The shape of the lesson
By the end of the lesson, learners can
- Count every bond: In the simulator, break the reactant molecules into atoms and then build the products.
- Worked examples: Draw out the displayed formulae first so that no bond is missed, and multiply by the numbers in the equation.
- Working backwards: If ΔH is known, the same equation gives one unknown bond energy.
- Draw displayed formulae; count every bond.
- Multiply by the coefficients in the equation.
- Mean bond energies are averages over many compounds.
- Gas phase only: liquids cause discrepancies.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Break, then make.. Every reaction is bonds breaking and new bonds forming. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Count every bond | In the simulator, break the reactant molecules into atoms and then build the products. Project tab 3 of the lab (Chemical energetics lab, bond energies) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Worked examples | Draw out the displayed formulae first so that no bond is missed, and multiply by the numbers in the equation. | Pairs |
| 21 to 30 min | Teach and check: Working backwards | If ΔH is known, the same equation gives one unknown bond energy. | Rally Coach |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Working backwards, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| 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: 2H 2 O contains four O–H bonds, and CO 2 contains two C=O double bonds, not two single C–O bonds. |
Bond energies: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Define bond energy and explain the term mean bond energy.
Calculate ΔH for H2(g) + Cl2(g) → 2HCl(g). (H–H 436, Cl–Cl 243, H–Cl 432 kJ mol−1)
Calculate ΔH for the combustion of ethene: C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(g). (C=C 610, C–H 410, O=O 496, C=O 805, O–H 460 kJ mol−1)
For N2(g) + 3H2(g) → 2NH3(g), calculate ΔH. (N≡N 944, H–H 436, N–H 390 kJ mol−1)
For H2(g) + F2(g) → 2HF(g), ΔH = −542 kJ mol−1. The H–H bond energy is 436 and the H–F bond energy is 562 kJ mol−1. Calculate the F–F bond energy.
Give two reasons why an enthalpy change calculated from mean bond energies differs from the value found by experiment.
Answer key and marking notes
- The energy needed to break one mole of a given covalent bond in gaseous molecules.
- A mean bond energy is an average value for that bond taken over many different compounds.
- Broken: 436 + 243 = 679 kJ; formed: 2 × 432 = 864 kJ.
- ΔH = 679 − 864 = −185 kJ mol−1.
- Broken: 610 + 4 × 410 + 3 × 496 = 610 + 1640 + 1488 = 3738 kJ.
- Formed: 4 × 805 + 4 × 460 = 3220 + 1840 = 5060 kJ.
- ΔH = 3738 − 5060 = −1322 kJ mol−1.
- Broken: 944 + 3 × 436 = 2252 kJ; formed: 6 × 390 = 2340 kJ.
- ΔH = 2252 − 2340 = −88 kJ mol−1.
- 436 + x − 2 × 562 = −542.
- x = −542 + 1124 − 436 = 146 kJ mol−1.
- Mean bond energies are averages; the actual bond energies in the specific molecules differ.
- Bond energies refer to gases; some reactants or products (for example water) are liquids or solids under standard conditions.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which equation represents the process whose enthalpy change is the bond energy of the H–Cl bond?
- AHCl(g) → H(g) + Cl(g)
- BHCl(aq) → H+(aq) + Cl−(aq)
- CH(g) + Cl(g) → HCl(g)
- D½H2(g) + ½Cl2(g) → HCl(g)
The C–H bond energy is quoted as a mean bond energy of 410 kJ mol−1. What does "mean" tell you?
- Ait is an average of the C–H bond energies measured in many different compounds, so the value in a particular molecule may differ slightly
- Bit is the energy needed to break all four C–H bonds in one mole of methane
- Cit is the average of the energy needed to break the bond and the energy released when it forms
- Dit is the value measured at the mean temperature of 298 K
Use bond energies to calculate ΔH for H2(g) + Cl2(g) → 2HCl(g). (H–H 436, Cl–Cl 243, H–Cl 432 kJ mol−1)
- A−185 kJ mol−1
- B+185 kJ mol−1
- C+247 kJ mol−1
- D−1.54 × 103 kJ mol−1
Use bond energies to calculate ΔH for CH4(g) + 2O2(g) → CO2(g) + 2H2O(g). (C–H 410, O=O 496, C=O 805, O–H 460 kJ mol−1)
- A−818 kJ mol−1
- B+818 kJ mol−1
- C+102 kJ mol−1
- D−13 kJ mol−1
Use bond energies to calculate ΔH for N2(g) + 3H2(g) → 2NH3(g). (N≡N 944, H–H 436, N–H 390 kJ mol−1)
- A−88.0 kJ mol−1
- B+88.0 kJ mol−1
- C+1.08 × 103 kJ mol−1
- D−960 kJ mol−1
Use bond energies to calculate ΔH for the hydrogenation of ethene: C2H4(g) + H2(g) → C2H6(g). (C=C 610, C–C 350, C–H 410, H–H 436 kJ mol−1)
- A−124 kJ mol−1
- B+124 kJ mol−1
- C+286 kJ mol−1
- D−560 kJ mol−1
For H2(g) + Br2(g) → 2HBr(g), ΔH = −103 kJ mol−1. Use H–H = 436 and Br–Br = 193 kJ mol−1 to calculate the H–Br bond energy.
- A366 kJ mol−1
- B732 kJ mol−1
- C263 kJ mol−1
- D526 kJ mol−1
The bond energy calculation for the combustion of methane gives −818 kJ mol−1, but the standard enthalpy change of combustion is −890 kJ mol−1. Which statement explains the difference?
- Athe bond energies used are mean values, and under standard conditions the water formed is a liquid, so energy is also released when it condenses
- Bbond energies are measured at 0 K, whereas ΔHc is measured at 298 K
- Csome of the methane undergoes incomplete combustion in the data book measurement
- Dthe bond energy method includes the activation energy, which makes the value less exothermic
Quiz key
Bond energy is the energy needed to break one mole of a particular covalent bond in gaseous molecules, forming gaseous atoms, so HCl(g) → H(g) + Cl(g), which is always endothermic (+432 kJ mol−1). Ionisation of HCl in water forms ions, not atoms, and involves the solvent, so it is not bond breaking in the gas phase. H(g) + Cl(g) → HCl(g) is bond making, which releases the same amount of energy; its enthalpy change is −432 kJ mol−1, not the bond energy. Making HCl from H2 and Cl2 is the enthalpy change of formation of HCl.
The energy of a C–H bond varies slightly from one molecule to another (and even between the successive C–H bonds in CH4), so data books give an average over a range of compounds. That is why calculations with mean bond energies give only approximate values of ΔH. Breaking all four C–H bonds in methane needs about 4 × 410 = 1640 kJ mol−1, not 410. Breaking and making the same bond involve the same amount of energy with opposite signs, so averaging them would give zero. "Mean" refers to averaging over compounds, not to temperature.
Bonds broken: H–H + Cl–Cl = 436 + 243 = 679 kJ mol−1 (endothermic). Bonds formed: 2 H–Cl = 2 × 432 = 864 kJ mol−1 (exothermic). ΔH = 679 − 864 = −185 kJ mol−1. +185 subtracts the wrong way round (formed − broken). +247 forgets that 2 mol of HCl form, using only one H–Cl bond. −1.54 × 103 adds the bonds broken and formed together as if both released energy (−679 − 864 = −1543).
Broken: 4(C–H) + 2(O=O) = 4(410) + 2(496) = 1640 + 992 = 2632 kJ mol−1. Formed: 2(C=O) + 4(O–H) = 2(805) + 4(460) = 1610 + 1840 = 3450 kJ mol−1. ΔH = 2632 − 3450 = −818 kJ mol−1. +818 is formed − broken, the wrong way round. +102 forgets the 2 in front of H2O and counts only 2 O–H bonds. −13 counts only one C=O bond in CO2, but O=C=O has two.
Broken: N≡N + 3(H–H) = 944 + 3(436) = 944 + 1308 = 2252 kJ mol−1. Formed: 2 × 3 = 6 N–H = 6(390) = 2340 kJ mol−1. ΔH = 2252 − 2340 = −88.0 kJ mol−1. +88.0 reverses broken and formed. +1.08 × 103 counts only 3 N–H bonds, forgetting that 2 mol of NH3 form (2252 − 1170 = +1082). −960 breaks only one H–H bond, forgetting the 3 in front of H2.
Broken: C=C + H–H = 610 + 436 = 1046 kJ mol−1. Formed: C–C + 2 C–H = 350 + 2(410) = 1170 kJ mol−1. ΔH = 1046 − 1170 = −124 kJ mol−1. (Counting all bonds gives the same answer: 610 + 4(410) + 436 = 2686 broken, 350 + 6(410) = 2810 formed.) +124 subtracts the wrong way round. +286 forms only one new C–H bond, but both hydrogen atoms of H2 end up bonded to carbon. −560 forgets to break the H–H bond.
−103 = (436 + 193) − 2x, so 2x = 629 + 103 = 732 and x = 366 kJ mol−1. 732 is the energy released forming both H–Br bonds; it has not been divided by 2. 263 uses the wrong sign for ΔH ((629 − 103) ÷ 2). 526 makes the same sign error and also forgets to divide by 2.
Bond energy calculations assume every species is a gas and use average values that do not exactly match the bonds in CH4, CO2 and H2O. The standard enthalpy of combustion forms liquid water; condensing 2 mol of steam releases about 88 kJ mol−1 more, so the data book value is more exothermic. Bond energies are quoted for standard conditions (298 K), not 0 K. Data book values are for complete combustion; incomplete combustion would make the value less exothermic, not more. ΔH depends only on the initial and final states, so the activation energy does not appear in either value.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.