2.2.1—Rate of reaction
- Syllabus
- First assessment 2025
- Objective
- 2.2.1
- Level
- SL
Reaction Rate
rate=changeinconcentration/time
An instantaneous rate is the gradient of a tangent at the stated point on a concentration–time, volume–time or mass–time graph. Keep the units consistent.
Choose two well-separated points on the tangent, not on the curve, to calculate its gradient. A reactant concentration has a negative gradient, so report its disappearance rate as a positive magnitude unless a signed change is requested.
| Requested rate | Graph operation | Evidence check |
|---|---|---|
| mean over an interval | secant gradient between interval endpoints | quote the interval and units |
| initial | tangent gradient at t = 0 | choose well-separated points on the tangent |
| instantaneous at time t | tangent gradient at that time | do not use two points on the curved trace |
A measured mass, pressure or gas volume is a rate proxy only when its change is tied to reaction progress under the stated conditions. Preserve reactant/product slope sign or report a positive disappearance/formation magnitude as requested.
Worked tangent example: on a concentration–time graph for HCl, two points on the tangent at t=0 are (0s,0.250moldm−3) and (14s,0.100moldm−3). The tangent gradient is (0.100−0.250)/(14−0)=−0.0107moldm−3s−1. For Mg+2HClMgClX2+HX2, divide the positive disappearance-rate magnitude by the HCl coefficient: v=0.0107/2=0.0054moldm−3s−1.
Finding an Instantaneous Rate
Assessment in practice
Representative question
Question 1
[Maximum number: 3]
Determine the instantaneous rate of reaction to two significant figures when [Br2]=0.0080 moldm−3.
tangent drawn on curve at 0.0080 moldm−3 «Rate =ΔtΔ[Br2]= » 2.8×10−5≪ moldm−3 s−1≫
two significant figures in final answer
Marking guidance:
Accept range of 2.6×10−5 to 3.1×10−5 « moldm−3 s−1 ».
Award [2 max] for 3.3×10−5∥moldm−3 s−1».
Award [1 max] for 3.33×10−5 « moldm−3 s−1 ».
Ignore negative sign for M2.
Award M3 for any numerical result with 2 significant figures.
Rate and Mechanisms Summary
Retrieve the route: measure a tangent rate, explain effective collisions, map rate factors, read Ea and energy profiles, evaluate mechanisms, determine molecularity and orders, calculate k, then use Arrhenius gradient and intercept for Ea and A.
Check tangent versus average slope, energy versus orientation, barrier labels, intermediate versus transition state, one-variable trial comparisons, order-dependent units, kelvin temperature and the signs of gradient and Ea.