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A 1.0 M Na2SO4 solution is to be electrolyzed in a U tube contianing carbon electrodes....

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A 1.0 M Na2SO4 solution is to be electrolyzed in a U tube contianing carbon electrodes. Before electrolysis begins, bromthymol blue indicator solution is added to the colorless solution and the color is adjusted to green (neutral). Predict the colors that will result at each electrode and support these predictions with anode and cathode half-reactions.


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Considering preferential electrolysis over neutral carbon electrodes in aqueous solution, the electrolysis of Na2SO4 solution gives oxygen at the anode and hydrogen at the cathode. Therefore, the process occurring at the carbon electrodes is, in essence, electrolysis of water.

The reactions are non-spontaneous, made to occur by the passage of electricity through the solution:

At the cathode, 2H2O + 2e- → H2 (g) + 2OH-

At the anode,  2H2O → O2 (g) + 4H+ + 4e-


So, there will be a surplus of OH- ions at the cathode and a surplus of H+ ions at the anode as the electrolysis proceeds. This will be manifested in the colour change of acid-base indicator bromothymol blue, already present there. The cathode chamber of the U-tube will turn blue (alkali colour), while the anode chamber will turn yellow (acid colour).


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