Grade 11 Physical Sciences · Chemical Change · Assign oxidation numbers

Assign oxidation numbers Grade 11

Work out the oxidation number of an atom in a molecule or ion (sulfur in H₂SO₄ is +6).

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Redox reactions: Assigning oxidation numbers Grade 11 ChemistryMiss Martins Maths and Science

How to Find Oxidation Numbers (Rules and Examples)Wayne Breslyn (Dr. B.)

How to Calculate Oxidation Numbers IntroductionTyler DeWitt

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Question 1.10 · September 2026 · Paper 2 · Provincial · 2 marks

1.10 The oxidation number of Cl in

HClO_{4}

is …(2)

A -1.
B +4.
C -7.
D +7.

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More past paper questions: assign oxidation numbers

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3. Question 6.2.3 · November 2019 · Paper 2 · National (DBE) · 3 marks
6.2 Consider the incomplete equation for the chemical reaction below.

$Zn(s) + H_{2} SO_{4} (aq) \to X + H_{2} (g)$

Answer the questions that follow.
6.2.3 Name the type of reaction illustrated above. Choose from ACID-BASE REACTION, PRECIPITATION REACTION or REDOX REACTION. Explain the answer.(3)
Show the memo (3 ticks)Memo
1. ✓
  RedoxRedox (reaction)
  1 mark · Describe ions and reactions in solution
2. ✓
  Charge of Zn in ZnThe charge of Zn changes from 0 in Zn
  1 mark · Assign oxidation numbers
3. ✓
  Charge of Zn in ZnSO₄to +2 in $ZnSO_{4}$ .
  1 mark · Assign oxidation numbers
4. Question 9.4.1 · November 2025 · Paper 2 · Provincial · 2 marks
A clean piece of aluminium metal strip is placed in a copper sulphate ( $CuSO_{4}$ ) solution. The reaction was observed again after 2 hours.

After 2 hours it was observed that the blue solution turned colourless and the shiny metallic aluminium turned brown.

The net ionic balanced equation for the reaction is given below:

$2 Al (s) + 3 Cu^{2 +} (aq) \to 3 Cu (s) + 2 Al^{3 +} (aq)$
9.4 Acidified potassium dichromate ( $K_{2} Cr_{2} O_{7}$ ) is an orange solution due to the presence of the dichromate ions ( $Cr_{2} O_{7}^{2 -}$ ).

Answer the questions that follow.
9.4.1 Determine the oxidation number of chromium (Cr) in $Cr_{2} O_{7}^{2 -}$ .(2)
Show the memo (2 ticks)Memo
1. ✓
  Set up the sum $Cr_{2} O_{7}^{2 -}$
  $2 x + 7 (- 2) = - 2$
  1 mark · Assign oxidation numbers
2. ✓
  Answer $x = + 6$
  1 mark · Assign oxidation numbers
5. Question 8.2.1 · November 2024 · Paper 2 · Provincial · 2 marks
8.2 The reaction between dichromate ions ( $Cr_{2} O_{7}^{2 -}$ ) and tin (II) ions ( $Sn^{2 +}$ ) in an acid medium is given below.

$Cr_{2} O_{7}^{2 -} + Sn^{2 +} + H^{+} \to Cr^{3 +} + Sn^{4 +} + H_{2} O (ℓ)$
8.2.1 Determine the oxidation number of Cr in $Cr_{2} O_{7}^{2 -}$ .(2)
Show the memo (2 ticks)Memo
1. ✓
  Set up the sum $2 x + 7 (- 2) = - 2$
  1 mark · Assign oxidation numbers
2. ✓
  Oxidation number $x = + 6$
  1 mark · Assign oxidation numbers
6. Question 9.2.1 · November 2023 · Paper 2 · Provincial · 2 marks
9.2 Sulphur dioxide gas ( $SO_{2}$ ) is bubbled into an acidified solution of potassium permanganate as shown in the diagram below.

It is observed that the solution turns from purple to colourless due to the reduction of $MnO_{4}^{-}$ ions to $Mn^{2 +}$ ions. During the reaction $SO_{2}$ is oxidised to sulphate ions, $SO_{4}^{2 -}$ .
9.2.1 Determine the oxidation number of sulphur, S in $SO_{4}^{2 -}$ .(2)
Show the memo (2 ticks)Memo
1. ✓
  Set up the sum $x + 4 (- 2) = - 2$
  1 mark · Assign oxidation numbers
2. ✓
  Answer $x = + 6$
  1 mark · Assign oxidation numbers

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