* Chemistry * Predictions / discussion / aftermath * (1 thread please)

EmacB

So glad i remembered to ask this..
When they specifically state the number of points for you to make in an answer, do you lose marks for stating more then that amount?
For example if a question says "give two differences between a nuclear and chemical reaction' and you go and put 3 or 4 differences do do you lose marks?
Or what if you put 2 right ones down and a wrong one will you still get full marks?

Just wondering because sometimes id know a few points but not be completely certain if they were all right so id like to just put them all down.

EmacB

macskanadrag wrote: »

This is from Revise Wise, isn't it? I came across that too and my calculator is not exactly cutting edge, definitely doesn't have the antilog function. It's ok though because accoring to the Chemistry syllabus:

"1. Use of calculators
Students will be expected to have an electronic
calculator conforming to the examination regulations
for the duration of the course and when answering
the examination paper. It is recommended that
students have available the following keys:
Ordinary level: +, –, x, ÷, 1/x , log10 x, 10^x, EE or EXP,
memory.
Higher level: as above, and x^2, (root)x, x^y.
In carrying out calculations, students should be advised to
show clearly all expressions to be evaluated using a calculator.
The number of significant figures given in the answer to a
numerical problem should match the number of significant
figures given in the question."

Better not see an antilog question on the paper!

Antilog is just 10 to the power of X !
I think:P

macskanadrag

EmacB wrote: »

Antilog is just 10 to the power of X !
I think:P

Hah, FACEPALM! You're right

And thank you

ride-the-spiral

I always hated the way chemistry used the term antilog, you'd think they could take the 5 minutes to explain the relation between log(x) and 10^x.

mpdg

macskanadrag wrote: »

This is from Revise Wise, isn't it?

The exact same question is in both my class notes and in my copy of Revise Wise XD. I copped that last week as it happens.

So, 10^x is antilog. Grand. What is the x though? I feel so stupid today..

*stares at notes

Zhavey

Best of luck to you all tomorrow doing Chemistry.

Gavarn

Major use of ethene = making plastic ??
Major use of ethyne = welding??

Exothermic

Yup! You could also say ethene is used in the commercial ripening of fruit such as bananas.

mpdg

Gavarn wrote: »

Major use of ethene = making plastic ??
Major use of ethyne = welding??

Yep, ethene is used to make PVC
yes to ethyne too

macskanadrag

mpdg wrote: »

So, 10^x is antilog. Grand. What is the x though? I feel so stupid today..

*stares at notes

In this example:

2.3= log[H+]
-2.3 = log[H+]
[H+] = antilog - 2.3
[H+] = 0.005

x is the pH value

mpdg

macskanadrag wrote: »

In this example:

2.3= log[H+]
-2.3 = log[H+]
[H+] = antilog - 2.3
[H+] = 0.005

x is the pH value

*bangs head against wall at own idiocy*

as in, the given pH? 10^2.3 - 2.3... No..
I'm going to stop asking now because I clearly am beyond saving :rolleyes:.

Thank you!

The best of luck tomorrow everyone!

soccymonster

Zhavey wrote: »

Best of luck to you all tomorrow doing Chemistry.

Thanks! 'coz I'll need it.

*runs off to learn atomic theory*

Sigh.

Exothermic

I'm seriously losing my motivation! :P Ugh.
This will be my last exam and it's gonna be such an anti-climax walking outside of school and it'll probably be raining. :P Throw about some more predictions and difficult questions. I need to get remotivated.

macskanadrag

mpdg wrote: »

as in, the given pH? 10^2.3 - 2.3... No..

The best of luck tomorrow everyone!

Aye, well:

pH = -log [H+]
2.3 = -log [H+]
-2.3 = log [H+]
[H+] = antilog 10^-2.3

Best of luck to all!

macskanadrag

Exothermic wrote: »

I'm seriously losing my motivation! :P Ugh.
This will be my last exam and it's gonna be such an anti-climax walking outside of school and it'll probably be raining. :P Throw about some more predictions and difficult questions. I need to get remotivated.

Go somewhere, dunno, do something you wanted to for a loooong time! Watch that movie, buy that game, go on holidays, stuff! But for now - do not think about that:rolleyes:

Ok, awkward questions, they do my head in.

1. How do you know what goes into the burette in different titrations and what goes into the flask? (eg. organic - sodium dichromate is upstairs in the ethanal one, downstairs in the ethanoic acid one)

2. Someone asked a great question before: do you know what happens if you write down more than the required number of answers (give two properies of x) and then if one of them is wrong, will that cancel out the whole answer?

3. Why is Fe^3+ green-brown in the redox halogen reaction but yellow in the Chatelier eqm with CNS?

Whatsername92

The more I read, the more I realise I don't know!
SCREWED for tomorrow!

Gavarn

In a steam distillation experiment 20.0g of plant material were heated in the presence of steam. Only 0.250g of pure organic liquid was obtained. Calculate the percentage yield.

Probably not a hard question but takes ages to go onto examinations.ie on an iPod. Thanks

Exothermic

:P What's the answer to the first one?

Exothermic

Gavarn wrote: »

In a steam distillation experiment 20.0g of plant material were heated in the presence of steam. Only 0.250g of pure organic liquid was obtained. Calculate the percentage yield.

Probably not a hard question but takes ages to go onto examinations.ie on an iPod. Thanks

It seems to easy to be true but...

0.25 / 20 = 0.0125 x 100 = 1.25%

partyatmygaff

Gavarn wrote: »

In a steam distillation experiment 20.0g of plant material were heated in the presence of steam. Only 0.250g of pure organic liquid was obtained. Calculate the percentage yield.

Probably not a hard question but takes ages to go onto examinations.ie on an iPod. Thanks

1.25% seeing as you only got 0.25g of the oil from 20g of plant material.

Gavarn

Exothermic wrote: »

It seems to easy to be true but...

0.25 / 20 = 0.0125 x 100 = 1.25%

Yeah that's what I thought at first but I think everything in chemistry is out to seriously confuse people haha

Thanks to both of yous for the reply

macskanadrag

Exothermic wrote: »

:P What's the answer to the first one?

You mean this?
1. How do you know what goes into the burette in different titrations and what goes into the flask? (eg. organic - sodium dichromate is upstairs in the ethanal one, downstairs in the ethanoic acid one)

Ethanol to ethanal, in burette: ethanol and sodium dichromate, ... DAMMIT! I just looked it up to make sure and in different notes i have different things!
In Revise Wise:
burette: ethanol + sodium dichromate + water
con. flask: dil. sulfuric acid + anti-bumping.
Is that correct?

Gavarn

macskanadrag wrote: »

You mean this?
1. How do you know what goes into the burette in different titrations and what goes into the flask? (eg. organic - sodium dichromate is upstairs in the ethanal one, downstairs in the ethanoic acid one)

Ethanol to ethanal, in burette: ethanol and sodium dichromate, ... DAMMIT! I just looked it up to make sure and in different notes i have different things!
In Revise Wise:
burette: ethanol + sodium dichromate + water
con. flask: dil. sulfuric acid + anti-bumping.
Is that correct?

That's correct for preparation of ethanal

macskanadrag

Gavarn wrote: »

That's correct for preparation of ethanal

cheers!

Do you know why the sodium dichromate is in the flask when preparing ethanoic acid and not in the dropping funnel...? That is, if it IS in the flask of course:rolleyes:

Exothermic

I got the impression that for ethanoic acid, because you need absolute full oxidation that it's the ethanol up in the dropping funnel so that each drop of it can be oxidised as it falls down? The other can just sorta flow through and be oxidised, but you don't want full oxidation? :P I'm kinda of rambling here.

Gavarn

macskanadrag wrote: »

cheers!

Do you know why the sodium dichromate is in the flask when preparing ethanoic acid and not in the dropping funnel...? That is, if it IS in the flask of course:rolleyes:

It is in the flask when preparing ethanoic acid.. And emmm give me half an hour to route through my notes and find out why it's not in the dropping funnel :P if you find out before me, please do share haha

Gavarn

"A primary alcohol can be oxidised to a carboxylic acid by refluxing with sodium dichromate solution that is acidified with sulphuric acid" so it must have to be acidified for some reason

macskanadrag

Gavarn wrote: »

"A primary alcohol can be oxidised to a carboxylic acid by refluxing with sodium dichromate solution that is acidified with sulphuric acid" so it must have to be acidified for some reason

It's acidified in the ethanal one as well. It acidifies upon landing into the sulfuric acid. Something along the line that Exothermic said... oxidising agent... in excess...

Actually, why do you need the sulfuric acid, why does the dichromate have to be acidified? Is a "to ensure the reaction goes to completion" sort of answer the right answer to that?

Exothermic

macskanadrag wrote: »

It's acidified in the ethanal one as well. It acidifies upon landing into the sulfuric acid. Something along the line that Exothermic said... oxidising agent... in excess...

Actually, why do you need the sulfuric acid, why does the dichromate have to be acidified? Is a "to ensure the reaction goes to completion" sort of answer the right answer to that?

Think back to the conditions necessary for oxidation. Acidic (ie. H+ ions) and the dichromate.
We don't use HCl because of the Cl- ions, so H2SO4 is used to satisfy the acidic part of oxidation.

Remember
Oxidation: Acid + Dichromate (and heat)
Reduction: H2 and a Ni catalyst (and heat)

Gavarn

" the sodium dichromate being in the reaction flask means the oxidising agent is always in excess" ...? Sounds right?

Have no idea why it needs to be acidified