Chemistry 3/4 2013 Thread

[lolipopper]

Sorry for being pedantic, I'm sure it was just a mistyping thing, as the strongest oxidant would be the highest on the left, the strongest reductant the lowest on the right.


Also, sorry for not posting the answer in my post, the V(02) is 86 mL. I think the concentration was just a distractor in the question, because they used n=v/vm for the volume. (also, (5 x 60)s x 4.5A = 1200 1350C)

Yeah, this makes sense. I thought that as the question didn't mention 'solution', there was only potassium fluoride in the cell. Is it the question's poor wording, or how am I supposed to recognise that there's also water in there?

lol yeah i wrote the ECS stuff wrong. the oxidants are on left and reductants on right. and yes im sorry for the concentration part. I would not make the mistakes on paper but i dont have a good phone to take photos and just post the working out. im stupid haha.

[lolipopper]

I was under the assumption that it changed depending on the type of cell and now I'm a bit lost. Can you clarify more on that?

Galvanic cell : strongest oxidant (positive electrode, cathode) , strongest reductant (negative electrode, anode)
Electrolytic cell : strongest oxidant (negative electrode, cathode) , strongest reductant (positive electrode, anode)

[lzxnl]

Sorry for being pedantic, I'm sure it was just a mistyping thing, as the strongest oxidant would be the highest on the left, the strongest reductant the lowest on the right.


Also, sorry for not posting the answer in my post, the V(02) is 86 mL. I think the concentration was just a distractor in the question, because they used n=v/vm for the volume. (also, (5 x 60)s x 4.5A = 1200 1350C)

Yeah, this makes sense. I thought that as the question didn't mention 'solution', there was only potassium fluoride in the cell. Is it the question's poor wording, or how am I supposed to recognise that there's also water in there?

I believe the question said 0.60 M KF. If a molarity is given, you can assume that we're speaking of aqueous solution.

This NEVER changes. What changes is the polarity of the anode and cathode.

I'm going to slightly rephrase what I said. Let's have two electrodes, A and B. In a galvanic cell, A is the positive cathode. Let's pretend that A consists of a Cu/Cu 2+ half cell. Then, B is the negative anode. Let's pretend that B consists of Fe/Fe 2+.
Now, even in an electrolytic cell, A is still positive and B is still negative. These positive and negative distinctions aren't that great IMO...I think of of them as having higher or lower electric potential and that electrons move from lower potential to higher potential (note potential is NOT quite the same as potential energy). However, the external voltage, if great enough, reverse the reaction and makes A an anode and B a cathode. We now have a positive anode and a negative cathode. It's not what my original statement implies; that A somehow becomes a negative cathode. A is still positive; it's now an anode.

[brightsky]

while we're at it, i have a question: is the negative electrode actually negatively charged? similarly is the positive electrode actually positively charged? I know that in electrolysis, the negative ions in solution always migrate towards the positive electrode, etc., which suggests that the anode is actually positively charged. but the same logic doesn't seem to apply for galvanic cells. we take electrons from the anode to the cathode, and deposit them at the cathode. but electrons move this way not because the anode is negatively charged and the cathode positively charged, but because the cathode has a higher reduction potential...and if you actually visualise what's happening in, say, the daniell cell, once the electrons reach the Cu rod, a Cu 2+ ion comes and snatches it to form Cu on the surface of the rod. so the rod doesn't actually become charged at all.

confused. :/

[clıppy]

while we're at it, i have a question: is the negative electrode actually negatively charged? similarly is the positive electrode actually positively charged? I know that in electrolysis, the negative ions in solution always migrate towards the positive electrode, etc., which suggests that the anode is actually positively charged. but the same logic doesn't seem to apply for galvanic cells. we take electrons from the anode to the cathode, and deposit them at the cathode. but electrons move this way not because the anode is negatively charged and the cathode positively charged, but because the cathode has a higher reduction potential...and if you actually visualise what's happening in, say, the daniell cell, once the electrons reach the Cu rod, a Cu 2+ ion comes and snatches it to form Cu on the surface of the rod. so the rod doesn't actually become charged at all.

confused. :/

The way I always looked at it was for galvanic cells:
The electrode producing electrons is negative
The electrode using electrons is positive

But since electrolytic cells are in reverse and we're pumping electrons in
The electrode producing electrons is positive
The electrode using electrons is negative

Right? Midway typing that I got lost as well... I need to revise my cells :/

[Edward21]

while we're at it, i have a question: is the negative electrode actually negatively charged? similarly is the positive electrode actually positively charged? I know that in electrolysis, the negative ions in solution always migrate towards the positive electrode, etc., which suggests that the anode is actually positively charged. but the same logic doesn't seem to apply for galvanic cells. we take electrons from the anode to the cathode, and deposit them at the cathode. but electrons move this way not because the anode is negatively charged and the cathode positively charged, but because the cathode has a higher reduction potential...and if you actually visualise what's happening in, say, the daniell cell, once the electrons reach the Cu rod, a Cu 2+ ion comes and snatches it to form Cu on the surface of the rod. so the rod doesn't actually become charged at all.

confused. :/

The basic rule of thumb is that the anions move towards the half cell, or region (if electrolytic cell) containing the anode..not necessarily the anode itself!! Thus cations move towards the half-cell/region containing the cathode, as positive charge is being lost via reduction, this restores the electroneutrality in this region by bringing more positive back in. It's also useful to remember exceptions, (well not really exceptions), but strange concepts relating to this, like the NO3- ion being a strong oxidant. How come this isn't reduced at the negative cathode in electrolysis, it's a strong oxidant, they get reduced really easy?? Well it's an anion, anions move towards the region containing the anode, plus the negative cathode and the negative anion would not be attracted towards each other! Just remember the basic rule that I stated at the top, because oxidation thus build up of excess positive will occur at the anode in both galvanic/electrolysis, and that the ions themselves are attracted electrostatically towards that region, but not necessarily the electrode in the region!! Like cations wouldn't be attracted towards the positive cathode in galvanic, just the half cell

[brightsky]

hmm okay but that doesn't really answer my question. is the negative electrode actually negatively charged? it seems that with the galvanic cell, the charge is in solution not at the electrode.

[lzxnl]

hmm okay but that doesn't really answer my question. is the negative electrode actually negatively charged? it seems that with the galvanic cell, the charge is in solution not at the electrode.

NO. Otherwise our salt bridge is malfunctioning. Its purpose is to maintain neutrality at both electrodes.
The "positive" electrode is just the electrode at a higher electric potential.

[brightsky]

NO. Otherwise our salt bridge is malfunctioning. Its purpose is to maintain neutrality at both electrodes.
The "positive" electrode is just the electrode at a higher electric potential.

okay so it's not electrostatic attraction that makes the negative ions in an electrolytic cell migrate to the anode...

[lzxnl]

okay so it's not electrostatic attraction that makes the negative ions in an electrolytic cell migrate to the anode...

hmm okay but that doesn't really answer my question. is the negative electrode actually negatively charged? it seems that with the galvanic cell, the charge is in solution not at the electrode.

Let's have a look at both of these together.
Whether the cell is electrolytic or galvanic makes no difference in this discussion.

In an electrochemical cell, at the anode, the site of oxidation, electrons are lost. Therefore, anions need to flow there to balance the charges; as soon as the electrons leave, there is a deficit of negative charge, which draws in other negative charges.
Likewise, at the cathode, the site of reductions, electrons are gained. Now, there is a net negative charge which attracts cations to balance out the charges.
These cations and anions generally come from the salt bridge.

It IS electrostatic attraction that makes the anions migrate to the anode.

[brightsky]

Let's have a look at both of these together.
Whether the cell is electrolytic or galvanic makes no difference in this discussion.

In an electrochemical cell, at the anode, the site of oxidation, electrons are lost. Therefore, anions need to flow there to balance the charges; as soon as the electrons leave, there is a deficit of negative charge, which draws in other negative charges.
Likewise, at the cathode, the site of reductions, electrons are gained. Now, there is a net negative charge which attracts cations to balance out the charges.
These cations and anions generally come from the salt bridge.

It IS electrostatic attraction that makes the anions migrate to the anode.

ahh ming bai le, thankyou!

[zvezda]

NO. Otherwise our salt bridge is malfunctioning. Its purpose is to maintain neutrality at both electrodes.
The "positive" electrode is just the electrode at a higher electric potential.

So whats the chemical basis behind the oxidation of chloride ions in preference to water at high concentrations during electrolysis???

[lzxnl]

So whats the chemical basis behind the oxidation of chloride ions in preference to water at high concentrations during electrolysis???

REALLY high concentrations of chloride ions, very low concentrations of chlorine gas => equilibrium for oxidation of chlorine shifts further to the right until it is more preferred.

[lolipopper]

So whats the chemical basis behind the oxidation of chloride ions in preference to water at high concentrations during electrolysis???

at really high concentrations, the standard conditions of the ECS aren't followed which include 1M solution, 25C, and 1 atm. you cant really draw an exact inference from the ECS. the chlorine case is just an exception you have to learn of by heart as we dont look at the reasons behind it in VCE. 

[lzxnl]

If you want figures...there IS an equation which does this for you.
Given an electrode potential E cell (i.e. reduction potential of cathode minus reduction potential of anode) at standard conditions
Your actual electrode potential is given by:
E cell = E cell standard - RT/nF * ln Q
R is the ideal gas constant
T is the temperature in Kelvin
n is the number of moles of electrons transferred per mole of reaction (so for instance, in Cu + F2 => CuF2, there are two electrons transferred per mole of reaction so n=2 for that reaction)
Q is the reaction quotient, or concentration fraction. Except you don't use concentrations for gases; you use the partial pressure of the gas in bars (atm or bars aka 100 kPa, depending on which convention you follow). Note that at standard states, when the pressures and concentrations are equal to 1 M or atm, Q=1, so ln Q = 0. As expected.
ln is the natural logarithm

So, if you have a LOT of chloride ion and not much chlorine gas, the reaction quotient will be very small, much smaller than 1. The natural log of this is hence negative, and the -RT/nF term becomes positive. You have an increase in cell potential which may be larger than the cell potential of the oxidation of water.