A student burned ethanol in a spirit burner to heat 50 cm3 of water in a boiling tube, using the apparatus in Fig. 5.1. The readings are shown in Table 5.1....

This is a calorimetry (energy from fuels) question. You find quantities burned and gained by subtraction, then judge a fair test and the reasons a school result is always too low.

(a) Mass of ethanol burned is the start mass minus the end mass: \( 92.40 - 91.75 = 0.65 \text{ g} \) [1]. Temperature rise is the final temperature minus the starting temperature: \( 49.0 - 21.0 = 28.0\ \degree\text{C} \) [1]. Always subtract in that order so the change is positive for heating.

(b) The 50 cm³ of water is measured with a measuring cylinder [1] (a pipette or burette is also acceptable, but a measuring cylinder is the standard glassware for this volume).

(c) To compare ethanol with propan-1-ol fairly, keep everything the same except the fuel (any three) [3]:

• use the same volume of water each time (50 cm³);
• use the same apparatus, for example the same boiling tube and thermometer;
• keep the distance from the burner flame to the tube the same;
• start at the same water temperature, and either heat until the same temperature rise then measure the mass burned, or burn for the same time then compare;

Only then does any difference in mass burned come from the fuel and not from the way it was heated.

(d) The measured energy is always lower than the true (data-book) value because heat is lost and combustion is imperfect (any two) [2]: heat is lost to the surrounding air instead of going into the water; heat is absorbed by the tube and thermometer; combustion is incomplete so soot forms and not all the fuel's energy is released; and some ethanol evaporates and escapes without burning. Each of these means less of the fuel's chemical energy reaches the water.