← Topic 5
⚡ Challenge Paper Preparation

Challenge Prep: Chemical Energetics

IGCSE Chemistry 0620 — Topic 5

Chemical energetics is deceptively simple on the surface — exothermic releases heat, endothermic absorbs heat. But challenge papers turn it into a minefield of sign errors, energy diagram mistakes, and bond energy miscalculations. This guide exposes every trap: the difference between ΔH and activation energy, why breaking bonds is always endothermic, how to draw energy diagrams correctly, and how to calculate ΔH from bond energies without forgetting to multiply by coefficients. Master these, and energetics becomes one of your easiest topics on exam day.

⚠️ Common Traps & Misconceptions

▼

Ten traps that cost students marks on energetics questions. Each one appears on challenge papers regularly.

⚠️ TRAP
Trap 1: Confusing temperature change direction with exo/endo
The TrapStudents think "if the thermometer goes up, it's endothermic because the system is gaining heat" or get confused because the surroundings get hotter while the reaction gives out energy.
The TruthThink from the reaction's perspective. Exothermic: the reaction releases energy to the surroundings → the surroundings (solution/air) get hotter → temperature rises. Endothermic: the reaction absorbs energy from the surroundings → the surroundings get colder → temperature falls.
Why It MattersChallenge papers give you thermometer readings and ask you to classify the reaction. If the temperature rises from 22°C to 35°C, it's exothermic. If it drops from 22°C to 16°C, it's endothermic. Getting this backwards invalidates the rest of your answer.
Example Question"When ammonium nitrate is dissolved in water, the temperature drops from 25°C to 18°C. Is this process exothermic or endothermic? Explain."
⚠️ TRAP
Trap 2: Drawing energy diagrams with products at the wrong level
The TrapStudents draw exothermic diagrams with products ABOVE reactants, or endothermic diagrams with products BELOW reactants.
The TruthExothermic: products are lower than reactants on the energy diagram (energy has been released, so products have less energy). Endothermic: products are higher than reactants (energy has been absorbed, so products have more energy). The arrow for ΔH always points from reactants' level to products' level.
Why It MattersEnergy profile diagrams are worth 3-4 marks on challenge papers. If the products are at the wrong height, you lose the ΔH direction mark, the label mark, and possibly the activation energy mark too.
Example Question"Draw an energy profile diagram for an exothermic reaction. Label the reactants, products, activation energy, and ΔH."
⚠️ TRAP
Trap 3: Thinking activation energy is the same as ΔH
The TrapStudents confuse the "hump" on the energy diagram (activation energy) with the overall energy change (ΔH), or think that a large activation energy means a large ΔH.
The TruthActivation energy (Ea) is the minimum energy needed to start the reaction — it's the height of the energy barrier (from reactants up to the peak of the curve). ΔH is the overall energy difference between reactants and products. They are completely independent: a reaction can have a high activation energy but a small ΔH, or vice versa.
Why It MattersChallenge papers might give you an energy diagram and ask you to identify Ea and ΔH separately, or ask "Does a catalyst change the activation energy, the enthalpy change, or both?" (Answer: only Ea.)
Example Question"On an energy profile diagram, label (i) the activation energy and (ii) the enthalpy change. Explain the difference between these two quantities."
⚠️ TRAP
Trap 4: Getting bond energy calculations backwards
The TrapStudents subtract the wrong way: they subtract bonds broken from bonds formed, or only calculate one side of the equation.
The TruthThe formula is: ΔH = energy to break bonds (reactants) − energy released making bonds (products). Or equivalently: ΔH = Σ(bonds broken) − Σ(bonds formed). Breaking bonds is always endothermic (requires energy). Forming bonds is always exothermic (releases energy). You must calculate BOTH sides — all bonds in reactants AND all bonds in products.
Why It MattersIf you subtract the wrong way round, your sign flips: you'll call an exothermic reaction endothermic, or vice versa. The method must be: break ALL bonds in reactants, make ALL bonds in products, then subtract.
Example Question"Use bond energies to calculate ΔH for: CH4 + 2O2 → CO2 + 2H2O. Bond energies: C–H = 412, O=O = 496, C=O = 743, O–H = 463 kJ/mol."
⚠️ TRAP
Trap 5: Confusion about the sign of ΔH
The TrapStudents write ΔH = −200 kJ/mol and then say "the reaction is endothermic because it has a negative enthalpy change."
The TruthNegative ΔH = exothermic (energy is released, so the system loses energy). Positive ΔH = endothermic (energy is absorbed, so the system gains energy). Think of it like a bank account: negative means you've given energy away (exo = "out"), positive means you've taken energy in (endo = "in").
Why It MattersChallenge papers give you a ΔH value and ask you to classify the reaction type. If ΔH = −394 kJ/mol, it's exothermic. If ΔH = +178 kJ/mol, it's endothermic. Getting the sign convention wrong is an instant lost mark.
Example Question"The enthalpy change for the decomposition of calcium carbonate is +178 kJ/mol. State and explain whether this reaction is exothermic or endothermic."
⚠️ TRAP
Trap 6: Forgetting to multiply bond energies by coefficients
The TrapIn CH4 + 2O2 → CO2 + 2H2O, students count 1 O=O bond instead of 2, or 2 O–H bonds instead of 4 (since there are 2 water molecules, each with 2 O–H bonds).
The TruthYou must count every single bond in the equation, using the balanced equation coefficients. CH4 has 4 C–H bonds. 2O2 has 2 O=O bonds. CO2 has 2 C=O bonds. 2H2O has 4 O–H bonds (2 per molecule × 2 molecules). Write them all out before calculating.
Why It MattersForgetting the coefficient "2" in front of O2 or H2O halves or doubles part of your calculation, giving a significantly wrong ΔH. Write out all bonds in a clear table before doing any arithmetic.
Example Question"How many O–H bonds are formed in the reaction: 2H2 + O2 → 2H2O?"
⚠️ TRAP
Trap 7: Thinking a catalyst changes ΔH
The TrapStudents say "a catalyst makes the reaction more exothermic" or "a catalyst changes the enthalpy change."
The TruthA catalyst provides an alternative reaction pathway with a lower activation energy. It does NOT change ΔH. The overall energy change between reactants and products is exactly the same whether a catalyst is present or not. The catalyst only makes it easier to get started.
Why It MattersChallenge papers show two energy diagrams — one with and one without a catalyst — and ask what's different. The peak is lower (lower Ea), but the start and end levels are the same (ΔH unchanged). Saying otherwise loses marks.
Example Question"On the energy profile diagram, draw a second curve to show the effect of adding a catalyst. Explain what has changed and what has stayed the same."
⚠️ TRAP
Trap 8: Thinking all endothermic reactions need constant heating
The TrapStudents believe endothermic reactions require a constant supply of heat to proceed, and will stop the moment you remove the heat source.
The TruthSome endothermic reactions only need enough energy to start (overcome the activation energy), and then continue on their own by absorbing heat from the surroundings. For example, dissolving ammonium nitrate in water is endothermic — the temperature drops — but it happens spontaneously without any heating. Thermal decomposition of CaCO3 does need sustained heating, but that's because of the very high activation energy, not because all endothermic reactions require it.
Why It MattersChallenge papers might describe dissolving a salt in water at room temperature and ask whether it could be endothermic. The answer is yes — the cooling sensation when ammonium nitrate dissolves proves it.
Example Question"An instant cold pack contains ammonium nitrate and water. When the barrier is broken, the pack becomes cold. Explain why, in terms of energy changes."
⚠️ TRAP
Trap 9: Thinking "breaking bonds releases energy"
The TrapStudents say "energy is released when bonds are broken" or "breaking bonds is exothermic."
The TruthBreaking bonds always requires energy (endothermic). You must put energy IN to pull atoms apart. Forming bonds always releases energy (exothermic). Energy comes OUT when atoms come together and form new bonds. A reaction is overall exothermic when more energy is released forming new bonds than is needed to break the old ones.
Why It MattersThis is one of the most frequently tested misconceptions on challenge papers. An answer that says "energy is released when bonds are broken" gets zero marks, even if the calculation is perfect.
Example Question"Explain, in terms of bond breaking and bond forming, why the combustion of methane is exothermic."
⚠️ TRAP
Trap 10: Ignoring the state of water in enthalpy calculations
The TrapStudents don't think about whether water is produced as liquid or as steam, and use the same ΔH for both.
The TruthWhen water is produced as liquid (condensed), additional energy is released compared to producing it as gas (steam). This means combustion reactions producing liquid water have a more negative (more exothermic) ΔH than those producing gaseous water. At IGCSE level, the question usually specifies the state, but challenge papers might ask you to explain the difference.
Why It MattersChallenge papers might give two ΔH values for the "same" reaction and ask why they differ. The answer is often: one assumes water is formed as liquid, the other as gas. Condensation releases extra energy.
Example Question"The enthalpy of combustion of hydrogen is −286 kJ/mol when water is formed as liquid, but −242 kJ/mol when water is formed as steam. Explain the difference."

🧩 Multi-Step Reasoning Walkthroughs

▼

Five challenging questions broken down step by step. Try each step yourself before revealing the next.

Walkthrough 1 — Bond Energy CalculationUse bond energies to calculate ΔH for the combustion of methane: CH4 + 2O2 → CO2 + 2H2O. Bond energies (kJ/mol): C–H = 412, O=O = 496, C=O = 743, O–H = 463.
1

Bonds broken

In the reactants:
CH4: 4 × C–H bonds = 4 × 412 = 1648 kJ
2O2: 2 × O=O bonds = 2 × 496 = 992 kJ
Total energy to break bonds = 1648 + 992 = 2640 kJ

2

Bonds formed

In the products:
CO2: 2 × C=O bonds = 2 × 743 = 1486 kJ
2H2O: 4 × O–H bonds (2 per molecule × 2 molecules) = 4 × 463 = 1852 kJ
Total energy released forming bonds = 1486 + 1852 = 3338 kJ

3

Apply the formula

ΔH = bonds broken − bonds formed = 2640 − 3338 = −698 kJ/mol

4

What does the sign tell us?

ΔH is negative, which means the reaction is exothermic. More energy was released making new bonds (3338 kJ) than was needed to break old bonds (2640 kJ). The extra 698 kJ is released as heat to the surroundings.

Final AnswerΔH = −698 kJ/mol. The reaction is exothermic.
Examiner's NoteThe most common errors: (1) counting 1 O=O instead of 2 (gives −1194 instead of −698). (2) Counting 2 O–H instead of 4 (gives −236). (3) Subtracting the wrong way round (gives +698, wrongly endothermic). Use a clear table with columns: bond type, number, energy per bond, total. Check your bond counts match the balanced equation.
Walkthrough 2 — Drawing an Energy Profile DiagramDraw and fully label an energy profile diagram for an exothermic reaction with a catalyst. Show: reactants, products, activation energy (without catalyst), activation energy (with catalyst), and ΔH.
1

Axes

x-axis = Progress of reaction (sometimes called "reaction coordinate" or "reaction pathway"). y-axis = Energy. No numbers needed — just relative positions.

2

Exothermic = products lower

Draw reactants at a higher energy level (left side). Draw products at a lower energy level (right side). The vertical gap between them is ΔH. Draw a downward arrow from reactants' level to products' level and label it "ΔH (negative)".

3

Without catalyst

Draw a smooth curve that starts at the reactants' level, rises to a peak above it (this peak is the transition state), then drops down to the products' level. Draw an upward arrow from the reactants' level to the peak and label it "Ea" (activation energy without catalyst).

4

With catalyst

Draw a second, lower curve (dashed line) that starts and ends at the same levels as the original curve, but has a lower peak. Label the new, shorter arrow "Ea (with catalyst)". Important: the start (reactants) and end (products) levels do NOT change — only the peak is lower. ΔH remains exactly the same.

Final AnswerThe diagram shows: reactants higher than products (exothermic), a hump representing Ea, a lower dashed hump for Ea with catalyst, and ΔH labelled as the vertical gap between reactant and product levels.
Examiner's NoteCommon errors: (1) products drawn above reactants (that's endothermic). (2) Catalyst curve starts or ends at different levels (it shouldn't — ΔH doesn't change). (3) Missing labels on Ea or ΔH. (4) Drawing a straight line instead of a smooth curve. Each error loses 1 mark.
Walkthrough 3 — Explaining Why Combustion is Exothermic"Explain, in terms of bond energies, why the combustion of hydrogen is exothermic. 2H2 + O2 → 2H2O."
1

Bonds must break first

Before any new substances can form, all the bonds in the reactants must be broken. This requires energy (endothermic step). Bonds broken: 2 H–H bonds and 1 O=O bond.

2

Bonds form and release energy

The atoms rearrange and new bonds are formed in the products. This releases energy (exothermic step). Bonds formed: 4 O–H bonds (2 in each of 2 water molecules).

3

Net energy change

The energy released by forming 4 O–H bonds is greater than the energy needed to break 2 H–H bonds and 1 O=O bond. The excess energy is given off as heat to the surroundings.

4

Therefore exothermic

Since more energy is released forming bonds than is absorbed breaking bonds, the overall reaction releases energy to the surroundings, making it exothermic (ΔH is negative).

Final AnswerThe energy released forming 4 O–H bonds is greater than the energy needed to break 2 H–H and 1 O=O bond, so the reaction is exothermic overall.
Examiner's NoteFor full marks, you MUST mention: (1) bond breaking requires energy, (2) bond forming releases energy, (3) energy released > energy absorbed. Saying "the reaction releases energy" without comparing bond breaking and forming gets 0-1 marks at challenge level. Also: never say "energy is released when bonds are broken."
Walkthrough 4 — Interpreting a Temperature-Time GraphA student mixes two solutions and records the temperature every 30 seconds. The temperature rises from 21°C to 34°C in the first 2 minutes, then gradually falls back towards room temperature. Explain these observations.
1

Temperature increase = exothermic

The temperature of the surroundings (the solution) increases by 13°C. This means the reaction is exothermic — it is releasing energy to the surroundings, heating them up.

2

Reaction completes

The temperature peaks at 34°C when the reaction is complete — all of one (or both) reactants have been used up. No more heat is being produced after this point.

3

Heat loss to the environment

After the reaction stops, the solution is warmer than the room. It gradually loses heat to the surrounding air by convection and radiation, so the temperature slowly falls back towards room temperature (about 21°C). This is just cooling, not an endothermic reaction.

4

Extrapolation

In careful experiments, the cooling starts before the reaction finishes (because heat is lost throughout). To get the true maximum temperature rise, you extrapolate the cooling curve backwards to the time of mixing. Challenge papers sometimes give you this graph and ask you to read the corrected ΔT.

Final AnswerTemperature rises because the reaction is exothermic. It peaks when the reaction is complete. It falls because the hot solution loses heat to the cooler surroundings.
Examiner's NoteStudents often say the temperature falls "because the reaction becomes endothermic" — this is wrong. The fall is simply heat loss to the environment, not a chemical change. Challenge papers test this distinction explicitly.
Walkthrough 5 — Bond Energy with an Unfamiliar ReactionUse bond energies to calculate ΔH for: N2 + 3H2 → 2NH3. Bond energies (kJ/mol): N≡N = 944, H–H = 436, N–H = 388.
1

Bonds broken (reactants)

N2: 1 × N≡N = 944 kJ
3H2: 3 × H–H = 3 × 436 = 1308 kJ
Total broken = 944 + 1308 = 2252 kJ

2

Bonds formed (products)

2NH3: Each NH3 has 3 N–H bonds. 2 molecules = 6 N–H bonds.
6 × N–H = 6 × 388 = 2328 kJ
Total formed = 2328 kJ

3

Subtract

ΔH = bonds broken − bonds formed = 2252 − 2328 = −76 kJ/mol

4

The reaction is exothermic

Negative ΔH means the reaction releases energy. Despite needing to break the very strong N≡N triple bond (944 kJ!), the formation of 6 N–H bonds releases enough energy to more than compensate. This is the Haber process for making ammonia.

Final AnswerΔH = −76 kJ/mol. The reaction is exothermic.
Examiner's NoteThe N≡N triple bond is one of the strongest bonds you'll encounter (944 kJ/mol). Students sometimes see this huge number and assume the reaction must be endothermic. But 6 N–H bonds are formed (6 × 388 = 2328 kJ), which more than compensates. Always do the full calculation before deciding.

🔍 Spot the Difference

▼

Pairs of questions that look nearly identical but have different answers. Spot the key distinction.

Question A
When NaOH is dissolved in water, the temperature rises. Is this exo- or endothermic?
Exothermic. Temperature of surroundings rises = energy released.
Question B
When NH4NO3 is dissolved in water, the temperature falls. Is this exo- or endothermic?
Endothermic. Temperature of surroundings falls = energy absorbed.
Key DifferenceBoth are dissolving processes, but they go in opposite directions energetically. Temperature rise = exothermic. Temperature fall = endothermic. Don't assume all dissolving is the same type.
Question A
What does activation energy represent on an energy diagram?
The minimum energy needed to start the reaction — measured from reactants' level to the peak of the curve.
Question B
What does ΔH represent on an energy diagram?
The overall energy difference between reactants and products — measured from reactants' level to products' level.
Key DifferenceEa is measured from reactants up to the peak. ΔH is measured from reactants to products. They measure different things and are completely independent of each other. A catalyst lowers Ea but does NOT change ΔH.
Question A
ΔH = −394 kJ/mol. Is this exothermic or endothermic?
Exothermic. Negative ΔH = energy released.
Question B
ΔH = +178 kJ/mol. Is this exothermic or endothermic?
Endothermic. Positive ΔH = energy absorbed.
Key DifferenceThe sign of ΔH tells you everything. Negative = exothermic (energy goes out). Positive = endothermic (energy goes in). Many students mix up the sign convention — think of it as a bank account: negative means you've spent (released) energy.
Question A
Breaking bonds: is this exo- or endothermic?
Endothermic (always). Energy must be put IN to pull atoms apart.
Question B
Forming bonds: is this exo- or endothermic?
Exothermic (always). Energy is released when atoms come together.
Key DifferenceBreaking bonds = endothermic. Forming bonds = exothermic. This is always true, no exceptions. The most common misconception in energetics is saying "breaking bonds releases energy." It doesn't. Ever.
Question A
A catalyst is added. How does the energy diagram change?
The peak is lower (lower Ea). Start and end levels stay the same (ΔH unchanged).
Question B
The temperature is increased. How does the energy diagram change?
The energy diagram doesn't change. Temperature gives particles more kinetic energy but doesn't alter Ea or ΔH. More particles have energy ≥ Ea, so the rate increases.
Key DifferenceA catalyst changes the pathway (lowers Ea). Temperature changes the particles' energy (more particles exceed Ea). Neither changes ΔH. But only the catalyst actually changes the energy profile diagram itself.

🔗 Chemical Energetics Concept Map

▼

Click each node to see how the subtopics connect.

⭐ CORE FRAMEWORK
Energy Changes in Reactions — everything connects through bonds
Exothermic vs Endothermic ▶
Exothermic: Energy released to surroundings. Temperature rises. ΔH negative. Products lower on energy diagram.
Endothermic: Energy absorbed from surroundings. Temperature falls. ΔH positive. Products higher on energy diagram.
Examples: Exo = combustion, neutralisation, oxidation. Endo = thermal decomposition, photosynthesis, dissolving NH4NO3.
Bond Breaking & Bond Forming ▶
Breaking bonds: ALWAYS endothermic (requires energy input)
Forming bonds: ALWAYS exothermic (releases energy)
Net result: If energy released (forming) > energy absorbed (breaking) → exothermic overall. Vice versa → endothermic overall.
Bond Energy Calculations ▶
Formula: ΔH = Σ(bonds broken) − Σ(bonds formed)
Method: (1) List ALL bonds in reactants with counts. (2) List ALL bonds in products with counts. (3) Multiply each by its bond energy. (4) Subtract.
Common error: Forgetting coefficients (e.g., 2H2O has 4 O–H bonds, not 2)
Energy Profile Diagrams ▶
Axes: y = Energy, x = Progress of reaction
Exothermic: Products below reactants. ΔH arrow points down.
Endothermic: Products above reactants. ΔH arrow points up.
Ea: Height from reactants to peak of curve. Lowered by catalyst.
Activation Energy & Catalysts ▶
Ea: Minimum energy needed to start a reaction. Independent of ΔH.
Catalyst: Provides alternative pathway with LOWER Ea. Does NOT change ΔH.
On diagram: Same start and end points, lower peak.
Practical Experiments ▶
Measuring ΔT: Use thermometer/temperature probe. Record initial and peak (or trough) temperatures.
Sources of error: Heat loss to surroundings, incomplete reactions, heat absorbed by container.
Improvement: Insulated cup (polystyrene), lid, extrapolation of cooling curve.

❌ "Why Is This Wrong?" Exercises

▼

Spot the error in each student's answer. Think before revealing.

Exercise 1: "Explain why combustion of methane is exothermic in terms of bond energies."
Student's Answer"It is exothermic because energy is released when the C–H and O=O bonds are broken."
The FlawBreaking bonds does not release energy — it requires energy (endothermic). Energy is released when new bonds (C=O and O–H) are formed.
Correct Answer"Energy is needed to break the C–H and O=O bonds (endothermic). Energy is released when C=O and O–H bonds form (exothermic). The energy released forming bonds is greater than the energy needed to break bonds, so the reaction is exothermic overall."
Key RuleBreaking = endothermic. Forming = exothermic. Always. The common phrasing "energy is released when bonds break" is the single most penalised error in IGCSE energetics.
Exercise 2: "Draw an energy profile for an exothermic reaction."
Student's AnswerThe student draws products ABOVE reactants with a downward ΔH arrow.
The FlawIf products are above reactants, the diagram shows an endothermic reaction, not exothermic. The student has the product level wrong.
Correct AnswerFor exothermic: products BELOW reactants. The ΔH arrow points downward from reactants' level to products' level, showing that products have less energy.
Key RuleExo = products lower (energy released, products have less). Endo = products higher (energy absorbed, products have more). Remember: "exit goes down" (exo = energy exits, level goes down).
Exercise 3: "A catalyst is added to a reaction. How does this affect ΔH?"
Student's Answer"The catalyst makes ΔH more negative because the reaction releases more energy."
The FlawA catalyst does not change ΔH. It only lowers the activation energy by providing an alternative pathway. The overall energy change between reactants and products stays exactly the same.
Correct Answer"A catalyst has no effect on ΔH. It provides an alternative reaction pathway with a lower activation energy, making the reaction faster, but the overall enthalpy change is unchanged."
Key RuleCatalyst: changes Ea (lowers it), does NOT change ΔH. On the energy diagram, the peak is lower but the start and end levels are identical.
Exercise 4: "Calculate ΔH for H2 + Cl2 → 2HCl using bond energies: H–H = 436, Cl–Cl = 242, H–Cl = 431 kJ/mol."
Student's Answer"Broken: 436 + 242 = 678. Formed: 431. ΔH = 678 − 431 = +247 kJ/mol."
The FlawThe student only counted 1 H–Cl bond formed, but the equation shows 2HCl — so 2 H–Cl bonds are formed. Energy released = 2 × 431 = 862 kJ.
Correct AnswerBroken: 436 + 242 = 678 kJ. Formed: 2 × 431 = 862 kJ. ΔH = 678 − 862 = −184 kJ/mol.
Key RuleThe coefficient in front of a molecule multiplies ALL bonds in that molecule. 2HCl = 2 H–Cl bonds. Always check your bond count against the balanced equation.
Exercise 5: "The temperature falls when ammonium chloride dissolves in water. A student says this is exothermic."
Student's Answer"Yes, dissolving is exothermic because it happens spontaneously."
The FlawSpontaneous does NOT mean exothermic. The temperature falls, which means the surroundings are losing energy to the reaction. The process is endothermic. Endothermic reactions can happen spontaneously.
Correct Answer"The temperature decreases, so the process is endothermic. The dissolving process absorbs energy from the water (surroundings), causing the temperature to fall."
Key RuleTemperature up = exothermic. Temperature down = endothermic. "Spontaneous" does not tell you which type — it only means the reaction happens without continuous external energy input.
Exercise 6: "The temperature of a reaction mixture rises, then slowly drops back to room temperature. A student says the reaction is first exothermic, then endothermic."
Student's Answer"The rise shows an exothermic stage and the fall shows an endothermic stage of the reaction."
The FlawThe temperature fall is NOT caused by an endothermic reaction. The reaction is complete. The fall is simply heat loss to the surroundings as the warm solution cools down to room temperature.
Correct Answer"The entire reaction is exothermic (temperature rises). After the reaction is complete, the solution loses heat to the cooler surroundings, so the temperature gradually returns to room temperature. This cooling is a physical process, not a chemical reaction."
Key RuleDon't confuse cooling (physical heat loss) with an endothermic reaction (chemical energy absorption). Check whether the reaction is still occurring.
Exercise 7: "Activation energy is the energy needed to break all the bonds in the reactants."
Student's Answer"Yes, activation energy is the total energy to break all bonds."
The FlawActivation energy is NOT the total bond-breaking energy. It is the minimum energy that reactant particles need to start the reaction — the energy needed to reach the transition state. Bond energies and activation energy are measured differently and have different values.
Correct Answer"Activation energy is the minimum energy that colliding particles must have for a reaction to occur. It is shown on the energy diagram as the height from reactants to the peak. It is NOT the same as total bond-breaking energy."
Key RuleEa is a kinetic barrier (how hard it is to start). Bond energy is a thermodynamic quantity (how much energy is stored in bonds). They are different concepts measured in different ways.
Exercise 8: "Photosynthesis is endothermic. A student says this means plants need sunlight because endothermic reactions only happen with continuous energy input."
Student's Answer"Photosynthesis needs light energy because endothermic reactions can't happen without a continuous supply of energy."
The FlawNot all endothermic reactions need continuous energy input. Some (like dissolving ammonium nitrate) happen spontaneously. Photosynthesis specifically needs light energy because the light provides the specific wavelengths absorbed by chlorophyll, not simply because it's endothermic.
Correct Answer"Photosynthesis is endothermic — it absorbs light energy from the Sun. It needs continuous sunlight because the energy source is light (not just thermal energy). However, not all endothermic reactions require continuous heating — some happen spontaneously at room temperature."
Key RuleEndothermic does not automatically mean "needs constant heating." It means energy is absorbed overall. The source of that energy varies: light (photosynthesis), heat from surroundings (dissolving), or sustained heating (thermal decomposition).

✍️ Ultra-Detailed Practice Questions

▼

Ten Cambridge-style challenge questions. Write your answer, then reveal the model answer with mark scheme and examiner's notes.

Question 1
[3 marks]
When magnesium ribbon is added to dilute hydrochloric acid, the temperature of the mixture rises from 22°C to 45°C. (a) State whether the reaction is exothermic or endothermic. (b) Explain your answer in terms of energy transfer.
Model Answer(a) Exothermic [1]
(b) The reaction transfers energy to the surroundings (the acid solution) [1], causing the temperature to rise [1].
Examiner's NotesMust use the correct terminology: "transfers energy to the surroundings" not "makes heat." Temperature rise = exothermic. The 23°C increase is large, consistent with a vigorous metal-acid reaction.
Question 2
[4 marks]
Draw a labelled energy profile diagram for an endothermic reaction. Label: (i) reactants, (ii) products, (iii) activation energy, (iv) ΔH. [Describe your diagram in words if you can't draw here.]
Model Answery-axis: Energy. x-axis: Progress of reaction [1]
Reactants on the left at a lower energy level, products on the right at a higher energy level [1]
Smooth curve rises from reactants to a peak, then drops to product level (but product level is ABOVE reactant level) [1]
Ea = arrow from reactants up to the peak. ΔH = arrow from reactants up to products, labelled as positive [1]
Examiner's NotesThe key distinction: products are HIGHER than reactants (endothermic). Common error: drawing products lower (that's exothermic). The ΔH arrow must point upwards and be labelled as positive or with a + sign.
Question 3
[4 marks]
Use bond energies to calculate ΔH for: H2 + Cl2 → 2HCl. Bond energies (kJ/mol): H–H = 436, Cl–Cl = 242, H–Cl = 431. State whether the reaction is exothermic or endothermic.
Model AnswerBonds broken: 1 × H–H = 436, 1 × Cl–Cl = 242. Total = 678 kJ [1]
Bonds formed: 2 × H–Cl = 2 × 431 = 862 kJ [1]
ΔH = 678 − 862 = −184 kJ/mol [1]
The reaction is exothermic (negative ΔH) [1]
Examiner's NotesThe trap is forgetting the "2" in 2HCl — you must form 2 H–Cl bonds, not 1. If you only count 1 H–Cl bond, you get ΔH = +247, which wrongly suggests endothermic. Always cross-check bond counts with the balanced equation.
Question 4
[3 marks]
Meera heats calcium carbonate in a science class in Bangalore. CaCO3 → CaO + CO2. The reaction is endothermic. Explain, in terms of bond breaking and bond forming, why this reaction is endothermic.
Model AnswerEnergy is needed to break the bonds in CaCO3 (endothermic) [1]
Energy is released when bonds in CaO and CO2 form (exothermic) [1]
The energy needed to break bonds is greater than the energy released forming new bonds, so the reaction absorbs energy overall and is endothermic [1]
Examiner's NotesThe three-point structure is essential: (1) breaking bonds needs energy, (2) forming bonds releases energy, (3) breaking > forming so endothermic. Missing any one of these points loses that mark. Never say "bonds release energy when broken."
Question 5
[4 marks]
Use bond energies to calculate ΔH for the combustion of ethanol: C2H5OH + 3O2 → 2CO2 + 3H2O. Bond energies (kJ/mol): C–H = 412, C–C = 348, C–O = 360, O–H = 463, O=O = 496, C=O = 743.
Model AnswerBonds broken in C2H5OH: 5 C–H (2060) + 1 C–C (348) + 1 C–O (360) + 1 O–H (463) = 3231 kJ
3O2: 3 × O=O = 1488 kJ. Total broken = 4719 kJ [1]
Bonds formed in 2CO2: 4 × C=O = 2972 kJ
3H2O: 6 × O–H = 2778 kJ. Total formed = 5750 kJ [1]
ΔH = 4719 − 5750 = −1031 kJ/mol [1]
Exothermic (negative ΔH) [1]
Examiner's NotesC2H5OH (ethanol) has 5 C–H bonds, not 6. The structure is CH3–CH2–OH. The O–H bond in the –OH group is separate from the C–H bonds. Missing the C–O or O–H bond is common. Draw out the structural formula to count bonds carefully.
Question 6
[3 marks]
Explain what happens to the activation energy and ΔH when a catalyst is added to a reaction. Use an energy profile diagram to support your answer.
Model AnswerThe catalyst provides an alternative reaction pathway [1] with a lower activation energy (lower Ea) [1]. The ΔH remains unchanged — the start and end energy levels are the same [1].
Examiner's NotesThree marks, three points: (1) alternative pathway, (2) lower Ea, (3) ΔH unchanged. On the diagram, draw a second (dashed) curve with a lower peak but the same start and end points. Many students draw the products at a different level with the catalyst — that's wrong.
Question 7
[3 marks]
A student mixes citric acid and sodium hydrogen carbonate in a beaker. The temperature drops from 24°C to 14°C. (a) What type of reaction is this? (b) Explain why the temperature drops even though no cooling device is used.
Model Answer(a) Endothermic [1]
(b) The reaction absorbs energy from the surroundings (the solution in the beaker) [1]. As energy is transferred from the solution to the reaction, the solution's temperature decreases [1].
Examiner's NotesThe phrase "absorbs energy from the surroundings" is essential. The surroundings (liquid) get colder because they are the energy source. This is a common practical example — instant cold packs use this principle with ammonium nitrate.
Question 8
[4 marks]
Burning hydrogen in oxygen produces water: 2H2 + O2 → 2H2O. Use bond energies to calculate ΔH. Bond energies (kJ/mol): H–H = 436, O=O = 496, O–H = 463.
Model AnswerBonds broken: 2 × H–H = 872 kJ. 1 × O=O = 496 kJ. Total = 1368 kJ [1]
Bonds formed: 4 × O–H = 1852 kJ (2 molecules of H2O, each with 2 O–H) [1]
ΔH = 1368 − 1852 = −484 kJ/mol [1]
Exothermic [1]
Examiner's NotesThe critical count: 2H2O = 4 O–H bonds, not 2. If you use 2 O–H bonds, you get ΔH = +442 (endothermic) — clearly wrong for combustion. Always multiply bonds per molecule by the number of molecules.
Question 9
[3 marks]
Rohan in Delhi uses an instant cold pack after a cricket injury. The pack contains ammonium nitrate and water separated by a barrier. When the barrier is broken, the pack becomes very cold. Explain why.
Model AnswerWhen the ammonium nitrate dissolves in water, the process is endothermic [1]. The dissolving process absorbs energy (heat) from the surroundings (the water and the pack itself) [1]. This causes the temperature of the pack to fall, making it feel cold [1].
Examiner's NotesMust explain: (1) endothermic process, (2) energy absorbed from surroundings, (3) temperature falls. Saying "it's cold because it's endothermic" without explaining the energy transfer mechanism is not enough for full marks.
Question 10
[5 marks]
The Haber process makes ammonia: N2 + 3H2 ⇌ 2NH3. The forward reaction is exothermic (ΔH = −92 kJ/mol). (a) Draw an energy profile diagram for the forward reaction. (b) On the same diagram, show the effect of adding a catalyst. (c) State what happens to ΔH when the catalyst is added.
Model Answer(a) x-axis: Progress of reaction. y-axis: Energy [1]
Reactants (N2 + 3H2) at a higher level, products (2NH3) at a lower level. Smooth curve with a peak between them. ΔH = −92 kJ/mol shown as a downward arrow [2]
(b) A second, lower curve (dashed) with a lower peak but the same start and end points [1]
(c) ΔH remains unchanged at −92 kJ/mol [1]
Examiner's NotesThis is a classic 5-mark question combining diagram drawing with catalyst understanding. The most common error is changing the product level when adding the catalyst. The catalyst ONLY lowers the peak — it does NOT change the overall energy difference. An iron catalyst is used industrially; the question tests whether you understand what it actually does.
IGCSE Chemistry 0620 — Topic 5 Challenge Prep Guide. Work through every section, revisit the traps the night before your exam.