An electrochemical cell consisting of half-cells A and B is assembled under standard conditions as shown below - NSC Physical Sciences - Question 8 - 2016 - Paper 2
Question 8
An electrochemical cell consisting of half-cells A and B is assembled under standard conditions as shown below.
Half-cell A
Pt, Cl2 (101,3 kPa) | Cl⁻ (1 mol-dm⁻³)
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Worked Solution & Example Answer:An electrochemical cell consisting of half-cells A and B is assembled under standard conditions as shown below - NSC Physical Sciences - Question 8 - 2016 - Paper 2
Step 1
8.1 At which half-cell, A or B, are electrons released into the external circuit?
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Answer
Electrons are released into the external circuit at half-cell B.
Step 2
8.2.1 Reduction half-reaction that takes place in this cell
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Answer
The reduction half-reaction taking place in this cell can be represented as:
Cl2(g)+2e−→2Cl−(aq)
Step 3
8.2.2 NAME or FORMULA of the substance whose oxidation number DECREASES
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Answer
The substance whose oxidation number decreases is Chlorine (Cl2).
Step 4
8.3 Calculate the initial cell potential of this cell when it is in operation.
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Answer
The initial cell potential (E°) can be calculated using the standard reduction potentials for the half-reactions:
E°_cell = E°_cathode - E°_anode
For half-cell A: E° = 1.36 V (reduction of Cl2)
For half-cell B: E° = -2.36 V (oxidation of Mg)
So,
E°_cell = 1.36 V - (-2.36 V) = 1.36 V + 2.36 V = 3.72 V.
Step 5
8.4 Write down an observation that will be made in half-cell B as the cell operates. Give a reason for the answer.
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Answer
The observation in half-cell B will be that the Mg electrode becomes smaller as the mass of the Mg electrode decreases. This occurs because the Mg is oxidized according to the reaction:
Mg(s)→Mg2+(aq)+2e−
Thus, as Mg is oxidized, the electrode loses mass.
