perthhigh.co.uk

[EuanHunter]

I take it the exam is a piece of cake no worries that we can help you with?!

[LisaCMarshall]

Hi just wondering if i could have some last minute advice for my prelim! I know my content well enough i think so hopefully thatll all go well, but its the way to answer them in exams that im worried about. I have some past papers but have no answers to them, so I dont know if i should just redo homework questions? any advice would be appreciated!

[EuanHunter]

Right first thing is dont panic Lisa! If you are happy wit the content then you've broken the back of the prelim already! My top tips are?

1) Jot down all the formulae for the calculations right at the start, somewhere on your exam paper. You know the triangles for mass = no. of moles x RFM and no. of moles = concentration x volume

2) Start on a question that looks straight forward, skip any questions your getting stuck on, you can always go back to them when you've done all you can.

3) Can you recognise all your functional groups and recognise all your reactions, e.g. can you pick out an ester from an amine? oxidation from condendsation?

Any specfic problems Lisa just post them and I'll keep a regular eye on the forum over the weekend.

All the best for Tuesday! remember give:

www.evans2chemweb.co.uk

a go
username: perthhigh
password: argon

[LisaCMarshall]

thank you Mr Hunter now i have some other questions for you
1) are cross links any type of hydrogen bonds?
2) do i need to remember every ppa that i did?
3) Avadgadro!! need help please! how do i go about this question: How many moles of hydrogen are in 8g of methane?
4) when a long chain carboxylic acid is added to glycerol does it always give an oil, then that oil has to be hydrogenated to get a fat?
thanks!

[EuanHunter]

Hi Lisa, just a few questions then eh?!!

1) Not quite sure what you mean in question 1, re clarify that one for me.

2) Yes! you never know what one might come up, if any!

3) Ah my favourite type of question... a calculation. Aye tell you Lisa you've made my Saturday night, nothing I'd rather do than a chemistry calculation

methane = CH4 therefore every one methane molecule contains 2 molecules of hydrogen (H2)

step 1: calculate no. of moles of methane in 8g using the triangle

no. of moles = mass/RFM of methane
................. = 8g/16g
................. = 0.5 moles

from what i said earlier 1 mole of methane --> 2 moles of H2
therefore 0.5 moles CH4 --> 1 mole of hydrogen

if you are to use Avogadro 1 mole = 6.02x10(23)
so you'd have 6.02x10(23) molecules of hydrogen
or 2 x 6.02x10(23) atoms of hydrogen = 1.204 x10(23)

4) No, remember glyerol (an alcohol) reacts with a fatty acid (a long chain carboxyic acid) to make an ester known as a fat or an oil.

The only difference between a fat an an oil is that an oil is unsaturated i.e. it has double bonds in the molecule.

So whether you get a fat or an oil when you react with gylcerol depends on whether the fatty acid molecule has double bonds.

If you use a fatty acid with double bonds in it then you'll get an oil.

If you use a fatty acid without double bonds in it then you'll get a fat.

Hope that helps. Sorry about question 1 but if you are a bit more specific i'll be of better help. Are you meaning something like DNA, whether the reason it forms the helix if because of hydrogen bonding?

[LisaCMarshall]

thanks that was a great help! i mean that, hydrogen bonds are cross links yes? so anyhting at all which contains hydrogen bonds, will it have cross links as well then ie water? or is it just certain things like kevlar which can have hydrogen bonds which are cross links? hope that makes more sense!

[EuanHunter]

Generally cross-linking refers to polymers where smallish amounts of another compound are added in to form bonds between the various polymer chains. This makes the more rigid, like bakelite.

---------------------------------- polymer chain
/ ... / .... / ..... / .... / .... /..../ cross link bonds
---------------------------------- polymer chain

Essentially hydrogen bonding and cross-linking are pretty similar just different degrees of strength. Hydrogen bonding being less strong at holding chains together than actually reacting them together with side chains.

[LisaCMarshall]

hi got some calculation questions for you! firstly, for hess' law calculations. When you have to 'use the data book' for getting your equations and enthalpy changes yourself, what page is it? i cant seem to find it. Also, redox reactions. Question: in an experiment to find the % of iron in a sample of iron wire, 0.70g of the wire was dissolved in excess Sulphuric acid.
reaction: [Fe(s) + 2H^+(aq) ---> Fe^2+(aq) + H2(g)]
The iron II solution was then tirtated against 0.1mol/l potassium permanganate soution, 24cm3 of which was required for complete reaction.
1) write an eqn for the reaction between iron II ions and the permanganate ions.
2) calculate number of moles of permanganate used, the number of moles of iron present and the % of iron in the wire.

If you would be able to give me a worked answer to this that would be extremely helpful thanks.

[EuanHunter]

Not sure of the page number in the new data booklets but the title is:

ENTHALPIES OF FORMATION AND COMBUSTION
SELECTED SUBSTANCES

Redox question:

first step write a balanced redox equation (use the electrochemical series in the databook to help here):

Fe2+(aq) --> Fe3+(aq).. +e-

MnO4-(aq) + 8H+(aq) +5e- --> Mn2+(aq) + 4H2O(l)

remember that the electrons must be on opposite sides of the arrow and must have the same number of them in order to cancel them out, therefore we must muliply the first eqn by 5 before combining them to give:

5Fe2+ .. + MnO4-(aq) + 8H+(aq) --> Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)

the no. of moles of permanganate is easy, standard grade!!

no of moles = concentration x volume

................. = 0.1 x (24/1000)
................. = 0.0024 moles

Looking at the balanced equation we see 1 mole of MnO4 reacts with 5 moles of Fe2+

therefore we do 5 x 0.0024

= 0.012 moles of Fe2+

mass of Fe = 0.012 x 55.8
................ = 0.6696g of iron

therefore the % = (0.6696/0.7) x 100

....................... = 95.7%


Hope that helps, any other problems Lisa just give me a shout ok!

[LisaCMarshall]

thanks that was a great help. Wondering about discrete molecules, is it the high mp and bp that makes sulphur and phosphorous discrete? Also, when naming benzene rings when do i know to use 'benzene' or 'phenyl'? And reducing and oxidising agents, if i was asked to identify from a reaction which was which, what is the best way to do this? write on charges?

[EuanHunter]

Discrete molecules usually have very low mp + bp becuase there are only weak van der waals bonds holding them together. However as the molecules get bigger there are more van der waals bonds. So although the bonds are weak there are loads of them which means higher mp + bp. The other factor which makes a difference is shape. The closer that molecules can pack together the stronger the van der waals bonds are and therefore the higher the mp + bp.

Sulphur forms 8 membered rings which are slightly puckered but essentially flat so like Ikea flatpacks you can get lots of them close together so you would expect the mp to be high --> its 115oC

Whereas phosphorus forms a kind of pyramid structure which means the molecules cant pack together as well, therefore you'd expect a lower mp in comparison to sulphur --> its 44oC

use of benzene and phenyl, well benezene you would use if you were just talking about C6H6 whereas phenyl is C6H5 because it has lost one of the hydrogens so that it can form a bond with another molecule.

Phenyl is just a functional group just like carboxyl or hydroxyl so for example if you have an alcohol with a phenyl group

C6H5OH it is called phenyol

Reducing agent, something that causes something to be reduced and therefore it has been oxidised.

Liken it to a cloth, it's a cleaning agent it causes something to be cleaned but in the process it will get dirty.

e.g.

Fe2O3 + CO --> Fe + CO2

iron oxide has been reduced because it has lost oxygen
carbon monoxide has been oxidised because it has gain oxygen

therefore iron oxide is the oxidising agent
and carbon monoxide is the reducing agent

so if it involves electrons remember OIL RIG and work it out from there.