Chemistry: Post your doubts here!

[williamcwz]

During the bromination of methane, the free radical •3
CH is generated and a possible terminating
step of this reaction is the formation of C2H6 by the combination of two free radicals.
What could be produced in a terminating step during the bromination of propane?
CH3

1 CH3CH2CH2CHCH3
CH3 CH3
2 CH3CHCHCH3
CH3
3 CH3CH2CHCH2CH3

 

[Abdel Moniem]

June 2007 question 34 and with explanation please to fully understand

 

[BreakingBad]

During the bromination of methane, the free radical •3
CH is generated and a possible terminating
step of this reaction is the formation of C2H6 by the combination of two free radicals.
What could be produced in a terminating step during the bromination of propane?
CH3

1 CH3CH2CH2CHCH3
CH3 CH3
2 CH3CHCHCH3
CH3
3 CH3CH2CHCH2CH3

Please check if you have the right options for this question because the product is supposed to be an alkane or a bromoalkane and none of the options given have the proper number of carbon and hydrogen atoms for it to be an alkane.

June 2007 question 34 and with explanation please to fully understand

1 equilibrium mixture N2F4 <=>2NF2
Enthalpy change is positive which means that the forward reaction is endothermic. When temperature is increased it favors the endothermic reaction and therefore the equilibrium will shift to the right. More moles (since ratio is 1:2) will be produced taking up greater volume and so, the mercury on LHS will be pushed downward and mercury on RHS will rise.

2 equilibrium mixture CH3NC <=>CH3CN
It won't bring about a change as the moles of the gas are same on both sides.

3 nitrogen
When the temperature rises, pressure will rise on both sides equally and so, no change will take place in mercury level.

Since only 1 is right, answer is D

 

[Abdel Moniem]

which ions are present in a solution of ethanol in an excess of concentrated sulphuric acid?
1) CH3CH2O^-1
2) CH3CH2^+OH2
3) HSO4^-1

 

[pikachu]

Can anyone explain this question?

 

[Isaac CKR]

What is the pH in a saturated solution of Ca(OH)2? Ksp = 5.5 x 10^-6 mol^3 dm^-9 for Ca(OH)2

Answer : pH = 12.35

I keep getting pH = 12.05 with my method.

Ksp = [Ca^2+][OH^-]^2

5.5 x 10^-6 = (x) (2x)^2

4x^3 = 5.5 x 10^-6

x = 0.0111 mol dm^-3

pOH = -log [0.0111]
pOH = 1.95

pH = 14 - 1.95 = 12.05. <----- X, answer is 12.35. WHY?

 

[sara kamal]

http://papers.xtremepapers.com/CIE/...d AS Level/Chemistry (9701)/9701_s03_qp_4.pdf
please explain Q2 part b

 

[ZaqZainab]

can anyone tell me the answer and explain why
Please

 

[BreakingBad]

which ions are present in a solution of ethanol in an excess of concentrated sulphuric acid?
1) CH3CH2O^-1
2) CH3CH2^+OH2
3) HSO4^-1

The ions present in a solution of ethanol and H2SO4 are:
2H+
OH-
HSO4-
and the intermediate:
CH3CH2O+

View attachment 34691
Can anyone explain this question?

First:
2AL + 3/2O2 --> AL2O3
Mole ratio is 2:1 so 0.02 mol of Al will give 0.01 mole of AL2O3.

Then:
Al2O3 + 6HCl --> 2AlCl3 + 3H2O
Mole ratio is 1:6 so 0.01 mol of Al2O3 will react with 0.06 mol of HCl completely.

Vol. = Mol/concentration = 0.06/2 = 0.03 dm^3 = 30 cm^3

can anyone tell me the answer and explain why
Please

i)
Ksp = [Mg2+][OH-]^2 (It's the formula of Ksp)
The unit is therefore (mol/dm^3)3 = mol^3dm^-9

ii)
Concentration of Mg = x
Therefore, concentration of OH- for it to be saturated = 2x (because 2 moles of OH- ions react with 1 mol of Mg2+ ion)

Use the formula aboxe:
(x)(2x)^2 = 2*10^-11
4x^3 = 2*10^11
x = 1.71*10^-4 moldm^-3

iii)
The magnesium is saturated y calcium sol. until it reaches saturation. This means that the saturated part is left out. Therefore, the left out part would be:
x/total *100 = (1.71*10^-4/0.054) * 100 = 0.317 %
The % amount taken = % total - % left out = 100 - 0.317 = 99.7 %

can anyone tell me the answer and explain why
Please

The answer is A.
When using atomization energies, we can write the equation as:
C + 2 (H-H) => 4 (C-H)
Atomization energy of C will be given
Atomization of H2 will be given which can be used to calculate H-H:
1/2 H-H = Atomization energy
According to equation:
Formation of methane = bond energy = product energy - reactant energy
B is not the answer because we can only calculate the enthalpy of reaction using those values.
C is not the answer because there is no formula connecting formation and combustion for this question.
D is not the answer because it doesn't make sense.

 

[ZaqZainab]

The answer is A.
When using atomization energies, we can write the equation as:
C + 2 (H-H) => 4 (C-H)
Atomization energy of C will be given
Atomization of H2 will be given which can be used to calculate H-H:
1/2 H-H = Atomization energy
According to equation:
Formation of methane = bond energy = product energy - reactant energy
B is not the answer because we can only calculate the enthalpy of reaction using those values.
C is not the answer because there is no formula connecting formation and combustion for this question.
D is not the answer because it doesn't make sense.

How doesn't D make sense we can just find the formation of CH4 and divide by 4?
Thanks

 

[ZaqZainab]

Can anyone help me out with this one?
Why can't the answer be B??

 

[ZaqZainab]

in the Ms they take Mass as the volume of acid used why is that?

 

[BreakingBad]

How doesn't D make sense we can just find the formation of CH4 and divide by 4?
Thanks

The definition of enthalpy energy of formation is defined as the energy change that takes place when 1 mole of a substance is formed from elements in it's standard state under standard conditions. It has nothing to do with bond energy. To make it clearer, if you take a look at the data booklet, the bond energy of CH bond is 413 kJ/mol while the enthalpy change of formation of CH4 is -75 kJ/mol.

 

[ZaqZainab]

The definition of enthalpy energy of formation is defined as the energy change that takes place when 1 mole of a substance is formed from elements in it's standard state under standard conditions. It has nothing to do with bond energy. To make it clearer, if you take a look at the data booklet, the bond energy of CH bond is 413 kJ/mol while the enthalpy change of formation of CH4 is -75 kJ/mol.

thanks so much could you help me with 2 other questions please

in the Ms they take Mass as the volume of acid used why is that?

Can anyone help me out with this one?
Why can't the answer be B??

 

[BreakingBad]

Can anyone help me out with this one?
Why can't the answer be B??

The answer cannot be B because if we use the combustion values we need 1 mole of PbO, in which case it will no longer have 1 mole of Pb3O4 and we will not be able to equate the formation and combustion values. The answer is D because when both values of formation are given we can simply use the formula and get our answer.

in the Ms they take Mass as the volume of acid used why is that?

Have they used 20 cm^3?

 

[ZaqZainab]

Have they used 20 cm^3?

they have used 30 cm^3 as the MASS but 30 cm^3 is the volume of the acid used? do we always just take volume instead of mass?

 

[BreakingBad]

they have used 30 cm^3 as the MASS but 30 cm^3 is the volume of the acid used? do we always just take volume instead of mass?

That is actually not so surprising because for liquids or aqueous substances, the formula for energy gained is in fact q=Vc(T2-T1).

 

[ZaqZainab]

That is actually not so surprising because for liquids or aqueous substances, the formula for energy gained is in fact q=Vc(T2-T1).

it was for me i did'nt know that as i just started my AS
thanks for your help

 

[Pwetty Angel]

they have used 30 cm^3 as the MASS but 30 cm^3 is the volume of the acid used? do we always just take volume instead of mass?

remember mr manoj's notes??? he said 1gm= 1cm^3 ... so 30cm^3 would be 30gms.
am soorry am late.

 

[SomeStudent]

I dont get moles.. of both AS and A2. My book has only got two pages of it.