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# Ocr f321 23 may 2013~official discussion thread Watch

1. (Original post by otrivine)
I made the question myself but the bond angle would be simply a tetrahedral because C(CH3)Si you can see the carbon atom is attached to 3 methyl groups and a silicon molecule and hence 4 bonding pairs and no lone pairs----> tetrahedral (109.5) because of the electron pair repulsion as into maximise separation and minimise repulsion
Ah that explains it. You can make questions up but you did infact make a mistake. I don't want to come across as a complete knob but i'll show you where you went wrong:
- The carbon isn't attached to 3 methyl groups, only 1
- You write the atoms in the incorrect order because by your example it would be C-CH3-Si, which is infact impossible. I think you meant to say CH3-C-Si but even then that would be impossible as silicon would not be able to get a full shell.
2. I thought that but if N-N has a triple bond that means it only has 2 electrons left to bond with.. so surely after bonding with the other 2 Hydrogens it wouldn't have any lone pairs?
3. (Original post by otrivine)
I have a good question for you guys to tackle

State the bond angle of C(CH3)3Si and explain why (4 marks)
The central atom carbon forms 4 bond pairs and no lone pairs. This means the bond angle will be 109.5 and have the shape tetrahedral. This is due to the fact, that bond pairs repel each other equally, giving a bond angle of 109.5
4. (Original post by zangorou)
The central atom carbon forms 4 bond pairs and no lone pairs. This means the bond angle will be 109.5 and have the shape tetrahedral. This is due to the fact, that bond pairs repel each other equally, giving a bond angle of 109.5

If you read my post above there would be no answer as the question itself is impossible to answer
5. (Original post by sladyy96)
how do you know it's (NH2)-(NH2)? does the question show you (is there a diagram)?
or does it say it has hydrogen bonding?
We know its bonded (NH2)-(NH2) because it tells us at the top of the question. Even if they told us N2H4 we would still be able to calculate that it is bonded in that way.
6. Reply to my new post chemistry guru please
7. When do you use nuclear charge
8. (Original post by Daniel__)
General question, in regards to systematic naming how do you know which element the roman numerals refers to as a rule of thumb? Also, can the roman numerals represent a negative oxidation number?
For example in Sodium Chlorate (V) the 5 refers to the oxidation number of chlorine but in say copper (II ) sulfate, the 2 refers to the oxidation number of copper. So the roman numeral generally refers to the element it is after.

Hope this helps.
9. (Original post by blackstarz)
I thought that but if N-N has a triple bond that means it only has 2 electrons left to bond with.. so surely after bonding with the other 2 Hydrogens it wouldn't have any lone pairs?
Okay when nitrogen bonds with just itself, so N2, it again wants to get a full shell. So it triple bonds to get a full shell.

In NH2-NH2 it is different. The aim for the molecule is basically to get every atom with a full shell. The way it does this is by the nitrogen bonding to the other nitrogen (single bond), then also each nitrogen single bonds with 2 hydrogens.

Nitrogen will almost always have a lone pair because it has 5 electrons in its outermost shell. Since electrons go in pair, you get 1 full pair and then 3 single electrons. In order to get a full shell it needs 3 bonds with each of those single electrons. When fully bonded it will therefore have 1 lone pair and 3 bond pairs. So it will almost always have a lone pair unless there is a dative covalent bond.
10. Hey, i've just been doing some past papers and i've come across a couple of questions where its like "calculate the volume of gas, in (dm3 or cm3), obtained by the student at RTP". When i've looked at the markscheme to work out moles of gas you times by 5/2 or 3/2.. why is that? this might be a stupid question :P
11. Is it acceptable to bullet point explanations when the pencil icon isn't there?
For things like an increase in ionisation energy/boiling point etc. It's so time consuming.
12. (Original post by theCreator)
Okay when nitrogen bonds with just itself, so N2, it again wants to get a full shell. So it triple bonds to get a full shell.

In NH2-NH2 it is different. The aim for the molecule is basically to get every atom with a full shell. The way it does this is by the nitrogen bonding to the other nitrogen (single bond), then also single bonds with the remaining hydrogens.

Nitrogen will almost always have a lone pair because it has 5 electrons in its outermost shell. Since electrons go in pair, you get 1 full pair and then 3 single electrons. In order to get a full shell it needs 3 bonds with each of those single electrons. So it will almost always have a lone pair unless there is a dative covalent bond.
Ahh, i see. Thanks a lot for your help. For a general rule with this type of question, do you always look at the central atom and how many bonds it has?
13. (Original post by jenny1995)
Hey, i've just been doing some past papers and i've come across a couple of questions where its like "calculate the volume of gas, in (dm3 or cm3), obtained by the student at RTP". When i've looked at the markscheme to work out moles of gas you times by 5/2 or 3/2.. why is that? this might be a stupid question :P
I think it's to do with the molar ratio of the equation
14. http://www.ocr.org.uk/Images/65868-q...and-groups.pdf
could someone plese help with 1 e part 4???
15. (Original post by jenny1995)
Hey, i've just been doing some past papers and i've come across a couple of questions where its like "calculate the volume of gas, in (dm3 or cm3), obtained by the student at RTP". When i've looked at the markscheme to work out moles of gas you times by 5/2 or 3/2.. why is that? this might be a stupid question :P
for example lets say in this imaginary reaction its 3H20 + 5H2, and lets also imagine we have 0.5 moles of H20.

In order to calculate the moles of hydrogen gas produced ( so we can then times it by 24 or 24,000) we need to work out the number of moles of hydrogen. To do this we use stoichiometry. So in order to work out the number of moles of hydrogen we do (0.05 / 3) * 5. If you don't understand or haven't heard of stoichiometry until know unfortunately I would say its too late
16. (Original post by Sara_A)
When do you use nuclear charge
use nuclear charge when talking about ionic bonding- e.g. MgCl2 has a stronger bond than NaCl. This is because the charge on the Magnesium ion (2+) is greater than on the Sodium ion (+). This causes there to be a stronger electrostatic attraction to the Cl- ion.
Also, you can use it when talking about metallic bonding, as this refers to the increased attraction between the positive metal ion and the negatively charged electrons.
17. (Original post by blackstarz)
Ahh, i see. Thanks a lot for your help. For a general rule with this type of question, do you always look at the central atom and how many bonds it has?
Generally yeah. You never look at hydrogens for example because they are never the central atom. As you said you look for atoms which would require multiple bonds, e.g. carbon requires 4, nitrogen requires 3. Then on the side I try to work out the shape of the molecule by drawing the atoms available and just playing around with the configuration until it works out.
18. (Original post by jenny1995)
Hey, i've just been doing some past papers and i've come across a couple of questions where its like "calculate the volume of gas, in (dm3 or cm3), obtained by the student at RTP". When i've looked at the markscheme to work out moles of gas you times by 5/2 or 3/2.. why is that? this might be a stupid question :P
I just did this question! Basically its to do with the stoichiometry. The number in front of the reactant involved was 2, (trying to remember off the top of my head) and the number in front of both of the products, so both of the gases produced, was 3 and 2. So you have to add 3 and 2 to find the total amount of moles of gas produced, and then divide by 2, which was the number in front of the reactant.
So you times it by 5/2 which is the same thing.

Hope that makes sense.
19. Thanks so much for your help! my teachers rubbish and never properly explained stoichiometry.. i get it now
20. What are ALL the uses we need to know for example magnesium hydroxide for the stomach pains and indigestion

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