A student wants to compare the energy released when two different acids, hydrochloric acid and sulfuric acid, are neutralised by the same alkali. Fig. 13.1 ...

This is a planning question about comparing the heat released in two neutralisation reactions. A fair comparison means changing only the acid and measuring the temperature rise each acid produces in the same alkali.

(a) A full method scores up to six marks, one for each clear step. [6]

1. Measure a fixed volume, for example \(25\ \text{cm}^3\), of the alkali into a polystyrene cup, which insulates the mixture and reduces heat loss. [1]
2. Record the starting temperature of the alkali with a thermometer. [1]
3. Add a fixed volume of the first acid (hydrochloric acid) and stir to mix thoroughly. [1]
4. Record the highest temperature reached during the reaction. [1]
5. Calculate the temperature rise as \(\text{highest temperature} - \text{starting temperature}\). [1]
6. Repeat the whole procedure with the second acid (sulfuric acid), using the same volumes, the same alkali and the same starting temperature. [1]

(b) To measure the acid volumes accurately you would use a volumetric pipette (a burette is also accepted), because these read to a much finer scale than a measuring cylinder. [1]

(c) The variable you change (independent variable) is the acid used. [1] Two variables to keep the same (control variables) are the volume or concentration of the alkali [1] and the volume or concentration of the acid (also acceptable: the starting temperature, or using the same cup). [1] Only then does any difference in temperature rise come from the acid itself.

(d) The acid that gives the larger temperature rise has released more energy. [1] So you compare the two temperature rises and pick the bigger one. [1]

(e) Two sources of error, for example heat lost to the surroundings, or the thermometer being read too late or inaccurately, or the volumes being measured wrongly. [1] One improvement, for example put a lid on the cup or insulate it to cut heat loss, or repeat and average the results. [1]

Exam tip: temperature rise, not final temperature, is the fair measure of energy released, because it removes the effect of the starting temperature.