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ResidentEvil
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#41
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Any ideas what might come up on friday? any suggestions? any hints?
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Chemistry Practical OCR A2 on Friday thread by ResidentEvil
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Baralian
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#42
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(Original post by cheesecakegirl)
I have no clue about the calculations for this and it has to be in friday!I think hess' law cycle should be the main reaction along the top and the other two reactions coming off either end to join in the middle underneath the main reaction.Similar examples should be in the course text book though i havent checked yet.Also i heard it was better to have an excess of HCl in the displacement reaction because its more realiable,is this true?Grrrrr i hate chemistry now,its just so annoying!I thought the biology plan was quite hard but its nothing compared to this!

xxx
well yes it is kinda annoying b'cas of the calculations, but i asked a chem teacher and he said use excess of HCl and excess of CuCl2 solution. and for the calculations i got the info from this message board.

Zn + 2HCl -----> ZnCl2 + H2

so the mole ratio is 1:2. if u r using 50 cm3 of HCl then there will b 0.05 moles of HCl in it and so 0.025 moles of Zn. using the simple formula for finding the number of moles we get, 0.025= mass(g)/65. the answer is 1.7g of zinc.

use the same calculation for 2nd reaction ... well in that case ratio is 1:1 so no big deal... just use excess of CuCl2. i hope it helps ...

now my question... can anyone help me with "Q=mc delta T" ?! i know c and delta T but how do i get "m" ?! wat i mean is , wat is the density of water?
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wizzkid
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#43
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(Original post by cobra01977)
It tells you on your instructions, all large blocks of text, not labels not equations or calculations and not in tables. it doesnt matter if you go over, our teacher who is an examiner reconed that you could go up to bout 700 without being penolised but if you went much further they might take 1 mark off for bad communication

When it says ' large blocks of text should be included' in the instructions, is'nt it reffering to the to the blocks of text on diagrams etc...
also it does not specifically say that calculations should not be included in the word count.
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fluffysheep2000
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#44
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(Original post by Baralian)
well yes it is kinda annoying b'cas of the calculations, but i asked a chem teacher and he said use excess of HCl and excess of CuCl2 solution. and for the calculations i got the info from this message board.

Zn + 2HCl -----> ZnCl2 + H2

so the mole ratio is 1:2. if u r using 50 cm3 of HCl then there will b 0.05 moles of HCl in it and so 0.025 moles of Zn. using the simple formula for finding the number of moles we get, 0.025= mass(g)/65. the answer is 1.7g of zinc.

use the same calculation for 2nd reaction ... well in that case ratio is 1:1 so no big deal... just use excess of CuCl2. i hope it helps ...

now my question... can anyone help me with "Q=mc delta T" ?! i know c and delta T but how do i get "m" ?! wat i mean is , wat is the density of water?
DONT USE EXCESS OF HCl AND CuCl2!!!!!!! You need to know that ALL of the HCl and CuCl2 have reacted as all of your enthalpy change figures are based on the number of moles of HCl and CuCl2 reacted, if you use an excess of them you dont know how much of them have reacted and therefore n = c x v and using molar ratios will be wrong. You need to use and excess of the metal you are using.

m is the mass of aqueous solution used. You assume that whatever solution you are using has the same density as water which is 1cm3 = 1g
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Baralian
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yeah now im totally lost... whom do i follow ...my chem teacher or fluffysheep ?! well in any case thanks for ur help ... i'm printing out both cases and i'll ask my teacher tomorow !
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