they get coldexothermic reactions transfer thermal energy to the surroundings, so the temperature rises
2
breaking bonds releases energybreaking bonds absorbs energy (endothermic)
3
activation energy is the energy needed for a reactionthe minimum energy colliding particles need to react
4
ΔH = bonds made − bonds brokenΔH = bonds broken − bonds made
5
endothermic ΔH is negativeendothermic ΔH is positive

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