DP Chemistry (first assessment 2025)
Question 19M.2.SL.TZ2.b
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.SL.TZ2.b |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Outline | Question number | b | Adapted from | N/A |
b.
[Maximum mark: 2]
19M.2.SL.TZ2.b
Outline, in terms of collision theory, how a decrease in pressure would affect the rate of reaction.
[2]
Markscheme
«rate of reaction decreases»
concentration/number of molecules in a given volume decreases
OR
more space between molecules [✔]
collision rate/frequency decreases
OR
fewer collisions per unit time [✔]
Note: Do not accept just “larger space/volume” for M1.
Examiners report
Few students were able to relate the effect of reduced pressure at constant volume with a decrease in concentration of gas molecules and mostly did not even refer to this, but rather concentrated on lower rate of reaction and frequency of collisions. Many candidates lost a mark by failing to explain rate as collisions per unit time, frequency, etc.
Syllabus sections
Structure 1. Models of the particulate nature of matter » Structure 1.5—Ideal gases » Structure 1.5.1—An ideal gas consists of moving particles with negligible volume and no intermolecular forces. All collisions between particles are considered elastic. Recognize the key assumptions in the ideal gas model.