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
Nearly half the students correctly calculated the enthalpy change and some of these went on to find the value in kJ g-1. The most common mistakes were to use the mass of starch rather than the mass of water and adding 273 to the temperature change.
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.