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AS Chemistry- helping each other out!

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Original post by kandykissesxox
Thank you very much! i understand :smile: It's a shame that you have to memorise that whole table :frown:


You're welcome :wink:

It'll become more easier to remember when you do more practise questions, trust me :smile:
Original post by nidah y
Can somebody help me with addition reactions and recations of alkenes i dont understand them. also if somebody could go over nucleophillic addition it would be greatly appreciated! :smile:


First and foremost:

Addition --- 2 reactants make 1 product
Subsitution --- 2 reactants make 2 products
Elimination --- 1 reactant makes 2 products

Electrophile = A atom that is attracted to an electron rich centre (e.g. double C=C bond), and accepts a pair of electrons, in order to form a new covalent bond.

Neuclophile = Basically opposite, so attracted to electron deficient centre and donates a pair of electron.


The mechanism for alkenes is that they react with electrophiles. E.g. Halogens or Hydrogen halide. Therefore they are known as electrophillic addition, since only 1 product is produced.

Other reactions of alkenes:

With hydrogen (Hydrogenation), Hydrogen opens up double bonds and makes saturated alkane. Nickel catalyst and 150 degrees Celsius needed.

With halogens, e.g F,Cl, Br, I
Halogens add across double bond, opening them UP = Saturated. Halogens would then replace two hydrogens. E.g - F2 would replace two hydrogens in ethene, forming 1,2-difluoroethane.

With steam
Produces an alcohol, since OH replaces H. Acid catalyst needed (E.g. - Phosphoric acid)

This is only SOME of the reactions, I would go on, but it will take forever to type. Hope things are starting to be clearer now. Feel free to ask any questions! :dancing2:
(edited 8 years ago)
Original post by nidah y
Can somebody help me with addition reactions and recations of alkenes i dont understand them. also if somebody could go over nucleophillic addition it would be greatly appreciated! :smile:




Posted from TSR Mobile
This pic should help.It shows the flowchart of alkene reactions.
Guys...
If CCl2 has a bond angle of 118 then why does it change so dramatically when you have two lone pairs... I thought if it was V-shaped then it would be 115 :frown:
How is everyone revising for the written empa?:smile:
I got 95% in my chem 1 mock but i find the empa's so hard because its really hard to prepare for :frown:
Original post by rm_27


Just to add:
H2 + Alkene = Hydrogenation; requires Ni catalyst + 150*C.
Halogen + Alkene = Halogenation.
Hydrogen Halide + Alkene = Halogenoalkane.
H20(g) + Alkene = Hydration; high pressure and temperature + strong acid catalyst (H3PO4/H2SO4).

Radical Substitution - Alkane
Initiation:
Cl2 -> Cl + Cl
Propagation:
CH4 + Cl -> CH3 + HCl
CH3 + Cl2 -> CH3Cl + Cl
Termination/Overall:
Cl + Cl -> Cl2
CH3 + CH3 -> C2H6
Cl + CH3 -> CH3Cl

(edited 8 years ago)
Yo guys who r doing aqa has anyone ever come across using CaCo3 in extracting iron ( Extracting Metals topic)


Posted from TSR Mobile
For OCR A we have to know how to read the results of mass spec analysis but not actually how these results are brought about :smile: So you only have to be able to read the graphs

Posted from TSR Mobile
Nucleophilic Substitution:

-The halogen is more electronegative than the Carbon atom and so becomes slightly negative, which induces the carbon atom to become slightly positively charged - electron deficient Carbon atom. This attracts nucleophiles such as OH. The lone pair of electrons from the nucleophile (formed due to heterolytic fission) are attracted and donated to the electron-deficient Carbon atom - this forms a new covalent bond between the Oxygen atom and Carbon atom. The C - Br breaks by heterolytic fission, and the electrons are donated to the halogen forming an ion.

Electrophilic Addition:

-This mechanism occurs during halogenation and halogenoalkane. The halogen is more electronegative and so becomes slightly negatively charged as the electron pair is attracted to the Bromine atom, this induces the Hydrogen atom to become slightly positively charged. The electron pair from the Carbon=Carbon is attracted by the slightly charged hydrogen atom which causes the double bond to break. A new bond forms between the first carbon atom and the hydrogen atom, and so the H - Br bond breaks by heterolytic fission and the electron pair goes to the bromine, forming a bromide (ion), which then reacts with the positively charged carbon atom.

[P.S, sorry for the poor drawing]
(edited 8 years ago)
Hey guys I'm really struggling.
I think O understand the Hess's cycle but when I'm solving past papers I never seem to solve them. So could someone please explain the hess's cycle? Thanks much appreciated. :smile:
Someone help me on the definiotn of bond angle and bond length??
Original post by TheMadHatteress
Hey guys I'm really struggling.
I think O understand the Hess's cycle but when I'm solving past papers I never seem to solve them. So could someone please explain the hess's cycle? Thanks much appreciated. :smile:

If you have an example of both Formation + Combustion, I'm happy to help!
Green chemistry bores me so much :boring:
I really don't understand hess's law or born harber cycles too so I would also appreciate any help on this topic thanks
Original post by SubwayLover1
Guys...
If CCl2 has a bond angle of 118 then why does it change so dramatically when you have two lone pairs... I thought if it was V-shaped then it would be 115 :frown:


If it has 2 lone pairs and 2 bonding pairs then it ought to have bond angle of 104.5o and it'll be a bent shape.
Original post by TeaAndTextbooks
I really don't understand hess's law or born harber cycles too so I would also appreciate any help on this topic thanks


Here's a site that should help you.
Original post by TeaAndTextbooks
I really don't understand hess's law or born harber cycles too so I would also appreciate any help on this topic thanks


Original post by TheMadHatteress
Hey guys I'm really struggling.
I think O understand the Hess's cycle but when I'm solving past papers I never seem to solve them. So could someone please explain the hess's cycle? Thanks much appreciated. :smile:


Just to mention, the cycle for Combustion and the route is ALWAYS the same for every combustion-related question. The values just differ from question to question.
1) Begin by forming an equation to form the products.
2) Substitute numbers into the cycle and calculate the Sum of Reactants.
3) As the cycle suggests to form the combustion products you can directly combust the products, we go against this arrow as suggested by the route which needs to be taken - and so, the value of the product changes from NEGATIVE to POSITIVE. Enthalpy change of reactants + products = answer.

Hope that helped, I'll try and find a question for Enthalpy change of Formation.
Original post by Disney0702
If it has 2 lone pairs and 2 bonding pairs then it ought to have bond angle of 104.5o and it'll be a bent shape.


I don't understand why it goes from 118 to 104.5 though?
Original post by S.Ahmad

Just to mention, the cycle for Combustion and the route is ALWAYS the same for every combustion-related question. The values just differ from question to question.
1) Begin by forming an equation to form the products.
2) Substitute numbers into the cycle and calculate the Sum of Reactants.
3) As the cycle suggests to form the combustion products you can directly combust the products, we go against this arrow as suggested by the route which needs to be taken - and so, the value of the product changes from NEGATIVE to POSITIVE. Enthalpy change of reactants + products = answer.

Hope that helped, I'll try and find a question for Enthalpy change of Formation.




Posted from TSR Mobile

Thankyou that helps alot I finally get the hess's law stuff, born haber cycles are still killing me though but thanks anyway :biggrin:
Original post by TeaAndTextbooks
Posted from TSR Mobile

Thankyou that helps alot I finally get the hess's law stuff, born haber cycles are still killing me though but thanks anyway :biggrin:

What specification are you doing?

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