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- 09-05-2015 23:22
Last edited by _NMcC_; 09-05-2015 at 23:24. -
samwillettsxxx
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- 10-05-2015 03:43
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- 10-05-2015 07:40
(Original post by samwillettsxxx)
Hi I've learnt carbocations with more alkyl groups are most stable I don't know why though
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- 10-05-2015 09:03
Tip for Hess Law and enthalpy changes
CRFP
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so Combustion = Reactants - Products
and Formation = Products - Reactants -
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- 10-05-2015 09:07
(Original post by C0balt)
That could work but the gas produced will have nowhere to go so bung could shoot out lol
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- 10-05-2015 09:25
(Original post by A84)
Tip for Hess Law and enthalpy changes
CRFP
Cristiano Ronaldo Football Player
so Combustion = Reactants - Products
and Formation = Products - Reactants
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- 10-05-2015 09:46
(Original post by samb1234)
This is taking a risk if you do it this way. I would strongly advise that you just learn the definitions of various enthalpy changes (which you need to know anyway) and get used to creating Hess cycles with them. If you just remember a formulaic way of doing questions you run the risk of being tripped up if they give you any data you're not familiar with (as they have in the past). Hess cycles drawn properly will also sort out any sign issues that again might lose marks
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- 10-05-2015 12:30
bruh the exam is in like 10 days for OCR A Unit 1
i'm beyond screwed
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- 10-05-2015 12:37
(Original post by samb1234)
This is taking a risk if you do it this way. I would strongly advise that you just learn the definitions of various enthalpy changes (which you need to know anyway) and get used to creating Hess cycles with them. If you just remember a formulaic way of doing questions you run the risk of being tripped up if they give you any data you're not familiar with (as they have in the past). Hess cycles drawn properly will also sort out any sign issues that again might lose marks
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- 10-05-2015 13:24
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- 10-05-2015 13:31
Could anyone explain to me why the mols of HCl are used as a basis to calculate mass og ZnCl2 and not the mols of ZnO.
I was thinking that due to the fact there are fewer mols of ZnO, this would be the limiting reactant however the mark scheme uses mols of HCl -
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- 10-05-2015 13:32
This is the mark scheme
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- 10-05-2015 13:58
(Original post by AndrewC19)
What are you struggling with in particular?I think the rest is okayish, i'm also weak with equations, you know when you need to work backwards, eg in water of crystallisation... it sucks
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- 10-05-2015 14:41
(Original post by VetVikki)
Could anyone explain to me why the mols of HCl are used as a basis to calculate mass og ZnCl2 and not the mols of ZnO.
I was thinking that due to the fact there are fewer mols of ZnO, this would be the limiting reactant however the mark scheme uses mols of HCl
Look at the equation and ratio. It is 1:2=ZnOCl
So moles of HCl is 0.12 and 0.060mol of ZnO is supposed to react but you have 0.0830
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- 10-05-2015 14:50
With water of crystallisation it's just practice and knowing what steps to follow, personally I take this approach:
1) Read the question and underline what information you are given.
2) Calculate the masses of anhydrous, and water. (Mass of hydrated - mass of anhydrous = mass of water).
3) Using this calculate the molar mass of water and the molar mass of anhydrous.
4) Divide the molar mass of water by anhydrous molar mass, to calculate your ratio and therefore you're numbers of waters of crystallisation.
This changes slightly dependant on the nature of the question, but if you use this method it shouldn't be too difficult to workout how to solve it!
With acid/base reactions you just have to remember that the ions switch around (sorta): e.g. HCl + NaOH -> NaCl + H2O.
Just remember the word equation and work from there: Acid + Base -> Salt + Water.
It's just a case of thinking about what you'd expect to form, so for more complex acids and bases. Try a load of practice questions, you'll pick up patterns!
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- 10-05-2015 15:16
(Original post by C0balt)
Zinc oxide is in excess.
Look at the equation and ratio. It is 1:2=ZnOCl
So moles of HCl is 0.12 and 0.060mol of ZnO is supposed to react but you have 0.0830
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- 10-05-2015 15:39
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samwillettsxxx
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- 10-05-2015 16:15
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- 10-05-2015 16:29
(Original post by samwillettsxxx)
Hey would that way of working it out apply to every question that asks which is in excess???
Solid is usually in excess though, because you would know that it is in excess visually (solid left) and when filtering interacted acid will not be let through
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- 10-05-2015 16:37
(Original post by C0balt)
Should do
Solid is usually in excess though, because you would know that it is in excess visually (solid left) and when filtering interacted acid will not be let through
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