Bond energies — return questions key

1. JR1 · Return

Model answer. ΔH = 600 − 720 = −120 kJ/mol: exothermic, so the products are 120 kJ/mol below the reactants and the ΔH arrow points down from the reactants' level to the products' level. The top of the hump is the activation energy above the reactants: 170 kJ/mol. Row B.

A full answer includes

• I worked out ΔH as 600 − 720.
• I got ΔH = −120 kJ/mol, so the products are 120 kJ/mol below the reactants.
• I put the top of the hump 170 kJ/mol above the reactants.
• I chose row B.
• I did not put the hump 600 kJ/mol above the reactants.

Also accept

• Row B

Watch for

• “Row A.” — 600 is the energy in for breaking every bond, a total for the sum. In the real reaction old bonds loosen as new ones form, so the hump is Ea, 170 kJ/mol.
• “Row C.” — More energy is given out than taken in: ΔH = 600 − 720 = −120 kJ/mol. The products are lower, and the ΔH arrow points down.
• “Row D.” — ΔH is measured from the reactants' level to the products' level. The top of the hump belongs to Ea, not to ΔH.

2. JR2 · Return

Options as numbered on the pupil sheet: 1 — The separate atoms; 2 — Enthalpy change, ΔH; 3 — Products; 4 — Reactants; 5 — Activation energy, Ea.

Model answer. The reaction starts on the left, so W is the reactants, CaCO₃, and ends on the right, so X is the products, CaO + CO₂. X is higher than W: the products have more energy, so the reaction is endothermic. Both arrows start at the reactants' level. Y ends at the top of the hump: it is the activation energy, Ea. Z ends at the products' level: it is the enthalpy change, ΔH, positive because it points up. No line shows the separate atoms: they belong to the bond-energy staircase, not to the real pathway.

A full answer includes

• I matched W, the line on the left, to the reactants.
• I matched X, the line on the right, to the products.
• I matched Y, the arrow that ends at the top of the hump, to the activation energy, Ea.
• I matched Z, the arrow that ends at the products' line, to the enthalpy change, ΔH.
• I did not match any feature to the separate atoms.

Also accept

• W reactants; X products; Y activation energy, Ea; Z enthalpy change, ΔH

Watch for

• “The higher line, X, is the reactants, because the reactants have the energy.” — The reaction starts on the left, so W is the reactants. In an endothermic reaction the products end higher than the reactants.
• “The arrow that ends at the top of the hump, Y, is ΔH.” — ΔH runs from the reactants' level to the products' level. Y ends at the top of the hump, so it is Ea.
• “Z is the activation energy, because it points up from the reactants.” — Z ends at the products' line, not at the top of the hump. It shows ΔH, which is positive here because it points up.
• “The top line of the diagram is the separate atoms.” — The separate atoms are only the top of the bond-energy staircase, a way of adding up the energy. No line on a real pathway diagram sits at the level of all bonds broken.

3. JR3 · Return

Model answer. Energy in: C–C, five C–H and C–Cl, 348 + 2060 + 338 = 2746 kJ/mol. Energy out: C=C, four C–H and H–Cl, 612 + 1648 + 431 = 2691 kJ/mol. ΔH = 2746 − 2691 = +55 kJ/mol. The reverse reaction breaks the bonds this one makes and makes the bonds it breaks, so the two totals swap places: ΔH = 2691 − 2746 = −55 kJ/mol.

Checked automatically

• (a) ΔH: 55 kJ/mol (exact value)
  • Common wrong answer -55 : Check the sign: ΔH = energy in − energy out = 2746 − 2691, which is positive.
  • Common wrong answer 319 : Ethene has a C=C double bond: use 612, not the C–C value, 348.
  • Common wrong answer 486 : Hydrogen chloride is a product too: add its H–Cl bond, 431, to the energy out.
• (b) ΔH for the reverse reaction: -55 kJ/mol (exact value)
  • Common wrong answer 55 : The reverse reaction swaps energy in and energy out: 2691 − 2746.

A full answer includes

• (a) I worked out the energy in: 348 + 5 × 412 + 338 = 2746 kJ/mol.
• (a) I worked out the energy out: 612 + 4 × 412 + 431 = 2691 kJ/mol.
• (a) I worked out 2746 − 2691.
• (a) I got ΔH = +55 kJ/mol.
• (b) I gave −55 kJ/mol: the same size with the opposite sign.
• (c) I wrote that the reverse reaction breaks the bonds that the forward reaction makes.
• (c) I wrote that the reverse reaction makes the bonds that the forward reaction breaks.
• (c) I wrote that the energy in and the energy out swap places.
• I did not give the reverse reaction the same ΔH, +55 kJ/mol.

Also accept

• +55 kJ/mol; −55 kJ/mol; the totals swap, so the sign changes

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 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.
• “A reaction and its reverse have the same ΔH.” — Running the reaction backwards breaks the bonds that were made and makes the bonds that were broken, so energy in and energy out swap and the sign of ΔH changes.