Answers · Energy changes in reactions

Energy changes in reactions — return questions key

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

1. NR1 · Return

Model answer. (a) The hand feels warm because it gains thermal energy. That energy comes from the setting plaster, so the reaction transfers thermal energy to its surroundings: it is exothermic. A reaction that took energy from the hand would leave the hand with less energy, and it would feel cold. (b) The thermometer stands in the water of the solutions, which is part of the surroundings, not the reacting particles. The water lost thermal energy, and that energy went into the reacting chemicals, which gained it. That is why the reaction is endothermic.

A full answer includes

(a) I wrote that thermal energy moves from the plaster into my hand.
(a) I wrote that the setting of the plaster is exothermic.
(a) I wrote that a reaction that took energy from my hand would make it feel cold.
(b) I wrote that the thermometer measures the water, part of the surroundings.
(b) I wrote that the water lost thermal energy to the reacting chemicals.
(b) I wrote that the reacting chemicals gained thermal energy.
(b) I did not write that the thermometer measures the reacting chemicals.

Also accept

(a) Energy goes from the plaster into the hand: exothermic; (b) the water lost energy and the chemicals gained it

Watch for

“Something that feels warm is taking heat from your hand.” — Your hand feels warm when it gains thermal energy. Here the energy comes from the setting plaster, so the reaction gives energy out.
“The thermometer measures the temperature of the reaction itself.” — The thermometer is in the surroundings: the water, the cup and the thermometer. It rises when the reaction gives thermal energy to them.

2. NR2 · Return

Model answer. (a) The product, 2NO, is on a higher level than the reactants, N₂ + O₂, so it stores more energy, taken in from the surroundings: the reaction is endothermic. (b) The activation energy is the minimum energy that colliding particles of the reactants, N₂ and O₂, must have to react. So it is measured from the reactants' level up to the top of the hump, not from the product.

A full answer includes

(a) I wrote endothermic.
(a) I wrote that the product, 2NO, is higher than the reactants.
(b) I wrote that Ea is the energy the colliding N₂ and O₂ particles need to react.
(b) I wrote that Ea is measured from the reactants' level up to the top of the hump.
(b) I did not measure Ea from the product's level.

Also accept

(a) Endothermic; (b) Ea is measured from the reactants, the particles that collide

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.
“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.

3. NR3 · Return

Model answer. ΔH is positive, so the products are 85 kJ/mol above the reactants: 70 + 85 = 155 kJ/mol, and the ΔH arrow points up from the reactants' level to the products' level. Ea is measured from the reactants' level, so the Ea arrow goes up from 70 kJ/mol to the top of the hump at 70 + 210 = 280 kJ/mol. Row A.

A full answer includes

I wrote 70 + 85 for the products' level.
I got 155 kJ/mol.
I wrote 70 + 210 for the top of the hump.
I got 280 kJ/mol.
I chose row A.
I did not start the Ea arrow at the products' level.

Also accept

Row A

Watch for

“Row B.” — Ea is what the colliding reactant particles need, so its arrow starts at the reactants' level: 70 + 210 = 280 kJ/mol, not 155 + 210.
“Row C.” — ΔH compares only the reactants and the products, so its arrow joins those two levels, not the top of the hump.
“Row D.” — 85 kJ/mol is ΔH itself, read as a level from zero. The products sit ΔH above the reactants: 70 + 85 = 155 kJ/mol.