Chemistry: Post your doubts here!

[raamish]

hey can any1 help me in drawing 3d stuctures of hydrocarbons. i just need some tips or some sites where i can learn to draw them

 

[soccerprinc3]

Can somebody please tell me the difference between Sn1 and Sn2 reactions for Halogenoalkanes?

 

[iFuz]

Can somebody please tell me the difference between Sn1 and Sn2 reactions for Halogenoalkanes?

Sn2 is single step reaction, rate of reaction depends on 2 molecules and is given by Primary and Secondary Alkyl Halides... tertiary alkyl halides dont give sn2 as they are bulky.....

Sn1 is two step and is given by tertiary and secondary alkyl halides.... Rate of reaction depends on one molecule .. and primary alkyl halides cant give it as their carbocation is highly unstable!

 

[Sky8]

Can somebody explain how to do ques 9 (a) for O/N 07

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w07_qp_4.pdf

& ques 7 (a) for O/N 09 variant 42 ?

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w09_qp_42.pdf

Thanks.

 

[Sky8]

Can we have a separate A2 chemistry thread ?

 

[RGBM211]

^Y not

 

[angelicsuccubus]

Why can't NaBr react with the water? I made that mistake at first too. :s

When you see that the question is asking you to make boromobutane from butanol ... you have to remember that an alcohol needs HBr for this reaction to occur. After that you have to think of an equation/reaction of NaBr that will make HBr ... and NaBr + water will not give you HBr..
Sodium bromide dissassociates into Na+ and Br- ions when it is dissolved in water.
NaBr + H2O = Na+ and Br- aq.

 

[raamish]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w11_qp_21.pdf

Qs3d) Why havent we taken the bond energies of Co2 and H2O to calculate the answer?

 

[touchedbyHisNoodliness]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w11_qp_21.pdf

Qs3d) Why havent we taken the bond energies of Co2 and H2O to calculate the answer?

Because they are in their standard states. We only take bond energies for chemical species that are not in their standard states.

 

[billy boy]

can any one explain me when to do product - reactant and when reactant - product in enthalpy change questions​

 

[angelicsuccubus]

can any one explain me when to do product - reactant and when reactant - product in enthalpy change questions​

enthalpy change= enthalpy of products - enthalpy of reactants... always when you have only these two or an enthalpy profile. If you have stuff like enthalpy of reaction or formation to find from enthalpy of combustion or enthalpy of formations... then use a Hess's diagram.

 

[Hassi123]

whats induced dipole-dipole and permanent dipole dipole? please help its bothering me alot.

 

[oldfashionedgirl]

and.. how come aluminium chloride is covalent but aluminium fluoride is ionic?
what's the metal used as a catalyst in a vehicle's catalytic converter? (palladium.. platinum..? can't remember)

platinum, NOT palladium.....just yesturday i saw this written in a mark scheme

 

[angelicsuccubus]

whats induced dipole-dipole and permanent dipole dipole? please help its bothering me alot.

We had a conversation about it earlier on the thread but I'll explain it again.
There are 2 main types of intermolecular forces (so these are between molecules not atoms..):

1. Van der Waal's forces
   • Instantaneous dipole- Induced dipole (London forces) - these are betwen non polar covalent molecules such as alkanes. They are the weekest and basically form if electron distribution becomes unever in a molecule. A temporary dipole is set up which can disturn the electron distribution in the adjacent molecule producing an opposite induced dipole.
   • Permanent dipole- Permanent dipole (Keesom forces)- polar molecules have permanent dipoles and are attached to each other by pd-pd forces where the permanently positive end of one molecule is attached to the permanently negative end of the other molecule. These forces are stronger than London Forces.
     
     Van der Waal's forces increase in strength with no. of electrons in the molecules and hence affect the melting and boiling points.
2. Hydrogen Bonding
   • These are the strongest of the intermolecular forces found between an electron deficient Hydrogen atom on one molecule and a lone pair of electrons on a highly electronegative atom of another molecule. So the force between the H atom of water and the N atom of ammonia is a hydrogen bond. It affects solubility of compounds.

 

[angelicsuccubus]

platinum, NOT palladium.....just yesturday i saw this written in a mark scheme

thanks =]

 

[oldfashionedgirl]

can any one explain me when to do product - reactant and when reactant - product in enthalpy change questions​

when ur given enthalpy change of formation of reactants and products, then its= product-reactant
if ur given enthalpy change of combustion of the elements and the product, then its= reactant-product
if ur asked to use bond energies from the data booklet, then too its= reactant-product

 

[oldfashionedgirl]

thanks =]

no prob

 

[ibadsiddiqi]

enthalpy change= enthalpy of products - enthalpy of reactants... always when you have only these two or an enthalpy profile. If you have stuff like enthalpy of reaction or formation to find from enthalpy of combustion or enthalpy of formations... then use a Hess's diagram.

Enthalpy change= Enthalpy of reactants - enthalpy of products when bond energies are given to you.

 

[hateexamz]

people wat to prepare fr while studyng fr p5?? should we jst go thru the syllabus nd solve past papers???

 

[angelicsuccubus]

people wat to prepare fr while studyng fr p5?? should we jst go thru the syllabus nd solve past papers???

there's a chem p5 tips thread too.. read that too.