Chemistry: Post your doubts here!

[qwertypoiu]

View attachment 54269

This was nicely explained thank you so much for this sir.
Thanks a lot for the help you provide to numerous students here as well

 

[tiki-taka]

How many decimal places should we take for burette readings
Shud it be 24.1 or 24.10??

 

[The Sarcastic Retard]

How many decimal places should we take for burette readings
Shud it be 24.1 or 24.10??

if u take 1st reading to 2dp then next readings should be in 2dp

 

[69ermalik007]

Yes.... princess anu......all the answers are correct.............but can u explain?

 

[Falconwing]

http://maxpapers.com/wp-content/uploads/2012/11/9701_w14_qp_11.pdf

40) A reaction pathway diagram is shown.
energy
extent of reaction
Which reactions would have this profile?
1 (CH3)3CBr + NaOH → (CH3)3COH + NaBr
2 CH3CH2Br + NaOH → CH3CH2OH + NaBr
3 (CH3)3CCH2CH2Cl + 2NH3 → (CH3)3CCH2CH2NH2 + NH4Cl

How does one even do this? Answer is D.

 

[Metanoia]

http://maxpapers.com/wp-content/uploads/2012/11/9701_w14_qp_11.pdf

40) A reaction pathway diagram is shown.
energy
extent of reaction
Which reactions would have this profile?
1 (CH3)3CBr + NaOH → (CH3)3COH + NaBr
2 CH3CH2Br + NaOH → CH3CH2OH + NaBr
3 (CH3)3CCH2CH2Cl + 2NH3 → (CH3)3CCH2CH2NH2 + NH4Cl

How does one even do this? Answer is D.

1 is a tertiary halogenoalkane, it will undergo SN1 reactions,

• the reaction will take place in 2 steps
• the reaction pathway will contain an intermediate state
• Such reaction paths will be a double hump.

2 and 3 are primary halogenoalkanes, they will undergo SN2 reactions,

• the reaction will take place in 1 step
• the reaction pathway will contain an transition state (not an intermediate).
• Such reaction paths will be a single hump.

 

[Falconwing]

I had no idea there was a connection! Thanks alot ^^

 

[Kool boy]

Any idea about paper 32

 

[Xaptor16]

Convert the masses of ethanol and ethanoic acid to moles to figure out the limiting reagent.

Limiting reactant is acid, 0.5 moles. Expected moles of ester is thus 0.5 moles, 44g.

%yield = 22/44 = 50%

thankyou

 

[Xaptor16]

n of alcohol = 30/46
n of acid = 30/60
limiting reagent is acid since its no of mole is smaller
so mass of theoretical yield of ester - 30/60 x 88 = 44
percentage = 22/44 x 100% = 50%

thanks!

 

[princess Anu]

Yes.... princess anu......all the answers are correct.............but can u explain?

last one : the rate increases at first as products are being formed ..(products act as catalyst here). but soon all of the substrate gets used up, products r no longer being formed therefore rate finally decreases.

21 A & B are wrong. in D, since v r using aqueous HCL there is a possibility of lone pair on O of H2O attacking the carbocation instead of the Cl.. therefore it may not give the best yield

27: I wasn't sure about it.. though I recognised the repeating unit ( I have marked it on the picture) & then I thought the groups on both ends of the repeating unit must possess a double bond in order to polymerise.

 

[princess Anu]

Q38 why is 2 wrong?

Q8 why do we need pH 7? both Al(no3)3 & Ba(No3)2 are neutral compounds right.. so even extremely high/low pH should not affect them? :s

 

[princess Anu]

help with this please

 

[princess Anu]

how to do it?

 

[princess Anu]

2 moles of water are formed, and the arrow is pointed in the correct direction, so it is +2 multiplied by the enthalpy of formation of water.

had they given an option like.. ' -2 * enthalpy change of combustion of H2'.. wud that be correct?

 

[nehaoscar]

Wouldn't a lower temperature increase the rate of the forward reaction since it's exothermic? So why is it A instead of D??

Did you find out why? If so please can you explain to me too?

 

[nehaoscar]

How to do this one?
I form the table but then I don't know what to do (which values for P and Q?)

 

[Autumn98]

someone please help me with this.. answer and reason? asap

 

[Mathemagical]

someone please help me with this.. answer and reason? asap

The answer is A.

From the diagram and the arrow showing the enthalpy change of formation, we can conclude that the bottommost layer is CO2(g).

Thus, the second from bottom level has to be Option C, which shows the elements in their standard states. Working our way upwards, we can see that the change from Option B to Option A is an enthalpy change of vaporisation, which is positive. Therefore, A is the top level.

 

[Mathemagical]

had they given an option like.. ' -2 * enthalpy change of combustion of H2'.. wud that be correct?

No, it would not be correct. The signs + and - are very important in Hess Law reactions, and it is important to keep them in mind while attempting questions of this sort.