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OCR AS Chemistry A Unit 1 retake - Friday

This is the place to post all your notes on the upcoming exam, what you think will come up, what topics you find difficult so that others can explain it.

Good luck! Let's hope we all do better than we did last time!

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I really hate electronegativity and the trends of the periodic table. Most of the other stuff is pretty basic; the only difference from the old 2811 past papers is the addition of salts.
Can someone explain the trends down Group 2 and Group 7 please?
I_Need_To_Revise
Can someone explain the trends down Group 2 and Group 7 please?


Group 2
Ionisation decreases; atoms become larger; increased sheilding outweighs increased nuclear charge; easier to lose electrons.

Group 7
Oxidising power decreases Cl < Br < I; atoms become larger and electronegativity decreases; increase nuclear charge outweighed my increase sheilding and increased atomic radius; halogens gain electrons; it is easier to gain electrons further up the group
Reply 4
Avatar for 786girl
786girl
someone please go through the chlorine stuff! and what we need 2 kno!
Reply 5
Avatar for Lottsie93
Lottsie93
Water treatment

H2O (l) + Cl2 (aq) = HClO (aq) + HCl (aq)

Kills bacteria
Toxic in large quantities/ Carcinogenic

Bleach

Cl2 (aq) + 2NaOH (aq) = NaCl (aq) + NaClO (aq) + H20 (l)
Reply 6
Avatar for RosePetal
RosePetal
11
The Phillip Allan unit guide for this unit and the next one is very helpful + the revision conference helped a lot too.
Predictions on what will come up:
- Show hydrogen bonding between 2 molecules of water,
- More about Group 2,
- Q's on metallic or covalent bonding.
Mr van der WAALs
Group 2
Ionisation decreases; atoms become larger; increased sheilding outweighs increased nuclear charge; easier to lose electrons.

Group 7
Oxidising power decreases Cl < Br < I; atoms become larger and electronegativity decreases; increase nuclear charge outweighed my increase sheilding and increased atomic radius; halogens gain electrons; it is easier to gain electrons further up the group


thanks :smile:
Can someone explain the reactions we need to know for group 2 metals and compounds, and also for group 7- is it just the halide tests and the redox reactions for group 7.
Reply 9
Avatar for RosePetal
RosePetal
11
I_Need_To_Revise
Can someone explain the reactions we need to know for group 2 metals and compounds, and also for group 7- is it just the halide tests and the redox reactions for group 7.

http://http://www.allinfo.plus.com/levelup/foundation.htm
This site outlines the reactions for both groups.
RosePetal
http://http://www.allinfo.plus.com/levelup/foundation.htm
This site outlines the reactions for both groups.


thanks :smile:
What do we reckon the chances of a salt calculation coming up are? It only has appeared once so far in all the F321 papers :|
Mr van der WAALs
What do we reckon the chances of a salt calculation coming up are? It only has appeared once so far in all the F321 papers :|


salt calculation??
I_Need_To_Revise
salt calculation??


Calculating hydrated salts etc. Such as MgO.XH2O.
Epsom salts can be used as bath salts to help relieve aches and pains.
Epsom salts are crystals of hydrated magnesium sulfate, MgSO4&#8226;xH2O.

A sample of Epsom salts was heated to remove the water. 1.57 g of water was removed
leaving behind 1.51 g of anhydrous MgSO4.

(i) Calculate the amount, in mol, of anhydrous MgSO4 formed.
(ii) Calculate the amount, in mol, of H2O removed.
(iii) Calculate the value of x in MgSO4&#8226;xH2O.
Mr van der WAALs
Calculating hydrated salts etc. Such as MgO.XH2O.


Oh that came up on the Jan 2010 paper- so i dont think it will come up on this one....too recent
Reply 16
Avatar for Gagannn_
Gagannn_
1
This is the oxidation number question that came up January:

2Sr(NO3)2 --> 2SrO + 4NO2 + O2

Using oxidation numbers, explain why the reaction involves both oxidation and reduction.


The answer for this question is:

The nitrogen, in Sr(NO3)2 has an oxidation number of +5. After the reaction the nitrogen, in NO2, has an oxidation number of +4. Nitrogen has gone from +5 to +4 so has been reduced. Oxygen, in Sr(NO3)2 has an oxidation number of -2. After the reaction, some of the oxygen retains the oxidation number of -2, but 2 atoms form O2 which has an oxidation number of 0. So some oxygen has gone from -2 to 0, it has been oxidised.




But, I don't get how nitrogen has an oxidation number of 5.
Gagannn_
This is the oxidation number question that came up January:

2Sr(NO3)2 --> 2SrO + 4NO2 + O2

Using oxidation numbers, explain why the reaction involves both oxidation and reduction.


The answer for this question is:

The nitrogen, in Sr(NO3)2 has an oxidation number of +5. After the reaction the nitrogen, in NO2, has an oxidation number of +4. Nitrogen has gone from +5 to +4 so has been reduced. Oxygen, in Sr(NO3)2 has an oxidation number of -2. After the reaction, some of the oxygen retains the oxidation number of -2, but 2 atoms form O2 which has an oxidation number of 0. So some oxygen has gone from -2 to 0, it has been oxidised.




But, I don't get how nitrogen has an oxidation number of 5.



It has an oxidation of +5 because; (NO3) has a 1- charge; hence O(2-) x 3 = -6 and 2 x NO3 gives -12; the two -1 charges on NO3 bring it back to -10; hence each nitrogen is +5.
I_Need_To_Revise
Oh that came up on the Jan 2010 paper- so i dont think it will come up on this one....too recent


The question I posted above was on the F321 paper last summer; if it came up in January as well it is probably not going to come up again; but you never know :rolleyes:
Reply 19
Avatar for Josh-H
Josh-H
14
Can someone explain when we would see the different colours of the halogens. And when to expect colour change observations during reactions, like which molecules will be involved?

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