HKDSE Chemistry — Rate of Reaction Lesson-Prep Guide (for tutors) | average rate, collision theory, graph slope

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students can recite that heat speeds things up, but freeze when asked how much faster and how you measure it. This lesson ties every factor to collision frequency and to a number they can compute.

Objectives — Students can: 1) compute an average rate as change in quantity divided by time, with the right unit (g s-1, cm3 s-1, mol dm-3 s-1); 2) explain how temperature, concentration, surface area and a catalyst change the rate through collision theory; 3) state that a catalyst changes rate, not yield.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Unit conversion	How many dm3 is 72.0 cm3?	0.072 dm3
Concentration	Concentration of 0.30 mol in 1.5 dm3	0.20 mol dm-3
Significant figures	Round 0.0450 g s-1 to two s.f.	0.045 g s-1

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Hook question	Ask whether crushed or whole magnesium reacts faster with acid, and why; take a show of hands	Vote and give one reason
5-15 min	Prerequisite check	Hand out three quick items; ask students to read out answers with units	Write answers, fix units
15-35 min	Three-act script	Deliver the rate formula, collision theory and the four factors; ask one student to restate each act	Restate each act in one sentence
35-45 min	Worked examples	Model one mass-loss question and one gas-volume question, numbering each step aloud	Copy steps and add units
45-55 min	Independent practice	Circulate; only ask whether the equation is written down yet	Do three items, self-check
55-60 min	Exit check	Collect the three answers and count wrong ones on the spot	Written work, no discussion

03 Teaching script (read it out)

Scene 1: Act 1: rate is quantity divided by time

“A rate is not a feeling, it is a number: the change you measured divided by the time it took. A marble chip goes from 5.00 g to 3.20 g in 40 s. First find the change, 1.80 g, then divide by 40 s, giving 0.045 g per second. Two things to watch: the unit follows what you measure, and this is an average over the whole period, so the early part is always faster.”

Which two quantities are divided, and what decides the unit?

Scene 2: Act 2: collision theory explains the speed-up

“Every factor comes down to one idea: how often reacting particles collide per second, and how hard. Higher temperature means higher average kinetic energy, so more collisions clear the activation energy. Higher concentration means more particles in the same volume, so more collisions. Greater surface area exposes more particles to the other reactant. A catalyst opens a different route with a lower activation energy. After each factor, say what changed about the collisions.”

What does each of the four factors change about collisions?

Scene 3: Act 3: catalysts and graph slopes

“A catalyst does not give you more product, it only gets you there sooner: the mass-loss curve ends at the same height with a steeper slope. As a rule of thumb the rate doubles for every 10 degrees C, so 20 to 50 C is 3 steps and roughly 8 times faster. Write roughly, because this is an empirical rule, not a law. On a graph, the slope is the rate; a flattening curve means the reactant is running out.”

Which part of the curve does a catalyst change, the slope or the final height?

04 Question types and worked steps

Type	Example	Steps	Answer
Average rate from mass loss	A 5.00 g chip weighs 3.20 g after 40 s. Find the average rate.	1) Subtract for a change of 1.80 g; 2) divide by 40 s; 3) state g s-1.	0.045 g s-1
Average rate from gas volume	72.0 cm3 of gas is collected in 24.0 s. Give the rate in cm3 s-1 and dm3 s-1.	1) 72.0 / 24.0 = 3.00; 2) convert to 0.072 dm3; 3) divide by time again.	3.0 cm3 s-1 = 0.0030 dm3 s-1
Rate from concentration change	A solution falls from 0.50 to 0.20 mol dm-3 in 40 s. Find the rate.	1) Change is 0.30; 2) divide by 40 s; 3) unit mol dm-3 s-1.	0.0075 mol dm-3 s-1
Comparing conditions	At 20 C the rate is 0.020 mol dm-3 s-1. Estimate it at 50 C.	1) Count 3 steps of 10 C; 2) double each time; 3) total factor 8.	about 0.16 mol dm-3 s-1

Sanity check (teach this): Sanity check: 0.045 g s-1 multiplied by 40 s must return 1.80 g. After multiplying by 8 the rate must be larger (0.020 x 8 = 0.160). In any catalyst question the final gas volume must equal the uncatalysed run; a different number means you changed the wrong factor.

05 Independent practice (3 questions)

Question	Answer	What to watch
Zinc granules go from 4.80 g to 3.00 g in 60 s. Find the average rate.	0.030 g s-1	Did they subtract before dividing, and write s-1?
96 cm3 of hydrogen is collected in 32 s. Find the rate in cm3 s-1.	3.0 cm3 s-1	Is time in the denominator, not the numerator?
The temperature rises from 15 to 35 C. By what factor does the rate change?	About 4 (two 10 C steps, 2 x 2)	Did they count the 10 C steps correctly?

06 Common mistakes and what to say

Mistake	Student writes	Say this
Inverted fraction	Writing 40 divided by 1.80	Rate is change divided by time, not time divided by change, so the answer should be below one here.
Wrong unit	Using mol dm-3 s-1 for a mass question	You measured grams, so the unit must carry grams: g s-1.
Catalyst increases yield	Saying a catalyst produces more gas	A catalyst is faster, not more. Look at the graph: the final height is the same.
Instantaneous used for average	Quoting the initial rate for the whole period	The question asks for the whole period, so use total change over total time.

07 Homework (paper, section and question numbers)

Secondary Chemistry chapter index (bonding, rates, acids and bases) — Work the rates chapter first
Lesson-prep overview — All subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
6.00 g falls to 3.60 g in 80 s. Average rate?	0.030 g s-1	Wrong: reteach the order of operations in Act 1
120 cm3 in 40 s. Rate?	3.0 cm3 s-1	Wrong: repeat unit tracking
From 25 to 45 C, by what factor does the rate rise?	About 4 (two steps)	Two or more wrong: return to collision theory

09 Tutor FAQ

Which year groups is this for?

S5 second term through S6; every DSE Paper 1 rates question fits this frame.

Is 60 minutes enough?

Yes: 20 minutes of teaching, the rest is student work and checking.

Is there practice attached?

The homework row links to the site chemistry index, so the rates questions can be worked in order.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.