Chemistry: Post your doubts here!

[slisjunknown]

For which equation is the enthalpy change correctly described as an enthalpy change of formation?

A C(g) + O2(g) → CO2(g)

B C(s) + 0.5 O2(g) → CO(g)

C 2N(g) + 4O(g) → N2O4(g)

D 2NO(g) + O2(g) → 2NO2(g)

The answer is B...but why not A?

 

[nehaoscar]

For which equation is the enthalpy change correctly described as an enthalpy change of formation?

A C(g) + O2(g) → CO2(g)

B C(s) + 0.5 O2(g) → CO(g)

C 2N(g) + 4O(g) → N2O4(g)

D 2NO(g) + O2(g) → 2NO2(g)

The answer is B...but why not A?

For enthalpy change of formation, the reactants have to be in their natural standard state
Carbon is given as a gas in A however it is found as a solid naturally, so it's B

 

[slisjunknown]

For enthalpy change of formation, the reactants have to be in their natural standard state
Carbon is given as a gas in A however it is found as a solid naturally, so it's B

I kind of forgot the definition....anyways thanks a lot..I perfectly understood

 

[princess Anu]

Why is it D... I thought it would be C because
- pale yellow ppt is formed by Br- when reacted with Silver nitrate
- white ppt for Cl-
- cream ppt for I-
and
- Cl- is soluble
- I- is soluble in concentrated ammonia
- Br- is insoluble

So if it is partly dissolved, and a darker yellow ppt is present, shouldn't Br- be present instead of I- because I- will also dissolve therefore there will be no ppt remaining after ammonia is added.....???

Cream ppt is of Br- or Agbr while Yellow ppt is of I- or AgI

 

[princess Anu]

View attachment 51706
Shouldn't the electronic configuration be
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
Since the 4s orbital is filled before the 3d orbital
But the mark scheme says
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2
But i have been taught that 4s fills before 3d so which one is correct??

yeah thats correct, its just that try to write the electronic configuration in the order.. even your way is acceptable, acc to my teacher.

 

[3l_general]

do all the organic chemistry reaction have only one kind of reaction path diagram ??????????
And can any one show it for me plzzzz

 

[immie.rose]

Please Please Please explain the concept behind this question, anyone?
Q4 all of part (c) in http://freeexampapers.com/A-Level/Chemistry/CIE/2007-Jun/9701_s07_qp_4.pdf
marking scheme: http://freeexampapers.com/A-Level/Chemistry/CIE/2007-Jun/9701_s07_ms_4.pdf

 

[Mujtaba Shar]

Hey guys where can i get simplest revision for As Chemistry..! P1 and P2

 

[Saif Qureshi]

Can anyone please help me, I dont understand this question.

 

[Xylferion]

Can anyone please help me, I dont understand this question.

15:30:15 is the ratio for N: P :K in 100g.
If the recommended usage is 14g of fertiliser, you need to change the ratios to match 14g.

15+30+15 = 60
NPK makes up 60g out of the 100g.
So how much will it make in 14g?
60/100 = x / 14

This gives you 8.4g out of the 14g.

15:30:15 is in the ratio 1:2:1, you need to split the NPK in this ratio.

You end up with 2.1 g / 4.2 g / 2.1 g

We're dealing with Nitrogen atoms, so we take 2.1 g and find the amount of moles present in 14g.
2.1g/14g = 0.15 mol

Now to find the concentration, we use the formula n = cV, where n is the number of moles, c is the concentration and V is the volume in dm^3.

n = 0.15
V = 5
c is what we're trying to find.

c = 0.15/5 = 0.03, hence your answer is A.

Hope that helped

 

[Saif Qureshi]

15:30:15 is the ratio for N: P :K in 100g.
If the recommended usage is 14g of fertiliser, you need to change the ratios to match 14g.

15+30+15 = 60
NPK makes up 60g out of the 100g.
So how much will it make in 14g?
60/100 = x / 14

This gives you 8.4g out of the 14g.

15:30:15 is in the ratio 1:2:1, you need to split the NPK in this ratio.

You end up with 2.1 g / 4.2 g / 2.1 g

We're dealing with Nitrogen atoms, so we take 2.1 g and find the amount of moles present in 14g.
2.1g/14g = 0.15 mol

Now to find the concentration, we use the formula n = cV, where n is the number of moles, c is the concentration and V is the volume in dm^3.

n = 0.15
V = 5
c is what we're trying to find.

c = 0.15/5 = 0.03, hence your answer is A.

Hope that helped

Thanks A Lot Mate

 

[Saif Qureshi]

OK one more doubt..This one..

 

[The Sarcastic Retard]

Hey guys where can i get simplest revision for As Chemistry..! P1 and P2

Everything in the textbook is fruitful, you have the best notes in there.

 

[The Sarcastic Retard]

OK one more doubt..This one..

moles KOH = 1.0 x 10^-2 mol/dm^3 x 0.025 dm^3 =0.00025

2 KOH + H2SO4 = K2SO4 + 2 H2O

moles H2SO4 = 0.00025/2 =0.000125

moles CaSO4 = 0.000125

initial concentration of CaSO4 = 0.000125 mol/ 0.050 dm^3 =0.0025 mol/dm^3

 

[shazmina]

http://maxpapers.com/wp-content/uploads/2012/11/9701_s13_qp_42.pdf
Can anyone please explain me Question 1 part b ( iii ) ...
like do i have to draw 2 tangents for both the graphs at time = 0 and find out the initial rate ie the gradient of these 2 lines or is there some other method for finding it??? i dont rele get the mark scheme .. so plz help

 

[ashcull14]

Which reactions have a coloured organic product?
1 ethanal + 2,4-dinitrophenylhydrazine reagent
2 ethanol + acidified potassium dichromate(VI)
3 ethene + cold dilute acidified potassium manganate(VII)
help plz

 

[Saif Qureshi]

Which reactions have a coloured organic product?
1 ethanal + 2,4-dinitrophenylhydrazine reagent
2 ethanol + acidified potassium dichromate(VI)
3 ethene + cold dilute acidified potassium manganate(VII)
help plz

I think it is reactions 1 and 2
1 will have an orange ppt.
2 will have a green ppt
3 the pale purple solution will turn colourless.

 

[asadalam]

Which reactions have a coloured organic product?
1 ethanal + 2,4-dinitrophenylhydrazine reagent
2 ethanol + acidified potassium dichromate(VI)
3 ethene + cold dilute acidified potassium manganate(VII)
help plz

Its 1 only,DNPH gives an ORGANIC product,rest do not.The main thing is ORGANIC

 

[Saif Qureshi]

Doubts. Can anyone tell the answer and explain it?

 

[ashcull14]

I think it is reactions 1 and 2
1 will have an orange ppt.
2 will have a green ppt
3 the pale purple solution will turn colourless.

thts wt i thought but its wrong...the ans above highlights the term the 'organic product' only formed in 1 nt in 2 nd 3