AS Chemistry- helping each other out!
Scroll to see replies
Original post by Dylann
When you are GIVEN enthalpies of formation, you use PRODUCTS - REACTANTS.
When you are GIVEN mean bond enthalpies, you use REACTANTS - PRODUCTS.
This is sort of because all values given in mean bond enthalpies are positive, i.e. energy is required to break a bond. So in effect you are doing [positive number] - [positive number] right? (since both sums are positive since all values given are positive).
If more energy is released, the sum of the products will be greater right? This makes sense because [small positive number] - [big positive number] = negative answer = exothermic, which makes sense if more energy was released.
You're probably a little confused because it says "formation of ammonia" so you instantly think "must be products - reactants", however, you are GIVEN mean bond enthalpies and not GIVEN formation enthalpies, so you cannot use that formula! You use products - reactants when you are GIVEN enthalpies of formation! (Basically, focus on the values you are GIVEN rather than what reaction is actually happening!)
When you are GIVEN mean bond enthalpies, you use REACTANTS - PRODUCTS.
This is sort of because all values given in mean bond enthalpies are positive, i.e. energy is required to break a bond. So in effect you are doing [positive number] - [positive number] right? (since both sums are positive since all values given are positive).
If more energy is released, the sum of the products will be greater right? This makes sense because [small positive number] - [big positive number] = negative answer = exothermic, which makes sense if more energy was released.
You're probably a little confused because it says "formation of ammonia" so you instantly think "must be products - reactants", however, you are GIVEN mean bond enthalpies and not GIVEN formation enthalpies, so you cannot use that formula! You use products - reactants when you are GIVEN enthalpies of formation! (Basically, focus on the values you are GIVEN rather than what reaction is actually happening!)
Can someone explain part b. It doesn't seem to be in the book.
Well I do CCEA so unfortunately I can't help you with the cyclohexane question as we don't do that until next year but, when testing for halogen ions (I assume that's the question) you simply add Silver Nitrate
Chloride Ions present - Colourless solution --> white precipitate forms then dissolves when Diluted Ammonia is added
Bromide Ions present - " "--> Cream precipitate forms which will dissolve if Concentrated Ammonia is added (not diluted)
Iodide Ions present- " "--> Yellow precipitate which will not dissolve in the presence of Concentrated Ammonia
Hope this helps mate!
Chloride Ions present - Colourless solution --> white precipitate forms then dissolves when Diluted Ammonia is added
Bromide Ions present - " "--> Cream precipitate forms which will dissolve if Concentrated Ammonia is added (not diluted)
Iodide Ions present- " "--> Yellow precipitate which will not dissolve in the presence of Concentrated Ammonia
Hope this helps mate!
(edited 3 years ago)
Original post by Super199
Can someone explain part b. It doesn't seem to be in the book.
I'm not completely certain, but I think coke (carbon) is the cheapest AND also produces the purest sample. I think coke is the cheapest because in order to get aluminium you have to electrolyse it from it's ore Al2O3 which is very expensive. Hydrogen is also quite expensive as it's very flammable. I don't think these are sufficient reasons for why coke is the cheapest though, so check with your teacher!
As for the purest sample of iron, carbon also produces the purest sample. This is because the products are in different states, i.e. one is a solid (iron) and the other is a gas (carbon monoxide) so the carbon monoxide doesn't really interfere with the solid at all! Al2O3 is also a solid and H2O may condense into water etc....CO has a very very low freezing point so it's the safest bet
Does anybody know where the January 2009as chemistry ed excel paper is?
I have been searching high and low, probably not doing a very good job. On the ed excel website it only has the examiner report and not the actual paper, can someone help me locate it?
I have been searching high and low, probably not doing a very good job. On the ed excel website it only has the examiner report and not the actual paper, can someone help me locate it?
https://chemrevise.files.wordpress.com/2013/08/mod-2-revision-guide-7-metal-extraction.pdf
I may be being daft but can some check this for me. If the link doesn't work it is the metal extraction notes AQA unit 2 on Chemrevise.
It is one of the equations for method 1:
It says Iron oxide + Carbon ---> Iron + Carbon monoxide.
But obviously the symbol equation
Isn't it supposed to be Carbon dioxide?
I may be being daft but can some check this for me. If the link doesn't work it is the metal extraction notes AQA unit 2 on Chemrevise.
It is one of the equations for method 1:
It says Iron oxide + Carbon ---> Iron + Carbon monoxide.
But obviously the symbol equation
Isn't it supposed to be Carbon dioxide?
Original post by AyshaU
I also do AQA we have a test on Friday on haloalkanes. So tying to remember as much as I can normally I do prefer organic chemistry though
Good luck with your test
Oh my! I hated Organic Chemistry at AS and I still hate it at A2, you must be those rare weirdos that like it, take it as a compliment mate
Original post by Super199
https://chemrevise.files.wordpress.com/2013/08/mod-2-revision-guide-7-metal-extraction.pdf
I may be being daft but can some check this for me. If the link doesn't work it is the metal extraction notes AQA unit 2 on Chemrevise.
It is one of the equations for method 1:
It says Iron oxide + Carbon ---> Iron + Carbon monoxide.
But obviously the symbol equation
Isn't it supposed to be Carbon dioxide?
I may be being daft but can some check this for me. If the link doesn't work it is the metal extraction notes AQA unit 2 on Chemrevise.
It is one of the equations for method 1:
It says Iron oxide + Carbon ---> Iron + Carbon monoxide.
But obviously the symbol equation
Isn't it supposed to be Carbon dioxide?
Yes you're right it is meant to be CO2.
But I believe the equation in the link is still right as everything is balanced and Fe has still been reduced from 3+ to 0.
But seeing as you're doing AQA the equation that you should know for the exam is: 2Fe2O3(s) +3C(s) -> 4Fe(l) + 3CO2(g)
I hope that helps
Original post by Georgiam247
Hey guys,
I was wondering, does anyone have a tutor for chemistry? I don't have any particular areas that I suck at but my teacher is so careless I feel like it'd be a good idea to get one. Anybody seen a lot of benefit from a tutor?
I was wondering, does anyone have a tutor for chemistry? I don't have any particular areas that I suck at but my teacher is so careless I feel like it'd be a good idea to get one. Anybody seen a lot of benefit from a tutor?
Hi there!
I'm an A2 AQA Chemsitry student and I got an A at AS.
I didn't get a tutor, all I had was my CGP Chemistry textbook.
It is absolutely amazing and I owe my grade to this book.
I really think you should get it even if you do opt to get a tutor.
AQA CGP Student book
Sample pages
Hello everyone please help me!
I have been asked to calculate enthalpy change of formation for N2O using data given.
table says:
C + N2O -> CO +N2 -193 KJ mol-1 (not specified whether C or F)
C +1/2O2 -> CO -111 KJ mol-1 (again not specified)
how to do I know whether to draw a Hess' cycle where the arrows point upwards from the elements (formation) or whether to draw one where they point down to combustion products (combustion) IF DONT KNOW WHETHER THE TABLE VALUES ARE FOR COMBUSTION OR FORMATION ENTHALPY CHANGE??? PLEASE HELP ME EVEN MY TACHER COULD NOT EXPLAIN THIS TO ME!!!!!!!!!!! a clear description would help
Can u tell me how to figure out if its a combustion cycle AND ALSO how to figure out if its a formation cycle! thanks!
and secondly,
in Q=MCdeltaT you determine the sign (-/+) by yourself by looking at whether the temperature has increased or decreased. I did a question where two solutions were mixed (NaOH and H2SO4) the temperature went from 20 degrees and rose to 29 degrees. The answer in the book said use a + sign as it's endothermic? can someone explain this?
another question mixed solid iron powder to copper sulphate in a polystyrene cup. The sign here needed was - when the temperature went from 21 to 25 degrees? here the temperature s increased but the sign is - , why is it EXOTHERMIC NOW??? I am so confused please explain clearly thanks.
I have been asked to calculate enthalpy change of formation for N2O using data given.
table says:
C + N2O -> CO +N2 -193 KJ mol-1 (not specified whether C or F)
C +1/2O2 -> CO -111 KJ mol-1 (again not specified)
how to do I know whether to draw a Hess' cycle where the arrows point upwards from the elements (formation) or whether to draw one where they point down to combustion products (combustion) IF DONT KNOW WHETHER THE TABLE VALUES ARE FOR COMBUSTION OR FORMATION ENTHALPY CHANGE??? PLEASE HELP ME EVEN MY TACHER COULD NOT EXPLAIN THIS TO ME!!!!!!!!!!! a clear description would help
Can u tell me how to figure out if its a combustion cycle AND ALSO how to figure out if its a formation cycle! thanks!and secondly,
in Q=MCdeltaT you determine the sign (-/+) by yourself by looking at whether the temperature has increased or decreased. I did a question where two solutions were mixed (NaOH and H2SO4) the temperature went from 20 degrees and rose to 29 degrees. The answer in the book said use a + sign as it's endothermic? can someone explain this?
another question mixed solid iron powder to copper sulphate in a polystyrene cup. The sign here needed was - when the temperature went from 21 to 25 degrees? here the temperature s increased but the sign is - , why is it EXOTHERMIC NOW??? I am so confused please explain clearly thanks.
Original post by haemo
There most likely is, however I don't know it, sorry.
Hi, I a a post where you said if combustion given you draw a certain cycle and if formation given you draw a certain Hess' cycle. I have found questions where only enthalpy changes are give when combustion or formation is not specified What o I do then???
Original post by Science100
Hello everyone please help me!
I have been asked to calculate enthalpy change of formation for N2O using data given.
table says:
C + N2O -> CO +N2 -193 KJ mol-1 (not specified whether C or F)
C +1/2O2 -> CO -111 KJ mol-1 (again not specified)
how to do I know whether to draw a Hess' cycle where the arrows point upwards from the elements (formation) or whether to draw one where they point down to combustion products (combustion) IF DONT KNOW WHETHER THE TABLE VALUES ARE FOR COMBUSTION OR FORMATION ENTHALPY CHANGE??? PLEASE HELP ME EVEN MY TACHER COULD NOT EXPLAIN THIS TO ME!!!!!!!!!!! a clear description would help Can u tell me how to figure out if its a combustion cycle AND ALSO how to figure out if its a formation cycle! thanks!
and secondly,
in Q=MCdeltaT you determine the sign (-/+) by yourself by looking at whether the temperature has increased or decreased. I did a question where two solutions were mixed (NaOH and H2SO4) the temperature went from 20 degrees and rose to 29 degrees. The answer in the book said use a + sign as it's endothermic? can someone explain this?
another question mixed solid iron powder to copper sulphate in a polystyrene cup. The sign here needed was - when the temperature went from 21 to 25 degrees? here the temperature s increased but the sign is - , why is it EXOTHERMIC NOW??? I am so confused please explain clearly thanks.
I have been asked to calculate enthalpy change of formation for N2O using data given.
table says:
C + N2O -> CO +N2 -193 KJ mol-1 (not specified whether C or F)
C +1/2O2 -> CO -111 KJ mol-1 (again not specified)
how to do I know whether to draw a Hess' cycle where the arrows point upwards from the elements (formation) or whether to draw one where they point down to combustion products (combustion) IF DONT KNOW WHETHER THE TABLE VALUES ARE FOR COMBUSTION OR FORMATION ENTHALPY CHANGE??? PLEASE HELP ME EVEN MY TACHER COULD NOT EXPLAIN THIS TO ME!!!!!!!!!!! a clear description would help
Can u tell me how to figure out if its a combustion cycle AND ALSO how to figure out if its a formation cycle! thanks!and secondly,
in Q=MCdeltaT you determine the sign (-/+) by yourself by looking at whether the temperature has increased or decreased. I did a question where two solutions were mixed (NaOH and H2SO4) the temperature went from 20 degrees and rose to 29 degrees. The answer in the book said use a + sign as it's endothermic? can someone explain this?
another question mixed solid iron powder to copper sulphate in a polystyrene cup. The sign here needed was - when the temperature went from 21 to 25 degrees? here the temperature s increased but the sign is - , why is it EXOTHERMIC NOW??? I am so confused please explain clearly thanks.
•
I can't tell you about Hess' cycle and that
•
But the endo/exo problem goes like this
•
If you are mixing NaOH and H2SO4, you're carrying out a neutralisation reaction. This is exothermic. So heat is given out into the surroundings, so if you're measuring the temperature of the solution, it will increase.
•
Exothermic reactions have a negative value for Enthalpy Change, because in an exothermic reaction, the enthalpy of the reactants is greater than that of the products.
•
Enthalpy change = enthalpy of products - enthalpy of reactants
so if reactants have more enthalpy than products, then enthalpy change will be negative
•
so deltaH is negative for exothermic reaction
•
when you add iron powder to copper sulphate, the iron displaces the copper because it is more reactive. if the copper sulphate solution increases in temperature when this happens, you know the reaction is exothermic.
•
so this reaction will also have negative deltaH
Hi guys. Anyone doing the AQA ISA (on group 2 metals and mole calculations) anytime soon? 
Original post by Science100
Hello everyone please help me!
I have been asked to calculate enthalpy change of formation for N2O using data given.
table says:
C + N2O -> CO +N2 -193 KJ mol-1 (not specified whether C or F)
C +1/2O2 -> CO -111 KJ mol-1 (again not specified)
how to do I know whether to draw a Hess' cycle where the arrows point upwards from the elements (formation) or whether to draw one where they point down to combustion products (combustion) IF DONT KNOW WHETHER THE TABLE VALUES ARE FOR COMBUSTION OR FORMATION ENTHALPY CHANGE??? PLEASE HELP ME EVEN MY TACHER COULD NOT EXPLAIN THIS TO ME!!!!!!!!!!! a clear description would help Can u tell me how to figure out if its a combustion cycle AND ALSO how to figure out if its a formation cycle! thanks!
I have been asked to calculate enthalpy change of formation for N2O using data given.
table says:
C + N2O -> CO +N2 -193 KJ mol-1 (not specified whether C or F)
C +1/2O2 -> CO -111 KJ mol-1 (again not specified)
how to do I know whether to draw a Hess' cycle where the arrows point upwards from the elements (formation) or whether to draw one where they point down to combustion products (combustion) IF DONT KNOW WHETHER THE TABLE VALUES ARE FOR COMBUSTION OR FORMATION ENTHALPY CHANGE??? PLEASE HELP ME EVEN MY TACHER COULD NOT EXPLAIN THIS TO ME!!!!!!!!!!! a clear description would help
Can u tell me how to figure out if its a combustion cycle AND ALSO how to figure out if its a formation cycle! thanks!It seems that you lack a fundamental understanding of what's going on here.
C + N2O -> CO +N2 -193 KJ mol-1
This is neither the enthalpy of combustion or formation. This is simply the enthalpy change for the reaction.
C +1/2O2 -> CO -111 KJ mol-1
This is just the enthalpy of formation for carbon monoxide
You (should) know that the enthalpy of formation is the enthalpy change for the formation of one mole of a compounds from its elements in their standard states.
i.e.
so you need to find a way to get to this equation from the two that you've been given
Spoiler
Original post by Disney0702
Good luck with your test
Oh my! I hated Organic Chemistry at AS and I still hate it at A2, you must be those rare weirdos that like it, take it as a compliment mate
Oh my! I hated Organic Chemistry at AS and I still hate it at A2, you must be those rare weirdos that like it, take it as a compliment mate
Thanks for the support. I have to say that organic chemistry is something I like as we then get other chapters like amount of substance and bonding (urgh!) that I just can not stand
What advice could you give someone like me
Original post by AyshaU
Thanks for the support. I have to say that organic chemistry is something I like as we then get other chapters like amount of substance and bonding (urgh!) that I just can not stand
What advice could you give someone like me
What advice could you give someone like me
WOAH!!! Amount of Substances is my baby, I love that topic!
The best advice I can give you is honestly to just do past paper questions.
This had helped me so much last year as not only had it helped strengthen my understanding of the topic it also taught me the language of the exam so I'm not completely clueless when I read a question.
I'm sure you may have noticed that the way AQA phrase their questions can be quite obscure sometimes that it makes you question yourself as to whether you were taught the topic in question.
Amount of Substances is DEFINITELY a topic that will come up in your Unit 1 exam so do as much questions as you can on it and I promise you it'll be a breeze in the end.
Original post by Hayley Williams
Just look through your textbook and make some notes! It's up to your what you get, i had a crap teacher and still got an A. Had to work my arse off but i got it 
I ended up watching some E.Rintoul videos which helped a lot, but haven't got my mock back yet for unit 2. I got a C in unit 1, I can easily bring that to a B I think or even an A, it's unit 2 that's the real problem for me now
Doing past papers has helped so much as well. I never realised how good they were.
Dsss
(edited 9 years ago)
Original post by Disney0702
WOAH!!! Amount of Substances is my baby, I love that topic!
The best advice I can give you is honestly to just do past paper questions.
This had helped me so much last year as not only had it helped strengthen my understanding of the topic it also taught me the language of the exam so I'm not completely clueless when I read a question.
I'm sure you may have noticed that the way AQA phrase their questions can be quite obscure sometimes that it makes you question yourself as to whether you were taught the topic in question.
Amount of Substances is DEFINITELY a topic that will come up in your Unit 1 exam so do as much questions as you can on it and I promise you it'll be a breeze in the end.
The best advice I can give you is honestly to just do past paper questions.
This had helped me so much last year as not only had it helped strengthen my understanding of the topic it also taught me the language of the exam so I'm not completely clueless when I read a question.
I'm sure you may have noticed that the way AQA phrase their questions can be quite obscure sometimes that it makes you question yourself as to whether you were taught the topic in question.
Amount of Substances is DEFINITELY a topic that will come up in your Unit 1 exam so do as much questions as you can on it and I promise you it'll be a breeze in the end.
Like you said to me when we were talking about organic, how the hell can you love amount of substance, its an okay topic and I'm sure I've revised pretty much every question that is out there as I've started doing the same papers again. I know it comes in every paper and that scares me. I've just got bonding to cover and I have finished my CHEM1 revision. then onto CHEM2 (yay!!)
If iI' not being nosy can I just ask what other A-levels did you do and how much time did you spend on each of these per week.
If you don't want to answer I honestly would understand
Quick Reply
Related discussions
- TSR Study Together - STEM vs Humanities!
- GCSE Exam Discussions 2024
- Chemistry Degree Lab Work
- Making chemistry notes
- Physical natural sciences
- !!! A level choices - Chem or Computer science
- A level chemistry for a biology degree? (help please!!)
- Do I have enough time to turn my grades around??
- Mass vs relative atomic mass on periodic table
- Official NATURAL SCIENCES applicants thread 2024
- What is it like doing chem vs natural sciences at uni?
- Study leave
- in need of urgent advice, would appreciate any help :)
- Lancaster uni timetable
- Need help on a Venn diagram question
- Biochemistry at University
- Cambridge Natural Sciences Interview Applicants 2023/24
- Higher chemistry
- I enjoy chem practicals more than bio practicals. Is a chem degree right for me?
- Save me from a level torture
Latest
Trending
Last reply 1 week ago
Im confused about this chemistry question, why does it form these productsTrending
Last reply 1 week ago
Im confused about this chemistry question, why does it form these products
