A-Level Chemistry 9701 / Topic 5 / AS

Break, then make.

Every reaction is bonds breaking and new bonds forming. Breaking costs energy and making releases it, so the enthalpy change is simply the difference between the two totals.

Full-screen labChemical energetics lab → Topic 5All chemical energetics lessons →

The key idea

Bond energy is the energy needed to break one mole of a particular covalent bond in gaseous molecules. Breaking bonds is endothermic and forming bonds is exothermic, so ΔH = Σ(bonds broken) − Σ(bonds formed). For bonds such as C–H the value depends on the molecule, so data books quote a mean bond energy averaged over many compounds. That, and substances that are not gases under standard conditions, is why bond energy answers differ slightly from experimental ΔH values.

Fig. 1: breaking the bonds in H₂ and Cl₂, then forming two H–Cl bonds

Section 01

Count every bond.

In the simulator, break the reactant molecules into atoms and then build the products. Watch the energy bar climb as bonds break and fall as bonds form.

Bond	kJ mol⁻¹	Bond	kJ mol⁻¹	Bond	kJ mol⁻¹
H–H	436	C–H	410	N≡N	944
Cl–Cl	243	C–C	350	N–H	390
H–Cl	432	C=C	610	O=O	496
Br–Br	193	C=O	805	O–H	460
H–Br	366

Section 02

Worked examples.

Draw out the displayed formulae first so that no bond is missed, and multiply by the numbers in the equation.

Combustion of methane, CH₄ + 2O₂ → CO₂ + 2H₂O. Broken: 4(C–H) + 2(O=O) = 1640 + 992 = 2632. Formed: 2(C=O) + 4(O–H) = 1610 + 1840 = 3450. ΔH = 2632 − 3450 = −818 kJ mol⁻¹ (experimental −890).
Haber process, N₂ + 3H₂ → 2NH₃. Broken: 944 + 3 × 436 = 2252. Formed: 6 × 390 = 2340. ΔH = −88 kJ mol⁻¹.
Hydrogenation of ethene, C₂H₄ + H₂ → C₂H₆. Only the changed bonds matter: broken C=C + H–H = 1046; formed C–C + 2 C–H = 1170. ΔH = −124 kJ mol⁻¹.

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.

Quick check+10 XP

Bond energy is the energy needed to break one mole of a bond in:

Quick check+10 XP

Breaking bonds is:

Quick check+10 XP

Which expression gives ΔH from bond energies?

Quick check+10 XP

Why are values such as C–H quoted as mean bond energies?

Section 03

Working backwards.

If ΔH is known, the same equation gives one unknown bond energy.

Example: H₂ + Cl₂ → 2HCl, ΔH = −185 kJ mol⁻¹. Then 436 + 243 − 2x = −185, so 2x = 864 and x = 432 kJ mol⁻¹ for H–Cl.
Why answers differ from experiment: mean bond energies are averages, and bond energies apply to gases, but water in a combustion equation is a liquid; its condensation releases extra heat not counted in the bond sum.

Examiner trap

It is broken minus formed, not the other way round; reversing it flips the sign. And count bonds per molecule and per mole of equation: 2H₂O contains four O–H bonds, and CO₂ contains two C=O double bonds, not two single C–O bonds.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.

Finish the four checks above to unlock the exam questions

Exam style+20 XP

Use bond energies (H–H 436, Br–Br 193, H–Br 366 kJ mol⁻¹) to calculate ΔH for H₂(g) + Br₂(g) → 2HBr(g).

Exam style+20 XP

Using bond energies, ΔH_c of methane is calculated as −818 kJ mol⁻¹, but the experimental value is −890 kJ mol⁻¹. The main reason is:

Exam style+20 XP

For N₂ + 3H₂ → 2NH₃, ΔH = −92 kJ mol⁻¹. Using N≡N 944 and H–H 436, what is the mean N–H bond energy?

Exam style+20 XP

Which bonds are broken when ethene reacts with hydrogen to form ethane?

Skill unlocked

Bond energies, mastered.

This skill is now recorded on your profile. Keep the chain going.

-Rank -Level -Score -Topics

Go deeper · practice

Six original Cambridge-style questions

Defining bond energy, calculating ΔH from bonds broken and formed, finding an unknown bond energy, and explaining differences from experimental values.