Pupil sheet · Energy changes in reactions
Hotter or colder? — questions
Name ______________________ Date ____________
Try every question before you use a nudge. Write in full sentences where you are asked to explain.
1. deduce
Aluminium powder is added to copper(II) chloride solution in a polystyrene cup, and the temperature of the solution rises. Which statement about the reaction is correct?
Circle one answer.
- The solution gives thermal energy to it, so it is exothermic
- It takes thermal energy from the solution, so it is endothermic
- It gives thermal energy to the solution, so it is endothermic
- It gives thermal energy to the solution, so it is exothermic
Nudge (only if you are stuck): Which part of the cup is the reaction, and which part is the surroundings?
2. deduce
Four reactions are carried out in polystyrene cups. Each record gives what was seen, then the start temperature and the highest or lowest temperature. Match each record to the kind of reaction.
Write the number of the matching option.
- Endothermic
- Exothermic
- 1. Turns cloudy; 22.5 °C → 29.0 °C: ______
- 2. Fizzes hard; 20.5 °C → 15.5 °C: ______
- 3. Fizzes; 18.0 °C → 26.5 °C: ______
- 4. Turns colourless; 23.0 °C → 19.5 °C: ______
Nudge (only if you are stuck): Which part of each record is about energy?
3. identify / explain
Dilute hydrochloric acid is poured onto potassium hydrogencarbonate powder in a test tube. The mixture fizzes, and the test tube feels cold in your hand. (a) Identify the surroundings of this reaction. (b) Explain why your hand feels cold.
(a) The surroundings
(b) Why your hand feels cold
Nudge (only if you are stuck): Split the test tube into the reacting particles and everything else.
4. calculate / identify
A pupil measures the temperature rise of an exothermic reaction in a polystyrene cup. In the first run the start temperature is 18.5 °C and the highest temperature is 30.5 °C. (a) Calculate the temperature rise in the first run. The pupil repeats the run three more times with the same amounts. The rises are 11.5 °C, 8.5 °C and 12.0 °C. (b) Identify the anomalous rise. (c) Calculate the mean rise of the first run and the repeats, leaving out the anomalous rise. Give your answer to 1 decimal place.
(a) Rise in the first run: ______________ unit ________
(b) Anomalous rise: ______________ unit ________
(c) Mean rise: ______________ unit ________
Nudge (only if you are stuck): Which rise does not agree with the other three?
5. identify
A reaction in a test tube is exothermic. A thermometer stands in the solution, and a pupil holds the test tube in one hand. Which row is correct?
| Row | Thermal energy moves | Thermometer reading | The hand |
|---|---|---|---|
| A | from the solution and hand into the reacting particles | rises | feels warm |
| B | from the reacting particles into the solution and hand | falls | feels warm |
| C | from the reacting particles into the solution and hand | rises | feels warm |
| D | from the solution and hand into the reacting particles | falls | feels cold |
Circle one answer.
- A
- B
- C
- D
Nudge (only if you are stuck): Is the thermometer in the reaction or in the surroundings?
6. deduce
Deduce whether each reaction is exothermic or endothermic from the way it behaves. Match each one.
Write the number of the matching option.
- Endothermic
- Exothermic
- 1. A small piece of sodium on water reacts so fast that the sodium melts into a ball: ______
- 2. Blue hydrated copper(II) sulfate turns white only while it is heated: ______
- 3. Hydrogen needs a lit splint to start burning, then burns on its own: ______
- 4. Sodium hydrogencarbonate in a cake mix gives off carbon dioxide only while it is in a hot oven: ______
Nudge (only if you are stuck): For each one, ask what would happen if the energy supply were taken away.
7. explain
A pupil mixes 20 cm³ of potassium hydroxide solution with 20 cm³ of dilute nitric acid, both at 19.5 °C. In a glass beaker the highest temperature is 24.5 °C. With fresh solutions in a polystyrene cup the highest temperature is 27.0 °C, and two minutes later the cup reads 26.0 °C. (a) Explain why the rise in the glass beaker is smaller. (b) Explain why the pupil uses 27.0 °C, not 26.0 °C, to calculate the rise in the cup.
(a) Why the rise in the beaker is smaller
(b) Why 27.0 °C, not 26.0 °C
Nudge (only if you are stuck): Where can thermal energy go once it has left the reacting particles?
8. identify / deduce / explain
A self-heating can of coffee has a sealed chamber of calcium oxide round the drink. Press the button and water reaches the calcium oxide, which reacts with it, and the coffee gets hot. (a) Identify the surroundings of this reaction. (b) A company wants to make a self-cooling can of juice in the same way. Deduce whether the reaction in its chamber must be exothermic or endothermic. Explain your answer.
(a) The surroundings
(b) The self-cooling can
Nudge (only if you are stuck): Find what gets warmer in the coffee can. Then ask which way energy must move to cool the juice.