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Edexcel - Chemistry Unit 2 - 4 June 2013

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Hey guys, I'm really stressing over this exam. Has anyone got any good revision notes or websites for last minute cramming. :smile:
Reply 981
Avatar for davcha
davcha
1
Original post by maryam1996
Hi, can someone please explain to me why the c-Br bond is more electronegative than the C-I bond. I also don't understand why thermal stability of metal carbonates increases down group 2 even-though that as the cation ion gets larger down the group doesn't that make it less stable as it can easily lose electrons now as it has more sheilding and so it's further away from the nucleus. I'm quite confused,
Thank you

The C-Br bond is more polar than the C-I bond because of the difference in electronegativities between I and Br. Br is more electronegative than I (electronegativity decreases down a group) and so the difference in electronegativites between C and Br is greater than that between C and I, meaning that the Br atom can "pull" the electrons closer to itself in a covalent bond with C.
Thermal stability increases down the group as the size of the cation increases. The charge remains constant and so as the size increases the charge density decreases. A small charge density distorts the nitrate, carbonate anion less and so puts less strain on it's bonds; this means it is less likely to break under heat.
Original post by maryam1996
Hi, can someone please explain to me why the c-Br bond is more electronegative than the C-I bond. I also don't understand why thermal stability of metal carbonates increases down group 2 even-though that as the cation ion gets larger down the group doesn't that make it less stable as it can easily lose electrons now as it has more sheilding and so it's further away from the nucleus. I'm quite confused,
Thank you


A bond isn't "electronegative" at all, it is polar. The C-Br and C-I bonds are polar due to the presence of an electronegative element. Iodine is far less electronegative than Bromine, hence that polar is far less polar.

As you go down the group, the size of the cation increases in size, but the charge remains the same (+2), hence the cation is less able to polarise and as a result distort the negative anion (carbonate/sulfate ion). Less distortion leads to increased thermal stability.

Original post by Linked
x


Covalent Bonds -> Hydrogen Bonds -> Dipole-Dipole Interactions -> London Forces
(edited 10 years ago)
Original post by Linked
arrange these in order of increasing strength -

covalent bonds
hydrigen bonds
london forces
dipole-dipole interactions


dipole-dipole interactions
london forces
hydrogen bonds
covalent bonds
Original post by geor
Horizontal test-tube containing ceramic fibres soaked in halogenoalkane + alcoholic KOH. Bung and delivery tube to an inverted test-tube under water. Heat the ceramic fibres! :smile:


Oh my god...you just make me very worried lol. It seems the holy CGP bible doesn't cover everything... :frown:

Tbh I've seen you post like 50 times giving answers on this thread, you must've absolutely smashed the revision for this lol, full UMS for you tomorrow!
(edited 10 years ago)
Original post by maryam1996
Hi, can someone please explain to me why the c-Br bond is more electronegative than the C-I bond. I also don't understand why thermal stability of metal carbonates increases down group 2 even-though that as the cation ion gets larger down the group doesn't that make it less stable as it can easily lose electrons now as it has more sheilding and so it's further away from the nucleus. I'm quite confused,
Thank you


:colone:

Bonds are never electronegative. Atoms are. Going down group 7, the electronegativity decreases. So, C-Br bond is more polar than C-I because Br is a more electronegative element. Moreover, it requires greater energy to break the C-Br bond than to break the C-I bond since the C-Br bond length is shorter. So, the activation energy is higher. :smile:

As for the 2nd question:

When they ask such question, simply answer in terms of polarisation:

Let's take MgCO3 and CaCO3 as examples. Mg2+ is more polarising than Ca2+ because its size is smaller. Therefore, it has higher charge density. This makes it more polarising. Hence, it causes more distortion of the carbonate electron cloud. (Write up to this and you will get full mark) :wink:
Original post by davcha
The C-Br bond is more polar than the C-I bond because of the difference in electronegativities between I and Br. Br is more electronegative than I (electronegativity decreases down a group) and so the difference in electronegativites between C and Br is greater than that between C and I, meaning that the Br atom can "pull" the electrons closer to itself in a covalent bond with C.
Thermal stability increases down the group as the size of the cation increases. The charge remains constant and so as the size increases the charge density decreases. A small charge density distorts the nitrate, carbonate anion less and so puts less strain on it's bonds; this means it is less likely to break under heat.

What do u expect for section C? I expect abt graphite and diamond
[QUOTE="pineapple78;42947930"]20 cm3 of sulfuric acid, concentration 0.25 mol dm–3, was neutralized in a titration with
barium hydroxide, concentration 0.50 mol dm–3. The equation for the reaction is
Ba(OH)2(aq) + H2SO4(aq) → BaSO4(s) + 2H2O(l)
(a) The volume of barium hydroxide required was (1)
A 10 cm3
B 20 cm3
C 25 cm3
D 40 cm3


HELP and an you please explain step by step please?[/QUOTE

20x0.25/1000 = 5x10-3
0.50/1000=5x10-4
5x10-3/5x10-4 =10cm
Reply 988
Avatar for davcha
davcha
1
Original post by Daniel Atieh
What do u expect for section C? I expect abt graphite and diamond

Yeah it's in the spec but doesn't seem to come up that often, so maybe. usualy it'd just be some green chemistry question asking us to compare processes...
Original post by James A
dipole-dipole interactions
london forces
hydrogen bonds
covalent bonds


London forces
Permanent dipole-permanent dipole interactions
Hydrogen bonds
Covalent bonds :cool:
I'm gonna start on the Jan 13 paper. I'll do Section A first and mark it.. Then go for a shower to take a break from revision, then attempt section B and C :woo:

Everyone should be aiming for full marks on section A.
Original post by HarryMWilliams
A bond isn't "electronegative" at all, it is polar. The C-Br and C-I bonds are polar due to the presence of an electronegative element. Iodine is far less electronegative than Bromine, hence that polar is far less polar.

As you go down the group, the size of the cation increases in size, but the charge remains the same (+2), hence the cation is less able to polarise and as a result distort the negative anion (carbonate/sulfate ion). Less distortion leads to increased thermal stability.



Covalent Bonds -> Hydrogen Bonds -> Dipole-Dipole Interactions -> London Forces


So what are the difference between London forces and Dipole Dipole interactions?
Does Dipole interactions occour only with polar molecules/polar bonds on molecules?
can someone please help me with this question
Benzene, which is needed for the new process of breaking down carbon dioxide, can
be made from coal. It is now usually made by catalytic treatment of one fraction of
crude oil at temperatures of around 500 °C and 20 atmospheres pressure.
Suggest the benefits and disadvantages of breaking down carbon dioxide using
benzene and the catalyst as described in the passage. You should consider
• the energy and resources needed
• the effects on the atmosphere
• whether it is a beneficial method for producing energy compared to direct
use of fossil fuels.
(
Reply 993
Avatar for Linked
Linked
6
Original post by James A
dipole-dipole interactions
london forces
hydrogen bonds
covalent bonds


why are covalent bonds stronger than hydrogen bonds? thx for the reply:smile:
Reply 994
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posthumus
OP
20
Original post by James A
I'm gonna start on the Jan 13 paper. I'll do Section A first and mark it.. Then go for a shower to take a break from revision, then attempt section B and C :woo:

Everyone should be aiming for full marks on section A.


I look back on that paper like " How on earth did I get a U :ashamed2: "
Original post by airheadbuster
There would be a major peak in the mass spectrum for butan-1-ol, CH3CH2CH2CH2OH, but not for butan-2-ol, CH3CH2CH(OH)CH3, at m/e value:

A = 15

B = 17

C = 29

D = 43

Which one will be correct and why?


Someone please answer this. :frown:
Original post by tahadamani
x


London forces are just intermolecular forces, the weakest, which arise from the spontaneous asymetrical arrangement of electrons in an atom or molecule. Dipoles can only occur when there is some degree of polarisation.
Reply 997
Avatar for Linked
Linked
6
do we need to know the equations for the reactions between halides and sulfuric acid?
Original post by Linked
do we need to know the equations for the reactions between halides and sulfuric acid?


You should know them yes, there was a question which required direct knowledge of the products in one past paper.
Reply 999
Avatar for posthumus
posthumus
OP
20
Original post by newyork newyork
can someone please help me with this question
Benzene, which is needed for the new process of breaking down carbon dioxide, can
be made from coal. It is now usually made by catalytic treatment of one fraction of
crude oil at temperatures of around 500 °C and 20 atmospheres pressure.
Suggest the benefits and disadvantages of breaking down carbon dioxide using
benzene and the catalyst as described in the passage. You should consider
• the energy and resources needed
• the effects on the atmosphere
• whether it is a beneficial method for producing energy compared to direct
use of fossil fuels.
(


Catalysts find alternative path with lower activation energy. Therefore molecules need less kinetic energy for reaction to occur and temperatures can be kept lower. Lower temperature means less energy wasted, which is good for the enviroment (less CO2 wasted). It's cheaper.... I don't know what else to add :tongue: How many marks is the question worth?

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