Chemical change

Rates of reaction

Collision theory: particles must collide with enough energy and the right orientation. Temperature, concentration, surface area and a catalyst change the rate, not the balanced equation.

CAPS

Grade 12 · Physical Sciences (Chemistry) · Chemical change

Chemistry is Physical Sciences Paper 2.

The paper
Grade 12 Paper 2 covers quantitative chemistry, energy and rates, chemical equilibrium, acids and bases, electrochemical cells, and organic nomenclature, reactions and polymers.
Working
Balance before you use a mole ratio. Show n = mM or c = nV with units. An IUPAC name needs the number on the functional group. A reaction that is not balanced cannot support a stoichiometric answer.
The relation
rate = Δc / Δt. Change in concentration over time
How a marker reads it
A catalyst lowers E_a. It is not used up, and it does not change the yield of a single reaction or the balanced equation. Say that, or the factor mark is lost.

Paper 2 is 150 marks. Tables of ions and the data sheet are part of the exam. Significant figures follow the data you were given.

Worked example

In 20 s the concentration of a reactant falls from 0.80 mol·dm⁻³ to 0.30 mol·dm⁻³. Calculate the average rate. Then say why warming the flask makes that rate larger, using collision theory.

Before any working — hunt first

Given

Numbers and units already in the sentence: 20 s · 0.80 mol·dm⁻³ · 0.30 mol·dm⁻³. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: the average rate. Circle that word — that is the missing variable.

Find

We are required to find: the average rate. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write rate = Δc / Δt with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives the average rate.

Simplified approach

Rate is how fast concentration changes. Heat does not add particles. It makes more collisions reach the activation energy.

Culture note

A zinc granule in a beaker of acid in a Durban lab fizzes slowly. Powdered zinc in the same acid fizzes hard — same chemistry, more surface, more collisions.

Formulas

rate=ΔcΔt

Change in concentration over time

Exam tricks you must know

These are the moves markers look for — not shortcuts that skip the method, but the habits that earn the method mark.

Never change a subscript to balance — only coefficients
How:The given formula is a substance. Fe₂O₃ is not FeO. Count atoms on each side. Put a number in front (coefficient) until both sides match. Changing a subscript invents a different compound and zeros the question.
When to use it:Balancing equations and mole-ratio questions.
Never write “the catalyst increases yield” — that sentence loses the mark in every province
How:A catalyst speeds both the forward and reverse rates equally. Equilibrium is reached faster; the yield (position of equilibrium) does not change. Write: “increases rate, yield unchanged.” On Haber, add the compromise sentence: rate versus yield.
When to use it:Rates, equilibrium, Haber and Contact processes.
pH to two decimals, with −log and square brackets — “it is strong because pH is 2” is a trap
How:Write pH = −log[H₃O⁺] as the formula line. Substitute the concentration with a division line if you had to convert cm³. Box the answer to two decimal places. Strength is about complete ionisation, not the pH number alone.
When to use it:Strong/weak acids, K_w, and titration pH.

Step 1 / 6

Average rate

The drop is 0.80 − 0.30 = 0.50 mol·dm⁻³. Rate = 0.5020=0.025 mol·dm⁻³·s⁻¹.

Rates of reaction and collision theory — Diagram

rate=0.5020=0.025 mol·dm⁻³·s⁻¹

Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

4. Step-by-step working

01
Average rate
The drop is 0.80 − 0.30 = 0.50 mol·dm⁻³. Rate = 0.5020=0.025 mol·dm⁻³·s⁻¹.
rate=0.5020=0.025 mol·dm⁻³·s⁻¹
02
Step F — Box the answer with its unit
Average rate = 0.025 mol·dm⁻³·s⁻¹. Heat increases the fraction of collisions that reach E_a.

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

The concentration of a reactant falls by 0.50 mol·dm⁻³ in 10 s. Calculate the average rate.

4. Step-by-step working

01
Write the formula
rate=ΔcΔt.
rate=ΔcΔt
02
Divide
0.5010=0.05.
0.05

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: The concentration of a reactant falls by 0.50 mol·dm⁻³ in 10 s. Calculate the average rate.

Before any working — hunt first

Given

Numbers and units already in the sentence: 0.50 mol·dm⁻³ · 10 s. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: the average rate. Circle that word — that is the missing variable.

Find

We are required to find: the average rate. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write rate = Δc / Δt with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives the average rate.

In other words — as if you have never seen this before

If you have never seen this before: Average rate of consumption is the change in concentration divided by the time. Quote the unit. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

Δc=0.50 mol·dm⁻³, Δt=10 s.

3. Which method — and why this one?

Average rate of consumption is the change in concentration divided by the time. Quote the unit.

4. Step-by-step working

01 / 05
Write the formula
rate=ΔcΔt.
rate=ΔcΔt
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Divide
0.5010=0.05.
0.05
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Sense-check
A steeper drop in the same time would be a larger rate. The unit is mol·dm⁻³·s⁻¹.
0.05 mol·dm⁻³·s⁻¹
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
04 / 05
Step F — Box the answer with its unit
0.05 mol·dm⁻³·s⁻¹
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

0.05 mol·dm⁻³·s⁻¹

Copy onto your answer sheet — in this order

Formula: rate=ΔcΔt

Answer: 0.05 mol·dm⁻³·s⁻¹

6. Check it like a marker

Dividing time by the concentration inverts the rate. The fall in concentration is the top of the fraction.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

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