SciPhyChem: (O Level Chem) An electrolysis set up with more than one cell

Saturday, 15 March 2014

(O Level Chem) An electrolysis set up with more than one cell

In the O Level exam, you may see electrolysis set ups that look intimidating because they contain more than one cell.

Let's look at diagram 1. It belongs to an MCQ that goes like this:

In the diagram, each cell contains an aqueous solution of a single salt and all four electrodes are platinum. Electrodes Q and S increase in mass during the electrolysis but no gas is given off at Q or S.

See diagram 1

If the increase in mass of Q is greater than the increase in mass of S in the same time, which statement is true?

(A) The cation of the solution in cell 1 is different from the cation of the solution in cell 2.
(B) The current flowing in cell 1 is greater than the current in cell 2.
(C) The cation in cell 1 is the same as in cell 2 but the solution in cell 1 is more concentrated than in cell 2.
(D) The loss of mass of electrode P is less than the loss of mass of electrode Q.

This is one of the few very tough questions that serve to distinguish the exceptional students from the average ones. If you're stuck with such questions, skip and waste no more time on it. But, now, we're not in an exam and so let's slowly analyze it.

Since the electrodes are Pt which is inert unlike Cu, we shouldn't expect the electrodes to be discharged. So, the +ve electrodes P and Q wouldn't dissolve in the electrolytes. ==> Eliminate option D.
You may think that the aqueous solutions are the same, but the question didn't say that. ==> Don't eliminate option A.
Assign charges to the electrodes: P = +ve, Q = -ve, R = +ve, S = -ve. Cations Xⁿ⁺ would diffuse to the negative electrodes Q and S where they're discharged and deposited: Xⁿ⁺(aq) + ne --> X(s).
At every point in the circuit, the same amount of charges passes through in a unit time. (If you've studied Physics, you would understand that the current at every point in a series circuit is equal.) ==> Eliminate option B.
Suppose the metal ion Xⁿ⁺ in cell 1 has a greater molar mass than the metal ion Yⁿ⁺ in cell 2 and that the two metal ions have the same charge, then for the same amount of current, the mass deposited on Q is greater than the mass deposited on S. ==> Accept option A.
Whether the concentration of the electrolytes is different would not affect the results. ==> Eliminate option C.

Don't assume that the electrolytes are the same if the question didn't say so...

Inert anodes such as platinum and graphite should not dissolve during electrolysis...

Current at every point in the series circuit is the same...

The mass of metal deposited on the cathode does not depend on the concentration of the solution...

Answer these questions before you check with the answers below.

Look at diagram 2. The grey electrodes are platinum and the reddish-brown electrodes are copper. Each cell contains aqueous copper(II) sulfate. In which cells is the intensity of the blue colour of the solution unchanged? Why?
Look at diagram 3.

(a) Which substance listed in the diagram has positive ions and mobile electrons? Explain your answer.
(b) (i) State the ions that are attracted to each electrode.
(ii) Predict the product at each electrode.

Answers: 1. In each cell, Cu²⁺(aq) is discharged and deposited as Cu(s) at the cathode. All the cathodes would increase in mass due to the Cu(s) deposited. The concentration of Cu2+(aq) would not decrease unless the anodes are made of copper such that copper dissolves: Cu(s) --> Cu²⁺(aq) + 2e. This occurs in cells Y and Z. 2. (a) Copper wire contains copper Cu²⁺ ions that are fixed in positions and surrounded by a sea of mobile electrons. (b)