A student investigates the reaction between excess magnesium ribbon and dilute hydrochloric acid. She places a measured volume of the acid in an uncovered polystyrene cup, records its starting temperature, adds the magnesium ribbon, stirs with the thermometer, and records the te…

Answer: ((a)) Maximum temperature rise = 32.5 − 19.0 = 13.5 °C. ((b)) Exothermic; the temperature of the mixture rises overall, showing that the reaction transfers thermal energy to the surroundings. ((c)) By 120 s the reaction has already finished releasing its thermal energy, so the warm mixture, being at a higher temperature than the surrounding air, starts to lose thermal energy to the surroundings, and its temperature falls. ((d)) Put a lid (with a small hole for the thermometer) on the polystyrene cup, or add extra insulation (lagging) around the cup; this reduces the rate of heat loss from the reaction mixture to the surrounding air while the reaction is happening, so the maximum recorded temperature is…. Part (a): Finding the maximum temperature rise The starting temperature, before the magnesium is added, is 19.0 °C. The highest reading in the table is 32.5 °C, recorded at 120 s. The temperature rise is the difference between these two va…

IGCSE Chemistry 0620: Reading and Evaluating a Temperature-Time Table

Time / s	Temperature / °C
0	19.0
30	24.5
60	29.0
90	32.0
120	32.5
150	31.5
180	30.0

Time / s

Temperature / °C

19.0

24.5

29.0

32.0

120

32.5

150

31.5

180

30.0

Part (a): Finding the maximum temperature rise

The starting temperature, before the magnesium is added, is $19.0\,^{\circ}\text{C}$ . The highest reading in the table is $32.5\,^{\circ}\text{C}$ , recorded at $120\,\text{s}$ .

The temperature rise is the difference between these two values, not the highest reading on its own:

$32.5 - 19.0 = 13.5\,^{\circ}\text{C}$

Part (b): Classifying the reaction

Overall, the temperature of the reaction mixture increases from $19.0\,^{\circ}\text{C}$ to a maximum of $32.5\,^{\circ}\text{C}$ . A rise in the temperature of the surroundings (here, the acid and the dissolving magnesium chloride solution) shows that the reaction is transferring thermal energy to the surroundings. This makes it an exothermic reaction.

Part (c): Explaining the fall in temperature after 120 s

The magnesium reacts completely well before $120\,\text{s}$ , so by the time the maximum reading is reached, the reaction itself has stopped producing any more thermal energy. However, the reaction mixture is now warmer than the surrounding air. A warm object placed in cooler surroundings always loses thermal energy to those surroundings over time, so heat escapes from the uncovered cup and its contents. This causes the recorded temperature to fall gradually after 120 s, even though no further reaction is taking place.

Part (d): Improving the experimental method

To reduce this heat loss and obtain a maximum temperature rise closer to the true value released by the reaction, the student could place a lid on the polystyrene cup (with a small hole for the thermometer to pass through), or add extra insulation (lagging) around the outside of the cup.

Reducing the surface through which thermal energy can escape, or adding an extra insulating layer, slows down the rate of heat loss from the warm reaction mixture to the surrounding air while the reaction is happening. This means less of the thermal energy released by the reaction escapes before it can be recorded, so the measured maximum temperature rise is a more accurate reflection of the total energy actually released by the reaction.

Final answers

(a) Maximum temperature rise $= \boxed{13.5}\,^{\circ}\text{C}$ .
(b) Exothermic. The temperature of the mixture rises overall, so thermal energy is transferred to the surroundings.
(c) The reaction has already finished by 120 s, so the warm mixture then loses thermal energy to the cooler surrounding air, causing the temperature to fall.
(d) Add a lid (or extra insulation/lagging) to the cup, to reduce heat loss to the surroundings and give a more accurate maximum temperature rise.