Chemistry: Post your doubts here!

[techgeek]

Can someone explain how is the answer B?

Use the ionic equations, for Sn2+ it's:
Sn2+ --> Sn4+ + 2e-
and for Mn2+ ions:
8H+ + MnO4^- + 5e- --> Mn2+ + 4H2O

but the problem is both equations are not balanced there are 2 electrons in the first one while 5 electrons in the 2nd one, this can be solved by multiplying them by such numbers so that the number of electrons i.e. charges, equal in both equations:
5(Sn2+ --> Sn4+ + 2e-)
2(8H+ + MnO4^- + 5e- --> Mn2+ + 4H2O)
------------------------ -------------------------- -----------------
The overall equation becomes:
16H+ + 5Sn2+ + 2MnO4^- --> 5Sn4+ + 2Mn2+ + 8H2O

Now you just have to use the molar ratios, like we use in chemical equations
No. of mol of SnCl2= 9.5/ 190
=0.05
since 5 mol of Sn2+ produce >>>2 mol of Mn2+
and 1 mol produce >>>>>>>>>>2/5 mol of Mn2+
similarly 0.05 mol produce >>>>>2/5 x 0.05
=0.02 mol

 

[Syed Umar]

Solid sulfur consists of molecules made up of eight atoms covalently bonded together.
The bonding in sulfur dioxide is O=S=O.
2232 enthalpy change of combustion of S8, S8(s)= –2376kJ mol-1
energy required to break 1 mole S8(s) into gaseous atoms = kJ mol-1
O=O bond enthalpy = 496 kJ mol–1
Using these data, what is the value of the S=O bond enthalpy?

 

[Rizwan Javed]

Need a little help in equilibrium. The ICE table is kinda confusing to me... could somebody please explain how do I make one for this question?
View attachment 60274

Sorry, for getting late

You know that the number of moles of Nitrogen, N2, at equilibrium are 2.32mol which are 0.32 moles greater than the number of moles initially. This tells us that, some amount of NH3 has broken down to give this increase in mole of N2 at equilibrium. So by using the mole ratios, find the number of moles of NH3 which have decomposed to give 0.32 mole of N2.
n(N2) : n(NH3) = 1 : 2
^ they are in this ratio, so the moles of NH3 broken are:
0.32 * 2 = 0.64mol

Subtract ^ this amount of mole from the initial number of moles NH3, to find the number of NH3 at equilibrium. 2.40-0.64 = 1.76mol

Now in a similar way, find the number of moles of H2 formed by the decomposition of NH3.
n(H2) : n(NH3) = 3 : 2
so moles of H2 formed = 0.64 * 3/2 = 0.96mol <----- add this amount to the initial number of moles of H2, to find the the no. of moles of H2 at equilibrium: 6 + 0.96 = 6.96mol

Now you have this information:


Since the volume is 1dm^3, the conc. of the substances at equilibrium will be equal to the moles of the substances.
Finally use the Kc expression, to get your final answer:


So the answer is A.

 

[Eugene99]

Sorry, for getting late

You know that the number of moles of Nitrogen, N2, at equilibrium are 2.32mol which are 0.32 moles greater than the number of moles initially. This tells us that, some amount of NH3 has broken down to give this increase in mole of N2 at equilibrium. So by using the mole ratios, find the number of moles of NH3 which have decomposed to give 0.32 mole of N2.
n(N2) : n(NH3) = 1 : 2
^ they are in this ratio, so the moles of NH3 broken are:
0.32 * 2 = 0.64mol

Subtract ^ this amount of mole from the initial number of moles NH3, to find the number of NH3 at equilibrium. 2.40-0.64 = 1.76mol

Now in a similar way, find the number of moles of H2 formed by the decomposition of NH3.
n(H2) : n(NH3) = 3 : 2
so moles of H2 formed = 0.64 * 3/2 = 0.96mol <----- add this amount to the initial number of moles of H2, to find the the no. of moles of H2 at equilibrium: 6 + 0.96 = 6.96mol

Now you have this information:
View attachment 60296

Since the volume is 1dm^3, the conc. of the substances at equilibrium will be equal to the moles of the substances.
Finally use the Kc expression, to get your final answer:
View attachment 60297

So the answer is A.

OOO thanks a lot, I get that

 

[techgeek]

Solid sulfur consists of molecules made up of eight atoms covalently bonded together.
The bonding in sulfur dioxide is O=S=O.
2232 enthalpy change of combustion of S8, S8(s)= –2376kJ mol-1
energy required to break 1 mole S8(s) into gaseous atoms = kJ mol-1
O=O bond enthalpy = 496 kJ mol–1
Using these data, what is the value of the S=O bond enthalpy?

Your question was incomplete but I found the full one.

First, we need to make the equation between S8 and O2:
S8 + O2 >>> SO2

Balancing it, gets us:

S8 + 8O2 >>> 8SO2


Since we know,

Enthalpy change of a reaction is = Bonds broken - bonds formed

fitting this into into our scenario:

-2376= [ΔHat (S) + 8 ( O=O)] - 16 (S=O)

(Note that one sulfur bonds with two Oxygens so for 8 sulfurs we will have 16 S=O bonds)

-2376= [2232 + 8x496] - [16 (S=O)]

16 (S=O) = 8576

so S=O = 8576/16

= 536

It's D

 

[Syed Umar]

OH

Your question was incomplete but I found the full one.

First, we need to make the equation between S8 and O2:
S8 + O2 >>> SO2

Balancing it, gets us:

S8 + 8O2 >>> 8SO2


Since we know,

Enthalpy change of a reaction is = Bonds broken - bonds formed

fitting this into into our scenario:

-2376= [ΔHat (S) + 8 ( O=O)] - 16 (S=O)

(Note that one sulfur bonds with two Oxygens so for 8 sulfurs we will have 16 S=O bonds)

-2376= [2232 + 8x496] - [16 (S=O)]

16 (S=O) = 8576

so S=O = 8576/16

= 536

It's D

OHHH RIGHTTT. I just got one step wrong. Thanks mate!

 

[areeba240]

plzzzzzzzzzzz helppppppppppppp

 

[areeba240]

plzzzz helppppppppppp

 

[ashcull14]

i got the reactions with reagent x and y and the reason why tollens is used for reagent Z
but y does it react with CH3O while the que asks us to assume tht the group CH3O doesnt react?

 

[ashcull14]

View attachment 60308
plzzzzzzzzzzz helppppppppppppp

H = sum of bonds broken - sum of bonds formed
H = 180 kJ/mol
180 = [Bond Energy of N2 + Bond Energy of O2] - [2 x Bond Energy of NO]
180 = [ 994 + 496] - [2x]
as.. x = NO
Solve x
-1310 = -2x
x= 655 kJ/mol

 

[The Sarcastic Retard]

View attachment 60310
i got the reactions with reagent x and y and the reason why tollens is used for reagent Z
but y does it react with CH3O while the que asks us to assume tht the group CH3O doesnt react?

which year paper is it?

 

[areeba240]

H = sum of bonds broken - sum of bonds formed
H = 180 kJ/mol
180 = [Bond Energy of N2 + Bond Energy of O2] - [2 x Bond Energy of NO]
180 = [ 994 + 496] - [2x]
as.. x = NO
Solve x
-1310 = -2x
x= 655 kJ/mol

thank u
i did exactly the same but due to the difference in the value of bond energy of nitrogen i couldn't get the ans....i have been using the the data booklet 2016 in that it is 944 instead of 994

 

[ashcull14]

thank u
i did exactly the same but due to the difference in the value of bond energy of nitrogen i couldn't get the ans....i have been using the the data booklet 2016 in that it is 944 instead of 994

no problem ..ure right

 

[ashcull14]

which year paper is it?

its nov 2014 p43

 

[ashcull14]

 

[Saad the Paki]

View attachment 60309
plzzzz helppppppppppp

You have to use trial and error.

Option A :
The equation: 2Ca + O2 --> 2CaO
Moles of oxygen : 0.3/24 = 1/80
Moles of calcium that will react : 2 × 1/80 = 1/40 (since 2 moles of calcium react with one mole of oxygen.
Moles of metal given in question : 1.15/40 = 23/800
The moles given and the moles that react aren't equal so this is incorrect

Now use this way for all of them. I'll do it for the correct option D

Option D :
The equation: 4Na + O2 --> 2Na2O
Moles of oxygen : 1/80
Moles of sodium that will react : 4 × 1/80 = 1/20
Moles of metal given in question: 1.15/23 = 1/20
They are the same thus D is the ans.

 

[areeba240]

You have to use trial and error.

Option A :
The equation: 2Ca + O2 --> 2CaO
Moles of oxygen : 0.3/24 = 1/80
Moles of calcium that will react : 2 × 1/80 = 1/40 (since 2 moles of calcium react with one mole of oxygen.
Moles of metal given in question : 1.15/40 = 23/800
The moles given and the moles that react aren't equal so this is incorrect

Now use this way for all of them. I'll do it for the correct option D

Option D :
The equation: 4Na + O2 --> 2Na2O
Moles of oxygen : 1/80
Moles of sodium that will react : 4 × 1/80 = 1/20
Moles of metal given in question: 1.15/23 = 1/20
They are the same thus D is the ans.

then why they have given the pressure and temperature

 

[Saad the Paki]

then why they have given the pressure and temperature

To let u know that the reaction is carried out at room temperature and pressure.
Because when we find the moles of oxygen we divide by 24. This is only applicable if the reaction is carried out at room temperature. See the first page of data booklet. There are different values of the molar gas constant under different conditions

 

[Saad the Paki]

Part (ii). I was wondering is it acceptable if the reagent is some nitrate salt of a metal more reactive than Mg like KNO3 or NaNO3 so a double displacement reaction takes place.

 

[techgeek]

View attachment 60328
Part (ii). I was wondering is it acceptable if the reagent is some nitrate salt of a metal more reactive than Mg like KNO3 or NaNO3 so a double displacement reaction takes place.

did you see the mark scheme?