Chemistry: Post your doubts here!

[qwertypoiu]

Any guess papers for the MCQ tomorrow??

Maximum 5 repeat questions.
There will be organic questions.
Wait lemme think...
Ooh there might be inorganic questions as well.
Oh yeah and there's definitely gonna be questions about moles somewhere there.
Make sure you know how to calculate enthalpy changes.
Make sure you know balanced equation for decomposition of metal nitrates...
Make sure you know the reaction of NaOH with cold and hot Nacl or something like that.

 

[qwertypoiu]

What about water and ammonium chloride?Thats endothermic

This isn't to do with hydration of an anhydrous salt. This is dissolving ammonium chloride in water, which is endothermic.
I believe in A2 we learn the details of dissolving.
I did a quick search on yahoo answer :

When dissolving a solid, you have two processes happening.

1/ The breaking up of the solid structure, which requires energy (endothermic)
2/ The hydration of the resulting ions or molecules, which releases energy (exothermic)

In the case of ammonium chloride, the breaking of the solid structure requires more energy than the hydration of the ions provides, meaning the overall temperature will drop.

 

[asadalam]

This isn't to do with hydration of an anhydrous salt. This is dissolving ammonium chloride in water, which is endothermic.
I believe in A2 we learn the details of dissolving.
I did a quick search on yahoo answer :

When dissolving a solid, you have two processes happening.

1/ The breaking up of the solid structure, which requires energy (endothermic)
2/ The hydration of the resulting ions or molecules, which releases energy (exothermic)

In the case of ammonium chloride, the breaking of the solid structure requires more energy than the hydration of the ions provides, meaning the overall temperature will drop.

So in anhydrous cases it will always be exo?

 

[qwertypoiu]

Isn't the thiosulphate ion (S2O3) donating 8 electrons in changing the state of 8 Cl atoms from O to -1? :s

Thiosulfate ions have two sulfur atoms.
They can take turns in giving electrons so it's not unfair
So 8 electrons given by two sulfur atoms so 4 each.

Edit: sorry this could be flawed.

 

[qwertypoiu]

So in anhydrous cases it will always be exo?

Yep.
Anhydrous ---> hydrous. = exothermic.
Hydrous ----> anhydrous. = endothermic. (you gotta heat it up to "evaporate" the water away)

 

[nehaoscar]

Oxidation state of sulfur in S2O3 2- = +2
If u reduce 4 moles of chlorine from 0 to -1, the sulfur will gain +4.
Only D has oxidation state of +6.

If it reduces 4Cl2, then doesn't that mean it forms 8Cl- ions so it donated 8 electrons??
But then it only gains +4 so it only donates 4 electrons? :/
Please can you explain??

 

[Metanoia]

Thiosulfate ions have two sulfur atoms.
They can take turns in giving electrons so it's not unfair
So 8 electrons given by two sulfur atoms so 4 each.

Edit: sorry this could be flawed.

This is actually correct.

The 8 electrons are "lost" by the two sulfur atoms, so each sulfur atom increased its oxidation state by 4 units.

 

[Abdul Hanan]

This isn't to do with hydration of an anhydrous salt. This is dissolving ammonium chloride in water, which is endothermic.
I believe in A2 we learn the details of dissolving.
I did a quick search on yahoo answer :

When dissolving a solid, you have two processes happening.

1/ The breaking up of the solid structure, which requires energy (endothermic)
2/ The hydration of the resulting ions or molecules, which releases energy (exothermic)

In the case of ammonium chloride, the breaking of the solid structure requires more energy than the hydration of the ions provides, meaning the overall temperature will drop.

What about water and ammonium chloride?Thats endothermic

C?

yes C. thanks

 

[ashcull14]

 

[Metanoia]

Is there an easier way to find out the number of isomers? and for this question how many are there?? Please help!!

It's 9701/13/0/N/14

Unfortunately, there are no short cuts except to sketch them out.

 

[princess Anu]

Unfortunately, there are no short cuts except to sketch them out.

why is it B?
I get that reaction one should be endothermic..but why is reaction 2 exothermic & not endo

 

[Metanoia]

View attachment 54819

1st statement is true as the complete ionization of H2SO4 provides 1 mol/dm3 of H+.
2nd statement is false as sulfate ions is contributed by 2nd reaction. And the clue of it being a weak acid means the forward direction is not favored.

 

[ashcull14]

can someoe please draw the diagrams of the mechanism above?

 

[Metanoia]

why is it B?
I get that reaction one should be endothermic..but why is reaction 2 exothermic & not endo

Hydrating of lime is an exothermic reaction.

You can also view it backwards, if you have Ca(OH)2, how would you drive away the water to form CaO and H2O?
You need to heat it. So the reverse reaction below is exothermic (the forward reaction is endothermic).

Ca(OH)2 + heat <--> CaO + H2O

 

[Metanoia]

View attachment 54821
can someoe please draw the diagrams of the mechanism above?

what year and variant is this from?

 

[ashcull14]

what year and variant is this from?

jue p12 june 2012

 

[asadalam]

View attachment 54821
can someoe please draw the diagrams of the mechanism above?

Its D.Its a straight chain but with the addition of NH3 a ring is formed with a propyl side chain,if i recall.

 

[Tasneem_m98]

Oohh okay thank you so much! Jazak Allah kheir!

No as in propanal the H atom is not bonded to O atom so there will be no H-bonding if you look closely.H bonding requires H to be bonded with a highly electronegative atom like O,but in both cases O is bonded to C not H.

 

[Metanoia]

jue p12 june 2012

It is not really the textbook display of the mechanisms, but here is an idea of how the changes occurred.

 

[ashcull14]