DSE Chemistry — Redox, Oxidation Numbers and Titration Lesson-Prep Guide (for tutors)

DSE Chemistry — Redox, Oxidation Numbers and Titration Lesson-Prep Guide (for tutors) — half-equations and electron balance

2026-11-03

Tutor lesson-prep guides · Secondary Chemistry

Students can say “losing electrons is oxidation” but cannot tell which atom changes in permanganate or how many electrons it takes. This lesson enters through oxidation numbers and closes the loop with electron balance and titration.

Objectives — Students can: (1) assign oxidation numbers with the four rules; (2) write an acidic half-equation and balance the overall equation by equal electrons; (3) find an unknown concentration from titre volume and mole ratio with correct significant figures.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Mole concentration	Moles in 250 cm3 of 0.10 M solution	0.025 mol
Oxidation number of an element	Value for an uncombined element	Zero
Significant figures	How many in 25.0 cm3	Three

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Rules	Four rules demonstrated one by one	Find Mn, Cr and S values
10-24 min	Half-equations	Atoms, then charge, then electrons	Write the manganate half-equation
24-38 min	Balancing	Lowest common multiple of electrons	Fe2+ to MnO4- becomes 5 to 1
38-50 min	Titration	n = cV, mole ratio, divide by sample volume	Gives 0.100 M
50-56 min	Errors and end point	End point versus equivalence point	Explain the effect of one drop
56-60 min	Exit check	Three questions collected and marked	Independent

03 Teaching script (read it out)

Scene 1: Oxidation numbers: four rules cover everything

One: an uncombined element is zero. Two: numbers in a compound sum to zero, in a polyatomic ion to the charge. Three: oxygen is usually -2, but -1 in peroxides such as H2O2. Four: hydrogen is usually +1, and -1 in metal hydrides.
MnO4-: Mn + 4(-2) = -1 so Mn is +7. Cr2O72-: 2Cr + 7(-2) = -2 so Cr is +6.
Sulphur is +6 in H2SO4 but only +4 in SO2, so every case must be calculated rather than memorised.

Ask the student to repeat: write the sum equation first, then solve for the unknown.

Scene 2: Half-equations: count the electrons properly

MnO4- + 8H+ + 5e- gives Mn2+ + 4H2O: balance the metal, balance oxygen with water, balance hydrogen with H+, then balance charge with electrons. The left side is -1 + 8 = +7 and the right is +2, a difference of five, hence five electrons.
Fe2+ gives Fe3+ + e-, so multiplying by five gives MnO4- + 5Fe2+ + 8H+ giving Mn2+ + 5Fe3+ + 4H2O.
In neutral or alkaline solution the same ion takes only three electrons and forms MnO2, so the acidic condition must be stated.

Ask the student to repeat: manganese falls from +7 to +2, a drop of five, so five electrons move.

Scene 3: Titration: from mole ratio to concentration

With 0.0200 M KMnO4 and 25.0 cm3 used, moles of MnO4- are 5.00e-4. The ratio is 1 to 5, so Fe2+ is 2.500e-3 mol. The sample was 25.0 cm3, so the concentration is 0.100 M.
The iron mass is 2.500e-3 times 55.85, which is 0.140 g, or 5.59 g per litre. If the sample is FeSO4.7H2O, iron is 20.1 percent by mass.

Ask the student to repeat: moles are an amount, molarity is per litre, so always divide by the sample volume.

04 Question types and worked steps

Type	Example	Steps	Answer
Assign	Oxidation number	Write the sum equation and solve	Mn +7, Cr +6, oxygen -1 in peroxide
Balance	Acidic half-equation	Atoms, then H+, then electrons	Manganate takes 5 electrons
Calculate	Concentration from titration	n = cV, mole ratio, divide by volume	Fe2+ is 0.100 M
Explain	Oxidising and reducing agents	Watch the change in oxidation number	Manganate is reduced so it is the oxidising agent

Sanity check (teach this): The electron count always equals the total change in oxidation number: manganese drops by five and iron rises by one, so the ratio must be 1 to 5, and a 1 to 1 answer means the half-equation was copied wrongly. Titration answers need units: 2.500e-3 mol is an amount while 0.100 M is a concentration. Dichromate takes six electrons, so the same titre of dichromate oxidises 1.5 times as much iron as manganate.

05 Independent practice (3 questions)

Question	Answer	What to watch
Oxidation number of Mn in MnO4-	+7	Was the ion charge treated as zero?
Moles of Fe2+ oxidised by 25.0 cm3 of 0.0200 M KMnO4	2.500e-3 mol	Was the 1 to 5 ratio used?
Mass percentage of Fe in FeSO4.7H2O	20.1 percent	Were the seven waters included?

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating peroxide oxygen as -2	Writes -2	“The two oxygens in hydrogen peroxide are bonded, so each is -1.”
Giving manganate only two electrons	Balances 2 to 1	“In acid, manganese falls from +7 to +2, which takes five electrons.”
Reporting moles as concentration	Stops at 2.500e-3 mol	“Moles are an amount, molarity is per litre, so divide by the sample volume.”
Confusing end point with equivalence point	Uses the first pink tinge directly	“The end point is what you see; the equivalence point is theoretical, and one drop separates them.”

07 Homework (paper, section and question numbers)

DSE Chemistry practice paper (Section A and Section B) — Section B structured questions on redox and titration
Secondary chemistry chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Oxidation number of Cr in Cr2O72-	+6	Three correct means move on to electrochemistry
Electrons gained by MnO4- in acid	5	One wrong means reteach half-equations
Next step after finding 0.0025 mol	Divide by the sample volume to get concentration	Two wrong means restart act one

09 Tutor FAQ

Are oxidation numbers always whole numbers?

In simple compounds and ions yes, but average values can be fractions, such as +8/3 for iron in Fe3O4.

Can five electrons be assumed without the acid condition?

No. In neutral or alkaline solution manganate takes three electrons and forms MnO2.

Why repeat a titration three times?

To reduce random error; if the titres differ by more than 0.10 cm3, look for bubbles or an unrinsed burette.

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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.