Chemistry - General Thoughts (1 Viewer)

[Bush Elephant]

*cries* studies so hard for chem and hardly anything was in it .. no CFC's etc.... o well.. bio was easy, chem not to bad....physics *shudders* i am hoping 2 be easy.. but aren't we all lol

 

[ceh]

Hmmmmyerss

I dont knwo about it was abit different from past papers, calculations were alot harder then previous and the lack of radioisotopes and CFC was rigged!!!
What was the correct answer to the Haber process q's, stumped me for like 10 minutes?

 

[Kd14]

The decrease in volume = increased pressure = more yield. NH3 went up, while H2 and N2 dropped down then everything went back to equilibrium again.

There's a thread for this question. =D

 

[AreYouAlright?]

For lead you definitely needed a confirmatory test, so i simply tested with HCl in which it forms a white ppt and confirmed with I^- in which it forms a yellow ppt.

 

[punki_pixi]

For lead you definitely needed a confirmatory test, so i simply tested with HCl in which it forms a white ppt and confirmed with I^- in which it forms a yellow ppt.

So if I said you add HCl and looked for a ppt if lead was present do you think that would earn at least one mark?

 

[alia]

Tests for cations and anions

could u have said aas to test for lead?

Yeh i hated having to remember tests for cations and anions.

My teacher is a HSC marker for like 20 yrs or something and they are very specific about those.

You have to have an initial test and a confirming test. You have to have the amounts and maybe the concentrations. You HAVE to say what you are puting them in e.g. test tube. and you have to state "if there is a yellow ppt then Pb is present if there is no ppt then Pb is not present"

I put add 1ml HCL in test tube (0.1mol/L) to 1ml of the sample, if there is no white ppt then no Pb2+ is present. if white ppt, then add NaI 1ml 0.1mol/L. If a yellow ppt forms then Pb2+ is present if no yellow ppt forms then no Pb2+ is in sample

 

[~ ReNcH ~]

Yeh i hated having to remember tests for cations and anions.

My teacher is a HSC marker for like 20 yrs or something and they are very specific about those.

You have to have an initial test and a confirming test. You have to have the amounts and maybe the concentrations. You HAVE to say what you are puting them in e.g. test tube. and you have to state "if there is a yellow ppt then Pb is present if there is no ppt then Pb is not present"

I put add 1ml HCL in test tube (0.1mol/L) to 1ml of the sample, if there is no white ppt then no Pb2+ is present. if white ppt, then add NaI 1ml 0.1mol/L. If a yellow ppt forms then Pb2+ is present if no yellow ppt forms then no Pb2+ is in sample

Considering the question asked for a "test" (no plural), how can students be expected to perform both an initial and a confirmatory test? - this, to me, implies the need for two tests, not one.

 

[~ ReNcH ~]

It's actually KI. Pb reacts with KI to form yellow PbI(sub)2 precipitate.

Adding hydroxide ions also produces a white precipitate.

 

[falltopieces]

Adding hydroxide ions also produces a white precipitate.

Um I put: 1. Add drops of Na2S into a sample of acidified water.
2. If precipitate forms then lead ions are present.

Is that wrong?. I got it from my notes on how ito identify the presence of heavy metals and since Pb is a heavy metal so I wrote that as the answer.

 

[seadonkey]

i just sed add NaI n it forms a yellow precip

 

[xiep]

lol, yeah.

Oh, was the Lead Acid Cell the big square one, or the little round one? Fuck I hope it was the big square one; I have absolutely no idea about the batteries, and I pretty much made it up using the std. potential reactions with Lead and MnO4, lol.

Yes... the big square one is the lead-battery. Although I thought giving us pictures doesnt do justice to people who know their batteries but not neccessarily recognising the pictures.

 

[Rax]

Well the dotpoint said you had to know structure thus you should be able to identify the structure of your battery from the pictures of them

 

[alia]

Pb2+

Considering the question asked for a "test" (no plural), how can students be expected to perform both an initial and a confirmatory test? - this, to me, implies the need for two tests, not one.

Yes well they are two separate tests but you need to have both of them to confirm it.
More like a two step process rather than two steps. Cos i think adding HCl to other cations forms a white ppt as well so u have to make sure it is lead.
Trust me, my teacher made sure we answered that one so specifically. She's been a marker for so long.

She said that people that say "add HCl to sample, if white ppt add NaI, if a yellow ppt forms then Pb is present" will get no marks. It's a procedure so you have to say what you are putting them in, how much you are putting in and im not sure about the concentrations. You also have to put in "if there is no white/yellow ppt then no Pb is present"

It's like another question for a qualitative test for hardness. You have to say "put x amount oif soap into a beaker with x amount of water, if a lather forms then water is soft, if no lather forms but a scum does then the water is hard"

 

[shobadaynature]

i guess the exam was generally ok except there's like 3 7 marks q for me!! (one of them in the shipwreck option)...argh!!!

for the lead test, we had to write 2 tests? I remembered only the HCl but all chlorides are soluble, then i got confused in the exam.

generally, i'd just say, i survived...:S

 

[Dumsum]

What's wrong with AAS for a lead test? Since you're only checking the spectrum that corresponds to lead. I was thinking if the lead concentration was uber small (which for some reason I got from the wording of the question...does anyone remember it exactly? has someone already posted it?) then no precipitate would be noticeable.

 

[alia]

What's wrong with AAS for a lead test? Since you're only checking the spectrum that corresponds to lead. I was thinking if the lead concentration was uber small (which for some reason I got from the wording of the question...does anyone remember it exactly? has someone already posted it?) then no precipitate would be noticeable.

"It is suspected that lead has contaminated the creek. Describe a CHEMICAL test that could be carried out on the water sample to determine the presence of lead" (2marks)

I assume that AAS is not really a chemical test. I'm not sure really.

 

[sarah kiss]

it was alrite

i guess da paper was ok i jus hope everyone else did crap so i can get scaled up Ha ha but