Chemical change

Electrochemistry

Galvanic vs electrolytic. Table of reduction potentials. E°cell = E°cathode − E°anode.

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
E°cell = E°cathode − E°anode. Standard cell potential
How a marker reads it
Write E°cathode − E°anode with the values in brackets, including the minus on zinc. 0.34 + 0.76 also works if you say why.

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

ZnZn²⁺ E° = −0.76 V and Cu²⁺ / Cu E° = +0.34 V. Find E°cell and which metal is oxidised.

Before any working — hunt first

Given

Numbers and units already in the sentence: 0.76 V · 0.34 V. 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: E°cell and which metal is oxidised. Circle that word — that is the missing variable.

Find

We are required to find: E°cell and which metal is oxidised. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write E°cell = E°cathode − E°anode with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives E°cell and which metal is oxidised.

Simplified approach

The more positive half is reduction (cathode): Cu. Zn is oxidised (anode). E°cell = 0.34 − (−0.76) = 1.10 V.

Culture note

A zinc-copper cell is the potato-clock trick at a science expo in Tshwane — same two half-cells.

Formulas

E°cell=E°cathode−E°anode

Standard cell potential

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

Pick the cathode

More positive E° is reduction: Cu²⁺ + 2e⁻ → Cu. That electrode is the cathode.

Electrochemistry / galvanic-electrolytic — Diagram

Cu²⁺+2e⁻→Cu

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
Pick the cathode
More positive E° is reduction: Cu²⁺ + 2e⁻ → Cu. That electrode is the cathode.
Cu²⁺+2e⁻→Cu
02
Oxidation
Zn → Zn²⁺ + 2e⁻. Anode. Electrons flow Zn to Cu in the wire.
Zn→Zn²⁺+2e⁻
03
Subtract
E°cell = E°cathode − E°anode = 0.34 − (−0.76) = 1.10 V. The two minuses become a plus.
E°cell=1.10 V
04
Step F — Box the answer with its unit
E°cell = 1.10 V. Zinc is oxidised.

More worked examples

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

Example 1 / 20

In a galvanic cell, zinc metal is the electrode written as Zn → Zn²⁺ + 2e⁻. Name the electrode and the process.

4. Step-by-step working

01
Identify the given
Electrons are on the product side. Zinc is losing them.
oxidation
02
Name the theorem
The electrode where oxidation happens is the anode.
anode

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: In a galvanic cell, zinc metal is the electrode written as Zn → Zn²⁺ + 2e⁻. Name the electrode and the process.

Before any working — hunt first

Given

Numbers and units already in the sentence: 2e⁻. 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: Name the electrode and the process. Circle that word — that is the missing variable.

Find

We are required to find: Name the electrode and the process. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write oxidation with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives Name the electrode and the process.

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

If you have never seen this before: Oxidation is loss of electrons. Oxidation happens at the anode. In a galvanic cell that anode is the negative electrode. 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?

Zinc loses two electrons. The cell is galvanic, so it runs by itself.

3. Which method — and why this one?

Oxidation is loss of electrons. Oxidation happens at the anode. In a galvanic cell that anode is the negative electrode.

4. Step-by-step working

01 / 05
Identify the given
Electrons are on the product side. Zinc is losing them.
oxidation
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
Name the theorem
The electrode where oxidation happens is the anode.
anode
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 galvanic anode is negative. An electrolytic anode is positive. This cell is galvanic.
negative anode
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
Oxidation at the anode. In this galvanic cell the anode is negative.
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

Oxidation at the anode. In this galvanic cell the anode is negative.

Copy onto your answer sheet — in this order

Formula: oxidation

Answer: Oxidation at the anode. In this galvanic cell the anode is negative.

6. Check it like a marker

Calling it reduction because zinc is a metal swaps the definition. Reduction is gain of electrons.

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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