cie chem doubts (again)

[Sue T]

all are paper 1:

mj03: Q33
33) The conversion of graphite into diamond is an endothermic reaction (?H = +3 kJ mol–1).

C(graphite) into C(diamond)

Which statements are correct?
1 The enthalpy change of atomisation of diamond is smaller than that of graphite.
2 The bond energy of the C–C bonds in graphite is greater than that in diamond.
3 The enthalpy change of combustion of diamond is greater than that of graphite.
ANS) 1,2&3 are correct
>none of the answers make sense 2 me

n03: Q16
16) In black and white photographic film, light converts silver chloride into metallic silver. After the film
has been developed, the unexposed silver chloride is removed by reaction with sodium
thiosulphate to produce a ‘fixed’ negative.

AgCl + 2Na₂S₂O₃ into 4Na⁺ + Cl^– + [Ag(S₂O₃)₂]^3–

What is the function of thiosulphate?

A to make the silver ions soluble
B to oxidise the silver ions
C to oxidise the silver metal
D to reduce silver ions

ANS) A
>isn't th ON of Ag increasing? how come thiosulphate isn't oxidising th silver but just simply making it soluble?

mj04: Q1
1) Which of these samples of gas contains the same number of atoms as 1g of hydrogen
(Mr : H₂, 2)?
A 22 g of carbon dioxide (Mr : CO₂, 44)
B 8 g of methane (Mr : CH₄, 16)
C 20 g of neon (Mr : Ne, 20)
D 8 g of ozone (Mr : O₃, 48)
ANS)C
>isn't the number of atoms of hydrogen 6.02*10²³/2

nov04: Q7 & 10
7) What are the bond angles in the PH₃ molecule likely to be?
A 90
B 104
C 109
D 120
ANS) B
>PH₃has 1 lone pair n 3 bond pairs just like NH₃how can its bond angle be like an H₂O molecule?isn't it pyramidal??

[nid404]

1) Ok. The question helps u to derive the ans
If the change to diamond from grpahite is endo....it means diamond is at a higher energy level...and it also implies that more energy needs to be supplied to break the bonds in diamond....since combustion is an exothermic process, it will have a greater enthalpy change than graphite.........
visualize an energy profile diagram......is u can't, I will draw one for u...should be easier to understand then
2) An oxidising agent is one that oxidises an element and gets reduced itself. And Ag+ is reduced, so the question of oxidation doesn't arise.
3)*atoms* not molecules....This should solve it
4)pyramidal is 4 bond pairs..
try elmination if ur confused
It's not octahedral(90)...It can't be 120 since planar is for 3 bond pairs
Using electron replusion theory....U should figure out that angle be less than 109* due to lone pair in the compound

Hope this helps

[Sue T]

thank you!