The method and the results are on the energy changes lab. Do the analysis yourself first, then check the answers below.
Quick answer: An exothermic reaction releases energy to the surroundings and its temperature rises; an endothermic reaction absorbs energy and its temperature falls. These answers go with the Chemistry 9-10 lab on energy changes: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Use the sample table to check your working. The signs matter: a temperature fall means energy was taken in, so ΔH is positive. Your own results will differ, but the order of the three salts and the large size of the calcium chloride value should appear in any careful data set.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-4
1. (26 − 20) ÷ 26 × 100 = 23%. 2. Energy flows into the cup from the warmer room during the experiment and the thermometer reading lags, so the temperature drop is smaller than it should be. About 23% smaller.
Question 6 Sources of error
The polystyrene and lid reduce energy transfer with the room, so ΔT is closer to the true value. A further error is that the water has a different mass than the 25 g assumed once the salt dissolves, or that the thermometer has limited accuracy. Insulation reduces energy exchange.
Question 7 Conclusion and use
Ammonium nitrate gives the largest fall and suits a cold pack; calcium chloride gives the largest rise and suits a hand warmer, although it is an irritant. Improve the method by adding a second lid layer or by repeating each test and taking a mean. Cold pack: ammonium nitrate; warmer: calcium chloride.
