CHEM UNIT 2!

[aligatorviper]

Jan 2012:
Q17- The answer is D since they have the same molecular mass of 58 and the statement in D says that their molecular ion peaks are different while they arent
Q20 C- iodine dissolved in water is yellow but when it is added to starch, it turns blue black. At the end point, the colour is then, colourless.
Q21 Bii- you need to know the mechanism for this. ( i have attached it) Just know that primary carbocations and tertiary carbocations follow different mechanisms since tertiary carbocations are stable while primary aren't. Please learn the steps in the uploaded file.
Jun 2012 Q 16- it's A and this cannot be explained cuz this is a property of CFCs and you need to know this. You are supposed to learn the properties of CFCs.

to part c q20: i guess iodine in water gives brown color not yellow!!!!!

 

[A-level student]

can someone plz help me with this one
jan 2011 chemistry unit 2 q15

And also q18 f ii jan 2010

 

[za Shadow]

Jan 2012:
question 17:
D, the molecular ion peak is the same because they have the same RFM

question 20c)
KI reacts with Chlorine and since chlorine is a better oxidising agent than Iodine, it displaces the iodide ion out of its salt causing it to be oxidised to iodine. The solution at this stage turns brown. As the thiosulfate ions reduce the iodine to iodide, the color fades and eventually it becomes pale yellow. The transition from pale yellow to colorless would be too "settled", so they wouldnt know exactly when the end point is reached. They use starch (which is the indicator for iodine), which turns blue to get a more observable color change
so,
from pale yellow
blue-black
colorless

21bii) Just read your notes or the book for the reaction mechanisms. The fact that they are different types of halogenoalkanes (primary and tertiary) means that they undergo nucleophilic substitution reactions through different mechanisms

June 2012:
16)hmm.. i think it's A
They are non-flammable since they were used in aerosols and as blowing agents and what not. i think they dont support combustion because of the strong C-F bonds but i am not sure
B+C+D are correct so it has to be A

thanks for this great explanation really helpful

Jan 2012:
Q17- The answer is D since they have the same molecular mass of 58 and the statement in D says that their molecular ion peaks are different while they arent
Q20 C- iodine dissolved in water is yellow but when it is added to starch, it turns blue black. At the end point, the colour is then, colourless.
Q21 Bii- you need to know the mechanism for this. ( i have attached it) Just know that primary carbocations and tertiary carbocations follow different mechanisms since tertiary carbocations are stable while primary aren't. Please learn the steps in the uploaded file.
Jun 2012 Q 16- it's A and this cannot be explained cuz this is a property of CFCs and you need to know this. You are supposed to learn the properties of CFCs.

thank you very much and thanks for the mechanisms you did really appreciate it

 

[saba1996]

thanks for this great explanation really helpful


thank you very much and thanks for the mechanisms you did really appreciate it

Do we have to know that halogeno alkanes undergo 2 types of mechanisms??? :'(

 

[IsraJ]

Do we have to know that halogeno alkanes undergo 2 types of mechanisms??? :'(

Yes we do and it has been seen in the past papers too.

 

[alex]

to part c q20: i guess iodine in water gives brown color not yellow!!!!!

When it dissolves and there is high concentration of iodine the color is brown. As the thiosulfate converts iodine to iodide the concentraiton of iodine decreases, so the color fades from brown to yellow and then pale yellow. When it turns pale yellow they add the indicator

 

[svenas]

When it dissolves and there is high concentration of iodine the color is brown. As the thiosulfate converts iodine to iodide the concentraiton of iodine decreases, so the color fades from brown to yellow and then pale yellow. When it turns pale yellow they add the indicator

am not getting this ... so when is the starch blue black / colourless / yellow ?

 

[alex]

am not getting this ... so when is the starch blue black / colourless / yellow ?

When it turns pale yellow
Otherwise, the color change would be from pale yellow to colorless so it would be too hard to distinguish when the end point is reached
So they add starch to get a more observable color change (from blue-black to colorless)

 

[qstar.inc]

how was exam, everyone?

 

[deathvalley69]

how was exam, everyone?

shit as usual

 

[aligatorviper]

good. hw wz urs???? n hw wz phy2