The diagram shows gas being given off at the positive carbon electrode during the electrolysis of aluminium oxide. Which gas is first formed at this electro...
Question 1 Report
The diagram shows gas being given off at the positive carbon electrode during the electrolysis of aluminium oxide. Which gas is first formed at this electrode?
Answer Details
The electrolyte is molten aluminium oxide, which contains only Al3+ and O2− ions. Negative ions move to the positive electrode, so oxide ions are discharged there by losing electrons: 2O2− → O2 + 4e−. The bubbles drawn at the positive rod are therefore oxygen.
Hydrogen and chlorine cannot form because no hydrogen or chlorine is present in the melt. Carbon dioxide is a tempting choice and some is indeed produced, but only afterwards, when the oxygen just released attacks the hot carbon rod and slowly burns it away. That is a secondary reaction, which is why the anodes have to be replaced regularly.
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