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OCR F321 - 23rd May

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Reply 40
I have this exam tomorrow... haven't done any revision... have such a bad flu that cant even move.. I got 68 in the Jan one.... 4th time retake...:colondollar:... hope its an easy paper
guys, they won't ask us anything not in the book

if they do, then we'll kill them.
Original post by sixthformer
guys, they won't ask us anything not in the book

if they do, then we'll hunt them.
fghh
Reply 43
anyone what do we need to know on water of crystallisation
Oxidation Numbers

All elements in their natural state have the oxidation number zero.
ie Hydrogen, H2 oxidation number is 0

Oxidation numbers of the atoms of any molecule add up to zero.
ie Water H2O, sum of oxidation numbers = 0

Oxidation numbers of the components of any ion add up to the charge of that ion.
ie Sulphate, SO4 2- sum of oxidation numbers = -2

The rules in order of priority

1. Oxidation numbers of elements in groups 1, 2 and 3 are always +1, +2, +3
2. The oxidation number of Fluorine is always -1
3. The oxidation number of Hydrogen is usually +1
4. The oxidation number of Oxygen is usually - 2
5. The oxidation number of Chlorine is usually -1

Examples

Deduce the oxidation number of Cl in NaCal

Sum of oxidation numbers must add up to zero.
Therefore Na must be +1
Oxidation number of Cl must balance this
Therefore oxidation state of Cl in NaCl is -1

N2O - O is -2, there are 2 N, so each must be +1

SO4 2- Four O at -2 each gives a total of -8. The ion has an overall charge of -2, so the Oxidation number of S is +6.

CO3 2- Three O at -2 each gives a total of -6. The ion has an overall charge of -2, so the Oxidation number of C is +4.

MnO4 - Four O at -2 each gives a total of -8. The ion has an overall charge of -1, so the Oxidation number of Mn is +7.

Cr2O7 2- Seven O at -2 each gives a total of -14. The ion has an overall charge of -2, so the Oxidation number of each Cr is +6.

NH3 - H is +1, there are 3 H, so N must be -3

KNO3 - O is -2 and K has its normal oxidation number of +1. KNO3 has no overall charge, so N must balance -6 from the 3 O and + 1 from the K. N is +5.
Original post by kingam
anyone what do we need to know on water of crystallisation


How to calculate the 'water of crystalisation' so work out the 'x' in •xH2O.
And just know the definitions of hydrous and anhydrous (with/without water respectively)
Original post by EternalDoom
Chlorine in cyclohexane is yellow (most reactive)
Bromine in cyclohexane is orange
Iodine in cyclohexane is purple (least reactive)


I thought chlorine was light green??
Original post by dream_merchant
I thought chlorine was light green??


Check the mark scheme, they accept 2-3 colours, my textbook taught me yellow
Reply 48
CAN SOMEBODY TEACH ME HOW TO WRITE IONIC EQUATIONS PLEASE?thanks x
Reply 49
Original post by dream_merchant
I thought chlorine was light green??


chlorine is pale green in both water and cyclohexane!
what colour is iodine in water. Brown?
Reply 51
Original post by dream_merchant
what colour is iodine in water. Brown?


its brown your right then in cyclohexane it pink
Original post by kingam
its brown your right then in cyclohexane it pink


Purple?!
Reply 53
Original post by dream_merchant
what colour is iodine in water. Brown?


in water:cl2 is pale green..Br2 is orange and I2 is brown
in cyclohexane:cl2 is pale green..Br2 is orange and I2 is violet. only colour of iodine is different the other two are the same in both water and cyclohexane :-)
Original post by sarian2
in water:cl2 is pale green..Br2 is orange and I2 is brown
in cyclohexane:cl2 is pale green..Br2 is orange and I2 is violet. only colour of iodine is different the other two are the same in both water and cyclohexane :-)


Thanks :smile:

and ions are colourless correct?
(edited 12 years ago)
Reply 55
Original post by dream_merchant
Thanks :smile:

and ions are colourless correct?


well depends how you look at ions for example when cl- reacts with Ag+ you get Agcl which is white.in case of Br2 AgBr is cream and AgI is yellow.
Reply 56
Original post by sarian2
CAN SOMEBODY TEACH ME HOW TO WRITE IONIC EQUATIONS PLEASE?thanks x


say you have a solution of bromine and you add it to a solution of sodium iodide you would get

Br2 + 2I^- =====> 2Br^- + I2

because the bromine is just in solution so you just write Br2, but as the iodine is in an ionic compound, it is an iodide ion so you write it as I^- . Then as bromine is more reactive than iodine, it displaces the iodide ions and becomes bromide ions Br^- leaving the iodine now in solution as I2.

The '2' in front of the ions is to balance the equation

hope this helps! :smile:
So...what do people think are going to come up in tomorrow's exam?
Original post by greenford
I have this exam tomorrow... haven't done any revision... have such a bad flu that cant even move.. I got 68 in the Jan one.... 4th time retake...:colondollar:... hope its an easy paper


how many attempts r u allowed? and if this s ur 4th attempt? does the highest out of da 4 attempts count or it is da highest out of the last 2 attempts? i want to know incase i have to resit this again as i need a minimum of an A on his to pull up my overall mark in the subject seeing as i got a U in practcal? :dontknow::cry2:
anyone know how much practical is weighted in this? (AS) Because i got a C...oops :/

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