Chemistry: Post your doubts here!

[Nibz]

Thanks Nibz

Another question
At which stage is the oxidation state of Nitrogen different to the changes in the other steps

NO3- (aq) -----------(A)----> NO2- (aq) ----(B)--> NO(g) ----(C)--> N2O(g) -----(D)--> N2(g)


so in A its +5 to +3
in B +3 to +2
in C it's +2 to +1
in D it's +1 to 0

How is it B?

It's A. Open your eyes and check the answer again.

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s11_er.pdf
Paper 11. MCQ number 2 - A

 

[DPratik]

WTH.....examiner report and ms are contradictory ....examiner report is ryt???or ms???​

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can't we just find the no of moles of CaSO4 reacting with the acid rain...but if we do so then the loss in mass comes 0.61kg but the mark scheme answer is 0.748kg !!

 

[DPratik]

Chemistry Equilibrium question pwease help !

Chemistry Equilibrium question pwease help !

Can anyone pretty please help me with this question ? ^.^
coz seriously , its driving me crazy >-> :%) :%)

A mixture contains 0.5 mol of ethanoic acid ,0.5 mol of ethanol,0.1 mol of ethyl ethanoate and 0.1 mol of water was set up and allowed to come to an equilibrium at 298K.
CH3CO2H + C2H5OH = CH3CO2C2H5
Calculate the amount of ,in moles, of each of the substance present at the equilibrium, if Kc is 4.0.
Note-(the reaction is reversible)

which year queestion is this????[/quote]

the possible answer to this question is.... ester=0.21 mol and alcohol and carboxylic acid-0.29 and 0.29 mol

 

[DPratik]

2 and 3 are correct.

the answer is 1 and 2 because the combustion of C6H12O6....can be found out by other ways like !!

 

[hassam]

why the intermediate ahud have +ve delta H

 

[étudiante]

why the intermediate ahud have +ve delta H

to form da intermidiate, bonds need 2 b BROKEN and breaking of bonds takes IN energy....... dats why delta H is postive....

 

[hassam]

BROTHER.....i thought
like this that if it was it must be positive delta H so that it is unstable and is converted further to products...........if it is already -ve delta H then it ll be already stable....well bond breaking explanation cn not be used in all situation ....it works perfectly well for SN1 rreaction profile....wat do ya say

 

[étudiante]

BROTHER.....i thought
like this that if it was it must be positive delta H so that it is unstable and is converted further to products...........if it is already -ve delta H then it ll be already stable....well bond breaking explanation cn not be used in all situation ....it works perfectly well for SN1 rreaction profile....wat do ya say

its sister acc
yes ur thinking is ryt... it IS unstable so it breaks down.... n dats cuz it has taken IN energy (+ve delta H)...
n no dis isnt 4 SN1 only.... its a general explanation

 

[DPratik]

why the intermediate ahud have +ve delta H

from the figure we can see that the reaction is endothermic which means obviously there will be delta H will be positive !!!

 

[Rabib_69]

Help with this question please.. It's from M/J 11 41

 

[DragonCub]

Help with this question please.. It's from M/J 11 41
"State and explain the variation in the oxidation numbers of the chlorides of the elements Na, Mg, Al and Si."

The oxidation number goes from +1 (for Na) to +4 (for Si) step by step.
You need to talk about the number of electrons in the outer shell of each element and thus their ions' bonding abilities to the chloride ions.

 

[DragonCub]

Hi guys
Can anyone tell me something about complex ions? One past paper question asked me to draw the displayed shape of d-orbitals of metal cation in complex ions, and to explain the energy differences among the d-orbitals. I felt like my teacher did not mention this part.

 

[Rabib_69]

The oxidation number goes from +1 (for Na) to +4 (for Si) step by step.
You need to talk about the number of electrons in the outer shell of each element and thus their ions' bonding abilities to the chloride ions.

ohh so they meant the oxidation number of the ions Mg, si and so on.. damn, i thought the Chloride ions

 

[smartangel]

in Q5 part b..why does the marking scheme give positive E values? im getting negative..
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_43.pdf

 

[Rabib_69]

Hi guys
Can anyone tell me something about complex ions? One past paper question asked me to draw the displayed shape of d-orbitals of metal cation in complex ions, and to explain the energy differences among the d-orbitals. I felt like my teacher did not mention this part.

yes, u just draw any one of the dx^2-y^2 or dxy orbitals on axes and label them.. then u explain the energy differences, by reference to the fact that there
is repulsion between the lone pairs of ligands and the orbitals which split them in two levels. three orbitals lower of two stay above of the d-orbital..

 

[Paradoxical]

Does anyone have the mark scheme for M/J 02 paper 4? If so, could you please upload it? Thanks!

 

[hmlahori]

Can someone help me with these questions?​

This is the third time im posting them.
Please and Thank you.....

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w07_qp_1.pdf
Q9(had very little idea about how to approach this question and ended up guessing the answer) and Q36

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_qp_1.pdf
Q3 What does 'ground state' mean? And r we supposed to rite the 1s2 2s2.... electronic configuration to know the answer?
Q21 Does an alcohol get oxidised by hot, concentrated manganate(7) ions?
Q31 Why is the answer B not A? all the three statements seem correct.
and Q38 Why cant the answer be B?​

​

 

[smartangel]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s08_qp_4.pdf
Q2 a iii.. how do we balance the equation using oxidation numbers?

 

[DragonCub]

​

Can someone help me with these questions?​

This is the third time im posting them.​

Please and Thank you.....​

​

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w07_qp_1.pdf ​

Q9(had very little idea about how to approach this question and ended up guessing the answer) and Q36​

​

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_qp_1.pdf​

Q3 What does 'ground state' mean? And r we supposed to rite the 1s2 2s2.... electronic configuration to know the answer?​

Q21 Does an alcohol get oxidised by hot, concentrated manganate(7) ions?​

Q31 Why is the answer B not A? all the three statements seem correct.​

and Q38 Why cant the answer be B?​

To W07 Q9, is the answer B?
This is a redox reaction, with the sulphite oxidised to sulphate. Oxidation and reduction always come together. With sulphur oxidised, the "metal" must be reduced. You can see the change in oxidation number of sulphur is plus 2 (from +4 to +6). So if 1 mole of "metal" reacts with 1 mole of sulphite, the "metal" should experience a change of minus 2 in its oxidation number in order to compensate the oxidation. In this case, according to the volumes and molar concentrations, the mole ratio of "metal" to sulphite is 2 : 1, so the "metal" is reduced by only 1 oxidation number, so it's from +3 to +2.
W07 Q36,
1 should not be correct. The reaction must happen since sulphuric acid is acidic and potassium iodide is basic.
2 is correct since concentrated sulphuric acid is a strong oxidising agent, and iodide ion is a fairly good reducing agent, the redox reaction shall take place.
3... I cannot be sure, but that 1 is wrong and 2 is correct shall make you able to deduce that the answer is C.

 

[smartangel]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w04_qp_4.pdf
in Q2 part e.. isnt the ratio of electrons to hydrogen 2:1.. hence when we get mol of electron by 345600/96500 we divide it by 2 to get get mol of hydrogen wich will also be its mass? PLEASE ANSWER