The shape of the lesson
By the end of the lesson, learners can
- Breaking costs, making pays: Extended content.
- 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.
- Draw displayed formulae and count every bond.
- Multiply by the numbers in the equation.
- ΔH = bonds broken − bonds made.
- Negative: exothermic. Positive: endothermic.
One picture every idea returns to
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.
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 pulls some bonds apart and puts new ones together. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Breaking costs, making pays | Extended content. 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 |
| 17 to 30 min | Teach and check: 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. | 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, 18 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 it out, 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 |
|---|---|
| Multiply each bond energy by the number of those bonds, including the big numbers in the equation: 2O 2 means two O=O bonds, and CO 2 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. |
Bond breaking, bond making and bond energies: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
(Extended) In terms of bond breaking and bond making, explain why some reactions are exothermic and others endothermic.
(Extended) Calculate ΔH for H2 + Cl2 → 2HCl. (Bond energies in kJ/mol: H–H 436, Cl–Cl 243, H–Cl 432)
(Extended) Calculate ΔH for the complete combustion of methane, CH4 + 2O2 → CO2 + 2H2O. (Bond energies in kJ/mol: C–H 410, O=O 496, C=O 805, O–H 460)
(Extended) Ethene reacts with bromine: C2H4 + Br2 → C2H4Br2. Calculate ΔH. (Bond energies in kJ/mol: C=C 610, Br–Br 193, C–C 350, C–Br 290; the C–H bonds are unchanged)
(Extended) Calculate ΔH for the decomposition 2NH3 → N2 + 3H2, and state whether it is exothermic or endothermic. (Bond energies in kJ/mol: N≡N 945, H–H 436, N–H 390)
(Extended) A student calculates ΔH for a reaction as −2632 kJ/mol by adding up only the energies of the bonds in the reactants. Explain the mistake.
Answer key and marking notes
- Bond breaking is endothermic and bond making is exothermic.
- If more energy is released making new bonds than is taken in breaking old bonds, the reaction is exothermic.
- If more energy is taken in breaking bonds than is released making bonds, the reaction is endothermic.
- Bonds broken: 436 + 243 = 679 kJ.
- Bonds made: 2 × 432 = 864 kJ.
- ΔH = 679 − 864 = −185 kJ/mol.
- Bonds broken: 4 × C–H + 2 × O=O = 1640 + 992 = 2632 kJ.
- Bonds made: 2 × C=O + 4 × O–H = 1610 + 1840 = 3450 kJ.
- ΔH = 2632 − 3450 = −818 kJ/mol.
- The value is negative, so the reaction is exothermic.
- Bonds broken: C=C + Br–Br = 610 + 193 = 803 kJ.
- Bonds made: C–C + 2 × C–Br = 350 + 580 = 930 kJ.
- ΔH = 803 − 930 = −127 kJ/mol.
- Bonds broken: 6 × N–H = 2340 kJ.
- Bonds made: N≡N + 3 × H–H = 945 + 1308 = 2253 kJ.
- ΔH = 2340 − 2253 = +87 kJ/mol: endothermic.
- Breaking the bonds in the reactants takes in energy, so it should be counted as positive.
- The student must also subtract the energy released when the bonds in the products are made: ΔH = bonds broken − bonds made.
10 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 expression gives the enthalpy change, ΔH, of a reaction from bond energies?
- AΔH = energy taken in to break bonds − energy released making bonds
- BΔH = energy released making bonds − energy taken in to break bonds
- CΔH = energy taken in to break bonds + energy released making bonds
- DΔH = energy released making bonds only
Ethene burns in oxygen: C2H4 + 3O2 → 2CO2 + 2H2O. Use the bond energies (kJ/mol) C=C 610, C–H 410, O=O 496, C=O 805 and O–H 460 to calculate ΔH.
- A−1322 kJ/mol
- B+1322 kJ/mol
- C−2314 kJ/mol
- D−402 kJ/mol
Ethene reacts with hydrogen: C2H4 + H2 → C2H6, ΔH = −124 kJ/mol. Using H–H 436, C–C 350 and C–H 410 kJ/mol, calculate the bond energy of the C=C bond.
- A610 kJ/mol
- B858 kJ/mol
- C734 kJ/mol
- D1046 kJ/mol
Propane burns completely: C3H8 + 5O2 → 3CO2 + 4H2O. Use the bond energies (kJ/mol) C–C 350, C–H 410, O=O 496, C=O 805 and O–H 460 to calculate ΔH.
- A−2050 kJ/mol
- B+2050 kJ/mol
- C−2750 kJ/mol
- D+365 kJ/mol
What is meant by the bond energy of a covalent bond?
- Athe energy needed to break one mole of that bond
- Bthe energy released when one mole of a substance burns
- Cthe energy needed to melt one mole of a covalent substance
- Dthe minimum energy particles need to react when they collide
How many covalent bonds must be broken to split one molecule of ethane, C2H6, completely into separate atoms?
- A7
- B6
- C8
- D2
For an endothermic reaction, which statement about bond breaking and bond making is correct?
- AThe energy taken in to break bonds is greater than the energy released making bonds
- BThe energy released making bonds is greater than the energy taken in to break bonds
- CNo bonds are broken; energy is only taken in to make new bonds
- DBreaking bonds releases energy, and more is released than is taken in
For 2C2H6 + 7O2 → 4CO2 + 6H2O the correct value is ΔH = −2868 kJ/mol, using C–C 350, C–H 410, O=O 496, C=O 805 and O–H 460 kJ/mol. A student gets −3568 kJ/mol. Which mistake did the student make?
- ALeft out the C–C bonds in the two ethane molecules
- BCalculated bonds made − bonds broken
- CCounted only one C=O bond in each CO2
- DCounted only one O=O bond instead of seven
Nitrogen, N2, is very unreactive. The N≡N bond energy is 945 kJ/mol, compared with 436 kJ/mol for H–H and 496 kJ/mol for O=O. Which is the best explanation for nitrogen's lack of reactivity?
- AA lot of energy is needed to break the triple bond, so reactions of N2 have a high activation energy
- BBreaking the N≡N bond releases a lot of energy, so N2 reacts only when cooled
- CNitrogen has no covalent bonds to break
- DA strong bond means that reactions of N2 are always exothermic
Which of these changes is exothermic?
- AH + H → H2
- BH2 → H + H
- CCl2 → Cl + Cl
- DO2 → O + O
Quiz key
Breaking bonds is endothermic (+) and making bonds is exothermic (−), so ΔH = bonds broken − bonds made. If more energy is released making bonds, ΔH comes out negative, as it should for an exothermic reaction. The second expression gives the right size but the wrong sign. Adding the two ignores that they act in opposite directions, and using only bonds made ignores the energy needed to break the reactant bonds.
Broken: 610 + 4 × 410 + 3 × 496 = 610 + 1640 + 1488 = 3738 kJ. Made: 4 × 805 + 4 × 460 = 3220 + 1840 = 5060 kJ. ΔH = 3738 − 5060 = −1322 kJ/mol. +1322 subtracts the wrong way round; −2314 breaks only one O=O bond instead of three; −402 counts only one O–H bond in each water molecule.
Bonds made: C–C + 2 C–H = 350 + 2 × 410 = 1170 kJ. Bonds broken: C=C + 436. So (C=C + 436) − 1170 = −124, giving C=C + 436 = 1046 and C=C = 1046 − 436 = 610 kJ/mol. 858 comes from using +124 instead of −124; 734 ignores ΔH altogether (1170 − 436); 1046 forgets to subtract the H–H bond, which is also broken.
Broken: 2 × 350 + 8 × 410 + 5 × 496 = 700 + 3280 + 2480 = 6460 kJ. Made: 6 × 805 + 8 × 460 = 4830 + 3680 = 8510 kJ. ΔH = 6460 − 8510 = −2050 kJ/mol. +2050 is made − broken; −2750 leaves out the two C–C bonds; +365 counts only one C=O bond in each CO2 (made = 2415 + 3680 = 6095).
Bond energy is the energy needed to break one mole of a particular covalent bond, in kJ/mol; the same amount is released when one mole of that bond is formed. Energy released on burning is an enthalpy of combustion. Melting a simple molecular substance overcomes weak intermolecular forces, not covalent bonds. The minimum energy for a reaction is the activation energy.
Ethane is H3C–CH3: one C–C bond joins the carbons and each carbon has three C–H bonds, so there are 1 + 6 = 7 bonds. 6 counts only the C–H bonds and forgets the C–C bond; 8 adds the numbers of atoms; 2 counts the carbon atoms.
An endothermic reaction has a positive ΔH. Since ΔH = energy to break bonds − energy released making bonds, the bond-breaking term must be larger. The second statement describes an exothermic reaction. Every reaction breaks bonds in the reactants and makes new bonds in the products, and making bonds releases energy rather than taking it in. Breaking bonds always takes energy in.
The student's answer is 3568 − 2868 = 700 kJ more negative, so 700 kJ is missing from the bonds broken. Two ethane molecules contain 2 C–C bonds, and 2 × 350 = 700 kJ, so the student left them out (broken = 9092 − 700 = 8392; 8392 − 11960 = −3568). Made − broken would give +2868. One C=O per CO2 would make the made total smaller and the answer positive (+352). One O=O instead of seven removes 6 × 496 = 2976 kJ, giving −5844.
Bond energy is the energy needed to break a bond. At 945 kJ/mol the N≡N triple bond is roughly twice as strong as H–H or O=O, so very few collisions have enough energy to break it: reactions of nitrogen have a high activation energy and need high temperatures or catalysts. Breaking bonds takes energy in; it never releases it. N2 contains a covalent triple bond. A strong bond in the reactants makes a reaction less exothermic, not more.
Joining two hydrogen atoms makes an H–H bond, and making a bond releases energy (436 kJ per mole of bonds), so it is exothermic. The other three each break a covalent bond, which always needs energy to be taken in, so they are endothermic.
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.