Answers · Energy changes in reactions

Reaction pathway diagrams — answer key

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

1. NQ8 · Practice

Model answer. The products, carbon dioxide and water, are on a lower level than the reactants, so they store less energy. The difference goes to the surroundings: the reaction is exothermic. The hump does not make it endothermic.

A full answer includes

I chose: the products have less energy than the reactants, so it is exothermic.
I compared the products' level with the reactants' level.
I did not use the hump to decide.

Also accept

The products have less energy: exothermic

Watch for

“The products have more energy than the reactants, so it is endothermic.” — Look again: the products' level on the right is lower than the reactants' level on the left.
“The hump shows that the reaction is endothermic.” — Every pathway diagram has a hump. Compare only the level of the products with the level of the reactants.
“The products are lower, so the reaction takes in thermal energy.” — Products lower means the chemicals have lost energy: it went out to the surroundings, not in.

2. NQ9 · Practice

Options as numbered on the pupil sheet: 1 — The products; 2 — The energy taken in from the surroundings; 3 — The reactants; 4 — The energy given out to the surroundings.

Model answer. The reactants are on the left, A, and the products on the right, B. The arrow C goes down from the reactants' level to the lower products' level: the energy given out to the surroundings, so the reaction is exothermic.

A full answer includes

I matched A to the reactants.
I matched B to the products.
I matched C to the energy given out to the surroundings.
I did not match C to the energy taken in.

Also accept

A reactants, B products, C energy given out

Watch for

“An arrow pointing down means energy is taken in.” — Down means the products have less energy: the difference has gone out to the surroundings.
“The products are on the left.” — The diagram is read from left to right, like the equation: reactants first, products last.

3. NQ10 · Practice

Model answer. (a) In Q the products are higher than the reactants, so the chemicals gain energy from the surroundings: Q is endothermic and makes the thermometer fall. In P the products are much lower: P gives out energy. (b) The activation energy is measured from the reactants' level to the top of the hump. For R, Ea = 210 − 180 = 30 kJ/mol; for S, Ea = 190 − 30 = 160 kJ/mol. Colliding particles of S need much more energy to react, so S needs more energy to start, even though the top of R's hump is higher on the scale.

A full answer includes

(a) I chose Q.
(a) I wrote that the products of Q are higher than its reactants.
(a) I wrote that Q takes in thermal energy from the surroundings, so the thermometer falls.
(b) I wrote 210 − 180 for R.
(b) I wrote 190 − 30 for S.
(b) I got Ea = 30 kJ/mol for R.
(b) I got Ea = 160 kJ/mol for S.
(b) I chose S, the reaction with the larger activation energy.
(b) I did not choose R because the top of its hump is higher.

Also accept

(a) Q: products higher; (b) S: Ea = 160 kJ/mol, against 30 kJ/mol for R

Watch for

“The hump shows that the reaction is endothermic.” — Every pathway diagram has a hump. Compare only the level of the products with the level of the reactants.
“R needs more energy to start, because the top of its hump is higher.” — Ea is measured from the reactants' level, not from zero. R's hump rises only 30 kJ/mol above its reactants; S's rises 160 kJ/mol above its reactants.
“P makes the thermometer fall, because its curve ends lower.” — Products lower means the chemicals lost energy to the surroundings, which get warmer.

4. NQ11 · Practice

Model answer. ΔH is the enthalpy change of the chemicals. Negative means they lose energy, which goes to the surroundings: the reaction is exothermic, and the surroundings get warmer.

A full answer includes

I chose: the chemicals lose energy, and the surroundings get warmer.
I read the minus sign as the energy of the chemicals going down.
I did not take the minus sign to mean that the temperature goes down.

Also accept

The chemicals lose energy, and the surroundings get warmer

Watch for

“ΔH is negative, so the temperature of the surroundings goes down.” — The sign belongs to the chemicals. A negative ΔH means the chemicals lose energy to the surroundings, which get warmer.
“The chemicals gain energy, and the surroundings get colder.” — That describes a positive ΔH, an endothermic reaction. A negative ΔH means the chemicals end with less energy.
“The chemicals gain energy, and the surroundings get warmer.” — Energy the chemicals gain must come from the surroundings, which would then get colder. And a negative ΔH means the chemicals lose energy.

5. NQ12 · Practice

Options as numbered on the pupil sheet: 1 — Endothermic; 2 — Exothermic.

Model answer. A negative ΔH means the chemicals lose energy to the surroundings: exothermic (1 and 3). A positive ΔH means they gain energy from the surroundings: endothermic (2 and 4). The size of the number shows how much energy, not which way it goes.

A full answer includes

I matched 1 and 3, with a negative ΔH, to exothermic.
I matched 2 and 4, with a positive ΔH, to endothermic.
I did not use the size of the number to decide.

Also accept

1 and 3 exothermic; 2 and 4 endothermic

Watch for

“ΔH is negative, so the temperature of the surroundings goes down.” — The sign belongs to the chemicals. A negative ΔH means the chemicals lose energy to the surroundings, which get warmer.
“A bigger number means the reaction is endothermic.” — Only the sign shows the direction. Reaction 1 has the biggest number, 1203, and it is exothermic.

6. NQ13 · Practice

Model answer. Particles react only if they collide with at least a certain energy. That minimum energy is the activation energy, Ea. On a reaction pathway diagram it goes from the reactants' level to the top of the hump.

A full answer includes

I chose: the minimum energy that colliding particles must have to react.
I noticed the word minimum in the definition.
I did not choose the difference between the reactants and the products, which is ΔH.

Also accept

The minimum energy that colliding particles must have to react

Watch for

“The energy that colliding particles give out when they react.” — The energy given out is ΔH. Ea is the energy colliding particles need before they can react.
“Ea is the height of the top of the hump read from the bottom of the graph.” — Measure Ea from the level of the reactants up to the top of the hump.
“The difference in energy between the reactants and the products.” — That difference is the enthalpy change, ΔH, not the activation energy.

7. NQ14 · Practice

Options as numbered on the pupil sheet: 1 — The enthalpy change, ΔH; 2 — The energy of the products; 3 — The activation energy, Ea.

Model answer. Arrow X goes from the reactants' level R up to the top of the hump: the activation energy, Ea. Arrow Y goes from the reactants' level down to the products' level P: the enthalpy change, ΔH, negative here.

A full answer includes

I matched X to the activation energy, Ea.
I matched Y to the enthalpy change, ΔH.
I did not match X to ΔH.

Also accept

X is Ea; Y is ΔH

Watch for

“Ea is measured from the products.” — Ea is the energy the reactants need, so it goes from the reactants' level to the top of the hump.
“The longer arrow is always ΔH.” — The length does not decide. Look at where each arrow ends: at the top of the hump, or at the products' level.

8. NQ15 · Practice

Model answer. The reactants are at 100 kJ/mol, the top of the hump at 250 and the products at 50. (a) Ea is measured from the reactants: 250 − 100 = 150 kJ/mol. (b) ΔH = products − reactants = 50 − 100 = −50 kJ/mol: negative, because the products are lower and the reaction is exothermic.

Checked automatically

(a) Activation energy: 150 kJ/mol (exact value)
- Common wrong answer 250 : That is the top of the hump read from zero. Subtract the reactants' level: 250 − 100.
- Common wrong answer 200 : That is measured from the products. Ea starts at the reactants' level: 250 − 100.
(b) Enthalpy change: -50 kJ/mol (exact value)
- Common wrong answer 50 : The size is right, but the products are lower: the chemicals lose energy, so ΔH is negative.

A full answer includes

(a) I read the reactants at 100 kJ/mol and the top of the hump at 250 kJ/mol.
(a) I wrote Ea = 250 − 100.
(a) I got Ea = 150 kJ/mol.
(b) I wrote ΔH = 50 − 100.
(b) I got ΔH = −50 kJ/mol, with the minus sign.
I did not read Ea as 250 kJ/mol from the bottom of the graph.

Also accept

150 kJ/mol; −50 kJ/mol

Watch for

“Ea is the height of the top of the hump read from the bottom of the graph.” — Measure Ea from the level of the reactants up to the top of the hump.
“ΔH is negative, so the temperature of the surroundings goes down.” — The sign belongs to the chemicals. A negative ΔH means the chemicals lose energy to the surroundings, which get warmer.

9. NQ16 · Practice

Model answer. ΔH is negative, so the reaction is exothermic and the products, 2H₂O + O₂, go on a lower level than the reactants, 2H₂O₂, 196 kJ/mol lower. The ΔH arrow points down from the reactants' level to the products' level. The hump rises 75 kJ/mol above the reactants, and the Ea arrow goes from the reactants' level up to its top. Energy goes up the side and the progress of the reaction along the bottom.

A full answer includes

(a) I put the products, 2H₂O + O₂, below the reactants, 2H₂O₂.
(a) I put them 196 kJ/mol lower.
(b) I described the ΔH arrow pointing down, from the reactants' level to the products' level.
(b) I gave its label as ΔH = −196 kJ/mol.
(c) I described the Ea arrow pointing up, from the reactants' level to the top of the hump.
(c) I gave its label as Ea = 75 kJ/mol.
(d) I labelled the y axis energy.
(d) I labelled the x axis progress of the reaction.
I did not start the ΔH arrow at the top of the hump.

Also accept

Products lower; ΔH arrow down; Ea arrow up from reactants to the top

Watch for

“Ea is measured from the products.” — Ea is the energy the reactants need, so it goes from the reactants' level to the top of the hump.
“The ΔH arrow goes from the top of the hump to the products.” — ΔH compares only the products with the reactants: the arrow joins those two levels.

10. NQ17 · Practice

Model answer. ΔH is negative, so the products are 70 kJ/mol below the reactants: 170 − 70 = 100 kJ/mol, and the ΔH arrow points down from the reactants' level to the products' level. Ea is measured from the reactants' level, so the top of the hump is at 170 + 90 = 260 kJ/mol. Row C.

A full answer includes

I wrote 170 − 70 for the products' level.
I got 100 kJ/mol.
I wrote 170 + 90 for the top of the hump.
I got 260 kJ/mol.
I chose row C.
I did not add Ea to the products' level.

Also accept

Row C

Watch for

“Row A.” — ΔH is negative, so the products are lower than the reactants: 170 − 70 = 100 kJ/mol, and the ΔH arrow points down.
“Row B.” — 190 kJ/mol is Ea added to the products' level. Ea is measured from the reactants' level: 170 + 90.
“Row D.” — The ΔH arrow joins only the reactants' and the products' levels. The top of the hump belongs to Ea.

11. NT2 · Apply

Model answer. (a) Burning butane gives out thermal energy, which heats the soup, so the reaction is exothermic: the products' level is lower than the reactants' level. (b) The chemicals lose energy, so ΔH is negative. (c) At room temperature too few butane and oxygen particles collide with at least the activation energy, so the gas does not light. The spark heats a little of the gas until its particles collide with enough energy to react. (d) That reaction gives out thermal energy, which heats the gas next to it until its particles collide with at least the activation energy, so the flame keeps itself going.

A full answer includes

(a) I wrote that the products' level is lower than the reactants' level.
(a) I wrote that burning gives out energy that heats the soup.
(b) I wrote negative.
(c) I wrote that, at first, the colliding particles do not have the activation energy.
(c) I wrote that the spark gives some particles at least the activation energy.
(c) I did not write that burning butane is endothermic because it needs a spark.
(d) I wrote that the burning gives out thermal energy.
(d) I wrote that this energy gives the next particles the activation energy.
(d) I did not write that the spark keeps supplying the energy.

Also accept

(a) Lower; (b) negative; (c) the spark supplies Ea; (d) the energy given out supplies Ea to the next particles

Watch for

“The hump shows that the reaction is endothermic.” — Every pathway diagram has a hump. Compare only the level of the products with the level of the reactants.
“ΔH is negative, so the temperature of the surroundings goes down.” — The sign belongs to the chemicals. A negative ΔH means the chemicals lose energy to the surroundings, which get warmer.
“The gas needs the spark because burning is endothermic.” — Burning gives out energy: it heats the soup, so it is exothermic. The spark is needed only to give the first particles the activation energy.