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OCR Chemistry F322~ 4th June 2013~ AS Chemistry

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Reply 1260
Avatar for Beths_7
Beths_7
7
Can someone help me with this question? Jan 2011, 3 b) (iii)
Why is the answer 23 and not 12? Its really confusing me :frown:

http://www.ocr.org.uk/Images/65360-question-paper-unit-f322-chains-energy-and-resources.pdf
Original post by Beths_7
Can someone help me with this question? Jan 2011, 3 b) (iii)
Why is the answer 23 and not 12? Its really confusing me :frown:

http://www.ocr.org.uk/Images/65360-question-paper-unit-f322-chains-energy-and-resources.pdf


The answer to (i) is 2310 but it needs to be converted to KJ so that will be 2.3

the answer to (ii) is 0.200 moles

(iii) is asking for the enthalpy change of reaction, so all you do is divide the answer to (i) 2.3 by the answer to (ii) 0.200 moles, then make sure to times this by 2 :smile:))

this comes out as 23
Reply 1262
Avatar for Ché.
Ché.
Original post by Beths_7
Can someone help me with this question? Jan 2011, 3 b) (iii)
Why is the answer 23 and not 12? Its really confusing me :frown:

http://www.ocr.org.uk/Images/65360-question-paper-unit-f322-chains-energy-and-resources.pdf


Ah!
I done this paper, yesterday evening...

Right, so!?
She equation that you must use - and I will share in written form - is: Q (which you calculated through mCAT) divided by the one marker question in the mole (which I believe was 0.2mol)

Then, I believe it is twice the amount due to the moles of Ammonia Thiocyanate = 2 due to the stoichiometric ration.

= 23.1 kJmol^-1.

To two significant figures, 23 works out just fine!
And, then it must always be a negative value with the enthalpy of combustion - as I am sure you know. :smile:

ImageUploadedByStudent Room1370081604.405460.jpg
Clear? :biggrin:



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Reply 1263
Avatar for Smko
Smko
[QUOTE="ellie2996;42900592"]The answer to (i) is 2310 but it needs to be converted to KJ so that will be 2.3

the answer to (ii) is 0.200 moles

(iii) is asking for the enthalpy change of reaction, so all you do is divide the answer to (i) 2.3 by the answer to (ii) 0.200 moles, then make sure to times this by 2 :smile:))

this comes out as 23[/QUOTE

why times 2?
[QUOTE="Smko;42900666"]
Original post by ellie2996
The answer to (i) is 2310 but it needs to be converted to KJ so that will be 2.3

the answer to (ii) is 0.200 moles

(iii) is asking for the enthalpy change of reaction, so all you do is divide the answer to (i) 2.3 by the answer to (ii) 0.200 moles, then make sure to times this by 2 :smile:))

this comes out as 23[/QUOTE

why times 2?


because of the stoichiometric ration :smile:
Reply 1265
Avatar for Smko
Smko
[QUOTE="ellie2996;42900678"]
Original post by Smko


because of the stoichiometric ration :smile:


I see! Thanks!! :biggrin: Good luck on Tuesday!
Original post by Ché.
Ah!
I done this paper, yesterday evening...

Right, so!?
She equation that you must use - and I will share in written form - is: Q (which you calculated through mCAT) divided by the one marker question in the mole (which I believe was 0.2mol)

Then, I believe it is twice the amount due to the moles of Ammonia Thiocyanate = 2 due to the stoichiometric ration.

= 23.1 kJmol^-1.

To two significant figures, 23 works out just fine!
And, then it must always be a negative value with the enthalpy of combustion - as I am sure you know. :smile:

ImageUploadedByStudent Room1370081604.405460.jpg
Clear? :biggrin:



Posted from TSR Mobile


When ever you are finding out the answer to these questions, and when the ask you for the number of moles of something, are you always finding one mole of the substance?


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Reply 1267
Avatar for Beths_7
Beths_7
7
[QUOTE="ellie2996;42900678"]
Original post by Smko


because of the stoichiometric ration :smile:


Okay I just didn't times it by two...I don't understand, why do you have to times by two? Is it because there's two moles of NH4SCN?
[QUOTE="Beths_7;42900705"]
Original post by ellie2996


Okay I just didn't times it by two...I don't understand, why do you have to times by two? Is it because there's two moles of NH4SCN?


yep :smile:
Reply 1269
Avatar for Beths_7
Beths_7
7
[QUOTE="ellie2996;42900710"]
Original post by Beths_7


yep :smile:


Okay thank you! I thought that enthalpy change of reaction was for one mole though...:confused:
I got Bio Unit 2 Edexcel exam tomorrow and then Chemistry the next day... WHY?!
Reply 1271
Avatar for Ché.
Ché.
Original post by Jimmy20002012
When ever you are finding out the answer to these questions, and when the ask you for the number of moles of something, are you always finding one mole of the substance?


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Typically, you would do one mole without even checking the stoichiometry of the equation provided.
Most, if not all except this one, supplies an equation with one mole per reactant at least.
If that makes sense?

That question was a little awkward hence why I had to check the question again to figure out if that was needed; the knowledge from F321 was creeping on me!
LOL.


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(edited 10 years ago)
Reply 1272
Avatar for Taffie
Taffie
Original post by scorpio22
thats an actually really good question.And one i find asking myself.
There is some areas such as mechanisms: electrophillic addition and nucleophillic sub.. learn the dipoles,movement of electrons etc
just learn it because it will come up somehow

boltzmann:temp or catalyst guranteed to come up
catalyst convertor process almost always comes up.

carbon capture almost comes up.
polymers;alcohol

I think this exam is about attention to detail, there is so many areas to make mistakes like writing incomplete instead of complete combustion.
drawing full displayed instead of sketal, significant figures. when it requests
There are also so many areas to get marks:

one thing that will definitely come up is definitions and enthalpy calculations.thats one i would encourgae to understand.
how to calculate formation from combustion
combustion from formation
enthalpy reaction
the reverse reaction activation etc


how to calculate formation from combustion
combustion from formation
enthalpy reaction
the reverse reaction activation I don't really know how to do that are there any questions I can do to practise on that please


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Can anyone help me with working out enthalpy changes

Enthalpy change of:

reaction when given formation values
reaction when given combustion values
reaction when given average bond enthalpies
formation when given reaction values
formation when given combustion values
formation when given average bond enthalpies

I never know what to do, either products - reactants or reactants - products
Anybody got Jan 2013 paper ?
Reply 1275
Avatar for ssflcb
ssflcb
12
Original post by Joey952
Anybody got Jan 2013 paper ?


http://www.scribd.com/doc/120653070/OCR-F214-Jan-2013-Paper
Anyone got any tips on how to learn the mechanisms? I just can't seem to get my head around them but they're worth so many marks!


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Completely confused myself :s-smilie: is an exothermic reaction bond breaking or bond making???
Original post by ellie2996
Completely confused myself :s-smilie: is an exothermic reaction bond breaking or bond making???


Endothermic = bond breaking (+ ve)
Exothermic = bond making (-ve)
:smile:


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i was just wondering..if our ISA was about oxidation of alcohols will that be in the exam??
thanks :smile:

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