Unit 4 questions

[bec]

Just did the STAV exam, and I've got a few questions from it

1. Q6 really confused me...I got the right answer but mostly though a bit of guessing

...Volta's pile consists of alternating zinc and copper discs separated by paper discs soaked in ammonium salt solution. However the device stopped producing electricity as hydrogen gas produced at the cathode polarised the electrode.
Identify the nature of the anode and cathode in Volta's pile.

My approach:
- according to the Q, hydrogen is produced and it has to come from somewhere...
2H2O + 2e- --> H2(g) + 2OH- is a possibility, since water is present, or 2H+ + 2e- --> H2(g) if H+ ions could come from the ammonia in salt bridge (can they?)
- Strongest oxidant out of what we've got is H+, strongest reductant is Zn
- Oxidation occurs at the anode: Zn(s) + 2e- --> Zn2+(aq) ----> ANODE IS ZINC
- Reduction occurs at the cathode: 2H+(aq) + 2e- --> H2(g) ------> CATHODE MUST BE COPPER (by elimination - ie, because it's not zinc)

How would you work this out?



2. When calculating enthalpy change using temperature change etc, how do you show calculations to make it negative? I've always gotten to a point where "x=992kJ" and then said "reaction is exothermic, therefore delta H = -992kJ/mol"....is this right?


3. The 2008 STAV exam asks for the "main equilibrium step involved" in H2SO4 production - badly worded question, or is there actually a property of a reaction that makes it more of a "main" equilibrium step?


4. Question 7 of STAV says that "...a student investigates the electrolysis of a mixture of three metal nitrates, 0.010M Al(NO3)3, 0.010M AgNO3, 0.0101M Cu(NO3)2. The total volume of solution is 100mL...."
One of the calculations requires knowing n(Ag+), and the solutions provided give n(Ag+) = 0.00100mol - presumably N = cV = 0.010*0.1
But...the way I read the question, I thought it was saying "we had a 0.01M solution of AgNO3 and mixed some of it with other solutions, making up a total of 100mL" - ie, [Ag+ in mixture]<0.01M
Is it an ambiguous question, or am I missing something?


That's all for now, thanks!

[Mao]

1) I would've done it that way.
by the way, that zinc half equation you got there, really should be
Zn(s) --> Zn2+(aq) + 2e-

On that note (about ammonium dissociating to supply the H+ for reduction), it is a spontaneous redox reaction, i.e. the H+ and Zn want to react (coblin would have more "correct" terminology for this concept), despite ammonium being a weak acid.
the reduction reaction consumes H+, remembering that dissociation is also a dynamic reaction, the continual removal of H+ (product) will drive the dissociation reaction to the right, hence ammonium can be a source of H+ for this reaction.

2) I would've done that also

3) the main step refers to the "critical step", aka "conversion", where sulfur dioxide is converted to sulfur trioxide.

4) it means that [Ag+]=0.01M, [Cu2+]=0.01M and [Al3+]=0.01M. (consequently, [NO3-]=0.06M)

[sorry, i lack the understanding to explain these things in detail like coblin does]

[bec]

1) I would've done it that way.
by the way, that zinc half equation you got there, really should be
Zn(s) --> Zn2+(aq) + 2e-

2) I would've done that also

3) the main step refers to the "critical step", aka "conversion", where sulfur dioxide is converted to sulfur trioxide.

4) it means that [Ag+]=0.01M, [Cu2+]=0.01M and [Al3+]=0.01M. (consequently, [NO3-]=0.06M)

[sorry, i lack the understanding to explain these things in detail like coblin does]

Thanks! And nope, succinct is good, you don't need to explain more (and I doubt you lack the understanding to anyway).

[bec]

What does it mean when current is "drawn" from a cell? Does it mean that it PRODUCES a current? Or does it imply that it REQUIRES a current - ie, it is an electroylitic cell?

[Collin Li]

It depends what context. Current can be drawn from a galvanic cell to supply energy to something.

Current can be drawn from an external battery for an electrolysis process.

[bec]

Hmmm ok. I can't remember where I saw a question that was ambiguous enough to make me ask about the "draw" thing, but if I find it I'll put it up here..

I've reached that point where I start questioning the things I've always understood, to the point where I just don't get it any more...eg. the equilibrium constant.
Why does this change with temperature? (I understand why K wouldn't change with changes to the system pressure, amount of product/reactant etc.)

Can anyone explain?

[Collin Li]

Equilibrium is about an energetic equilibrium (going into thermodynamic concepts that you don't really want to know -- trust me). When you change the average kinetic energy of the surroundings (the temperature), you change the chemical equilibrium that will achieve the energetic equilibrium.

[bec]

Oh...so I probably never understood it in that case. Well, if I "don't want to know"....I'll just drop it.

[Collin Li]

Just in case you do want to know... here's opening Pandora's box (for VCE Chemistry):

If you think exothermic reactions imply spontaneity, then consider what it means for the atmosphere. An exothermic reaction must be an endothermic process for the atmosphere. So is the process spontaneous or not? What is special about the atmosphere that makes it not want to lose energy?

The fact is that an exothermic reaction is not a sufficient condition for spontaneity. It is slightly more complicated than that. For VCE Chemistry, you only need to believe that exothermic reactions imply spontaneity.

[Collin Li]

One way you can think about it (although it is slightly baseless), is just accepting Le Chatelier's principle:

If a change in temperature has occurred, the system will want to oppose it. Since a change in temperature doesn't change the reaction quotient (immediately), then the disequilibrium (system wanting to change) must be as a result of a change in , so that the reaction quotient is no longer equal to .

This doesn't really explain why changes, but it deduces that must change.

(Every other change that causes disequilibrium actually changes the reaction quotient, rather than .)

This is probably the best way for you to understand it.

[bec]

... here's opening Pandora's box (for VCE Chemistry):

One way you can think about it (although it is slightly baseless), is just accepting Le Chatelier's principle:

If a change in temperature has occurred, the system will want to oppose it. Since a change in temperature doesn't change the reaction quotient (immediately), then the disequilibrium (system wanting to change) must be as a result of a change in , so that the reaction quotient is no longer equal to .

This doesn't really explain why changes, but it deduces that must change.

(Every other change that causes disequilibrium actually changes the reaction quotient, rather than .)

This is probably the best way for you to understand it.

Couldn't you argue with the same logic, though, that if you increase the pressure of a system, the reaction quotient doesn't change immediately either and therefore the disequilibrium "must mean that K has changed"?

I know what you mean though, that I should just accept it and move on. 3/4 chem's a bit like that...consider Pandora's Box closed

[Collin Li]

Couldn't you argue with the same logic, though, that if you increase the pressure of a system, the reaction quotient doesn't change immediately either and therefore the disequilibrium "must mean that K has changed"?

If you hold temperature constant, a change in pressure is a change in concentration.

[bec]

But even if the pressure changes, the reaction quotient doesn't change immediately does it? I thought that the proportion of reactants:products would stay the same (in that exact instant when pressure was changed) and THEN it would shift to the side with more/less moles to restore equilibrium?

[Collin Li]

is a ratio of concentrations.

If each concentration is determined by:

(because of the above derivation)

Then a change in pressure is a change in the (given both sides of the reaction are not equimolar, and there is constant temperature).

That is the disequilibrium. changes so that

[Collin Li]

But even if the pressure changes, the reaction quotient doesn't change immediately does it? I thought that the proportion of reactants:products would stay the same (in that exact instant when pressure was changed) and THEN it would shift to the side with more/less moles to restore equilibrium?

I guess what I am saying is that an increase in pressure is synonymous with an increase in the concentration, for constant temperatures (and for an ideal gas, of course).