Edexcel AS Chemistry Unit 2 Thread Watch

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killfestab
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#121
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#121
Halogens get less reactive down the group.
They are oxidising agents - i.e. they GET REDUCED themselves... ( they gain electrons)

All the halogens need 1 electron to get a complete outerorbital however, flourine is the most reactive because there are fewer electron shells so greater force of attraction between nucleus and outer shell of electrons. Plus there is also less shielding from inner shells.


Halides are reducing agents, and they become BETTER REDUCING AGENTS down the group and this can be seen with the reactions of KX (x = Cl, Br, I ) with concentrated sulfuric acid. If they are reducing agents, they GET OXIDISED...they lose electrons. Flourine doesnt want to lose its electrons
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killfestab
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#122
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@TS

Because tertiary halogenoalkanes make tertiary carbocations that are more stable than the primary carbocations produced by primary halogenoalkanes, they must have different mechanisms - i.e. different reaction pathways. In fact, Primary halogenalkanes undergo the SN2 mechanism while, tertiary halogenoalkanes undergo the SN1 mechanism, and each has a different rate determining step.

I have not done A2 chem, but i think we will learn about the details of the rate equations next year. However, because they undergo different mechanisms they will have different rates of reaction. not sure about the details but hope it helped
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pervy_sage
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#123
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#123
why are double bonds shorter then single bonds?

and is dipole-dipole interaction and permanent dipoles the same thing?

what are the bond angles in methanal?
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manwithmidi
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#124
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(Original post by hazu121)
Okay, this is what I got from the spec, but dont hold me to it.

Group 7
1. Some of their properties i.e they are diatomic, they are oxidising agents whose strength decreases down the group, they get darker in colour down the group, bp/mp increases down the group, solubility and reactivity decrease down the group etc.
2. The appearances of the elements normally, in water and in cyclohexane
3. The trends in ionisation energy
4.Their reactions
a)Oxidation with metals, non-metals, and with ions in solution.
b)Displacement reactions
c)Disporpotionation with hot alkali and with cold alkali
d)With water

Halides
Reactions
1. With Silver Nitrate (Tests for the halide ions)
a) the action of sunlight on the ppts produced in reaction 1
2. With Concentrated Sulphuric Acid (reduction)

Hydrogen Halides
Reactions
1. With water
2. With Ammonia

Hope this is helpful
Sounds good. Guess it's time to cram that into a sunday afternoon, and try and do 3 or more exam papers :/.
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killfestab
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#125
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(Original post by pervy_sage)
why are double bonds shorter then single bonds?

and is dipole-dipole interaction and permanent dipoles the same thing?

what are the bond angles in methanal?
The bond length depends on the strength of the bond. Its logical, if the atoms are attracted to each other more strongly, the bond will be stronger, and they will be closer to each other. In a double bond, the bond is extremely strong, and so shorter than a single bond because the force of attraction of the nuclei and the shared pair of electrons is much greater.

yes, permanent dipole-permanent dipole = dipole - dipole interaction

Methanal is trigonal planar, therefore bond angles are 120 degrees
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manwithmidi
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#126
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(Original post by pervy_sage)
why are double bonds shorter then single bonds?

and is dipole-dipole interaction and permanent dipoles the same thing?

what are the bond angles in methanal?
There are two types of dipole interactions (but dont quote me on that)
permanent-permanent, and induced dipole-dipole.

Then there are also the London Forces which are Instantaneous-induced dipole interactions.
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fareshta jan
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#127
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#127
can someone please post the unit 2 chemistry edexcel jan 210 examiner report?????????!!!!! please urgent !!!!!
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Sade67
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#128
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(Original post by fareshta jan)
can someone please post the unit 2 chemistry edexcel jan 210 examiner report?????????!!!!! please urgent !!!!!
yeah if anyones has got that it would be helpful! june 09 as well!
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SkyNinja
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#129
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#129
Dont forget guyz that Green chemistry can hold up to 10 marks in the exam, especially the essay question on the back, thats worth like 6 marks. It can make that difference between a grade, most people tend to ignore it. Also there is sure to be an acid base titration calculation which can score up to 7 marks (not on iodine-thiosulphate because thats already came up, probably something like the estimation of chlorine in bleach).
Redox and chemical equilibrium can score some marks on the multiple choice so be sure to revise them. (especially the effects of temperature and pressure seem to be most common). You should judge whats common and whats already come up then from there revise the reaction that are likely to come up, instead of learning every single reaction. I know there are only 2 past papers and it will be hard to predict questions but it worked for me on unit 1 and in biology. Remember only 18 hours left till the exam, good luck everyone that is taking it
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jonathan3909
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#130
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#130
A really good titration question at the end page of the book!
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jonathan3909
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#131
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#131
DO read this for Green Chemistry Question

http://www.scribd.com/doc/29240039/G...-Study-EDEXCEL
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SK-mar
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#132
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#132
(Original post by SkyNinja)
Dont forget guyz that Green chemistry can hold up to 10 marks in the exam, especially the essay question on the back, thats worth like 6 marks. It can make that difference between a grade, most people tend to ignore it. Also there is sure to be an acid base titration calculation which can score up to 7 marks (not on iodine-thiosulphate because thats already came up, probably something like the estimation of chlorine in bleach).
Redox and chemical equilibrium can score some marks on the multiple choice so be sure to revise them. (especially the effects of temperature and pressure seem to be most common). You should judge whats common and whats already come up then from there revise the reaction that are likely to come up, instead of learning every single reaction. I know there are only 2 past papers and it will be hard to predict questions but it worked for me on unit 1 and in biology. Remember only 18 hours left till the exam, good luck everyone that is taking it
how do you know there will be an acid-base titration question?? pluss could you expand on that chlorine bleach thing pleaseee.. cheers
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jonathan3909
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#133
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#133
Q Polar liquids are affected by electric fields. For which of the following liquids would a jet
of the liquid be affected by an electric field?
A hexane
B cyclohexane
C cyclohexene
D cyclohexanol
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fareshta jan
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#134
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#134
(Original post by SkyNinja)
Dont forget guyz that Green chemistry can hold up to 10 marks in the exam, especially the essay question on the back, thats worth like 6 marks. It can make that difference between a grade, most people tend to ignore it. Also there is sure to be an acid base titration calculation which can score up to 7 marks (not on iodine-thiosulphate because thats already came up, probably something like the estimation of chlorine in bleach).
Redox and chemical equilibrium can score some marks on the multiple choice so be sure to revise them. (especially the effects of temperature and pressure seem to be most common). You should judge whats common and whats already come up then from there revise the reaction that are likely to come up, instead of learning every single reaction. I know there are only 2 past papers and it will be hard to predict questions but it worked for me on unit 1 and in biology. Remember only 18 hours left till the exam, good luck everyone that is taking it
thanks
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SkyNinja
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#135
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#135
That basically covers it all up, just get the major points from the web page and youve got yourself 5-10 marks before the exam has begun. I must admit there sure is alot of stuff to remember for this exam, just have to remember the basics and first principles to figure stuff out and im sure it wont be as bad.
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Phalange
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#136
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#136
(Original post by jonathan3909)
Q Polar liquids are affected by electric fields. For which of the following liquids would a jet
of the liquid be affected by an electric field?
A hexane
B cyclohexane
C cyclohexene
D cyclohexanol
D
All others are non-polar.
If you put an electrostatically charged rod or substance near a jet of water or a polar liquid, the liquid moves towards the rod (or away) but it's not important the direction just that it will move

Edit: Think of it like magnets... XD

On a side note..........
Which methods do people have to make haloalkanes? ( WIll use the shortest carbon chain to save time xD)
I have these:

CHLORO ALKANES
1) Add PCl5
CH3OH + PCl5 ---> CH3Cl + POCl3 + HCl

2) Add HCl to a TERTIARY alcohol (normal rxn)

3) Add PCl3
3CH3OH + PCl3 ----> 3CH3Cl + H3PO3

BROMO/IODO ALKANES:
1) Add damp red phosprous to the halogen (e.g. Br2), then add alcohol
2P + 3Br2 ----> 2PBr3 *see how we have PBr3 like PCl3 before??
CH3OH + PBr3 ---> CH3Br + H3PO3

2) Add phosphoric (V) acid to metal halide then add alcohol
3NaBr + H3PO4 ---> Na3PO4 + 3HBr
CH3OH + HBr ----> CH3Br + H2O

Ok here is where I am confused. My CGP book says NOT to use sulphuric acid for bromo/iodo but phosphoric because:
We do not use H2SO4 beacuse HBr/HI are oxidised by it so you end up with a range of products .e.g Br2 and so a reduced yield of the desired haloalkane
BUT my text book ("endorsed by edexcel") says you can but only 50% for a bromoalkane

1) Add 50% H2SO4 + NaBr, then add an alcohol
NaBr + H2SO4 ---> NaHSO4 + HBr
CH3OH + HBr ----> CH3Br + H2O

Any insight??
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dimi3
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#137
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#137
hi guys, umm quick question. do we have rates of reactions for unit 2? as in like calculations fr that stuff...?
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killfestab
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#138
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#138
no
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jonathan3909
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#139
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#139
(Original post by Phalange)
D
All others are non-polar.
If you put an electrostatically charged rod or substance near a jet of water or a polar liquid, the liquid moves towards the rod (or away) but it's not important the direction just that it will move


Edit: Think of it like magnets... XD

On a side note..........
Which methods do people have to make haloalkanes? ( WIll use the shortest carbon chain to save time xD)
I have these:

CHLORO ALKANES
1) Add PCl5
CH3OH + PCl5 ---> CH3Cl + POCl3 + HCl

2) Add HCl to a TERTIARY alcohol (normal rxn)

3) Add PCl3
3CH3OH + PCl3 ----> 3CH3Cl + H3PO3

BROMO/IODO ALKANES:
1) Add damp red phosprous to the halogen (e.g. Br2), then add alcohol
2P + 3Br2 ----> 2PBr3 *see how we have PBr3 like PCl3 before??
CH3OH + PBr3 ---> CH3Br + H3PO3

2) Add phosphoric (V) acid to metal halide then add alcohol
3NaBr + H3PO4 ---> Na3PO4 + 3HBr
CH3OH + HBr ----> CH3Br + H2O

Ok here is where I am confused. My CGP book says NOT to use sulphuric acid for bromo/iodo but phosphoric because:
We do not use H2SO4 beacuse HBr/HI are oxidised by it so you end up with a range of products .e.g Br2 and so a reduced yield of the desired haloalkane
BUT my text book ("endorsed by edexcel") says you can but only 50% for a bromoalkane

1) Add 50% H2SO4 + NaBr, then add an alcohol
NaBr + H2SO4 ---> NaHSO4 + HBr
CH3OH + HBr ----> CH3Br + H2O

Any insight??
Cyclohexene is not polar:confused: !!!!
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jonathan3909
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#140
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#140
(Original post by killfestab)
no
Can you please answer my above Questions.Thanks
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