Grade 12 Physical Sciences · Chemical Change

Calculate the emf of a cell

Calculate a cell's emf using E°cell = E°cathode − E°anode, and use a positive value to show the reaction is spontaneous.

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Galvanic Cell Electrochemistry: Calculating the EMF of the galvanic cell | Grade 12 ChemistryMiss Martins Maths and Science

Electrochemistry Gr 12: Galvanic Exam QuestionKevinmathscience

Cell Potential Problems - ElectrochemistryThe Organic Chemistry Tutor

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Question 9.1 · November 2024 · Paper 2 · National (DBE) · 5 marks

A strip of silver is added to a

1

mol·dm

^{- 3}

solution of

Pb(NO_{3})_{2}

25

°C.

9.1 Consider the reaction below.

2 Ag(s) + Pb^{2 +} (aq) \to 2 Ag^{+} (s) + Pb(s)

By means of a calculation, determine whether this reaction is SPONTANEOUS or NON-SPONTANEOUS.(5)

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

More past paper questions: calculate the emf of a cell

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3. Question 8.2.3 · November 2022 · Paper 2 · National (DBE) · 4 marks
8.2 A standard electrochemical cell is set up as shown below.
8.2.3 Calculate the initial emf of this cell.(4)
Show the memo (4 ticks)Memo
1. ✓
  Formula $E °_{cell} = E °_{reduction} - E °_{oxidation}$
  1 mark · Calculate the emf of a cell
2. ✓
  Cathode value $= - 0, 27 - (\dots)$
  1 mark · Calculate the emf of a cell
3. ✓
  Anode value $= - 0, 27 - (- 1, 18)$
  1 mark · Calculate the emf of a cell
4. ✓
  Answer with unit $E °_{cell} = 0, 91$ V
  1 mark · Calculate the emf of a cell
4. Question 1.9 · May/June 2022 · Paper 2 · National (DBE) · 2 marks
1.9 An electrochemical cell was set up using a $Hg (ℓ) ∣ Hg^{2 +} (aq)$ half-cell and another half-cell under standard conditions.

Which ONE of the following half-cells, when connected to the $Hg (ℓ) ∣ Hg^{2 +} (aq)$ half-cell, will result in the HIGHEST cell potential?(2)
1. A $A ℓ (s) ∣ A ℓ^{3 +} (aq)$
2. B $Zn(s) ∣ Zn^{2 +} (aq)$
3. C $Co(s) ∣ Co^{2 +} (aq)$
4. D $Pt(s) ∣ H_{2} (g) ∣ H^{+} (aq)$
Show the memo (1 tick)Memo
1. ✓
  Most negative anode gives the largest emf $Hg^{2 +} / Hg$ has $E^{θ} = + 0, 85$ V, so it is the cathode with each option. $E_{ce ll}^{θ} = E_{c a t h o d e}^{θ} - E_{an o d e}^{θ}$ is largest when $E_{an o d e}^{θ}$ is the most negative.
  Al: $0, 85 - (- 1, 66) = 2, 51$ V
  Zn: $0, 85 - (- 0, 76) = 1, 61$ V
  Co: $0, 85 - (- 0, 28) = 1, 13$ V
  H $_{2}$ : $0, 85 - 0, 00 = 0, 85$ V
  
  Answer: A
  2 marks · Calculate the emf of a cell
5. Question 8.3 · May/June 2023 · Paper 2 · National (DBE) · 5 marks
Learners want to identify an unknown metal X using a standard half-cell, X| $X^{2 +}$ .

They set up an electrochemical cell under standard conditions using two half-cells, as shown in the diagram below.

The initial emf of this cell is $1, 20$ V.
After the cell has operated for some time, it is found that the mass of electrode X has increased.
8.3 Identify X by means of a suitable calculation.(5)
Show the memo (5 ticks)Memo
1. ✓
  Formula $E_{cell}^{θ} = E_{cathode}^{θ} - E_{anode}^{θ}$
  1 mark · Calculate the emf of a cell
2. ✓
  Substitute emf $1, 20 = E_{cathode}^{θ} - ...$
  1 mark · Calculate the emf of a cell
3. ✓
  Substitute E of the hydrogen anode $1, 20 = E_{cathode}^{θ} - 0$
  1 mark · Calculate the emf of a cell
4. ✓
  E of cathode $E_{cathode}^{θ} = 1, 20$ (V)
  1 mark · Calculate the emf of a cell
5. ✓
  Identify XX is Pt / platinum
  1 mark · Compare oxidising and reducing agent strength
6. Question 8.1.2 · May/June 2022 · Paper 2 · National (DBE) · 5 marks
8.1 An electrochemical cell is set up using an aluminium rod, A $ℓ$ , and a gas X.

The initial emf measured under standard conditions is $2, 89$ V.

Answer the questions that follow.
8.1.2 Use a calculation to identify gas X.(5)
Show the memo (5 ticks)Memo
1. ✓
  Formula $E_{ce ll}^{θ} = E_{re d u c t i o n}^{θ} - E_{o x i d a t i o n}^{θ}$
  1 mark · Calculate the emf of a cell
2. ✓
  Substitute 2,89 $2, 89 = E_{re d u c t i o n}^{θ} - E_{o x i d a t i o n}^{θ}$
  1 mark · Calculate the emf of a cell
3. ✓
  Substitute −1,66 $2, 89 = E_{re d u c t i o n}^{θ} - (- 1, 66)$
  1 mark · Calculate the emf of a cell
4. ✓
  Answer $E_{re d u c t i o n}^{θ} = 1, 23$ (V)
  1 mark · Calculate the emf of a cell
5. ✓
  Identify gas XX is $O_{2}$ /oxygen
  1 mark · Compare oxidising and reducing agent strength

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