IGCSE Chemistry 0620 / Topic 5 / IGCSE

Break, then make.

Every reaction pulls some bonds apart and puts new ones together. Breaking costs energy, making pays it back. Whether a reaction is exothermic comes down to which bill is bigger, and with a table of bond energies you can work it out. Extended content.

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The key idea

Extended. Bond breaking is endothermic: energy must be taken in to separate atoms. Bond making is exothermic: energy is released when atoms join. ΔH = energy needed to break bonds − energy released making bonds. If more energy is released making the new bonds than is needed to break the old ones, ΔH is negative and the reaction is exothermic.

Fig. 1: breaking and making bonds in H₂ + Cl₂ → 2HCl

Section 01

Breaking costs, making pays.

Extended content. Pick a reaction in the simulator. It breaks every bond in the reactants, shows the energy going in, then forms every bond in the products and shows the energy coming out.

Bond energy: the energy needed to break one mole of a particular covalent bond, in kJ/mol. The same amount is released when that bond forms.
Breaking bonds: endothermic, energy in, counted as positive.
Making bonds: exothermic, energy out, counted as negative.
Overall: ΔH = (sum of bonds broken) − (sum of bonds made).

Stage 1 · Learn

Check what the sim just showed you

Four quick checks. Each correct answer earns XP and records this skill on your profile.

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(Extended) Breaking chemical bonds is:

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(Extended) Making chemical bonds is:

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(Extended) In a reaction, 1200 kJ is needed to break bonds and 1500 kJ is released making bonds. What is ΔH?

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(Extended) How many O=O bonds are broken in the equation CH₄ + 2O₂ → CO₂ + 2H₂O?

Section 02

Working it out.

Draw the displayed formula of every molecule so that you can count every bond, including the ones that look the same on both sides.

Bond	Energy / kJ/mol	Bond	Energy / kJ/mol
H–H	436	C–H	410
Cl–Cl	243	O=O	496
H–Cl	432	C=O	805
O–H	460	C=C / C–C	610 / 350

H₂ + Cl₂ → 2HCl. Broken: H–H + Cl–Cl = 436 + 243 = 679. Made: 2 × H–Cl = 2 × 432 = 864. ΔH = 679 − 864 = −185 kJ/mol.
CH₄ + 2O₂ → CO₂ + 2H₂O. Broken: 4 × C–H + 2 × O=O = 1640 + 992 = 2632. Made: 2 × C=O + 4 × O–H = 1610 + 1840 = 3450. ΔH = 2632 − 3450 = −818 kJ/mol.
C₂H₄ + H₂ → C₂H₆. Only the changed bonds matter: broken C=C + H–H = 610 + 436 = 1046; made C–C + 2 × C–H = 350 + 820 = 1170. ΔH = −124 kJ/mol.

Examiner trap

Multiply each bond energy by the number of those bonds, including the big numbers in the equation: 2O₂ means two O=O bonds, and CO₂ has two C=O bonds. And always subtract in the order broken − made. Doing it the other way round gives the right number with the wrong sign.

Stage 2 · Exam

Exam-style questions

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(Extended) Use the bond energies H–H 436, Br–Br 193 and H–Br 366 kJ/mol to calculate ΔH for H₂ + Br₂ → 2HBr.

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(Extended) Use the bond energies H–H 436, O=O 496 and O–H 460 kJ/mol to calculate ΔH for 2H₂ + O₂ → 2H₂O.

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(Extended) Why is the combustion of methane exothermic, in terms of bonds?

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(Extended) Use the bond energies N≡N 945, H–H 436 and N–H 390 kJ/mol to calculate ΔH for N₂ + 3H₂ → 2NH₃.

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Bond energies, mastered.

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Six original Cambridge-style questions

Bond breaking and bond making, explaining exothermic and endothermic reactions in terms of bonds, and calculating ΔH from bond energies. Extended content.