Equilibria, Energetics and Elements (F325) - June 2011 Exam.

Yeh, it is, cheers for help though

Anytime!


FAO: Everyone

When a question asks you to write the rate equation for this reaction: I wrote
Rate =

according to the mark scheme it's K= Rate over reactants?

Looking at wrong part of the mark scheme, silly me

Suggest why the lattice enthalpy of an ionic solid cannot be measured directly?!

Ms:
Impossible/difficult to get gaseous ions (without them reacting)/difficult to vapourise ions and measure the enthalpy change at the same time/


Textbook: Its impossible to form one mole of ionic lattice from gaseous ions

2 different things and not sure I know what they are talking about!

Shed some light

No I think it's all legacy!

Thanks...do you know if MgCO3 is a gaint ionic lattice?

Not trying to patronise you by writing out the definition of Lattice Enthalpy but it i'm trying to show you how to see it like I do...

Say for instance NaCl(s)

Na+ (g) + Cl-(s) -> NaCl(s)

Trying to get the exact volumes of these two gaseous ions to form one mole of NaCl is near enough impossible to do, add to the fact that Na+ and Cl- will be extremely reactive with anything else, and it makes it virtually impossible to directly measure lattice enthalpy.

Does that sort of help?

Stop patronising meee!:P am the GOD of chemistry
Perhaps u meant cl(g)+..
So ur saying gaseous ions are too reactive and might react with something else when we measuring enthalpy change
Makes more sense now
But if you had looked at the other answer is totally different to this. That one is talking

about vaporising ions which is more of a problem in measuring atomisation enthalpy

TBH, this unit is a piece of p*ss. Yet i have been unable to score well in any past paper!!

It is very easy, the thing is you don't have to do a lot to go wrong. what kind of marks are you getting?

FeO4 2- + H+ -------> O2 + H20 + Fe 3+
Fe6+ Fe3+ - reduced by 3, so you want an even number really as O2- -> O and you can't have 1.5O2- lol
so if you try
2Fe6+ -> 2Fe3+ overall reduction of 6
you have 8 O2- ions now, but you only want 3 of them to be oxidized to O2
so that means you'll have 1.5 O2, and 5H2O
you don't really want halves though, so double it again to...
4Fe6+ ->4Fe3+ overall reduction of 12
16O2-, so you want 6 O2- to be reduced, giving you 3O2 molecules
this leaves you with 10 O2-, which stay as O2-
10 O2- means you must have 10H2O, so to balance that you need 20H+

giving overall equation 4FeO42- + 20H+ -> 4Fe3+ + 10 H2O + 3O2

probably would've taken me a LOT longer if you didn't post the answer haha

Okay, I didn't actually try to do it before, and you're right it's really hard!
Obviously I can see that the answer is balanced, but i have no idea how you'd go about doing that! If your teacher can't do it, i've got no chance i'm afraid! Sorry I wasn't more help.

why is the h-o-h bond angle in the water ligand 107 rather than 104.5?

In water, there is 2 bondings pairs and 2 lone pairs (on the oxygen) this increases repulsion between electrons,
Whereas in the ligand there, is 3 bonding pairs and 1 lone pair, which minimizes the repulsion between electrons, hence why bond angle is 2.5 degrees more!