chemistry unit 4, 17 june 2010

why has this thread suddenly died?? is there another thread for this now or what lol?

I think people may be preparing for an afternoon exam, I know I am.

did you get partial pressures from methanol or whatever to be 4.9 in the beginning of the Kp question?

no i somehow managed to muck that one up but that is the right answer!

for the second part did you get 46.8 moles for ethanoic acid and 3.2 for the other two? then 28.08 atm pressure for ethanoic acid?

Yes I was really hesitant about some of my calculations but I checked them over a number of times and they seemed legit. Hopefully it turns out nice, but I missed large elements of the MgCl2 stuff because I cba to labour through that data booklet (it was wasting too much time and i did what I could without the data). Was the enthropy total like -110? and the one about the half equivelance - was the pH about 4.7 or something ? - that one I took a bit of an educated gamble

EDIT: I wont be replying for a while - Im off for D2

Re: the half-equivalence point - I went for 4.7ish as well (the pH=pKa thing) but I think a few people have been saying pH7 and others for that question so I was debating whether I read the question right - educated gamble as you say.

Yeah I got dStotal as -110 too, and as for finding those ionic radius values, what a joke....

How did you work out the question about partial pressure that went as follows:
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Reaction occurs 150-200'c

CH3OH + CO <===> CH3COOH

50.0 mol CH3OH and 50.0 mol CO allowed to reach equilibrium. Total pressure of 32.0 atm. At 175'c, partial pressure of CH3COOH is 22.2atm. Calculate partial pressure for CH3OH and CO.
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My idea was to work backwards from the partial pressure for CH3COOH, e.g. Mole fraction = 22.2atm x 32.0 atm. However, that then gave:

no moles CH3COOH / total number of moles

But how do you know total number moles at equilibrium or number of moles CH3COOH?? Can someone please explain how you are mean't to work it out?

ermm yeh i got -110. and i think the half equivalence was 7 (neutral) because when you have equal quantities of H+ and OH- the solution is always neutral. good luck in D2!!

Re: the half-equivalence point - I went for 4.7ish as well (the pH=pKa thing) but I think a few people have been saying pH7 and others for that question so I was debating whether I read the question right - educated gamble as you say.

Yeah I got dStotal as -110 too, and as for finding those ionic radius values, what a joke....

*Sad Face* also for the hydration of CoCl2.6H20 i got 185.6 as my deltaS(system)

for anyone who sat unit 4 on 17 june 2010, can you remember whether the enthalpy change for the thermal decomposition of Co(2)Cl2.6H20 to water and CoCl2 was exo or endothermic and what the orders of reaction were for the methanol and hydroxide?

order of reaction with respect to methanol was 1st order and zero order with respect to OH- which suggested that OH- reacted in after rate determining step so the mechanism had more than 1 step. thermal decomposition of Co(2)Cl2.6H20 to water and CoCl2 was endothermic, delta S surroundings was negative, delta S system was positive, but delta S surrounding was more negative than delta S system was positive so delta S total was negative something about -181 JK-1mol-1 I believe so reaction was not thermodynamically feasible at room temperature, therefore it was stable at room temperature it would decompose at higher temperatures.