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

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can someone help me please?

on page 153 of the text book, the worked example, says that there is about 20 times more hydrogen carbonate ions in the blood than there is carbonic acid. but then the ratio at the bottom says 10.8/1 ?
?
surely that would mean there was only TEN times as much?

Help appreciated!
:smile:

Reply 1101

CoventryCity

Original post by student777

But surely you can just get that from oxidation numbers? Eg if it's Cl- it has oxisation state -1

And what about this question, from june 2010 question 2:

BrO3- + ....Br- + ....H+ --> ....Br2 + ....H2O

There aren't any charges on the RHS?

Agreed so you must make the charge on the LHS 0 as well

Reply 1102

CoventryCity

Original post by KnuckleheadNinja

I asked this earlier, I have no idea :confused:

Reply 1103

cws121

Thanks i thought that but i just wanted to make sure, sorry about all the questions...can someone explain enthalpy change of solution and its relationship with lattice enthalpy and hydration enthalpy please ??

Reply 1104

KnuckleheadNinja

Original post by CoventryCity

I asked this earlier, I have no idea :confused:

maybe its a mistake? or im hoping so....id rather think the textbook is wrong than i dont understand stuff the day before the exam :eek:

Reply 1105

raj16

do we have to know the manganate + iron, cu+I equations? also wen ur asked to draw a half cell wen do use a Pt electrode instead?

Thanks

Reply 1106

haydyb123

Original post by student777

That's where your cheeky H+ come in handy :wink:

So you know from oxidation numbers Br is +5 and is going to gain 5 electron as it is being reduced to form Br2

So therefore you need 5 electrons provided by Br- so

BrO3^- + 5Br^- + 6H^+ -> 3Br2 + 3 H2O

Charges balances because charges on the left cancel out :smile:

do you see now?

Reply 1107

BA1

Original post by KnuckleheadNinja

maybe its a mistake? or im hoping so....id rather think the textbook is wrong than i dont understand stuff the day before the exam :eek:

Yes, it's a mistake and the ratio is approximately 10. There are LOADS of mistakes in the textbook unfortunately.

Good luck to everyone anyway! I need 79/150 UMS for a grade A, which should equate to approx. 45-55 raw marks. I'm still revising a lot though, you never know what OCR will throw at us (especially after January's F324 paper...)

Reply 1108

monkeyDace

Original post by M_I

Which one is it?

How do you figure it out?

the colour change is from purple ((MnO4)--charge -1) to yollow (Fe3+), the very pale pink of the Mn2+ does not show

edit: -ve??? what did i do wrong???

(edited 12 years ago)

Reply 1109

CoventryCity

Original post by cws121

Enthalpy change of solution is the enthalpy change when 1 mole of a compound dissolves in water

This is done by breaking the ionic compound into its gaseous ion, this is the opposite of lattice enthalpy so has the same magnitude just a different sign

The the gaseous ions are dissolved into water so become aqueous ions in water (this is hydration)

Enthalpy change of solutions can be worked out using born haber cycles if you have the values for Lattice enthalpy and hydration

Reply 1110

KnuckleheadNinja

Original post by BA1

that my friend, is AWESOME! you going for A* then? I wish you luck! Not that you need it of course, because you have brains. :wink:

Reply 1111

hellosarah

Original post by raj16

do we have to know the manganate + iron, cu+I equations? also wen ur asked to draw a half cell wen do use a Pt electrode instead?

Thanks

Yeaaaah you do need to know those equations and i think you use a platinum electrode when you involve ions or non-metals. But when you have to investigate a solid, you use those rod things. I think?

Reply 1112

CoventryCity

Platinum electrodes are used when you have a mixture of two ions like Fe2+/Fe3+. Or when we are using non metals like hydrogen

Reply 1113

sportycricketer

Original post by BA1

Good luck to everyone anyway! I need 79/150 UMS for a grade A, which should equate to approx. 45-55 raw marks. I'm still revising a lot though, you never know what OCR will throw at us (especially after January's F324 paper...)

What are you doing to revise?? I've done all old past papers and the new ones as well :s-smilie:

read through my notes a couple of times as well

Reply 1114

Medifield

I don't understand how to work out the question at the end of the OCR unit 5 exam June 2010. 7b :frown:

can someone please help?

Reply 1115

raj16

Original post by hellosarah

Yeaaaah you do need to know those equations and i think you use a platinum electrode when you involve ions or non-metals. But when you have to investigate a solid, you use those rod things. I think?

surely all the half cells involve ions?
what about the Iodine + thiosulphate, and manganate + hydrogen peroixde??

Reply 1116

haydyb123

Hey I have a question but don't have the mark scheme for it I have the correct answer but I have no idea how to get there using maths rather than just working it out trial and error:

a 167 mg sample of iron reacts with a stream of dry chlorine to form 487mg of solid X the molar mass of Solid X was determned to be 324.6 g mol-1

Calculate the molecular formula of X

Reply 1117

BA1

Original post by KnuckleheadNinja

that my friend, is AWESOME! you going for A* then? I wish you luck! Not that you need it of course, because you have brains. :wink:

I can't get an A*, I had 78/90 in F324 (which is just under 90%) and I'm not bothering to resit it as it clashes with my holiday dates end of next week :P I also did badly on the A2 practicals so had less than 90% in total for them. I just need a grade A for university so I'm not bothering to try and get the A*.

PS: That's why you should try and do well in the AS modules, to take the pressure off A2! I'm walking into an A2 maths exam next week needing 23/100 raw marks for a grade A, or 90/100 for an A* lmao!