DP Chemistry (first assessment 2025)
Question 19M.2.SL.TZ1.c
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.SL.TZ1.c |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Calculate | Question number | c | Adapted from | N/A |
c.
[Maximum mark: 2]
19M.2.SL.TZ1.c
Calculate the energy released, in kJ g−1, when 3.49 g of starch are completely combusted in a calorimeter, increasing the temperature of 975 g of water from 21.0 °C to 36.0 °C. Use section 1 of the data booklet.
[2]
Markscheme
q = «mcΔT = 975 g × 4.18 J g–1 K–1 × 15.0 K =» 61 100 «J» / 61.1 «kJ» [✔]
«heat per gram = =» 17.5 «kJ g–1» [✔]
Note: Award [2] for correct final answer.
Examiners report
The incorrect mass was frequently used when calculating energy released from combustion of starch in a calorimeter. Those who used the mass of water correctly frequently stopped when energy in kJ or J was calculated, and did not seem to notice that the question asked for the energy to be calculated in kJg−1 so a further calculation was required.
Syllabus sections
Reactivity 1. What drives chemical reactions? » Reactivity 1.1—Measuring enthalpy changes » Reactivity 1.1.4—The standard enthalpy change for a chemical reaction, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states. It can be determined from the change in temperature of a pure substance. Apply the equations Q = mcΔT and ΔH = − Qn in the calculation of the enthalpy change of a reaction.