Chemistry: Post your doubts here!

[Xaptor16]

Are you sure we're looking at the same thing cuz the table I send has ionisation energies of 950, 1800 and 2700

 

[The Sarcastic Retard]

When a primary alcohol is heated with aqueous acidified potassium dichromate, it is, in general, oxidized.
With gentle warming, it is first oxidized to an aldehyde, and if the aldehyde product is further allowed to react in heated conditions where the oxidant (dichromate, in this case) is in excess, the carboxylic acid of that alcohol will be produced. So, in this case, it is not heated excessively AND the aldehyde is removed immediately by distillation after it is formed. Therefore, we can say that the product obtained in the reaction is, indeed, an aldehyde. If the yield of this is 70%, let's see what we can find out from the equation:

Ethanol produces Ethanal on reaction with dichromate, and the pure reaction with the oxidant removed to simplify things is

CH3CH2OH + [O] ==> CH3CHO(NOT CH3COOH) + H2O (Now do it)

(Note that the equation is not balanced as in reality - this is only to show us that the ratio of Ethanol to Ethanal is ideal conditions is 1:1. This is not the right reaction, which is itself very complex, and can be found here).


Now u can do it.

 

[The Sarcastic Retard]

Are you sure we're looking at the same thing cuz the table I send has ionisation energies of 950, 1800 and 2700

1800 - 950 = 850
2700 - 1800 = 900
next diff is larger that is 2100.

 

[Xaptor16]

When a primary alcohol is heated with aqueous acidified potassium dichromate, it is, in general, oxidized.
With gentle warming, it is first oxidized to an aldehyde, and if the aldehyde product is further allowed to react in heated conditions where the oxidant (dichromate, in this case) is in excess, the carboxylic acid of that alcohol will be produced. So, in this case, it is not heated excessively AND the aldehyde is removed immediately by distillation after it is formed. Therefore, we can say that the product obtained in the reaction is, indeed, an aldehyde. If the yield of this is 70%, let's see what we can find out from the equation:

Ethanol produces Ethanal on reaction with dichromate, and the pure reaction with the oxidant removed to simplify things is

CH3CH2OH + [O] ==> CH3CHO(NOT CH3COOH) + H2O (Now do it)

(Note that the equation is not balanced as in reality - this is only to show us that the ratio of Ethanol to Ethanal is ideal conditions is 1:1. This is not the right reaction, which is itself very complex, and can be found here).


Now u can do it.

1800 - 950 = 850
2700 - 1800 = 900
next diff is larger that is 2100.

Oh okay thankyou!

 

[DESTROYER1198]

i have another moles question that would be nice if you helped me with s12 qp21 Q5) B)ii & iii:
why in ii when we wanted to find the moles of hydrogen atom we multiplied by 2??

 

[Farhad221]

i have another moles question that would be nice if you helped me with s12 qp21 Q5) B)ii & iii:
why in ii when we wanted to find the moles of hydrogen atom we multiplied by 2??

because the answer we get is the mole of hydrogen molecules , to find the moles of H atoms we times it by two as each hydrogen molecule has two H atoms.

 

[DESTROYER1198]

because the answer we get is the mole of hydrogen molecules , to find the moles of H atoms we times it by two as each hydrogen molecule has two H atoms.

if we take H moecule as Y and H atom as X then Y=2X
so X=Y/2 isnt it?
so why we have to multiply instead of dividing??

 

[nehaoscar]

Practicals!
Anyone have any points and tips for all the reactions and reagents and stuff needed to know for practicals?
And any tips on finishing the practicals fast cause they take time..
I like the titrations but then heating and all takes too much time
And how do you know which gas to test for when? Like it takes too much time to test for each and every gas
and by the time you do the test, the gas surely must have escaped so sometimes the result is wrong...
And tips please and perhaps a revision guide note if anyone has??
Also any predictions on this year's practical for variant 33?

 

[The Sarcastic Retard]

if we take H moecule as Y and H atom as X then Y=2X
so X=Y/2 isnt it?
so why we have to multiply instead of dividing??

1 mole of H2 = ? mol of H atoms
Metanoia's style.

 

[The Sarcastic Retard]

i have another moles question that would be nice if you helped me with s12 qp21 Q5) B)ii & iii:
why in ii when we wanted to find the moles of hydrogen atom we multiplied by 2??

ii) 1 mol = 24dm^3
--> ? = 160 * 10^-3 dm^3
---> n of H2 = 6.67 * 10^-3 mol <----------- This is the moles of molecule. So we know 1 mol of H2 molecule = 2 mol of H atoms. So 2 * n of H2 = 0.0133 mol of H atoms.
iii)If there is 1 OH- then one H+ will be produced and in X; for 1 mole of X, 2 moles of H atoms are produced. So there are two OH- in each molecule of X.

 

[Farhad221]

Practicals!
Anyone have any points and tips for all the reactions and reagents and stuff needed to know for practicals?
And any tips on finishing the practicals fast cause they take time..
I like the titrations but then heating and all takes too much time
And how do you know which gas to test for when? Like it takes too much time to test for each and every gas
and by the time you do the test, the gas surely must have escaped so sometimes the result is wrong...
And tips please and perhaps a revision guide note if anyone has??
Also any predictions on this year's practical for variant 33?

 

[The Sarcastic Retard]

Practicals!
Anyone have any points and tips for all the reactions and reagents and stuff needed to know for practicals?
And any tips on finishing the practicals fast cause they take time..
I like the titrations but then heating and all takes too much time
And how do you know which gas to test for when? Like it takes too much time to test for each and every gas
and by the time you do the test, the gas surely must have escaped so sometimes the result is wrong...
And tips please and perhaps a revision guide note if anyone has??
Also any predictions on this year's practical for variant 33?

http://labskills.co.uk/a-level-labskills-trial/biology/index.html Register yourself here. Its very helpful.
Personal tip, Do the titration practical first as you good at it. Leave the calculations, go to heating stuffs question or the ones you are left and find difficult. Then when u get free time like 2 or 3 minutes heating stuffs and thing move on to calculations. Moles stuffs are very easy... so u'll finish it in very short time. If u master Enthalpy changes, your heating practical gets easier in calculations. And like we use to do in our IGCSE or O levels remember the tests given in last two pages of our every practical paper which will save time instead of recording the observation u know it directly, it doesnt happen in many cases but its good for you only...

 

[DESTROYER1198]

ii) 1 mol = 24dm^3
--> ? = 160 * 10^-3 dm^3
---> n of H2 = 6.67 * 10^-3 mol <----------- This is the moles of molecule. So we know 1 mol of H2 molecule = 2 mol of H atoms. So 2 * n of H2 = 0.0133 mol of H atoms.
iii) It will show that 1 mole of the compond produces one mole of H2 hence it must contain two OH groups

Thanks alot man

 

[The Sarcastic Retard]

Thanks alot man

Look at my edited post for part iii if not understood properly.

 

[nehaoscar]

Thanks a lot!

 

[nehaoscar]

http://labskills.co.uk/a-level-labskills-trial/biology/index.html Register yourself here. Its very helpful.
Personal tip, Do the titration practical first as you good at it. Leave the calculations, go to heating stuffs question or the ones you are left and find difficult. Then when u get free time like 2 or 3 minutes heating stuffs and thing move on to calculations. Moles stuffs are very easy... so u'll finish it in very short time. If u master Enthalpy changes, your heating practical gets easier in calculations. And like we use to do in our IGCSE or O levels remember the tests given in last two pages of our every practical paper which will save time instead of recording the observation u know it directly, it doesnt happen in many cases but its good for you only...

Thankyou so much! Yeah i like enthalpy too, it's just i hate the reactions in which you have to test for gases etc... and then sometimes you get it wrong and then the whole thing goes wrong! Hopefully no redox reactions this year *fingers crossed*

 

[nehaoscar]

Q = mCT
m = grams
C = J/kg/K
T = celcius
Then how is Q is Joules?
Shouldn't the temperature be in Kelvin so that the K's cancel from T and C? :S

 

[Tasneem_m98]

Someone please help (M/J 2011 paper 22 question 5)
In part b, how come the answer is dilute H2SO4 not concentrated H2SO4??

Also part e(i) , the aldehyde products of the partial oxidation of CH3(CH2)7CH--CH(CH2)7X (where X represents the rest of the molecule)
the mark scheme wrote two products, one of them being OHC(CH2)7CX .. I dont understand why is there an additional C beside the X???

Thank you :3

 

[lazyhits_5122]

Salam I need examiner reports of all past papers ... plz send me the link where i can get

 

[Zash Riyash]

Someone please help (M/J 2011 paper 22 question 5)
In part b, how come the answer is dilute H2SO4 not concentrated H2SO4??

Also part e(i) , the aldehyde products of the partial oxidation of CH3(CH2)7CH--CH(CH2)7X (where X represents the rest of the molecule)
the mark scheme wrote two products, one of them being OHC(CH2)7CX .. I dont understand why is there an additional C beside the X???

Thank you :3

Since there has to be 8 carbon in total