Chemical equilibrium is where the NSC exam is won or lost in Paper 2. The chemistry section of Paper 2 carries about 92 marks. Read that number again. Ninety-two. And equilibrium is right at the heart of it, carrying around 20 to 25 marks. Students who do not understand Le Chatelier's Principle properly leave those marks behind. Not because the questions are hard. Because they memorised the principle without understanding it. And memorising without understanding is a recipe for disaster when the DBE changes the wording even slightly. This post fixes that.
In This Post You Will Learn
✓ What chemical equilibrium actually means and what a reversible reaction looks like
✓ The meaning of Kc and how to write an equilibrium expression
✓ Le Chatelier's Principle in plain English and how to apply it to every scenario
✓ What happens when you change concentration, temperature, or pressure
✓ Why catalysts do NOT shift equilibrium (and why the exam keeps asking this)
✓ How to handle equilibrium calculations step by step
What is Chemical Equilibrium?
Most chemical reactions you learned about in earlier grades go in one direction. Reactants turn into products. Done.
But some reactions go both ways. The products can turn back into reactants at the same time. These are called reversible reactions.
Chemical equilibrium is the point where the forward reaction and the reverse reaction happen at the same rate.
Critical point that students miss: At equilibrium, the reaction has NOT stopped. Both reactions are still happening. Reactants are still becoming products. Products are still becoming reactants. But they are doing it at exactly the same speed, so the overall amounts do not change.
This is called a dynamic equilibrium. Dynamic means things are still moving. Equilibrium means the net result is balanced.
Forward reaction: A + B → C + D (happening)
Reverse reaction: C + D → A + B (also happening)
At equilibrium: Rate forward = Rate reverse
Concentrations stay constant (but NOT necessarily equal)The Equilibrium Constant (Kc)
For any reversible reaction at equilibrium, we can write an equilibrium expression using the concentrations of products and reactants.
For the general reaction: aA + bB ⇌ cC + dD
[C]^c x [D]^d
Kc = ─────────────────────
[A]^a x [B]^bProducts on top. Reactants on bottom. Each concentration raised to the power of its coefficient.
Example: Write the Kc expression for:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
[NH₃]²
Kc = ─────────────
[N₂][H₂]³What Does the Value of Kc Tell You?
| Kc Value | What It Means |
|----------------|-------------------------------------------|
| Kc >> 1 (large) | Products are favoured. Lots of product at equilibrium. |
| Kc << 1 (small) | Reactants are favoured. Very little product formed. |
| Kc ≈ 1 | Neither side strongly favoured. Roughly equal amounts. |Important: Kc only changes when TEMPERATURE changes. Changing concentration or pressure does NOT change Kc. The equilibrium shifts, the concentrations adjust, but the ratio stays the same. This is a common exam question.
Le Chatelier's Principle: The One Rule That Runs Everything
Here it is in plain English.
If you disturb a system at equilibrium, the system will shift in the direction that reduces the disturbance.
Think of it like a stubborn system. Whatever you do to it, it pushes back. Add something? It uses it up. Remove something? It makes more. Heat it up? It absorbs the heat. The system always fights the change.
Now let us apply this to every possible scenario the NSC can throw at you.
What Happens When You Change Concentration
Adding More Reactant
You add more of a reactant. There is now "too much" reactant. The system shifts FORWARD (to the right) to use up the extra reactant and make more product.
Removing a Product
You remove some product. There is now "too little" product. The system shifts FORWARD (to the right) to make more product.
Adding More Product
You add more product. Too much product. The system shifts in REVERSE (to the left) to use up the extra product and make more reactant.
Removing a Reactant
You remove some reactant. Too little reactant. The system shifts in REVERSE (to the left) to make more reactant.
| Change | Direction of Shift | Effect on Products |
|----------------------|-------------------|--------------------|
| Add reactant | Forward (right) | More products |
| Remove reactant | Reverse (left) | Less products |
| Add product | Reverse (left) | More reactants |
| Remove product | Forward (right) | More products |For full live lessons on this topic, see our Grade 12 Physical Science tuition page.
What Happens When You Change Temperature
This is where students start making errors. Temperature changes depend on whether the reaction is exothermic or endothermic.
First, you need to know which direction releases heat and which absorbs it. The question will tell you. Look for ΔH values or the word "exothermic"/"endothermic."
Think of heat as a product (in exothermic reactions) or as a reactant (in endothermic reactions). Then apply Le Chatelier's Principle exactly like you would for concentration changes.
Exothermic Reaction (ΔH < 0): Heat is a "product"
A + B ⇌ C + D + heat
Increase temperature (add heat): System shifts LEFT (reverse) to absorb the extra heat. Fewer products. Kc decreases.
Decrease temperature (remove heat): System shifts RIGHT (forward) to replace the heat. More products. Kc increases.
Endothermic Reaction (ΔH > 0): Heat is a "reactant"
heat + A + B ⇌ C + D
Increase temperature (add heat): System shifts RIGHT (forward) to absorb the extra heat. More products. Kc increases.
Decrease temperature (remove heat): System shifts LEFT (reverse) to replace the heat. Fewer products. Kc decreases.
| Change | Exothermic Reaction | Endothermic Reaction |
|--------------------|----------------------|----------------------|
| Increase temp | Shift LEFT, Kc ↓ | Shift RIGHT, Kc ↑ |
| Decrease temp | Shift RIGHT, Kc ↑ | Shift LEFT, Kc ↓ |Temperature is the ONLY factor that changes Kc. The exam will try to trick you. If they ask "does Kc change when concentration is added?" the answer is NO. Kc only changes with temperature.
What Happens When You Change Pressure (Gases Only)
Pressure changes only affect reactions involving gases. And they only matter when there are different numbers of gas moles on each side.
Increase pressure: The system shifts to the side with FEWER moles of gas.
Decrease pressure: The system shifts to the side with MORE moles of gas.
Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Left side: 1 + 3 = 4 moles of gas
Right side: 2 moles of gas
Increase pressure? System shifts RIGHT (towards 2 moles, fewer gas molecules).
Decrease pressure? System shifts LEFT (towards 4 moles, more gas molecules).
What if Both Sides Have Equal Moles of Gas?
Then pressure changes have NO effect on the equilibrium position. The system has nowhere to shift to reduce the pressure because both sides are equal.
Example: H₂(g) + I₂(g) ⇌ 2HI(g)
Left: 2 moles. Right: 2 moles. Changing pressure does nothing.
The Catalyst Question (They Ask This Every Year)
A catalyst does NOT shift the equilibrium. It does NOT change Kc. It does NOT change the amounts of products or reactants at equilibrium.
What a catalyst does: it speeds up BOTH the forward and reverse reactions equally. The system reaches equilibrium faster, but the equilibrium position is the same.
The exam asks this almost every year in some form. "Does a catalyst affect the equilibrium position? Explain." The answer is always no, and the reason is always that it speeds up both reactions equally.
Equilibrium Calculations: The ICE Table Method
For calculation questions, use an ICE table. ICE stands for Initial, Change, Equilibrium.
Worked Example
At a certain temperature, the following reaction reaches equilibrium:
H₂(g) + I₂(g) ⇌ 2HI(g)
Initially, 1 mol of H₂ and 1 mol of I₂ are placed in a 1 dm³ container. At equilibrium, 0.8 mol of HI is present. Calculate Kc.
H₂ + I₂ ⇌ 2HI
Initial: 1 1 0
Change: -0.4 -0.4 +0.8
Equilibrium: 0.6 0.6 0.8How did I get the changes? 0.8 mol of HI was formed. The ratio is 1:1:2. So for every 2 mol HI formed, 1 mol H₂ and 1 mol I₂ are used. Half of 0.8 = 0.4.
Since volume = 1 dm³, concentration = moles (convenient).
[HI]² (0.8)² 0.64
Kc = ──────────── = ──────────── = ────── = 1.78
[H₂][I₂] (0.6)(0.6) 0.36Kc = 1.78
If you want to build a strong physics foundation alongside your chemistry, read our guide on Doppler Effect and Waves Grade 12 - Everything Explained.
Common Mistakes Students Make
- Saying the reaction "stops" at equilibrium
It does not stop. Both reactions are still happening. Equilibrium is dynamic. The rates are equal, so the concentrations stay constant, but the reactions continue. If you write "the reaction stops" in the exam, you lose the mark.
- Thinking Kc changes when concentration changes
Kc does NOT change when you add or remove reactants or products. The equilibrium shifts, the concentrations adjust, but the Kc value stays the same. Kc only changes with temperature. This is tested almost every year.
- Forgetting to use molar ratio in ICE table calculations
If the equation says 2HI, and 0.8 mol of HI is formed, then the change for H₂ is 0.4 (half of 0.8), not 0.8. Students forget the stoichiometric ratio and use the same number for all species. Always go back to the balanced equation and check the ratios.
- Confusing "shift left" with "Kc decreases"
A shift left because you added product does NOT decrease Kc. The equilibrium adjusts, but Kc stays the same. Only a temperature change alters Kc. "Shift direction" and "Kc change" are two different things. The exam will test whether you know the difference.
- Getting the pressure effect backwards
Increase pressure = shift to fewer moles. Students sometimes think "increase pressure = more force = more product" which is wrong. The system goes to the side that relieves the pressure, which is the side with fewer gas molecules.
How This Topic Appears in the NSC Exam
Chemical equilibrium appears in Paper 2 of the Grade 12 Physical Science NSC exam.
It typically carries between 20 and 25 marks, making it one of the biggest chemistry topics in Paper 2.
This topic usually appears as Question 7 or Question 8 in Paper 2. The DBE structures it with a good mix of theory and calculations.
A typical question gives you a reversible reaction and asks you to:
Write the Kc expression (2 marks). State Le Chatelier's Principle in words (2 marks). Predict the direction of the shift when a specific change is made (2 marks each, usually 3 scenarios). Explain the effect on Kc for each change (2 marks). Calculate Kc using an ICE table or given equilibrium concentrations (4 to 6 marks).
In the 2023 NSC exam, chemical equilibrium included a Le Chatelier's question with three different disturbances (concentration change, temperature change, and addition of a catalyst), followed by a Kc calculation using equilibrium concentrations provided in the question.
Typical mark breakdown:
| Question Type | Marks |
|----------------------------------------|-------|
| Write Kc expression | 2 |
| State Le Chatelier's Principle | 2 |
| Predict shift for concentration change | 2 |
| Predict shift for temperature change | 2-3 |
| Explain effect of catalyst | 2 |
| Effect on Kc | 2-3 |
| Kc calculation | 4-6 |
| Graph interpretation | 2-3 |
| TOTAL | 20-25 |The theory marks here are the easiest marks in all of chemistry. Learn the CAPS definitions word for word. Practise predicting shifts using the table method above. Then nail the ICE table calculation. That is 20+ marks locked in.
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