Hotter or colder? — answer key

1. NQ1 · Practice

Model answer. The solution, the water around the reacting particles, is the surroundings. Its temperature rises, so it has gained thermal energy, and that energy came from the reaction. A reaction that transfers thermal energy to the surroundings is exothermic.

A full answer includes

• I chose: it gives thermal energy to the solution, so it is exothermic.
• I used the solution as the surroundings.
• I did not choose "it takes thermal energy from the solution".

Also accept

• It gives thermal energy to the solution, so it is exothermic

Watch for

• “It takes thermal energy from the solution, so it is endothermic.” — If the reaction took thermal energy from the solution, the solution would get colder. It got warmer.
• “The solution gives thermal energy to the reaction, so it is exothermic.” — Exothermic means thermal energy goes out of the reaction. The solution gained energy, so the energy came from the reaction, not from the solution.
• “It gives thermal energy to the solution, so it is endothermic.” — Endo means "in". A reaction that gives thermal energy to its surroundings is exothermic: exo means "out".

2. NQ2 · Practice

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

Model answer. Only the temperature shows which way thermal energy goes. Records 1 (22.5 → 29.0 °C) and 3 (18.0 → 26.5 °C) rise: the reaction transferred thermal energy to the surroundings, so they are exothermic. Records 2 (20.5 → 15.5 °C) and 4 (23.0 → 19.5 °C) fall: the reaction took in thermal energy from the surroundings, so they are endothermic. Fizzing, cloudiness and a colour change only show that a reaction happened.

A full answer includes

• I matched 1 and 3 to exothermic.
• I matched 2 and 4 to endothermic.
• I used only the two temperatures in each record.
• I did not call record 2 exothermic because it fizzes.

Also accept

• 1 and 3 exothermic; 2 and 4 endothermic

Watch for

• “Fizzing shows that a reaction is exothermic.” — Bubbles show that a gas is made. Citric acid and sodium hydrogencarbonate fizz and get colder. Only the temperature change shows which way the energy goes.
• “A colour change shows that a reaction gives out energy.” — A colour change shows that a new substance has formed. In record 4 the temperature falls, so that reaction takes energy in.
• “An endothermic reaction gives out cold.” — Cold does not flow. Thermal energy flows from the surroundings into the reacting chemicals, so the surroundings are left with less energy and get colder.

3. NQ3 · Practice

Model answer. (a) The reaction is the reacting particles: the acid and the potassium hydrogencarbonate. The surroundings are everything else: the water the acid is dissolved in, the test tube, your hand and the air. (b) The reaction takes in thermal energy from its surroundings. Some of that energy comes from your hand, through the glass, so your hand is left with less energy and its temperature falls. Nothing "cold" comes out of the mixture: the reaction is endothermic.

A full answer includes

• (a) I named at least one thing that is not reacting: the water, the test tube, my hand or the air.
• (a) I did not count the reacting acid or the potassium hydrogencarbonate as surroundings.
• (b) I wrote that the reaction takes in thermal energy.
• (b) I wrote that the energy comes from my hand, through the test tube.
• (b) I wrote that my hand loses energy, so its temperature falls.
• (b) I did not write that cold flows out of the mixture.

Also accept

• (a) The water, the test tube, your hand and the air; (b) the reaction takes thermal energy from your hand

Watch for

• “An endothermic reaction gives out cold.” — Cold does not flow. Thermal energy flows from the surroundings into the reacting chemicals, so the surroundings are left with less energy and get colder.
• “The potassium hydrogencarbonate is part of the surroundings.” — It is reacting, so it is part of the reaction. The surroundings are everything that is not reacting.

4. NQ4 · Practice

Model answer. (a) Rise = highest − start = 30.5 − 18.5 = 12.0 °C. (b) The four rises are 12.0, 11.5, 8.5 and 12.0 °C. Three agree closely; 8.5 °C is far from them, so it is anomalous. (c) Mean = (12.0 + 11.5 + 12.0) ÷ 3 = 35.5 ÷ 3 = 11.833… = 11.8 °C to 1 decimal place.

Checked automatically

• (a) Rise in the first run: 12.0 °C (exact value)
  • Common wrong answer 30.5 : That is the highest temperature. Subtract the start temperature.
  • Common wrong answer 49.0 : You added the two temperatures. Subtract the start from the highest.
• (b) Anomalous rise: 8.5 °C (exact value)
  • Common wrong answer 11.5 : 11.5 °C is close to 12.0 °C. Look for the rise that is far from all the others.
• (c) Mean rise: 11.8 °C (3 significant figures)
  • Common wrong answer 11.0 : That mean includes the anomalous 8.5 °C. Leave it out and divide by 3.

A full answer includes

• (a) I wrote 30.5 − 18.5.
• (a) I got a rise of 12.0 °C.
• (b) I chose 8.5 °C, the rise far from the others.
• (c) I added 12.0, 11.5 and 12.0 and divided by 3.
• (c) I got 11.8 °C, to 1 decimal place.
• (c) I did not include 8.5 °C in the mean.

Also accept

• 12.0 °C; 8.5 °C; 11.8 °C

Watch for

• “Every result goes into the mean.” — An anomalous result does not fit the pattern of the others, so leave it out. With 8.5 °C in, the mean is 11.0 °C, lower than the true rise.
• “The temperature rise is the highest temperature.” — The rise is the highest temperature minus the start temperature.

5. NQ5 · Practice

Model answer. Exothermic means thermal energy moves out of the reacting particles into the surroundings: the solution, the glass and the hand. The thermometer is in the solution, so its reading rises, and the hand feels warm. Row C.

A full answer includes

• I chose row C.
• I chose energy moving out of the reacting particles.
• I took the thermometer to read the solution, part of the surroundings.
• I did not choose row B, where the thermometer falls.

Also accept

• Row C

Watch for

• “Row A.” — If thermal energy moved into the reacting particles, the solution and the hand would lose energy and get colder. In an exothermic reaction energy moves out.
• “Row B.” — The thermometer is in the solution, which is part of the surroundings. The surroundings gain the energy, so the reading rises.
• “Row D.” — That row describes an endothermic reaction. Exothermic means energy goes out of the reaction, so the surroundings warm up.

6. NQ6 · Practice

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

Model answer. 1: the energy that melts the sodium comes from the reaction itself, so it gives out thermal energy: exothermic. 2: the change goes on only while the crystals are heated, so it takes in thermal energy all the time: endothermic. 3: hydrogen needs a lit splint to start, but then burns on its own and gives out thermal energy: exothermic. 4: sodium hydrogencarbonate breaks down only while the hot oven heats it, so it takes in thermal energy: endothermic.

A full answer includes

• I matched 1, sodium on water, to exothermic.
• I matched 2, heating hydrated copper(II) sulfate, to endothermic.
• I matched 3, burning hydrogen, to exothermic.
• I matched 4, sodium hydrogencarbonate in the oven, to endothermic.
• I did not call burning hydrogen endothermic because it needs a lit splint to start.

Also accept

• 1 and 3 exothermic; 2 and 4 endothermic

Watch for

• “Anything that needs heating to start is endothermic.” — Hydrogen needs a lit splint to start, but then gives out thermal energy and keeps burning. An endothermic reaction keeps taking energy in.
• “The sodium melts because the water is warm.” — The water starts cool. The energy that melts the sodium comes from the reaction, so it is exothermic.
• “The cake gives off a gas, so the reaction gives out energy.” — A gas shows that a new substance forms, not which way energy goes. In 4 the gas comes off only while the oven heats the cake, so the reaction takes in thermal energy: it is endothermic.

7. NQ7 · Practice

Model answer. The reaction gives out the same energy in both containers. (a) Glass lets thermal energy pass through it much more easily than polystyrene, a good thermal insulator, so in the beaker some of the energy leaves for the room while the reaction goes on: the rise is only 5.0 °C, against 7.5 °C in the cup. (b) Once the reaction has finished, the warm cup loses thermal energy to the room and slowly cools. 27.0 °C is the reading before that cooling; 26.0 °C is too low.

A full answer includes

• (a) I wrote that glass lets more thermal energy pass through than polystyrene.
• (a) I wrote that, in the beaker, some of the energy given out is lost to the room.
• (a) I did not write that the beaker stops the mixture getting too hot.
• (b) I wrote that the reaction has finished by the later reading.
• (b) I wrote that the cup then loses thermal energy to the room and cools.
• (b) I did not write that 26.0 °C is better because it is the last reading.

Also accept

• (a) Glass lets more energy escape to the room; (b) the cup cools after the reaction has finished

Watch for

• “The polystyrene cup makes the reaction give out more energy.” — The same reaction gives out the same energy. The cup keeps more of it in the solution, so the thermometer shows more of it.
• “The last reading is best, because the reaction is over.” — Once the reaction is over the cup cools towards room temperature, so a later reading is too low.

8. NT1 · Apply

Model answer. (a) The reaction is the calcium oxide and the water reacting with it. The surroundings are everything else: the coffee, the metal of the can and the air around it. The coffee warms up because the reaction transfers thermal energy to it. (b) To cool the juice, the reaction must take in thermal energy from its surroundings, the juice, so the temperature of the juice falls: the reaction must be endothermic.

A full answer includes

• (a) I named the coffee as part of the surroundings.
• (a) I did not count the calcium oxide and the water reacting with it as surroundings.
• (b) I chose endothermic.
• (b) I wrote that the reaction must take in thermal energy from the juice.
• (b) I wrote that the temperature of the juice then falls.
• (b) I did not write that the reaction gives cold to the juice.

Also accept

• (a) The coffee, the can and the air; (b) endothermic: it takes thermal energy from the juice

Watch for

• “An endothermic reaction gives out cold.” — Cold does not flow. Thermal energy flows from the surroundings into the reacting chemicals, so the surroundings are left with less energy and get colder.
• “The calcium oxide is part of the surroundings, because it gets hot.” — The calcium oxide is reacting, so it is part of the reaction. The surroundings are everything else: the coffee, the can and the air.