These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. Substances with single-letter names (W, Z) are fictional. Assume every cup is insulated, and that thermometers read to ±0.2 °C unless a question says otherwise.
Question 1 (multiple choice · foundation)
Which of the following processes is exothermic for the substance named?
- (A) Ice melting on a warm plate
- (B) Acetone evaporating from a cotton pad
- (C) Water vapour forming frost on a car window overnight
- (D) Solid carbon dioxide (dry ice) turning directly into a gas
Answer and explanation
Answer: (C). Frost forms by deposition (gas → solid). The water molecules come together and attract each other, so energy is released to the window and the air.
- (A) Melting needs energy to overcome attractions between water molecules, so it is endothermic.
- (B) Evaporation (liquid → gas) is endothermic; it is why the pad feels cool.
- (D) Sublimation (solid → gas) is endothermic for the carbon dioxide.
Question 2 (multiple choice · core)
A student dissolves a solid in water in an insulated cup. The temperature of the solution falls from 22.0 °C to 17.5 °C. Which statement is correct?
- (A) The dissolving is exothermic, because the solution lost energy to the surroundings.
- (B) The dissolving is endothermic, because energy moved from the water into the dissolving solute particles.
- (C) The dissolving is endothermic, because cold was released by the solid into the water.
- (D) The dissolving is exothermic, because the temperature change is negative.
Answer and explanation
Answer: (B). ΔT = 17.5 − 22.0 = −4.5 °C. The water (the surroundings) lost energy, and that energy went into the dissolving system, so the process is endothermic.
- (A) treats the water as the system. The water did lose energy, but the process being classified is the dissolving, which gained it.
- (C) has the right label but the wrong reason: there is no such thing as cold being released. Only energy moves.
- (D) confuses the sign of the temperature change of the surroundings with the energy change of the system. A negative ΔT in the water means the system gained energy.
Question 3 (multiple choice · core)
Which condition makes the dissolving of an ionic solid in water exothermic?
- (A) The ion–water attractions formed are weaker overall than the attractions between ions in the solid and between water molecules.
- (B) The ion–water attractions formed are stronger overall than the attractions between ions in the solid and between water molecules.
- (C) The ions in the solid are held by very strong attractions, whatever the strength of the ion–water attractions.
- (D) No attractions are broken when an ionic solid dissolves.
Answer and explanation
Answer: (B). Separating ions and making room between water molecules both need energy. Forming ion–water attractions releases energy. If the energy released is larger, the system loses energy overall: exothermic.
- (A) describes an endothermic dissolving process.
- (C) looks at only one step. Strong ion–ion attractions make separating the ions cost more energy, which pushes towards endothermic; on its own it cannot make the process exothermic.
- (D) is false: the attractions in the solid must be overcome for the ions to move apart.
Question 4 (multiple choice · core)
A gas burner heats a beaker of water from 20 °C to 60 °C. Which row correctly classifies the process for each system?
- (A) Burning gas: exothermic. Water: endothermic.
- (B) Burning gas: endothermic. Water: exothermic.
- (C) Burning gas: exothermic. Water: exothermic.
- (D) Burning gas: endothermic. Water: endothermic.
Answer and explanation
Answer: (A). The burning gas loses energy, which flows into the water. Heating is endothermic for the substance being heated.
- (B) reverses the direction of energy transfer.
- (C) treats the water as giving out energy because it gets hot. A substance that gets hotter has gained energy.
- (D) treats the flame as taking in energy because combustion needs a spark to start. The starting energy does not change the overall energy loss.
Question 5 (constructed response · core)
A student carries out three experiments in insulated cups, each containing 50 mL of liquid.
| Experiment | What is mixed | Initial temperature (°C) | Final temperature (°C) |
|---|---|---|---|
| 1 | Dilute hydrochloric acid + dilute sodium hydroxide solution | 20.8 | 27.3 |
| 2 | Solid W + water | 21.0 | 18.6 |
| 3 | Magnesium ribbon + dilute hydrochloric acid (bubbles of hydrogen form) | 21.2 | 34.9 |
(a) Calculate ΔT for each experiment. (b) Classify each process as endothermic or exothermic. For Experiment 2, justify your answer by stating the system, the surroundings and the direction of energy transfer. (c) In Experiment 3, the hydrogen gas pushes back the air above the cup. Explain why this means the temperature rise slightly underestimates the energy lost by the reacting system.
Worked solution and suggested Marlbridge rubric
(a) Experiment 1: 27.3 − 20.8 = +6.5 °C. Experiment 2: 18.6 − 21.0 = −2.4 °C. Experiment 3: 34.9 − 21.2 = +13.7 °C.
(b) Experiment 1 (neutralisation): exothermic. Experiment 3 (magnesium with acid): exothermic. Experiment 2: endothermic. The system is solid W dissolving; the surroundings are the water. The water’s temperature fell, so energy moved from the water into the dissolving system.
(c) Some of the energy released by the reaction leaves the system as work: the hydrogen produced pushes back the atmosphere. The thermometer detects only energy that warms the solution (heat). So the energy lost by the system equals the heat transferred plus the work done, which is a little more than the temperature rise shows.
| Point | What earns it |
|---|---|
| 1 | All three ΔT values with signs |
| 1 | Experiments 1 and 3 exothermic and Experiment 2 endothermic |
| 1 | Experiment 2: identifies the system (dissolving W) and surroundings (water) and states energy moved from the water into the system |
| 1 | Part (c): energy also leaves as work done by the gas on the surroundings, which the thermometer does not record |
Do not award the third point for “the solid absorbed cold” or for “the solution lost energy” with no system named.
Question 6 (constructed response · stretch)
A reaction that produces a gas is carried out in a cylinder fitted with a light, freely moving piston. The cylinder stands in a small, insulated water bath. As the reaction happens, the piston rises steadily. A thermometer in the water bath shows no change in temperature (it reads to ±0.1 °C).
A student claims: “Because the water bath did not change temperature, the energy of the reacting system did not change.”
(a) Identify the way in which energy crossed the boundary of the system in this experiment. (b) Evaluate the student’s claim.
Model answer and suggested Marlbridge rubric
(a) As the piston rises, the gas made by the reaction pushes the piston and the air above it through a distance. The system does work on the surroundings.
(b) The claim is not justified. Energy can leave a system as heat or as work. The steady temperature shows that no heat transfer was large enough to detect, but the rising piston shows that the system did work on its surroundings. Energy that leaves as work is lost by the system, so the evidence shows the energy of the reacting system decreased even though the bath did not warm up. (A very small heat transfer could also be hidden inside the ±0.1 °C precision.)
| Point | What earns it |
|---|---|
| 1 | Identifies work done by the system as the gas pushes the piston |
| 1 | States that energy can be transferred as heat or work, so a constant temperature does not rule out an energy change |
| 1 | Concludes that the system lost energy (claim rejected), linked to the work done |
A comment about the precision of the thermometer is a good addition but does not replace the second or third point.
Question 7 (constructed response · core)
Compound Z is a fictional ionic solid used in a cold pack. When the pack is squeezed, water mixes with solid Z and the pack becomes cold.
(a) Is dissolving Z endothermic or exothermic? Explain your answer in terms of the attractions that are broken and formed. (b) A person holds the activated pack. Describe the direction of energy transfer between the hand and the pack, and explain why the hand feels cold. (c) Z is dissolved in a different solvent in which the attractions between the ions of Z and the solvent molecules are much weaker than in water. All other attractions are about the same, the same amount of Z dissolves completely, and the same mass of solvent with about the same heat capacity is used. Predict whether the temperature drop would be larger or smaller than in water, and justify your prediction.
Model answer and suggested Marlbridge rubric
(a) Endothermic. Dissolving Z breaks the attractions between its ions and some attractions between water molecules, which needs energy. Ion–water attractions then form, which releases energy. Because the pack cools, the energy released by the new attractions must be less than the energy needed to break the old ones, so the system gains energy overall.
(b) Energy moves from the hand into the pack. The hand loses energy, so its temperature falls and it feels cold. Nothing called “cold” moves into the hand.
(c) The temperature drop would be larger. The energy needed to separate the ions is unchanged, but less energy is released when the weaker ion–solvent attractions form. So more energy, overall, must be taken in from the surroundings per amount of Z dissolved.
| Point | What earns it |
|---|---|
| 1 | Endothermic, with breaking attractions needing energy and forming attractions releasing it |
| 1 | Links the cooling to the energy released being smaller than the energy needed |
| 1 | Energy flows from the hand to the pack (no “cold flows”) |
| 1 | Larger drop, justified by less energy released when the weaker attractions form |
In (c), accept “more endothermic” in place of “larger temperature drop” if the justification is the same.
How did you do?
- Q1 or Q4 wrong: re-read “Three kinds of process” and Worked example 2 in the study guide.
- Q2 or Q5 wrong: practise “Reading temperature changes”: the thermometer reads the surroundings.
- Q3 or Q7 incomplete: your answer needs both halves, attractions broken (energy in) and attractions formed (energy out).
- Q6 incomplete: revisit “Heat and work: two ways energy moves”.
Then tick off the topic checklist.