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AS Chemistry- helping each other out!

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Can someone please give me a step by step guideline on how to predict the shapes and bond angles in molecules and ions? I find this really difficult and the book only confuses me
Anyone have tips on how to revise for Chemistry?
Original post by Super199
Anyone have tips on how to revise for Chemistry?


I read a cgp book
Original post by frozo123
I read a cgp book

Do you do like a chapter at a time or? I'm with AQA and there are quite a few chapters and doing a chapter a day or so will be rather time consuming.
Original post by lukejoshjames
Mr means molecular mass, which is defined as the mass per mole of a molecule. If you numbers within the empirical formula itself, for example in MgCl2, then you would add the Mr of Magnesium to twice the Mr of Chlorine, and that would give the you the Mr of MgCl2.
However, if there is a number before the empirical formula itself, for example 2MgCl2, that is the same as 2(MgCl2), which is 2 moles of MgCl2. As Mr is mass per mole, finding the mass of 2 moles of the substance would be incorrect unless you divided that by whatever number of moles there are of the substance.


In short: every single atom in a molecule has to be count in calculating the molecular mass. Example by myself: FeO3. To get the molecular mass, the mass of one iron atom must be added to the mass of three oxygen atoms.

Makes sense, as a molecule consists of atoms.
Original post by Super199
Do you do like a chapter at a time or? I'm with AQA and there are quite a few chapters and doing a chapter a day or so will be rather time consuming.

I'm with edexcel and I assume all the exam boards are pretty much the same anyway, heard edexcel and salters B are the hardest though.
Hm not really?
I think if you thoroughly revise, you can get through 3/4 of the chapters a day. I read the whole book a couple days ago over the two days, guess it took me 4 hours to understand 90% of it.

Just do questions from the book, and past papers, read the markscheme, so you don't make similar mistakes again.
For the 'Green Chemistry' topic of the Unit 2 Exam in AS OCR A, how much detail do we need to go into? Im a little lost as to whether we need to know lots of examples etc.

Thanks


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OCR A
Original post by jerseyalevel
For the 'Green Chemistry' topic of the Unit 2 Exam in AS OCR A, how much detail do we need to go into? Im a little lost as to whether we need to know lots of examples etc.

Thanks


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In past papers, examples have been of the type e.g.: "Give 2 ways of disposing of waste polymers" to which you could reply "combustion of plastics to give energy, and 'sorting+moulding"

and the subtly different question type e.g. "give 2 ways chemists have developed new techniques to reduce waste" to which you could reply "developing photodegradable polymers and developing use as chemical feedstock in cracking"

look at past papers for examples of how much detail you need to go into, but just to be safe, learn 1 level of detail beyond this :smile:
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Ugh,I have to revise Green chemistry over the two week break. ozone layer calculations are a nightmare.
Can someone explain the displacement reactions for the halogens. Is the reason why bromine cannot displace chlorine due to chlorine being more reactive than it. Also when bromine displaces say potassium iodide do you get an orange or brown solution?
Does anyone know a resource with model answers to typical past paper questions?
Original post by Super199
Can someone explain the displacement reactions for the halogens. Is the reason why bromine cannot displace chlorine due to chlorine being more reactive than it. Also when bromine displaces say potassium iodide do you get an orange or brown solution?


the oxidising ability decreases as you go down the halogens, chlorine is a smaller atom than bromine, which means that when it oxidises the bromide ion the incoming electron from that ion feels an overall stronger attraction so the electron would be pulled more strongly to the chlorine forming the chloride ion. So you'd get orange bromine.
You know how we say that to calculate the number of electrons a shell can hold, we use the formula 2n2??? e.g the 3rd shell has a maximum capacity of 18 electrons according to the equation, so why do we draw shells in sets of 8 electrons? (apart from the 1st shell)
Original post by Georgiam247
You know how we say that to calculate the number of electrons a shell can hold, we use the formula 2n2??? e.g the 3rd shell has a maximum capacity of 18 electrons according to the equation, so why do we draw shells in sets of 8 electrons? (apart from the 1st shell)


you don't, you follow the sub level idea behind the shells, so for example say you're looking at sodium which has 11 electrons, you'd say that its electronic configuration would be 1S2 2S2 2P6 3S1 so the first shell has 2 electrons, the second has 8 and the third one. You can have shells that are occupied by even 18 electrons.. i.e caesium :smile:
Original post by ThatMadClown
you don't, you follow the sub level idea behind the shells, so for example say you're looking at sodium which has 11 electrons, you'd say that its electronic configuration would be 1S2 2S2 2P6 3S1 so the first shell has 2 electrons, the second has 8 and the third one. You can have shells that are occupied by even 18 electrons.. i.e caesium :smile:


So really the octet rule is inaccurate and not advanced enough? Thank you :biggrin:
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Avatar for HToo
HToo
4
hi
i just wanted to know if anyone has got or knows where to find the june 2014 chemistry aqa unit 1 and 2 question papers???
Original post by HToo
hi
i just wanted to know if anyone has got or knows where to find the june 2014 chemistry aqa unit 1 and 2 question papers???


Sharing last years papers is prohibited, as is asking for them.
Can someone explain something to me. It is to do with bond angles.

4a). PH4+ . The plus represents it has lost an electron. Now is the electron being lost from the phosphorous or the hydrogen.

another one 4c) PF6- . So one phosphorous and 6 flourines. Has the phosphorous gained an electron or the chlorine. Slight confused on how it works.

Thanks :smile:
Original post by Super199
Can someone explain something to me. It is to do with bond angles.

4a). PH4+ . The plus represents it has lost an electron. Now is the electron being lost from the phosphorous or the hydrogen.

another one 4c) PF6- . So one phosphorous and 6 flourines. Has the phosphorous gained an electron or the chlorine. Slight confused on how it works.

Thanks :smile:


a) think of an ammonium ion? It is dative covalent bonding a PH3 molecule is trigonal pyramidal in shape. Bond angle of 107' When a proton comes along, the PH3 will donate it's lone pair of electrons to the proton and dative covalently bonds therefore the new bond angle is 109.5, ygm?

c) The bond angle will be 90', octahedral shape

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