Stankovic123's chem q's

[zvezda]

Mao's point is pretty clever.
I'll illustrate with an example.

Let's say we want to compare ethane and ethanol, or C2H6 and C2H5OH
The reactions are:
C2H6 + 3.5 O2 => 3H2O + 2CO2     Reaction 1
C2H5OH + 3O2 => 3H2O + 2CO2    Reaction 2

The products are exactly the same.
So if we want to see which is more stable, we consider the reaction C2H6 + 0.5 O2 => C2H5OH
By Hess's law, the enthalpy change of this reaction is equal to the enthalpy of formation of the right hand side, minus the enthalpy of formation of the left hand side. As the enthalpy of formation of oxygen gas is zilch, the enthalpy change of this reaction allows us to compare the enthalpies of ethane and ethanol.
This would be done by, saying, flipping reaction 2 around and adding that to reaction 1, so that the enthalpy change of ethane to ethanol is simply the enthalpy of combustion of ethane minus the enthalpy of combustion of ethanol.

Ah right. I see now.
Cheers nliu

[zvezda]

Apparently ethanoate anions can act as an acid or a base?
There was a question in the AN guide which asked "which species has the capacity to act as a base or an acid in this reaction flask?" The answer was ethanoate amongst sodium cations and chloride
Help with this is much appreciated

[lzxnl]

I will need to see the question. Ethanoate ions cannot act as an acid to any noticeable extent.

You may have misinterpreted the question; if it was asking for the species with any acid OR base properties, ethanoate would be preferred over chloride or sodium +.

[zvezda]

I will need to see the question. Ethanoate ions cannot act as an acid to any noticeable extent.

You may have misinterpreted the question; if it was asking for the species with any acid OR base properties, ethanoate would be preferred over chloride or sodium +.

Yeah it could have been that.
Also, are the molar masses in the data book considered for significant figures??

[zvezda]

In the last question of exam 2 2010 part a,
It mentions that the electrolyte is pure phosphoric acid as a reason why the predicted voltage of the phosphoric acid fuel cell is not what its real one is??
Also, why didnt they accept a reference to the potential for the gases to have a higher pessure than 1 atm??
Cheers

[brightsky]

In the last question of exam 2 2010 part a,
It mentions that the electrolyte is pure phosphoric acid as a reason why the predicted voltage of the phosphoric acid fuel cell is not what its real one is??
Also, why didnt they accept a reference to the potential for the gases to have a higher pessure than 1 atm??
Cheers

remember standard conditions: 298 K, [ions] = 1.0 M, Pgas = 1 atm. if the electrolyte is PURE phosphoric acid, then the concentration of phosphoric acid would go through the roof. hence, the cell does not operate at standard conditions, and so the electrochemical series is useless (well probably not useless, but you get what I mean).

don't put that...usually it is assumed that gases getting pumped into the fuel cell have a pressure of 1 atm (since the gases are derived from literally 'the atmosphere' which is 1 atm).

[jgoudie]

Quite right, in general you can pretty much say if in an experiment you do not get what you predict from the electrochemical series you have two choices:
1) It is not at SLC as described above
2) The fact that the EC series does not explain the rate of a reaction.

The second choice here is normally used when the you don't OBSERVE a reaction.

Hope this little extra helped a bit and didn't confuse the reason given by brightsky.

remember standard conditions: 298 K, [ions] = 1.0 M, Pgas = 1 atm. if the electrolyte is PURE phosphoric acid, then the concentration of phosphoric acid would go through the roof. hence, the cell does not operate at standard conditions, and so the electrochemical series is useless (well probably not useless, but you get what I mean).

don't put that...usually it is assumed that gases getting pumped into the fuel cell have a pressure of 1 atm (since the gases are derived from literally 'the atmosphere' which is 1 atm).

[zvezda]

Ahh yes i see.
Totally disregarded the implications of "liquid phosphoric acid".
Thanks to the both of you

[lzxnl]

There are other problems too.
Firstly, the electrochemical series only works for aqueous solutions. Liquid phosphoric acid has no water in it, so everything falls apart. The H+ refers to a proton solvated in WATER. Not phosphoric acid; the H+ there would be H4PO4+, and I have no idea how good phosphoric acid is at protonating itself (pure H2SO4 has an auto-dissociation constant of 10^-4 ish, in contrast to water's 10^-14)

Secondly, phosphoric acid is not a liquid at 101.3 kPa and 298 K. We already don't have standard conditions there.

Thirdly, if you had liquid phosphoric acid, phosphoric acid is considered "pure". Then, we wouldn't use concentrations anymore, but chemical activities, and those are slightly more trippy to deal with, especially as phosphoric acid has undergone a phase change.

[zvezda]

What's the chemical basis for the connection of two electrolytic cells? Doubt and explanation of one will be needed for the exam, but i was just curious.
Thanks

[brightsky]

What's the chemical basis for the connection of two electrolytic cells? Doubt and explanation of one will be needed for the exam, but i was just curious.
Thanks

what do you mean...?

[lzxnl]

I believe it's a typo, asking "doubt an explanation will be needed for the exam"

If you connect two cells together, it's like connecting resistors in series.

[zvezda]

Sorry i shouldve used some proper english lol.
What i meant was that i doubt that we will be asked to explain the chemistry behind the 2 connected electroyltic cells, but i was just curious as to how they work anyway.

Also, in a 1-hexene molecule, would there be 6 hydrogen environments? Just need some clarification here.
Thanks

[lzxnl]

Sorry i shouldve used some proper english lol.
What i meant was that i doubt that we will be asked to explain the chemistry behind the 2 connected electroyltic cells, but i was just curious as to how they work anyway.

Also, in a 1-hexene molecule, would there be 6 hydrogen environments? Just need some clarification here.
Thanks

The chemistry is relatively simple; a large voltage drives non-spontaneous reactions in both cells like how current flows in a series circuit.

And yes. Each hydrogen environment is different

[zvezda]

The chemistry is relatively simple; a large voltage drives non-spontaneous reactions in both cells like how current flows in a series circuit.

And yes. Each hydrogen environment is different

I just cant see how theyre strictly connected though. So youre saying that electrons flow through the solutions of each cell as well??

Also, according to TSSM, fuel cells can be rechargeable, like the vanadium redox cell apparently?

Cheers nliu