DSE Chemistry | Rates of Reaction Tutor Prep Guide | Collision Theory and Rate Calculations

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students hear the word rate and write faster, but marks come from a number with a unit plus a reason. This lesson welds the two together: every explanation must return to collision frequency or to the fraction of effective collisions.

Objectives — Students can: (1) compute an average rate from data and read a tangent for an instantaneous rate; (2) explain the effects of concentration, temperature, surface area, catalyst and pressure using collision theory; (3) design a controlled experiment comparing two rates; (4) judge from gas volume data whether a reaction has finished.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Moles and gas volume	Volume of 1 mol of gas at room conditions	about 24000 cm3
Slope	What the slope of a concentration-time graph means	the rate, with a minus sign
Fair test	How many variables may change between two runs	one

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Defining rate	change per unit time	0.40 cm3 s-1
8-22 min	Average and instantaneous	two points against a tangent	the first 30 s is faster
22-36 min	Collision theory	two conditions for an effective collision	energy and orientation
36-48 min	The five factors	frequency or energy, for each	30 C gives 8 times
48-56 min	Experiment design	change one variable only	name the control
56-60 min	Exit check	three questions collected	independent work

03 Teaching script (read it out)

Scene 1: A rate always carries a unit, and a time interval

Magnesium reacts with dilute hydrochloric acid: 24 cm3 of gas is collected in the first 30 s and 48 cm3 in 120 s. The first-30-second average is 24 over 30 = 0.80 cm3 s-1, while the whole run averages 48 over 120 = 0.40 cm3 s-1.
Both numbers are right; the interval differs. Concentration is highest at the start, so the instantaneous rate is largest and the curve flattens. Ask the student: why must the whole-run average be smaller than the first 30 s? Student task: compute the average between 30 s and 120 s (0.27 cm3 s-1).

Scene 2: Collision theory: only fast enough and well aimed collisions work

Molecules collide constantly yet most collisions achieve nothing. Raising the temperature lifts the average kinetic energy, so far more molecules exceed the activation energy, which is why the rate climbs faster than concentration alone would suggest. A rise of 10 C roughly doubles the rate, so 20 C to 50 C is 3 such steps, about 8 times.
Ask the student: does a catalyst release more heat? (No, it lowers the activation energy and leaves delta H unchanged.) Student task: state which reactions pressure affects (those with gaseous reactants).

Scene 3: The five factors in one list: concentration, temperature, area, catalyst, pressure

Each answer must say whether collision frequency or the fraction of effective collisions changed. Concentration, pressure and surface area raise the frequency; temperature raises both frequency and the energy fraction; a catalyst opens a path with a lower activation energy. With 0.50 g of magnesium the maximum yield is about 494 cm3 of hydrogen (0.50 over 24.3 times 24000), so 48 cm3 at 120 s means the reaction is still running and the curve has not flattened. Ask the student: does powdering the magnesium change the final gas volume? (No, only how fast it gets there.) Student task: write the powder against ribbon experiment and name three conditions to hold fixed.

04 Question types and worked steps

Type	Example	Steps	Answer
Average rate	gas volumes at two times	delta V over delta t, with units	0.40 cm3 s-1
Explain a factor	one change given	frequency or fraction of effective collisions	heating raises both
Read a gradient	concentration-time curve	tangent, slope taken negative	steepest at the start
Design an experiment	one variable to compare	hold everything else fixed	same mass, concentration and temperature

Sanity check (teach this): Rates need units, and cm3 s-1 cannot be swapped for mol dm-3 s-1 without converting. A rate from volume data is an average over that interval, never an instantaneous value. The ten-degree rule is an approximation for simple reactions near room temperature. The maximum gas volume here is about 494 cm3, so any reading above that points to a leak or a misread. A catalyst shifts neither the position of equilibrium nor delta H.

05 Independent practice (3 questions)

Question	Answer	What to watch
Average rate when 24 cm3 collects in the first 30 s	0.80 cm3 s-1	delta V over delta t
Average rate over 120 s when 48 cm3 collects	0.40 cm3 s-1	same formula, longer interval
Rate multiple when 20 C rises to 50 C, doubling every 10 C	8 times	two cubed

06 Common mistakes and what to say

Mistake	Student writes	Say this
Giving a rate with no unit	answers 0.4	A rate is an amount per second, so a bare number loses the mark.
Treating the average as instantaneous	says the rate at 120 s is 0.40	The whole-run average is not the final rate; the curve is almost flat by then.
Heating only makes particles move faster	never mentions activation energy	Say that more particles now carry enough energy to pass the activation energy.
Saying a catalyst raises the yield	answers more product	A catalyst only arrives at the same end point sooner.

07 Homework (paper, section and question numbers)

Secondary Chemistry chapter index — rates set, questions 3 and 4
Prep guide home — other chapters in the subject
DSE Chemistry practice papers — rates set, questions 1 to 5

08 Exit check (last 5 minutes)

Question	Answer	Decision
Average rate when 24 cm3 collects in 30 s	0.80 cm3 s-1	three correct means move on to collision theory
Rate multiple for a 30 C rise	about 8 times	one wrong means reteach the ten-degree rule
Effect of a catalyst on the final yield	none	two or more wrong means back to act one

09 Tutor FAQ

Why does temperature matter more than concentration?

Heating raises both collision frequency and the fraction of molecules above the activation energy, and that fraction grows exponentially.

How is an initial rate read from a curve?

Draw the tangent at time zero, take the magnitude of its slope and state the units.

Why does surface area not change the final volume?

The final volume is set by the moles of the limiting reactant; area only changes how long the reaction takes.

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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.