[xiankai]

i've stumbed upon a past-year question:

When conc. aq. NaCl is electrolysed using carbon electrodes, H2 is collected at the cathode and Cl2 at the anode. When iron electrodes are used instead, much less Cl2 is collected at the anode.

Explain why much less chlorine is collected when iron electrodes are used.

my only guess is that iron is prefferably discharged into ions instead of chlorine being oxidised... can anyone help me on this? thanks.

[xiankai]

i've checked the answer to that particular question, and it said,

"When iron anode is used, some oxygen gas is produced at the same time. Some of the electrical energy is used to liberate oxygen. Hence, less Cl2 is produced."

now to the extent of my knowledge, i've dont know how iron anodes make it such that oxygen is produced instead. Can anyone provide me with a logical explaination  

[woelen]

If you have a sufficiently high voltage, oxygen will be produced (besides other stuff), regardless of what type of electrode is used.

At the anode, several things are present, which can be oxidized:

1) the anode material
2) chloride ions
3) water

The ease at which these things are oxidized depends on concentration, temperature and the actual anode material. But, if the voltage is high, then almost certainly water is oxidized (look up the redox potential for the reaction 2H2O ---> 4H(+) + O2 in a table). Add the voltage loss due to resistive effects and add the voltage, needed for the cathode reaction (2H2O +2e --> H2 + 2OH(-)). If you apply more than this total voltage, then you'll get oxygen. In general a few volts is sufficient already, unless a large current is used.

[xiankai]

i've looked up the tables... and they show that;

2H2O         --> O2 + 4H+ + 4e- (-1.23V)
Fe             --> Fe3+ + 3e-        ( 0.04V)
2Cl-           --> Cl2 + 2e-          (-1.36V)

how does a high voltage ensure that water is prefferably oxidised over chlorine? from what it seems, chlorine is still easier to oxidise and is also in high concentration

[woelen]

This is one of the funny things of chemistry. If multiple reactions can occur at the same time, then they will occur at the same time. At high-school pupils learn all kinds of chemical reactions with perfect stoichiometry, but in real life, things are much more complex and there hardly is any reaction, which runs quantitatively with a certain stoichiometry.

So, if you have sufficient voltage available, then all reactions can occur at the same time and this is what you observe. Besides that, formation of oxygen becomes easier and easier during the electrolysis. At the cathode, OH(-) ions are formed and the reaction 4OH(-) --> 2H2O + O2 + 2e is much more facile than the formation of oxygen from water or the formation of chlorine. So, over time, OH(-) will be the main supplier of oxygen at the anode.

But even if there were no OH(-), oxygen would still be formed, simply because it can be formed.