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A-level Chemistry Revision Squad!

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Original post by High Stakes
No...

In terms of increasing strength:

1) van der Waals / London forces.
2) Dipole-dipole forces.
3) Hydrogen bonding.

This is basic chem 101. Get these solid otherwise it's going to make you shakey at the higher stuff.


--
@RonnieRJ - How's chem going for you? :wink: Are you with AQA too?

yeah lol i see what u mean.. but in my textbook it is mentioned that the strength of the van der waal forces are more important than the increase in dipole dipole interactions
((in determining the boiling point ) ..as the halogen gets larger there is more electron density so the van der waal forces get stronger outweighing the dipole dipole interactions;; this was written when explaining the example for why 1-iodobutane has a higher boiling point than 1-chlorobutane
Original post by NoorL
yeah lol i see what u mean.. but in my textbook it is mentioned that the strength of the van der waal forces are more important than the increase in dipole dipole interactions
((in determining the boiling point ) ..as the halogen gets larger there is more electron density so the van der waal forces get stronger outweighing the dipole dipole interactions;; this was written when explaining the example for why 1-iodobutane has a higher boiling point than 1-chlorobutane


Nevermind. That's referring to the size of the halogen itself! But in terms of purely haloalkane vs alkane the halogen will bring about a greater bpt...
(edited 8 years ago)
Reply 62
Avatar for osayuki
osayuki
4
Original post by Xenon17
Well the two central atoms in H3NBF3 are N and B there exists a dative covalent bond between the N and B as N has a lone pair and B accepts this so both these atoms now have 4 bonding regions of electrons thus all electron pairs repel and the shape is tetrahedral with a bond angle of 109.5.


Thank you :smile:
As far as I'm aware, colour changes of chemicals vary from board to board, so not everything is going to be the same.

I did CCEA and have practiced with AQA questions, the structure is different and a lot of the information is different too :smile:
Original post by High Stakes
Are you with aqa or?

nope edexcel..
Original post by High Stakes
Nevermind. That's referring to the size of the halogen itself! But in terms of purely haloalkane vs alkane the halogen will bring about a greater bpt...

alright .. thank u a lot for helping
Original post by High Stakes
No...

In terms of increasing strength:

1) van der Waals / London forces.
2) Dipole-dipole forces.
3) Hydrogen bonding.

This is basic chem 101. Get these solid otherwise it's going to make you shakey at the higher stuff.


--
@RonnieRJ - How's chem going for you? :wink: Are you with AQA too?


Yeah AQA! Chem is good other than organic because I've really started to hate it this year lol
Hi, what does react with nitric acid to form cream solution and when ammonia is added form colourless solution? The same solution also reacts with dilute nitric acid followed by barium chloride to form milky solution. I am confused because 1st test means that it is bromide but 2nd test means that it is sulfate. I forgot to mention that in the flame test it gave orange colour. Question taken from Edexcel AS-Level Core Practical (New Specification)
(edited 8 years ago)
Original post by RonnieRJ
Yeah AQA! Chem is good other than organic because I've really started to hate it this year lol


Organic chem is bae. How are you doing overall though in past papers? Do you feel confident this year?
Original post by High Stakes
Organic chem is bae. How are you doing overall though in past papers? Do you feel confident this year?


Ugh not anymore synthesis is really annoying :frown:

And I'm the wierd one who doesn't do past paper remember 🌚 So idk really but I always get A*s in mocks when I have to do them in lesson so think I'll be fine hopefully :biggrin:
Original post by RonnieRJ
Ugh not anymore synthesis is really annoying :frown:

And I'm the wierd one who doesn't do past paper remember 🌚 So idk really but I always get A*s in mocks when I have to do them in lesson so think I'll be fine hopefully :biggrin:


For synthesis you need to know all the mechanisms down to a T.

Chem is my fav subject atm probably. And Maths.
Original post by High Stakes
For synthesis you need to know all the mechanisms down to a T.

Chem is my fav subject atm probably. And Maths.


Yeah they're not too bad but I just find them soo boring it's unreal

And nah maths is my ultimate Bae atm, and physics too tbf
Original post by RonnieRJ
Yeah they're not too bad but I just find them soo boring it's unreal

And nah maths is my ultimate Bae atm, and physics too tbf


Chem vs Physics!!!!!! :colone:

I'm waiting for the easter holiday. School's such a waste of time at the moment. I just wanna go home and do my thing and study lmao.


Do you find Unit 4 or Unit 5 easier? I think Unit 4 is pretty forward because the first three big topics: equilibria, kinetics and acid/bases are basically maths lol.
Original post by High Stakes
Chem vs Physics!!!!!! :colone:

I'm waiting for the easter holiday. School's such a waste of time at the moment. I just wanna go home and do my thing and study lmao.


Do you find Unit 4 or Unit 5 easier? I think Unit 4 is pretty forward because the first three big topics: equilibria, kinetics and acid/bases are basically maths lol.

OMG I knowwww!! And my new school hasn't got study leave :| guess I'm just gonna have to not come anyway lmao, I'll be taking study leave end of May whether they allow it or not :angry:

And probs unit 5 bc thermodynamics and it has hardly any topics, I counted subtopics and unit 4 has 40 while unit 5 only 20??? But yeah I like the first half of unit 4 then it just goes downhill :/
What is the uncertainty for a 250cm3 grade B volumetric flask and a 50cm3 grade B volumetric flask? Lol, I tried looking online I couldn't find it; it's either that I'm blind LOL or that it isn't necessary??
Reply 75
Avatar for Flask
Flask
1
I really struggle remembering how to do redox equilibria questions. Does anyone have a list of like 1. Work out equations 2. Balance redox equation 3. Work out moles... Something like that? I remember lists a lot easier for some reason
Original post by Flask
I really struggle remembering how to do redox equilibria questions. Does anyone have a list of like 1. Work out equations 2. Balance redox equation 3. Work out moles... Something like that? I remember lists a lot easier for some reason


There is two methods of doing them
- The oxidation number method
- Half equations method
find which one is the most suitable for you, for example I prefer the half equations method, a list of things to do would be like

1.

create the half equations

2.

-For each half equation:

3.

Balance it (except for oxygen and hydrogen molecules, those leave them till the next step)

4.

Balance the oxygen molecules first by adding H2O and then the hydrogen molecules by adding H+

5.

balance the overall charge by adding electrons so it would be the same on each side of the half equation

6.

make sure the number of electrons for each half equation is the same to cancel them out, if not then balance it (like if one half equation has 2 electrons and the other has 4 electrons then multiply the first half equation by 2)

7.

Add the two equations

if the reaction is basic make sure to add OH- ions for the equation which would be step 8
There are soooo many reactions to remember in edexcel chemistry, does anyone have any good ways of revising/remembering them all?
Reply 78
Avatar for SANTR
SANTR
8
Original post by PlayerBB
There is two methods of doing them
- The oxidation number method
- Half equations method
find which one is the most suitable for you, for example I prefer the half equations method, a list of things to do would be like

1.

create the half equations

2.

-For each half equation:

3.

Balance it (except for oxygen and hydrogen molecules, those leave them till the next step)

4.

Balance the oxygen molecules first by adding H2O and then the hydrogen molecules by adding H+

5.

balance the overall charge by adding electrons so it would be the same on each side of the half equation

6.

make sure the number of electrons for each half equation is the same to cancel them out, if not then balance it (like if one half equation has 2 electrons and the other has 4 electrons then multiply the first half equation by 2)

7.

Add the two equations

if the reaction is basic make sure to add OH- ions for the equation which would be step 8


Just to clarify, you work out the overall charge on both sides i.e including any H+ ions, before balancing using electrons?
Original post by SANTR
Just to clarify, you work out the overall charge on both sides i.e including any H+ ions, before balancing using electrons?


Yes

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