Chemical change

Chemical equilibrium

Closed system, equal forward and reverse rates. Le Chatelier. K_c.

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
K_c = [C]ᶜ [D]ᵈ / [A]ᵃ [B]ᵇ. Equilibrium constant
How a marker reads it
Never write “the catalyst increases yield”. That sentence loses the mark in every province.

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.

Diagram

Worked example

N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g) ΔH < 0. Haber: how does high pressure and a catalyst affect yield?

Simplified approach

4 gas moles → 2. High pressure favours the side with fewer moles: more NH₃. A catalyst speeds both directions equally — same yield, faster.

Culture note

Sasol and the Haber process feed Limpopo maize with fertiliser — this is the equilibrium that makes ammonia.

Formulas

K_c=[C]ᶜ [D]ᵈ[A]ᵃ [B]ᵇ

Equilibrium constant

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 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.
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.
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 / 5

Count gas moles

Left: 1 + 3 = 4 mol gas. Right: 2 mol gas. High pressure → fewer moles → products.

Chemical equilibrium / le-chatelier — Diagram

Concentration-time graph for the Haber equilibrium with nitrogen added at t1 — Diagram

4 mol→2 mol

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.

More worked examples

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

Example 1 / 20

N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. State what happens to the yield of ammonia when the pressure is increased at constant temperature, and what a catalyst does to Kc.

4. Step-by-step working

01
Count the outcomes
4 mol of gas become 2 mol of gas.
right side has fewer moles
02
Apply the law
Increasing pressure shifts the equilibrium to the right. The yield of ammonia rises. Kc itself is not changed by pressure.
yield increases

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: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. State what happens to the yield of ammonia when the pressure is increased at constant temperature, and what a catalyst does to Kc.

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

If you have never seen this before: Higher pressure favours the side with fewer moles of gas. A catalyst speeds both directions and does not change Kc. 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?

4 mol of gas on the left, 2 mol on the right. Forward reaction exothermic. Catalyst added in a second part.

3. Which method — and why this one?

Higher pressure favours the side with fewer moles of gas. A catalyst speeds both directions and does not change Kc.

4. Step-by-step working

01 / 05
Count the outcomes
4 mol of gas become 2 mol of gas.
right side has fewer moles
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
Apply the law
Increasing pressure shifts the equilibrium to the right. The yield of ammonia rises. Kc itself is not changed by pressure.
yield increases
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 catalyst lowers the time to reach equilibrium. It does not change Kc or the yield.
Kc unchanged
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
Higher pressure increases the ammonia yield. A catalyst does not change Kc.
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

Higher pressure increases the ammonia yield. A catalyst does not change Kc.

Copy onto your answer sheet — in this order

Formula: right side has fewer moles

Answer: Higher pressure increases the ammonia yield. A catalyst does not change Kc.

6. Check it like a marker

Heating this mixture would shift it left, because the forward reaction is exothermic. Do not use that answer for a pressure change.

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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Acids and bases

Physical Sciences — Chemistry

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