**Chemistry...Before/After

reznov

hollingr wrote: »

Kc= [I3] / [I2][I-]

initially: [I2]= 0.08, [I-] = 0.24, [I3] = 0
change: [I2]= -0.0793, [I-] = -0.0793, [I3] = +0.0793
@equilibrium: [I2]= 0.0007, [I-] = 0.1607, [I3] = 0.0793

So, Kc at equilibrium = (0.0793) / (0.0007)(0.1607)

Kc = 704.95

I thought that Kc was wild initially but I got that too.

DepoProvera

What a beautiful paper!

montown

hollingr wrote: »

Kc= [I3] / [I2][I-]

initially: [I2]= 0.08, [I-] = 0.24, [I3] = 0
change: [I2]= -0.0793, [I-] = -0.0793, [I3] = +0.0793
@equilibrium: [I2]= 0.0007, [I-] = 0.1607, [I3] = 0.0793

So, Kc at equilibrium = (0.0793) / (0.0007)(0.1607)

Kc = 704.95

is this definitely right? i got the same answer and same method but was sure i messed it up?

also, did anyone get Q10 part c out? the one about the silver bracelet/necklace whatever it was, i was stumped by it altogether!

ivanh

partyatmygaff wrote: »

They're not isomers unfortunately.

Leaving aside the carbon backbone, there's an extra oxygen atom in a carboxylic acid.

Definitely not

Propanoic Aid - C2H5COOH (C3H6O2)

Propanone - CH3COOCH3 (C3H6O2)

DepoProvera

Answered all Qs bar Q5..

Why does propanone not react with Fehlings reagent? I said as it isn't oxidised as easily but I assumed you had to give an explanation of why it wasn't?

Alkenes more reactive than alkanes? I was unsure but talked of the negativity of the bond being reactive etc..

Also raging as I had looked at the definition for 'bond energy' last night as I saw it in a paper and noticed it wasn't in my list of definitions, but got distracted and ended up not learning it.. Still had my 8 in Q4 but still, sucks!

DepoProvera

ivanh wrote: »

Definitely not

Propanoic Aid - C2H5COOH (C3H6O2)

Propanone - CH3COOCH3 (C3H6O2)

That's not the formula for propanone.

RedTexan

ivanh wrote: »

Definitely not

Propanoic Aid - C2H5COOH (C3H6O2)

Propanone - CH3COOCH3 (C3H6O2)

There's only one oxygen in propanone

ChemHickey

ivanh wrote: »

Definitely not

Propanoic Aid - C2H5COOH (C3H6O2)

Propanone - CH3COOCH3 (C3H6O2)

The second one you have mentioned is methyl ethanoate.

reznov

DepoProvera wrote: »

Answered all Qs bar Q5..

Why does propanone not react with Fehlings reagent? I said as it isn't oxidised as easily but I assumed you had to give an explanation of why it wasn't?

Alkenes more reactive than alkanes? I was unsure but talked of the negativity of the bond being reactive etc..

Also raging as I had looked at the definition for 'bond energy' last night as I saw it in a paper and noticed it wasn't in my list of definitions, but got distracted and ended up not learning it.. Still had my 8 in Q4 but still, sucks!

Ketones don't possess hydogen bonds. Thus, it makes them less reactive and it's practically impossible to oxidise a ketone without breaking it's carbon-carbon chains.

finality

Did anyone do question 10(c)? I found it so confusing, what did ye get for it?

FaoiSin

finality wrote: »

Did anyone do question 10(c)? I found it so confusing, what did ye get for it?

Didn't do it in the exam but I'll do it out tomorrow after Ag. Was it Stoichiometry?

finality

YeaaahFaoiSin wrote: »

Didn't do it in the exam but I'll do it out tomorrow after Ag. Was it Stoichiometry?

yeah, it was a bit of an odd one though.

My answers were
i) Aluminium is oxidized (though I now have a feeling I wrote silver in the exam )
ii) 0.0003 moles
iii) 0.0054 g
iv) 0.0744 g

though they could be completely wrong, I did the question because I really liked the acids/bases part :L

Chuchoter

I didn't do anything crazy for the KC question and I got 4..which seems more reasonable than 705 or w/e

Hayezer

So for Q1 e) did you just have to do the M1 V1/N1 =M2 V2/N2? Did you have to multiply or divide your answer by 2 or something?

finality

Chuchoter wrote: »

I didn't do anything crazy for the KC question and I got 4..which seems more reasonable than 705 or w/e

(0.0793)/(0.08)(0.24) gives 4.13

but that would mean there was the same concentration of I2 and I- in the vessel at equilibrium as when you added them, so where would the I3 come from?

Hayezer

Also whats a consequence of suck back :pac:? Don't actually know what suck back is, I just know how to prevent it :L :pac:

Hayezer

Instrumentation technique to measure concentration of a heavy metal ion in water?

finality

Hayezer wrote: »

Also whats a consequence of suck back :pac:? Don't actually know what suck back is, I just know how to prevent it :L :pac:

I said the boiling tube containing the glass wool could shatter.

DepoProvera

Hayezer wrote: »

Instrumentation technique to measure concentration of a heavy metal ion in water?

AAS

DepoProvera

finality wrote: »

I said the boiling tube containing the glass wool could shatter.

Also said this. Also the stage in the experiment I said when disassembling the apparatus

Hayezer

Not too dissapointed now that I look back over the test, would be happy overall if I didn't mess up the pink-green-pink colour in Q9, I said green-pink-green, and if I had just had a quick look at heat of formation...12 marks gone straight away, think I got all of the rest that question right

RedTexan

Hayezer wrote: »

So for Q1 e) did you just have to do the M1 V1/N1 =M2 V2/N2? Did you have to multiply or divide your answer by 2 or something?

Yeah it was just a simple titration equation, the same as comes up ever year. Don't understand why people were attempting to confuse the issue, the answers were .0012 and 4. something I think. For the first part about how much Sodium carbonate was necessary you needed to divide by two and get the mass of .0025 moles

Hayezer

RedTexan wrote: »

Yeah it was just a simple titration equation, the same as comes up ever year. Don't understand why people were attempting to confuse the issue, the answers were .0012 and 4. something I think. For the first part about how much Sodium carbonate was necessary you needed to divide by two and get the mass of .0025 moles

Do you mean 0.12? I got that, and then 4. something alright

Heatproof

Can colorimetry be a valid answer for the "Instrumentation technique to measure concentration of a heavy metal ion in water?" Question

Coeurdepirate

finality wrote: »

I said the boiling tube containing the glass wool could shatter.

I wrote "danger of explosion" cause I was thinking of this but I didn't know how to phrase it :L

RedTexan

Heatproof wrote: »

Can colorimetry be a valid answer for the "Instrumentation technique to measure concentration of a heavy metal ion in water?" Question

Atomic Absorption Spectrometry, what you said is for measuring the amount of free chlorine

montown

finality wrote: »

yeah, it was a bit of an odd one though.

My answers were
i) Aluminium is oxidized (though I now have a feeling I wrote silver in the exam )
ii) 0.0003 moles
iii) 0.0054 g
iv) 0.0744 g

though they could be completely wrong, I did the question because I really liked the acids/bases part :L

could you explain how you got those answers? think i got something similar but not too sure

finality

montown wrote: »

could you explain how you got those answers? think i got something similar but not too sure

i) I figured that sulfur was more electronegative than aluminium, so aluminium was + something on the right but had an oxidation number of 0 on the left so it must have lost electrons. Not sure about it though.
ii) divided the mass of sulfur they gave by the atomic mass of sulfur
iii) got the ratio of moles of aluminium to moles of sulfur, found the number of moles of aluminium, multiplied by atomic mass of aluminium
iv) Got the ratio of moles of sulfur to moles of silver, found the mass of silver in the silver sulfide, added the mass of sulfur.

partyatmygaff

ivanh wrote: »

Definitely not

Propanoic Aid - C2H5COOH (C3H6O2)

Propanone - CH3COOCH3 (C3H6O2)

That's not propanone's formula.

CH3-CO-CH3 is the structural formula of propanone.

DepoProvera

partyatmygaff wrote: »

That's not propanone's formula.

CH3-CO-CH3 is the structural formula of propanone.

Bit late :P off topic but partyatmygaff you're doing Pharmacy right? Would ye recommend doing it?

It neny!

hey! one question...
i made a mistake on the 15 mark calculation in Q2 from the very start but the rest of the calculation was correct, does that mean i lose all 15 marks?

Just Like Heaven

No. You'll probably get like, 9.

hollingr

depends on what kind of mistake you made..

It neny!

I forgot the density formula ...thought d=v/m
so how much marks would i lose if i only got that part wrong?

hollingr

-3 I think, assuming you got everything else correct.. i think that part would be a 5 x 3 marks.

I think it's a bit late for analysing marks though, how can you even remember at this stage!

Darksider

hollingr wrote: »

-3 I think, assuming you got everything else correct.. i think that part would be a 5 x 3 marks.

I think it's a bit late for analysing marks though, how can you even remember at this stage!

this! i don't even remember what q2 was about much less fine details about how i answered them haha

Bittersweet Disaster

Hey, does anyone know where I can get the marking scheme for Chem HL 1996?

checkcheek

Bittersweet Disaster wrote: »

Hey, does anyone know where I can get the marking scheme for Chem HL 1996?

its kinda pointless doing exam papers from before 2002 because it was a different syllabus so the things examined are not the same.