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# OCR F321 - 23rd May

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1. 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...... hope its an easy paper
2. guys, they won't ask us anything not in the book

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

if they do, then we'll hunt them.
fghh
4. anyone what do we need to know on water of crystallisation
5. 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.
6. (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)
7. (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??
8. (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
9. CAN SOMEBODY TEACH ME HOW TO WRITE IONIC EQUATIONS PLEASE?thanks x
10. (Original post by dream_merchant)
I thought chlorine was light green??
chlorine is pale green in both water and cyclohexane!
11. what colour is iodine in water. Brown?
12. (Original post by dream_merchant)
what colour is iodine in water. Brown?
its brown your right then in cyclohexane it pink
13. (Original post by kingam)
its brown your right then in cyclohexane it pink
Purple?!
14. (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 :-)
15. (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

and ions are colourless correct?
16. (Original post by dream_merchant)
Thanks

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.
17. (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!
18. So...what do people think are going to come up in tomorrow's exam?
19. (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...... 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?
20. anyone know how much practical is weighted in this? (AS) Because i got a C...oops :/

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