Grade 12 Physical Sciences · Chemical Change

Combine half-reactions into net equation

Combine two half-reactions so the electrons cancel to give the balanced overall reaction (Zn + Cu²⁺ → Zn²⁺ + Cu).

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Galvanic Cell Electrochemistry: Net Overall Cell Reaction | Grade 12 ChemistryMiss Martins Maths and Science

How to Balance Redox Equations in Acidic SolutionTyler DeWitt

Balancing Redox Reactions in Acidic and Basic ConditionsProfessor Dave Explains

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Question 8.2.1 · November 2024 · Paper 2 · National (DBE) · 3 marks

8.2 A galvanic cell is represented by the following cell notation:

Pb(s) ∣ Pb^{2 +} (aq) ∥ Fe^{3 +} (aq), Fe^{2 +} (aq) ∣ Pt(s)

Answer the questions that follow.

8.2.1 Write down the balanced net ionic equation for this cell.(3)

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: combine half-reactions into net equation

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3. Question 9.2.4 · May/June 2022 · Paper 2 · National (DBE) · 3 marks
The simplified diagram below represents an electrochemical cell used for the electrolysis of a concentrated sodium chloride solution, NaC $ℓ$ (aq). X and Y are carbon electrodes.
9.2 Chlorine gas, $C ℓ_{2}$ (g), is released at electrode X.

Write down the:
9.2.4 Balanced equation for the net (overall) cell reaction that takes place in the cell(3)
Show the memo (3 ticks)Memo
1. ✓
  Reactants $2 H_{2} O (ℓ) + 2 C ℓ^{-} (aq)$
  1 mark · Combine half-reactions into net equation
2. ✓
  Products $\to C ℓ_{2} (g) + H_{2} (g) + 2 OH^{-} (aq)$
  1 mark · Combine half-reactions into net equation
3. ✓
  Balancing $2 H_{2} O (ℓ) + 2 C ℓ^{-} (aq) \to C ℓ_{2} (g) + H_{2} (g) + 2 OH^{-} (aq)$
  1 mark · Combine half-reactions into net equation
4. Question 8.2.4 · November 2022 · Paper 2 · National (DBE) · 3 marks
8.2 A standard electrochemical cell is set up as shown below.
8.2.4 Write down the balanced equation for the net cell reaction taking place.(3)
Show the memo (3 ticks)Memo
1. ✓
  Reactants $Ni^{2 +} + Mn$
  1 mark · Combine half-reactions into net equation
2. ✓
  Products $\to Mn^{2 +} + Ni$
  1 mark · Combine half-reactions into net equation
3. ✓
  Balancing $Ni^{2 +} + Mn \to Mn^{2 +} + Ni$
  1 mark · Combine half-reactions into net equation
5. Question 9.4.3 · November 2025 · Paper 2 · Provincial · 3 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.
9.4.3 Write down the balance net ionic equation for this reaction.(3)
Show the memo (3 ticks)Memo
1. ✓
  a) Multiply the oxidation half-reaction by 6 $Cr_{2} O_{7}^{2 -} + 14 H^{+} + 6 e^{-} \to 2 Cr^{3 +} + 7 H_{2} O$
  $6 (Fe^{2 +} \to Fe^{3 +} + e^{-})$
  1 mark · Combine half-reactions into net equation
2. ✓
  b) Reactants $Cr_{2} O_{7}^{2 -} + 6 Fe^{2 +} + 14 H^{+} \to$
  1 mark · Combine half-reactions into net equation
3. ✓
  b) Products $\to 2 Cr^{3 +} + 6 Fe^{3 +} + 7 H_{2} O$
  1 mark · Combine half-reactions into net equation
6. Question 8.2.3 · November 2024 · Paper 2 · Provincial · 4 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.

$Cr_{2} O_{7}^{2 -} + Sn^{2 +} + H^{+} \to Cr^{3 +} + Sn^{4 +} + H_{2} O (ℓ)$
8.2.3 Use the Table of Standard Reduction Potentials and write down the balanced net ionic equation.(4)
Show the memo (4 ticks)Memo
1. ✓
  Oxidation half-reaction $Sn^{2 +} \to Sn^{4 +} + 2 e^{-}$
  $Cr_{2} O_{7}^{2 -} + 14 H^{+} + 6 e^{-} \to 2 Cr^{3 +} + 7 H_{2} O$
  1 mark · Write half-reactions
2. ✓
  Reactants $3 Sn^{2 +} + Cr_{2} O_{7}^{2 -} + 14 H^{+}$
  1 mark · Combine half-reactions into net equation
3. ✓
  Products $\to 3 Sn^{4 +} + 2 Cr^{3 +} + 7 H_{2} O$
  1 mark · Combine half-reactions into net equation
4. ✓
  Balancing $3 Sn^{2 +} + Cr_{2} O_{7}^{2 -} + 14 H^{+} \to 3 Sn^{4 +} + 2 Cr^{3 +} + 7 H_{2} O$
  1 mark · Combine half-reactions into net equation

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