Chlorine

[woelen]

a positive ion is called a cathode, while a positive electrodes is called the anide.

 

Positive ion: cation

Negative ion: anion

Positive electrode: anode

Negative electrode: cathode

[Pat Says]

I think you exchanged the role of anode and cathode. Cl2 is formed at the anode and H2 is formed at the cathode.

Another point is that you WILL get gaseous contaminants. At the anode you also will have formation of oxygen. Formation of chlorine gas and formation of oxygen are two competing reactions. The conditions must be carefully controlled in order to minimize the production of oxygen at the anode.

 

Ahh yeah the O2 contamination makes sense now because it has to be produced when the water is split. How do you carefully control the conditions though? Put a lot of salt in and lower the voltage or what?

 

I don't understand why the H2 is formed at the cathode though (an Cl2 at the anode for that matter). I thought since the H2 was slightly positive due to its polar-covalence with 02 it would be attracted to the negatively charged anode. Can you explain this to me please?

 

edit: Doesn't the Oxygen combine in the liquid wqith the salt to form hydroxl ions?

[woelen]

Ahh yeah the O2 contamination makes sense now because it has to be produced when the water is split. How do you carefully control the conditions though? Put a lot of salt in and lower the voltage or what?

 

I don't understand why the H2 is formed at the cathode though (an Cl2 at the anode for that matter). I thought since the H2 was slightly positive due to its polar-covalence with 02 it would be attracted to the negatively charged anode. Can you explain this to me please?

 

edit: Doesn't the Oxygen combine in the liquid wqith the salt to form hydroxl ions?

If you have NaCl in solution, then this is split in ions Na(+) and Cl(-). The negative ions are attracted towards the anode, where they give off electrons and combine to Cl2. The positive Na(+) ions move towards the cathode. At the cathode, however, no metallic Na is formed, but hydrogen. The positive ions only take care of conduction of electricity, but it is the water, which is reduced.

 

If you want to produce Cl2 without too much O2 by means of electrolysis, then you need to use a graphite anode (copper dissolves) and a voltage, which is not too high.

[Pat Says]

Ahh sweet... so I was right about lowering the voltage (that was just a guess because it would lower the rate of water being split but ionic bonds are easier to split).

 

I still don't understand... why would the negatively charged chlorine be attracted to the negatively charged anode? (and vice versa w/the Na+)

[akcapr]

the cl- ion(anion) is attracted to the positive anode, the Na+ ion (cation) goes to the cathode.

[Pat Says]

Yeah, that's what I was saying... maybe me and woelen are debating over a silly semantics error. Anode = -electrode and cathode = +electrode

 

But, it still hasn't been explained completely as to why oxygen will be released with the chlorine because, like I said (and read), the oxygen combines in the liquid with the salt to form hydroxl ions. So I stil think, maybe, that it will be pure chlorine afterall.

 

(does anyone else know for sure?)

[akcapr]

the oxygen bonded to the hydrogen is negative wehn seperated, its o -2. so it travels to the positive anode, where it is oxidzed?/reduced? to normal O nutral. same for hydrogen except its positive so it goes to the negative cathode. why the gas is reduced.oxidized im not sure, but i belive it has to do with electrons from electrisity fulfilling the charge.

[Pat Says]

I think it would be reduced (where you put the question marks). Oh jeez I am sooo stupid. The anode is positive... not negative... that was my error (it was semantics afterall). I got it confused with anion (negatively charged ion) and cation (positively charged ion). WHy do they mix it up like that?

 

And I am pretty sure it is reduced because it is picking up free electrons in the system (from the electrodes).

[jdurg]

Chlorine and Oxygen are produced at the same electrode. Based upon their potentials, oxygen should be produced and chlorine should remain as the chloride ion, but that's going under perfect conditions. In reality, the difference between the two is so small that chlorine production actually predominates as long as there is a large concentration of chloride ions. So if you have NaCl in your solution, it will produce chlorine gas in preference to oxygen. Once the chloride ion becomes a low enough concentration, oxygen gas takes preference and oxygen is produced in place of chlorine.

[Pat Says]

Ahhh sweet.. so when I said you need a higher amount of salt to produce the chlorine and keep that oxygen 'at bay' I was right?

[jdurg]

Somewhat. The oxygen and water don't 'combine' to form hydroxide ions. During electrolysis, the bonds between hydrogen and oxygen are broken as the H+ ions get reduced to elemental hydrogen and the oxygen atoms are ripped from the OH- ions and become oxidized to elemental oxygen gas. The problem is, the oxidation potential for the formation of oxygen is about -1.27 volts while that for chloride to become chlorine gas is about -1.36. That is so freaking close that if there are enough chloride ions present, the chloride becomes more easily oxidized and will form chlorine gas. When there isn't a lot of chloride present, the conditions that cause chlorine to be formed more favorably dissipate. As a result, the expected output of oxygen gas occurs.

[Pat Says]

Ah, ok, that makes sense now. Thanks for clearing this up . Oh btw I was just wondering what school you go/went to because I was thinking of going into chemistry (or atomic physics) and was wondering what school would be good. (I'm assuming that you have some sort of chemistry degree.)

[jdurg]

I earned my Bachelor's of Science in Forensic Chemistry from West Chester University of Pennsylvania in 2002.

[Pat Says]

Forensics eh? Sounds really cool but, I've always wanted to know, is CSI actually realistic or not?

[jdurg]

CSI is glamorized, hollywood altered garbage. They glamorize everything to make it entertaining. If you went and visited an actual forensics laboratory, you'd be bored to death watching all the typical chromatography, GC/MS, and other 'analytical techniques' going on. Most of the stuff they do would result in mistrials and evidence being thrown out of court for improper handling and improper tracking.