Post by: crayolé on September 20, 2010, 05:42:00 pm
So before i start going kenhung ( :smitten: )on your asses by spamming up this board with individual question threads I thought I might stick it all in here for everyones convenience ;]
Much appreciation and many karmas coming your way
First up
How would one go about finding the enthalpy change of H1?
Post by: vexx on September 20, 2010, 06:27:41 pm
And the rule of thumb is if bonds break it is endothermic, and so it would be =+1726kj/mol
Post by: crayolé on September 20, 2010, 10:21:38 pm
Okay, For this question, i have all the concentrations and stuff right its just my equilibrium constant equation that I can't get right.
So in this closed system, the products are [Cl2] and [CO] right? So they should be the numerator with the [CoCl2] being the denominator.
The answers do it the other way around though?
Also, would the reduction reaction during the corrosion of Fe(s) use the
O2 +4H+ +4e- ----> 2H2O
or
O2 +2H2O +4e- ----> 4OH-
Post by: Edmund on September 23, 2010, 10:42:59 pm
Cheers ;]The equilibrium constant should be derived from the given reaction. The question does say that phosgene gas is the product of the reaction:
Okay, For this question, i have all the concentrations and stuff right its just my equilibrium constant equation that I can't get right.
So in this closed system, the products are [Cl2] and [CO] right? So they should be the numerator with the [CoCl2] being the denominator.
The answers do it the other way around though?
Also, would the reduction reaction during the corrosion of Fe(s) use the
O2 +4H+ +4e- ----> 2H2O
or
O2 +2H2O +4e- ----> 4OH-
The reduction reaction would be
Post by: crayolé on September 27, 2010, 11:34:56 pm
Just got semi-fucked over with NEAP 09, need these questions answered ;3
Okay, so for the first one.
For the cathode reaction, is O2 + 4H+ + 4e- -----> 2H2O correct also?
Second one, Can someone please run through the method on how to derive individual half reactions from a full one?
Cheers
Post by: fady_22 on September 28, 2010, 12:03:41 am
For the cathode reaction, is O2 + 4H+ + 4e- -----> 2H2O correct also?No, that isn't right.
Where is the source of H+? You have to look at what reactants are available to you.
Second one, Can someone please run through the method on how to derive individual half reactions from a full one?
I usually look at the oxidation number changes in the reactants. From there, you can see if reduction/oxidation has occurred, and you can write half equations easily (just in the normal way).
So, for the anode, look for oxidation. Oxidation is an increase in oxidation number, which occurs with zinc.
Post by: crayolé on October 14, 2010, 10:21:40 pm
For this one, there's the question and I've also included the answer
So let me get this right, if an iron electrode is used as the anode, we would see the reaction
but this reaction is beneath the
Is it possible for an electrolytic cell to go from a non-spontaneous reaction to a spontaneous one just like that?
EDIT: thanks Studyinghard
Post by: Studyinghard on October 14, 2010, 10:23:26 pm
Post by: stonecold on October 14, 2010, 10:25:26 pm
Post by: Studyinghard on October 14, 2010, 10:27:55 pm
According to my teacher, you balance oxygen with water for an acidic cell, then if it is an alkaline cell, you neutralize the H+ with OH-. If it is neutral, then just O2 + 4e- > 2O2-
my tutor explained a bit differently. He followed the original steps for balancing redox reactions and then told us to add OH- ions to which ever side that didnt have the OH- ions and then combine them with H+ to form water. (if there were any H+ present)
Post by: stonecold on October 14, 2010, 10:34:24 pm
Cheers guys, heres a couple more to bug you about.
Just got semi-fucked over with NEAP 09, need these questions answered ;3
Okay, so for the first one.
For the cathode reaction, is O2 + 4H+ + 4e- -----> 2H2O correct also?
Second one, Can someone please run through the method on how to derive individual half reactions from a full one?
Cheers
But would that work for this question?
Post by: crayolé on October 17, 2010, 01:29:37 am
Just for clarification, can spontaneous reactions still occur in an electrolytic cell?
And theres another question that I'm stumped on, not sure if its because of the wording or what but I just can't seem to get my head around it
Quote
Water and the iron(II) ion are both able to act as either an oxidant or a reductant during a chemical reaction. Select the equation representing a reaction that would be expected to proceed substantially to the right spontaneously.
A. Fe (s) + 2 Fe3+ (aq) -----------------> 3 Fe2+ (aq)
B. 2 H2O (l) -----------------> O2 (g) + 2 H2 (g)
C. 2 H2O (l) + 2 Fe2+ (aq) -----------------> O2 (g) + 4 H+ (aq) + 2 Fe (s)
D. 2 H2O (l) + 4 Fe3+ (aq) -----------------> O2 (g) + 4 H+ (aq) + 4 Fe2+ (aq)
Post by: vexx on October 17, 2010, 02:14:14 am
ie: negative gradient when connecting the two reactants
look at A)
Fe(s) is a relatively strong reductant that can react with the relatively strong oxidant Fe3+(aq). They have a negative gradiant when connected, and so are spontaneous. So balance charges and check to see if that is the correct answer.
'to the right' is basically just in the forward direction so it doesn't really mean anything being there..
B-D If you look at reactions with H20, you can see that water is a weak reductant or a weak oxidant when it is on it's own (ie not with O2) and so will not be likely to react spontaneous as it will form a positive gradient with most reactants (that being it is not spontaneous), but check each of the above using the electrochemical series, balancing out charges and such to see if they are there.
Then you should get the correct answer, which though i haven't done the entire question seems to be somewhat obvious.
Post by: crayolé on October 18, 2010, 12:50:13 am
How would one go about separating this equation into two half-reactions?
Post by: vexx on October 18, 2010, 01:41:33 am
Cheers vexxyboy :smitten:^^
How would one go about separating this equation into two half-reactions?
Well, you can see that Zn ---> Zn0
And Ag2O ----> 2Ag
So just balance them by adding in the waters, H+ ions and electrons in the correct way.
Pretty sure that's it.
Post by: crayolé on October 22, 2010, 10:25:37 am
For the first MC, the solutions says its C but I got D? I'm confused, in their '10 paper, they included the solids in the K value but they disregard it in this instance?
The full reaction is this
AgCl(s) + 2NH3(aq) ----> Ag(NH3)2(aq) + Cl-(aq)
For the second MC, the solutions say its A but would C also be a viable answer?
It would donate its proton to the NH3 ions thus causing a net back reaction and increasing the amount of Cl- and Ag+ thus less AgCl(s) would need be dissolved?
For the third MC - this sounds like a pretty stupid question (it probably is) but why cant it be assumed that
Post by: physics on October 22, 2010, 11:04:37 am
Post by: Mao on October 22, 2010, 07:44:03 pm
For the second question, I don't see how A can work. An increased volume generally mean more salt can be dissolved, not an 'increase' in undissolved salt. Addition of Cl- might work for some cases, but generally when you have a low Ag+ concentration and there's little free Ag+, adding Cl- won't do much at all. (The competing equation with ammonia is quite strong, unless you add VERY concentrated NaCl, you won't really get the AgCl back.) I don't think any of the options are really correct.
For the third question, you can only assume that for weak acids because initially there's only water, no conjugate salts or H+ present. For most system however, you cannot assume the system is 'clean' initially, and there may be some products already present.
Post by: crayolé on October 23, 2010, 02:35:53 am
Quote
Question 2
A student calibrated a solution calorimeter containing 100 mL of water using electrical apparatus. The specific heat capacity of water is 4.18 J °C-1 g-1. Assuming that the student’s measurements were reliable, which calibration factor is most likely for a well-insulated calorimeter?
A. 240 J K-1
B. 480 J K-1
C. 960 J K-1
D. 1418 J K-1
Also, are we supposed to learn fusion/fission reaction formulas and derive it's properties? (refer to attachment)
Post by: Mao on October 23, 2010, 01:15:45 pm
As for fusion/fission, yeah, you are meant to know how to balance those equations. You are also meant to know fusion is exothermic up until Fe, and is endothermic after that. Fission is endothermic up until Fe, and is exothermic after that.