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# Edexcel Chemistry Unit 1 (23/05/2013) Watch

1. Also, can someone explain the principles of bond enthalpy to me please?
2. Hey

guys quick questions... can sumone plzzzz explain me question 6,9 and 13.... in the january 2013 unit 1 chemistry paper
these are the mcq questions (clearly)
thanks alot....
3. (Original post by amber206)
How do you calculate percentage errors?!

%error= difference between experimental value and data book value / data book value x 100%
4. how do you do question 17 on jun 09. Which of the following gas samples occupies the greatest volume at the same temperature and pressure: 1 gram of ethane, 1 gram of oxygen, 1 gram of fluorine or 1 gram of neon? RAM (H=1 C=12 O=16 F=19 Ne=20 . thanks
5. On jan 09 q23cii why is the bond enthalpy reaction that is more exothermic more likely to happen and also why is the h-f bond shorter than h-cl, h-, and h-br?
6. (Original post by rainerised)
how do you do question 17 on jun 09. Which of the following gas samples occupies the greatest volume at the same temperature and pressure: 1 gram of ethane, 1 gram of oxygen, 1 gram of fluorine or 1 gram of neon? RAM (H=1 C=12 O=16 F=19 Ne=20 . thanks
its 1 gram of neon
any gas occupies 24 dm^3 at standard room temp and pressure
what you need to do is find the moles and x24dm^3
fluorine and oxygen are diatomic therefore you have to do 1/(16x2) for o2 and 1/(19x2) for F2

you should find that neon has a volume of 1.2 dm^3
7. (Original post by Dragonette)
Hey

guys quick questions... can sumone plzzzz explain me question 6,9 and 13.... in the january 2013 unit 1 chemistry paper
these are the mcq questions (clearly)
thanks alot....
6) the answer is A. Usually you would keep both temperature and pressure constant but because it's an experiment to measure the enthalpy change of a reaction, therefore you can't keep the temperature constant because that's what you're measuring.
9) another A. The temperature change will be the same because while you are going from 0.02 mol to 0.10 mol by increasing the volume of the solutions by the same amount, the reaction mole ratio remains the same? (I'm not 100% sure about this one).
13) C because... Well just because, we have to know that for ionic compounds contours of the electron density maps are isolated to either the cation or the anion: there is no/very very little shared charge, unlike in the case of covalent compounds where there is a lot of shared charge because electron pairs are shared in covalent bonds.
8. (Original post by felicity95)
Hi!

Just another bond enthalpy question. why is the answer to this question A and not C. Surely the mean bond enthalpy is the energy required to break one mole of bonds in the gas phase averaged over many compounds. so then isn't CH4 1 mole, and 1/4CH4 is just a 1/4 mole?

Also if the question had said bond enthalpy as opposed to mean bond enthalpy would it then have been C?

Sorry to ramble on, last minute panic.
You want the mean bond enthalpy of the C-H bond... CH4 has four of these... so the others are just finding the bond enthalpy of CH4 (4 x C-H bonds). So you want a quarter of a mole so you get the average enthalpy of one C-H bond.

(Original post by amber206)
Also, can someone explain the principles of bond enthalpy to me please?
Bond enthalpy value is pretty much the potential energy stored within the bonds

The definition you may need to know for the exam though is...

'Bond enthalpy is the enthalpy change when a bond in a gaseous molecules is broken'
9. (Original post by snowy173)
its 1 gram of neon
any gas occupies 24 dm^3 at standard room temp and pressure
what you need to do is find the moles and x24dm^3
fluorine and oxygen are diatomic therefore you have to do 1/(16x2) for o2 and 1/(19x2) for F2

you should find that neon has a volume of 1.2 dm^3
ok thanks but i thought you always had to say the RFM of oxygen is 16 not 32?
10. How much on intermolecular forces is required for this exam guys?
Thanks for everyone's help so far
11. (Original post by rainerised)
On jan 09 q23cii why is the bond enthalpy reaction that is more exothermic more likely to happen and also why is the h-f bond shorter than h-cl, h-, and h-br?
The HF bond is so short because of the great difference in electronegativity between the hydrogen and fluorine (which is the most EN element in the periodic table). This results in the covalent bond being very polar: the bonding pairs of electrons are closer to the more EN atom, which makes it electron rich and the hydrogen electron deficient. The hydrogen will be closer to the fluorine.
i always picture it as the two atoms holding hands (this reminds you that they share pair of electrons). If the EN's of the two atoms are the same (ie they are the same element) then their hands are straight and the electrons that they are holding onto are perfectly in the middle between them : no polarity in the bond. If one has a higher EN then it will be pulling the electrons close to itself, thereby also pulling the other atom closer to itself.
Hope this helps.
12. (Original post by amber206)
How much on intermolecular forces is required for this exam guys?
Thanks for everyone's help so far
Not much... that's covered in Unit 2.

You'll need to know your intramolecular forces obviously, such as ionic bonding, covalent bonding...
13. can someone help me with this question please. When 10 cm3 of 2 mol dm–3 hydrochloric acid is reacted with10 cm3 of 2 mol dm–3 sodium hydroxide solution, the temperature change is T.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
When the reaction is repeated with 50 cm3 of each solution, the temperature change is
A T
B 5 × T
C
1
5
× T
D 10 × 2 × T
the question is from the jan 2013 paper.
thank you
14. (Original post by rainerised)
ok thanks but i thought you always had to say the RFM of oxygen is 16 not 32?
RAM(O) = 16
RFM(O2) =16x2=32
15. (Original post by acer0951)
The outer electron of Al occupies the 3P sub shell which is a higher energy level compared to Magnesium in which the outer electron occupies the 3S sub shell. The outer electron in Al is further away from the attraction of the nucleus due to greater distance and also greater inner shielding from electrons which again reduces the attractive force of the nucleus on the outer electron therefore the first ionisation energy for Al is lower than Mg.

Hope this helps

Posted from TSR Mobile
thank you
16. How much of the organic do we need to know for unit 1?
17. (Original post by newyork newyork)
can someone help me with this question please. When 10 cm3 of 2 mol dm–3 hydrochloric acid is reacted with10 cm3 of 2 mol dm–3 sodium hydroxide solution, the temperature change is T.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
When the reaction is repeated with 50 cm3 of each solution, the temperature change is
A T
B 5 × T
C
1
5
× T
D 10 × 2 × T
the question is from the jan 2013 paper.
thank you
Temperature is directly proportional to the volume (under constant pressure of course), since both have increased by the same amount.... the temperature change will be the same
18. (Original post by Pirateprincess)
The HF bond is so short because of the great difference in electronegativity between the hydrogen and fluorine (which is the most EN element in the periodic table). This results in the covalent bond being very polar: the bonding pairs of electrons are closer to the more EN atom, which makes it electron rich and the hydrogen electron deficient. The hydrogen will be closer to the fluorine.
i always picture it as the two atoms holding hands (this reminds you that they share pair of electrons). If the EN's of the two atoms are the same (ie they are the same element) then their hands are straight and the electrons that they are holding onto are perfectly in the middle between them : no polarity in the bond. If one has a higher EN then it will be pulling the electrons close to itself, thereby also pulling the other atom closer to itself.
Hope this helps.
thanks thats a great way of looking at it
19. Does anybody know what diagrams of experiments we need to know? - the ones that we have to draw
20. Guys are we supposed to know how fractional distillation works , do we have to learn the process

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