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InkFlaskCambridge IGCSE
Contains the answers

Answers · Bond energies

Breaking and making bonds — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. JQ1 · Practice

Model answer. The shared pair of electrons is attracted to both nuclei. Pulling the atoms apart works against that attraction, so breaking a bond takes energy in: it is endothermic. When atoms join and share a pair, energy is given out: making a bond is exothermic.

A full answer includes

  • I chose: breaking a bond takes energy in; making a bond gives energy out.
  • I did not choose a statement in which breaking a bond gives energy out.

Also accept

  • Breaking takes energy in; making gives energy out
  • Breaking absorbs energy; making releases energy

Watch for

  • “Breaking bonds releases energy.” — Breaking a bond always takes energy in: the atoms are pulled apart against the attraction of the shared pair of electrons. Energy is given out when new bonds are made.
  • “Breaking a bond and making a bond both give energy out.” — If both gave energy out, a molecule could be broken and remade again and again to give out energy for ever. Breaking takes energy in; making gives the same amount out.
  • “Breaking a bond and making a bond both take energy in.” — Atoms coming together are pulled into the bond by the shared pair, so making a bond gives energy out. Only breaking takes energy in.

2. JQ2 · Practice

Model answer. In the bond the two iodine atoms share a pair of electrons, and the pair is attracted to both nuclei. To separate the atoms you must work against that attraction, so energy is taken in. Taking energy in makes bond breaking endothermic.

A full answer includes

  • (a) I wrote that the two iodine atoms share a pair of electrons.
  • (a) I wrote that the shared pair is attracted to both nuclei.
  • (a) I wrote that energy is needed to pull the atoms apart against this attraction.
  • (b) I stated endothermic.
  • I did not write that breaking the bond gives energy out.

Also accept

  • (a) Work against the attraction of the shared pair to both nuclei; (b) endothermic
  • (a) Energy is needed to overcome the attraction between the shared electrons and the nuclei; (b) endothermic

Watch for

  • “Breaking bonds releases energy.” — Breaking a bond always takes energy in: the atoms are pulled apart against the attraction of the shared pair of electrons. Energy is given out when new bonds are made.

3. JQ3 · Practice

Model answer. The bond energy is the energy to break one mole of the bond: 338 kJ/mol. Making one mole of the same bond gives out the same amount, 338 kJ/mol. 2CCl4 is two moles of molecules with four C–Cl bonds each, so 8 moles of C–Cl bonds: 8 × 338 = 2704 kJ/mol.

Checked automatically

  • (a) To break 1 mol of C–Cl bonds: 338 kJ/mol (exact value)
  • (b) Given out as 1 mol of C–Cl bonds forms: 338 kJ/mol (exact value)
    • Common wrong answer -338 : Give the amount of energy given out: 338 kJ/mol. A sign comes in only when you write ΔH.
  • (c) To break all the bonds in 2CCl4: 2704 kJ/mol (exact value)
    • Common wrong answer 338 : That is one C–Cl bond. 2CCl4 has 2 × 4 = 8 of them.
    • Common wrong answer 1352 : That is one CCl4 molecule. There are two: 8 × 338.
    • Common wrong answer 676 : Each CCl4 has four C–Cl bonds, not one: 8 × 338.

A full answer includes

  • (a) I stated 338 kJ/mol.
  • (b) I stated 338 kJ/mol.
  • (c) I counted 2 × 4 = 8 C–Cl bonds in 2CCl4.
  • (c) I worked out 8 × 338.
  • (c) I got 2704 kJ/mol.
  • I did not use only the four C–Cl bonds of one molecule in (c).

Also accept

  • 338; 338; 2704 kJ/mol

Watch for

  • “Making a bond gives out less energy than breaking it takes in.” — Making a bond gives out exactly the same amount of energy as breaking the same bond takes in: 338 kJ/mol for C–Cl either way.
  • “The number in front of a formula can be ignored when you count bonds.” — Count the bonds in one molecule, then multiply by the number in front of the formula: every bond in the molecule is counted that many times.

4. JQ4 · Practice

Options as numbered on the pupil sheet: 1 — Energy given out (exothermic); 2 — Energy taken in (endothermic).

Model answer. Breaking a bond takes energy in: the C–Cl bond breaking (1) and the C–H bond breaking (4). Making a bond gives energy out: two bromine atoms joining (2) and an N–H bond forming (3).

A full answer includes

  • I matched 1, the C–Cl bond breaking, to energy taken in.
  • I matched 2, two bromine atoms joining, to energy given out.
  • I matched 3, an N–H bond forming, to energy given out.
  • I matched 4, a C–H bond breaking, to energy taken in.
  • I did not match atoms joining to energy taken in.

Also accept

  • 1 and 4 taken in; 2 and 3 given out

Watch for

  • “Breaking bonds releases energy.” — Breaking a bond always takes energy in: the atoms are pulled apart against the attraction of the shared pair of electrons. Energy is given out when new bonds are made.
  • “Joining two atoms takes energy in, because work is needed to push them together.” — The two atoms are pulled into the bond by the attraction of the shared pair to both nuclei, so energy is given out as the bond forms.

5. JQ5 · Practice

Model answer. ΔH = 6482 − 8534 = −2052 kJ/mol. Making the bonds in CO2 and H2O gives out 2052 kJ/mol more than breaking the bonds in propane and oxygen takes in. The extra goes to the surroundings, so the reaction is exothermic and ΔH is negative.

Checked automatically

  • (a) ΔH: -2052 kJ/mol (exact value)
    • Common wrong answer 2052 : Check the sign: ΔH = energy in − energy out = 6482 − 8534.
    • Common wrong answer 15016 : Subtract, don't add: ΔH = 6482 − 8534.

A full answer includes

  • (a) I wrote ΔH = 6482 − 8534.
  • (a) I got ΔH = −2052 kJ/mol.
  • (b) I deduced that the reaction is exothermic.
  • (b) I explained that more energy is given out making the bonds in CO2 and H2O than is taken in breaking the bonds in propane and oxygen.
  • I did not write that energy is given out when the bonds in propane break.

Also accept

  • −2052 kJ/mol; exothermic, more given out making bonds than taken in breaking them
  • −2052 kJ/mol; exothermic, more energy released forming bonds than absorbed breaking them

Watch for

  • “ΔH = energy out − energy in.” — That flips the sign. ΔH = energy taken in breaking bonds − energy given out making bonds, so an exothermic reaction comes out negative.
  • “A fuel gives out energy because its bonds break.” — Breaking the bonds in the fuel takes energy in. The energy comes out when the new bonds in the products form, and those give out more than the breaking took in.

6. JQ6 · Practice

Model answer. The reaction breaks the strong C=O and O–H bonds in carbon dioxide and water, which takes energy in, and makes the bonds in glucose and oxygen, which gives energy out. Less energy is given out than is taken in, so the difference must come from the surroundings, here as light: the reaction is endothermic.

A full answer includes

  • I wrote that breaking the bonds in carbon dioxide and water takes energy in.
  • I wrote that making the bonds in glucose and oxygen gives energy out.
  • I wrote that more energy is taken in breaking bonds than is given out making bonds.
  • I wrote that the difference is taken in from the surroundings, as light.
  • I did not write that making the bonds in glucose takes energy in.

Also accept

  • Energy taken in breaking bonds is more than energy given out making bonds
  • More energy is absorbed breaking bonds than is released forming bonds

Watch for

  • “Breaking bonds releases energy.” — Breaking a bond always takes energy in: the atoms are pulled apart against the attraction of the shared pair of electrons. Energy is given out when new bonds are made.
  • “A positive ΔH means the reaction gives out energy.” — A positive ΔH means the chemicals gain energy: more is taken in breaking bonds than is given out making bonds. The reaction is endothermic.

7. JQ7 · Practice

Model answer. Up: 300 + 200 = 500 kJ/mol. Down: 220 + 220 = 440 kJ/mol. ΔH = 500 − 440 = +60 kJ/mol, so the products, 2AB, are 60 kJ/mol above the reactants on both drawings: the staircase and the pathway diagram start and end at the same levels. The hump is the activation energy, 130 kJ/mol above the reactants, far below the separate atoms, 2A + 2B, at 500 kJ/mol. The staircase only adds up the energy.

A full answer includes

  • I added the steps up: 300 + 200 = 500 kJ/mol taken in.
  • I added the steps down: 220 + 220 = 440 kJ/mol given out.
  • I found that the products end 60 kJ/mol above the reactants, as the staircase does.
  • I chose the statement that the hump is far below 2A + 2B.
  • I did not put the top of the hump at the level of 2A + 2B.

Also accept

  • It starts and ends at the same levels as the staircase; its hump is far below 2A + 2B

Watch for

  • “The top of the hump is at the level of the separate atoms, 2A + 2B.” — The staircase breaks every bond first only to add up the energy. Ea is 130 kJ/mol, so the hump is 370 kJ/mol below 2A + 2B.
  • “The staircase ends going down, so the products are below the reactants.” — Compare the totals, not the last steps: up 300 + 200 = 500, down 220 + 220 = 440. The end is 60 kJ/mol above the start.
  • “The ΔH arrow runs from the top of the hump to the products.” — ΔH runs from the reactants' level to the products' level. The arrow from the reactants up to the top of the hump is Ea.

8. JT1 · Apply

Model answer. For the reaction to start, bonds in hydrogen and oxygen have to start breaking (loosening), and breaking bonds takes energy in: the flame supplies this activation energy. It does not need to break every bond: only enough molecules need to start reacting. Making the O–H bonds in water then gives out more energy than was taken in. That energy starts the neighbouring molecules reacting, which give out energy in turn, so the reaction spreads through all the hydrogen in a flash.

A full answer includes

  • (a) I wrote that bonds in the hydrogen and oxygen molecules must start to break for the reaction to begin.
  • (a) I wrote that breaking bonds takes energy in.
  • (a) I wrote that the flame supplies this energy, the activation energy.
  • (a) I did not write that the splint breaks every bond in the hydrogen.
  • (b) I wrote that making the O–H bonds in water gives out more energy than breaking the bonds takes in.
  • (b) I wrote that this energy starts the next hydrogen and oxygen molecules reacting.
  • (b) I did not write that energy is given out when the H–H bonds break.

Also accept

  • (a) Bond breaking must start and takes energy in: the flame supplies the activation energy; (b) bond making gives out more, which starts the next molecules

Watch for

  • “Breaking bonds releases energy.” — Breaking a bond always takes energy in: the atoms are pulled apart against the attraction of the shared pair of electrons. Energy is given out when new bonds are made.
  • “The energy needed to break every bond is the activation energy.” — Breaking every bond is only a total for the sum. In a real reaction old bonds loosen while new ones form, so the activation energy is much smaller.