⚡ HIGHER TIER ONLY
SPEC 4.5.1.3 • PAPER 1 • ⭐ PRIORITY
The Energy Change of Reactions
Bond energies. Three labelled steps. Every Higher Tier Paper 1 carries this — and the marks are won or lost on technique, not chemistry.
The Two Principles
Every chemical reaction is a two-step energetic process: bonds in the reactants are broken, then new bonds in the products are formed. Each step has a fixed energetic direction.
Breaking bonds
REQUIRES energy. Pulling atoms apart costs energy — this is an endothermic step. Energy IN.
Making bonds
RELEASES energy. Forming new bonds is energetically favourable — exothermic step. Energy OUT.
"During a chemical reaction: energy must be supplied to break bonds in the reactants; energy is released when bonds in the products are formed... The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction." — AQA Specification 4.5.1.3
The two outcomes
Exothermic: energy released forming new bonds is GREATER than energy needed to break old bonds → net energy released
Endothermic: energy needed to break old bonds is GREATER than energy released forming new bonds → net energy absorbed
The 3-Step Method
Every bond energy calculation follows the same labelled three-step structure. Skipping the labels is the single biggest reason students lose marks.
Energy change = (sum of bonds BROKEN) − (sum of bonds MADE)
STEP 1
Bonds broken = sum of all bond energies in the REACTANTS. Add up every bond that needs to be broken to start the reaction. Label this clearly: "bonds broken = ...".
STEP 2
Bonds made = sum of all bond energies in the PRODUCTS. Add up every bond that forms in the products. Label this clearly: "bonds made = ...".
STEP 3
Energy change = bonds broken − bonds made. Label this clearly: "energy change = ...". The sign tells you the type:
•
Negative answer → exothermic (released more than was needed). The magnitude is the positive value.
Positive answer → endothermic (needed more than was released).
Sign convention note
AQA accepts both sign conventions. When a question gives a positive energy change for an exothermic reaction (treating it as a magnitude), the equivalent set-up is "energy released = bonds made − bonds broken". Either approach is fine — Alert 3 covers both.
Worked Example — H₂ + Cl₂ → 2HCl
The textbook starting point. A clean forward calculation showing every step.
Bond energies given: H–H = 436, Cl–Cl = 243, H–Cl = 432 kJ/mol
Calculate the energy change for: H₂ + Cl₂ → 2HCl
Count the bonds first
Reactants: 1 × H–H bond (in H₂) + 1 × Cl–Cl bond (in Cl₂) = 2 bonds broken
Products: 2 × H–Cl bonds (in 2HCl) = 2 bonds made
Step 1 — Bonds broken:
H–H + Cl–Cl = 436 + 243 = 679 kJ
Step 2 — Bonds made:
2 × H–Cl = 2 × 432 = 864 kJ
Step 3 — Energy change:
bonds broken − bonds made = 679 − 864 = −185 kJ/mol
Interpretation
Negative result → reaction is exothermic. Energy released forming H–Cl bonds (864 kJ) was greater than energy needed to break H–H and Cl–Cl bonds (679 kJ). Magnitude of energy released = 185 kJ/mol.
Explaining "Why Exothermic" — 2-Mark Pattern
You'll be asked to explain why a reaction releases energy without doing the full calculation. The mark scheme has a tight expected structure.
Mark scheme requires both ideas: "energy is needed to break bonds AND energy is released when bonds form" — plus "the energy released is greater than the energy needed". — P1 Jun 2023 Q04.2
The 2-mark structure
Mark 1: "Energy is needed to break bonds and energy is released when bonds form" — both ideas in one sentence.
Mark 2: "The energy released (forming bonds) is greater than the energy needed (to break bonds)" — the comparison.
Mark scheme also accepts
For Mark 2, examiner accepts:
"Energy transferred in bond making is greater than energy transferred in bond breaking"
Numerical comparison: e.g., "2 × 432 (kJ/mol) is greater than (436 + 243)" — quoting actual numbers from the table
Counting Bonds — Don't Skip This
The single biggest source of lost marks isn't the maths — it's miscounting bonds. Always draw out the structure first.
MoleculeStructureBonds
H₂H — H1 × H–H
O₂O = O1 × O=O (double bond — counts as one bond at one bond energy)
H₂OH — O — H2 × O–H
CH₄C with 4 × H around it4 × C–H
CO₂O = C = O2 × C=O
C₃H₈ (propane)3-carbon chain, ends each have 3 H, middle has 2 H2 × C–C and 8 × C–H
H₂SH — S — H2 × H–S
SO₂O = S = O2 × S=O
Don't forget the coefficient
If the equation has "2H₂O", that means TWO water molecules — so 2 × 2 = 4 O–H bonds total. Always multiply the bonds-per-molecule by the coefficient in the balanced equation.
Worked Example 2 — Methane Combustion
A more complex example with multiple bond types and coefficients. The same three-step structure applies.
CH₄ + 2O₂ → CO₂ + 2H₂O
Bond energies: C–H = 412, O=O = 498, C=O = 805, O–H = 463 kJ/mol
Calculate the energy change.
Bond inventory
Reactants (broken): CH₄ has 4 × C–H = 4 bonds. 2O₂ has 2 × O=O = 2 bonds. Total: 4 C–H + 2 O=O.
Products (made): CO₂ has 2 × C=O = 2 bonds. 2H₂O has 2 × 2 = 4 × O–H = 4 bonds. Total: 2 C=O + 4 O–H.
Step 1 — Bonds broken:
4 × C–H + 2 × O=O= 4(412) + 2(498)= 1648 + 996 = 2644 kJ
Step 2 — Bonds made:
2 × C=O + 4 × O–H= 2(805) + 4(463)= 1610 + 1852 = 3462 kJ
Step 3 — Energy change:
2644 − 3462 = −818 kJ/mol
Interpretation
Negative → exothermic. Magnitude = 818 kJ/mol. (This is why combustion is a major energy source — large negative energy changes.)
Alert 1 — Label EVERY Step
Examiner Alert · The biggest mark-loser
The examiner described typical low-scoring scripts as "a page full of additions and subtractions with no words to explain what these represented". When the answer is wrong, unlabelled working cannot earn partial-credit marks — the examiner can't see which step you got right.
Source: P1 Jun 2024 Q08.3 + P1 Nov 2021 Q08.4 examiner reports.
✗ Loses partial credit
436 + 243 = 6792 × 432 = 864679 − 864 = −185= −185 kJ/molIf 864 was wrong, examiner can't tell what 864 represents → no method marks awarded.
✓ Earns partial credit
Bonds broken = 436 + 243 = 679Bonds made = 2 × 432 = 864Energy change = 679 − 864= −185 kJ/mol (exothermic)Even with one arithmetic slip, the labelled method earns method marks.
Alert 2 — Bond Counting Errors
Examiner Alert
In P1 Jun 2024 Q08.3 (propane combustion), the most common errors were: missing one of the C–C bonds entirely (counting 1 instead of 2), counting the wrong number of C–H bonds (often 9 instead of 8), and forgetting the coefficient on H₂O (counting 4 O–H bonds when there are 8). Drawing the structural formula first prevents almost all of these.
The fix — work systematically
Draw the structural formula of every molecule in the equation.
List each bond type for each molecule (e.g., "propane has 2 C–C and 8 C–H bonds").
Multiply by the coefficient in the balanced equation.
Sum bond-by-bond type before plugging into the energy calculation.
Quick worked check — propane
For C₃H₈ + 5O₂ → 3CO₂ + 4H₂O:
Bonds broken: 2 × C–C + 8 × C–H (in propane) + 5 × O=O (in 5O₂)
Bonds made: 6 × C=O (in 3CO₂, each has 2 C=O) + 8 × O–H (in 4H₂O, each has 2 O–H)
Alert 3 — Direction of Subtraction
Examiner Alert · Two equivalent conventions
AQA mark schemes use both sign conventions interchangeably (the spec just says "difference between bonds broken and bonds made"). Either is accepted — but they give different signs, so you need to know which you're using:
Signed convention: energy change = bonds broken − bonds made (negative for exothermic, positive for endothermic)
Magnitude convention: energy released = bonds made − bonds broken (positive for exothermic; this is the "magnitude" of the energy change)
✗ Wrong reasoning
"Got −185, that's negative, must be wrong, change to +185."
Negative IS the answer in the signed convention. Don't change signs to make it look like a positive number.
✓ Correct interpretation
"Got −185 → reaction is exothermic. Energy change = −185 kJ/mol (signed) OR energy released = 185 kJ/mol (magnitude)."
State both: the calculated value AND the type of reaction.
Match the question framing
If the question gives a positive energy change for an exothermic reaction (e.g., "the energy change is 2219 kJ/mol exothermic"), it's using the magnitude convention — set up your equation as energy released = bonds made − bonds broken. If the question doesn't specify a value, either convention works — just be consistent.
Conceptual trap
P1 Jun 2023 Q04.2 examiner caught students writing "energy is required in both breaking and making bonds" — wrong. Breaking bonds requires energy. Making bonds RELEASES energy. Never the reverse, regardless of which sign convention you use.
Reverse Calculations — When X Is Unknown
The 5-mark questions in 2024 and 2021 weren't standard forward calcs — they gave you the energy change and asked you to find a missing bond energy. Same method, just rearranged.
The setup
Step 1: Calculate bonds broken (with X if unknown bond is in reactants).
Step 2: Calculate bonds made (with X if unknown bond is in products).
Step 3: Set up the equation using the given energy change. For exothermic: energy released = bonds made − bonds broken.
Step 4: Solve algebraically for X.
For exothermic with energy released given as positive:
energy released = bonds made − bonds broken
Set it up like an equation
Don't try to solve in your head. Write it out: "energy released = X bonds made − Y bonds broken", substitute numbers, then rearrange to find X. Algebra mark is awarded for correct setup; arithmetic mark for the final value.
Common Errors — Bond Energy Calcs
What students doCorrect version
Working with no labels — "page full of additions"Always: "bonds broken = ...", "bonds made = ...", "energy change = ..."
Counting 1 C–C bond in propane (instead of 2)Draw C–C–C chain — there are 2 C–C bonds between 3 carbons
Forgetting the coefficient — "4H₂O has 4 O–H bonds"4H₂O has 4 × 2 = 8 O–H bonds
"Energy is required in both breaking and making bonds"Breaking bonds: REQUIRES energy. Making bonds: RELEASES energy
Writing magnitude as negative for exothermicEnergy change = −185 kJ/mol; magnitude = 185 kJ/mol
Using "exothermic loses mass" / "loses matter"Exothermic loses ENERGY, not mass. Conservation of mass still applies
Subtracting bonds made from bonds broken in reverse calc without setting up the equationSet up equation: energy released = bonds made − bonds broken; solve for X
Practice — Why Exothermic
The reaction between hydrogen and chlorine is exothermic. Bond energies: H–H = 436, Cl–Cl = 243, H–Cl = 432 kJ/mol.
Explain why this reaction releases energy to the surroundings.2 MARKSHT
Source: P1 Jun 2023 Q04.2 (verbatim)
MP1
"Energy is needed to break bonds AND energy is released when bonds form" — both ideas, one sentence.
MP2
"The energy released (forming H–Cl bonds) is greater than the energy needed (to break H–H and Cl–Cl bonds)".
ALT
Numerical version: "2 × 432 (kJ/mol) is greater than 436 + 243 (kJ/mol)" — also accepted for MP2 if comparison is explicit.
⚠️
Watch out: writing only "the products are at lower energy than reactants" describes the profile shape (1-mark answer for the previous lesson) — but here you need the bond-energy explanation.
Practice — Pick The Right Expression
H–H + Cl–Cl → 2 H–Cl
Which expression shows how to calculate the overall energy change for this reaction?
1 MARK
Source: P1 Jun 2023 Q04.1 (verbatim)
A) 436 + 346 + 432 kJ/mol
B) 436 + 346 + (2 × 432) kJ/mol
C) 436 + 346 − 432 kJ/mol
D) 436 + 346 − (2 × 432) kJ/mol
ANS
D — 436 + 346 − (2 × 432) kJ/mol
WHY
Bonds broken (in reactants H₂ + Cl₂): 1 × H–H + 1 × Cl–Cl = 436 + 346. Bonds made (in 2HCl): 2 × H–Cl = 2 × 432 (note the coefficient!). Energy change = bonds broken − bonds made = 436 + 346 − (2 × 432).
📝
Why each wrong answer is wrong:
A: adds all values — doesn't subtract bonds made
B: adds 2 × 432 instead of subtracting it
C: forgets the coefficient (uses 432 instead of 2 × 432) — common error
⚠️
Note on the Cl–Cl value: the standard textbook value is 243 kJ/mol, but this question uses 346. Always use the values given in the question, never values you've memorised.
Practice — Find Bond Energy X
Equation: 2H₂S(g) + 3O₂(g) → 2H₂O(g) + 2SO₂(g)
Bond energies: H–S = 364, O=O = 498, O–H = 464, S=O = X kJ/mol
The energy released forming new bonds is 1034 kJ/mol greater than the energy needed to break existing bonds.
Calculate the bond energy X.5 MARKSHT
Source: P1 Nov 2021 Q08.4 (verbatim)
COUNT
Bonds in 2H₂S: 2 × (2 H–S) = 4 H–S. Bonds in 3O₂: 3 × O=O. Bonds in 2H₂O: 2 × (2 O–H) = 4 O–H. Bonds in 2SO₂: 2 × (2 S=O) = 4 S=O.
MP1
Bonds broken = 4(364) + 3(498) = 1456 + 1494 = 2950 kJ
MP2
Bonds formed = 4(464) + 4X = 1856 + 4X kJ
MP3
Set up equation (energy released = bonds formed − bonds broken):
(1856 + 4X) − 2950 = 1034
MP4
Rearrange: 4X = 1034 + 2950 − 1856 = 2128
MP5
X = 2128 ÷ 4 = 532 kJ/mol
📝
Mark scheme allows: "correct use of incorrectly calculated values of bonds broken and/or bonds formed" — partial credit for arithmetic slips if the method is correct.
Practice — Propane Combustion (X)
Equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Bond energies: C–C = 347, C–H = X, O=O = 498, C=O = 805, O–H = 464 kJ/mol
The overall energy change of this exothermic reaction is 2219 kJ/mol.
Calculate the bond energy of the C–H bond (X).5 MARKSHT
Source: P1 Jun 2024 Q08.3 (verbatim)
COUNT
Reactants: propane has 2 C–C + 8 C–H bonds; 5O₂ has 5 O=O bonds. Products: 3CO₂ has 6 C=O bonds; 4H₂O has 8 O–H bonds.
MP1
Bonds broken = 2(347) + 8X + 5(498) = 694 + 8X + 2490 = 3184 + 8X kJ
MP2
Bonds made = 6(805) + 8(464) = 4830 + 3712 = 8542 kJ
MP3
Set up equation (energy released = bonds made − bonds broken):
8542 − (3184 + 8X) = 2219
MP4
Rearrange: 8X = 8542 − 3184 − 2219 = 3139
MP5
X = 3139 ÷ 8 = 392 kJ/mol (allow 392.375 rounded to 3 s.f.)
📝
Compare to data book value: the standard C–H bond energy is around 412 kJ/mol; for propane specifically the average is closer to 392. Mark scheme awards full marks for correctly setting up and solving the equation.
Calculation Checklist
Run through this list before you put your pen down on any 5-mark bond energy question.
The 5-point check
1
Drew the structural formulae — every reactant and product, every bond visible ✓
2
Multiplied by coefficients in the balanced equation ✓
3
Wrote "Bonds broken = ..." with the sum, then the value ✓
4
Wrote "Bonds made = ..." with the sum, then the value ✓
5
Wrote "Energy change = bonds broken − bonds made = ..." with the final value AND interpretation (exo/endo) ✓
Reverse calc check
For unknowns: write the equation BEFORE solving. "energy released = bonds made − bonds broken" → substitute → solve. The algebra mark depends on a clean equation setup, not on getting the final number.
Checkpoint — Lesson 5.03
You should now be able to:
Concepts
State that breaking bonds REQUIRES energy and making bonds RELEASES energy
Explain why a reaction is exothermic in terms of bond energies (energy released forming bonds > energy needed to break bonds)
Use the sign of the energy change to identify exo (negative) or endo (positive)
Calculations
Apply the 3-step method with labels: bonds broken, bonds made, energy change
Count bonds correctly, including coefficients on molecules in balanced equations
Solve reverse calculations for an unknown bond energy by setting up an equation
Examiner-proof technique
Label every step with words — "bonds broken =", "bonds made =", "energy change =" — to protect partial-credit marks
Draw structural formulae before counting bonds
For "explain why exothermic" questions, give BOTH ideas (break/make) AND the comparison (released > needed)
Connection back to Lesson 5.02
The energy change you've just calculated is exactly the same quantity shown by the "overall energy change" arrow on a reaction profile (Lesson 5.02). Bond energies give you the quantitative answer in kJ/mol; reaction profiles give the visual picture. Both should agree on whether the reaction is exo or endothermic — and a Higher Tier exam can ask you to translate between them.
Next up
Lesson 5.04 — Chemical Cells and Fuel Cells [chemistry only]. We move from bond energies to electrochemistry: how chemical reactions can generate voltage, what makes a hydrogen fuel cell different from combustion, and why students keep drawing the wrong cell diagram.
Next: The Two Principles