AS Chemistry- helping each other out!

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braindead1997

The enthalpy change when the amount of reactants shown in the equation react together under standard condition 298K, 100kPa

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Thank you soooooo much!!! Now i understand it!!! God! My teacher gave it in a really messed up way and you just explained it perfectly!!! Thank you!!!!!! :biggrin:

Reply 361

danconway

Original post by braindead1997

I guess you could say that the teachers kinda forced us to take IAL since it was a new syllabus and usually the new syllabus papers are easier, but god they were dead wrong cause the paper format was nothing like I've ever laid eyes on! And I have no idea if it was only me but the papers felt real thick too... Then they also went onto say that IAL is gonna be more recognized than GCE and stuff like that, but they're both equally recognized.... *sigh* biggest mistake in my life... i have to resit for a couple of papers now....

I might be mistaken, but I thought the two specifications (and exam papers) were identical apart from your Unit 3 and 6 being written papers...

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Reply 362

A84

well, as far as I know, many universities now want to accept only IAL...

Reply 363

GretaP

How can you identify whether an equation is demonstrating bond enthalpy or mean bond enthalpy? ImageUploadedByStudent Room1420536934.465084.jpg

ImageUploadedByStudent Room1420536934.465084.jpg

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Reply 364

C6H12O6

www.a-levelchemistry.co.uk
So useful for AQA and OCR AS and A2 chemistry, really worth looking at when revising, got me an A :biggrin:

..you're welcome

Reply 365

langlitz

Original post by GretaP

How can you identify whether an equation is demonstrating bond enthalpy or mean bond enthalpy? ImageUploadedByStudent Room1420536934.465084.jpg

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You can see that in the first equation, one C-H bond is broken so p in this case represents bond enthalpy. In the second equation then all 4 C-H bonds on the methane are broken and the overall enthalpy of the reaction is 4q, so in this case to get q you would need to divide the enthalpy change by 4. So q is the average bond enthalpy of the 4 bonds that were broken.

So p represents, bond enthalpy and also enthalpy of the reaction
q represents just the mean bond enthalpy

(edited 9 years ago)

Reply 366

A84

Can somebody explain how the standard enthalpy change of combustion may be used to calculate the standard enthalpy change of formation of a given compound indirectly?
Thanks.

Reply 367

braindead1997

Original post by danconway

I might be mistaken, but I thought the two specifications (and exam papers) were identical apart from your Unit 3 and 6 being written papers...

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It's is the same syllabus for both but the thing is the paper format... It's so confusing and some really weird questions came as well... You can check out the May/June 2014 physics unit 1,2 and 3 papers... most of us messed it up cause it seemed to ask answers in a non understandable indirect way... no idea what they were asking for half the time i was writing the paper... the chem was also messed up but not as much as the physics i guess... maths was ok except for S2...

Reply 368

braindead1997

Original post by A84

well, as far as I know, many universities now want to accept only IAL...

Well, I attended a seminar and most of the people asked what the advantages and disadvantages were of doing GCE and IAL... They were both equal, I guess, not many disadvantages for both... Then they also said that both are equally recognized as well... The woman who explained it to us was someone from the edexcel board too. :wink:

Reply 369

A84

Original post by braindead1997

hey
well that is good, but now GCES r only offered in UK right? so IAL has replaced the in other countries

Reply 370

braindead1997

Original post by A84

hey
well that is good, but now GCES r only offered in UK right? so IAL has replaced the in other countries

GCE being applied only in UK? Well, I'm in SL and I have some friends who do GCE... in SL... so not sure about that.... but they said stuff about cancelling Jan exams for I think GCE in 2015... not sure if they have done it by now... apparently Jan exams will be held only for IAL...

Reply 371

A84

Original post by braindead1997

yea, ur friends r probably resitting or maybe in a2... yea, jan exams r cancelled since this month this yr...

Reply 372

username1626859

Hi guys
I do ocr b

Does any know any links for revision notes for elements from the sea or atmosphere? Any A2 salters students?

Reply 373

braindead1997

Original post by A84

yea, ur friends r probably resitting or maybe in a2... yea, jan exams r cancelled since this month this yr...

Ahh... maybe... eish... jan exams cancelled... jeez.... are you doing AS or A2?? Make sure to pull through in one go! Then you're done and freeeeee!!! I had my freedom ripped away from me... :frown:

.... I should've done better... and now hopefully i can make up for it...

Reply 374

A84

Original post by braindead1997

Ahh... maybe... eish... jan exams cancelled... jeez.... are you doing AS or A2?? Make sure to pull through in one go! Then you're done and freeeeee!!! I had my freedom ripped away from me... :frown:

.... I should've done better... and now hopefully i can make up for it...

Im doin AS. this may
Good luck 2 u

Reply 375

langlitz

Original post by A84

Can somebody explain how the standard enthalpy change of combustion may be used to calculate the standard enthalpy change of formation of a given compound indirectly?
Thanks.

Suppose you had something like:

C_2 H_6(g) + 3\frac{1}{2} O_2(g) \rightarrow 2CO_2(g)+3H_2O(g)

\Delta H^{\circ} = –1560.7 kJ mol^{-1}

If you then reverse this equation and flip the sign of the enthalpy of combustion...
You can then use the standard enthalpy of combustion of hydrogen and carbon i.e.

C(g) + O_2(g) \rightarrow CO_2(g)

H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(g)

You then work towards the equation for the formation of 1 mole of ethane from it elements in their standard states

2C(g) + 3H_2(g) \rightarrow C_2H_6(g)

Make sure you get the right sign on all the enthalpies.

If you don't understand from this then I could work through it properly on paper

Reply 376

charco

Study Forum Helper

Original post by A84

Can somebody explain how the standard enthalpy change of combustion may be used to calculate the standard enthalpy change of formation of a given compound indirectly?
Thanks.

By using Hess' law.

Check out this interactive on enthalpy changes ...

Reply 377

A84

Original post by langlitz

Suppose you had something like:

C_2 H_6(g) + 3\frac{1}{2} O_2(g) \rightarrow 2CO_2(g)+3H_2O(g)

\Delta H^{\circ} = –1560.7 kJ mol^{-1}

If you then reverse this equation and flip the sign of the enthalpy of combustion...
You can then use the standard enthalpy of combustion of hydrogen and carbon i.e.

C(g) + O_2(g) \rightarrow CO_2(g)

H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(g)

You then work towards the equation for the formation of 1 mole of ethane from it elements in their standard states

2C(g) + 3H_2(g) \rightarrow C_2H_6(g)

Make sure you get the right sign on all the enthalpies.

If you don't understand from this then I could work through it properly on paper

I understand this, but in the edexcel book, they want us toexplain this question. Could u write all this in a couple of sentences please?

Reply 378

braindead1997

Original post by A84

Im doin AS. this may
Good luck 2 u

Thnx.... I'll be doing a few AS subjects starting tomorrow and then A2 in may/june.... Study hard!!! Good luck to you as well!!! :smile:

Reply 379

Paintballs

Hello. I really need help in simple chemistry problems. I cannot figure it out the solution. Thank you in advance
1.How many kg 25% NaOH solution you need to neutralise 1.96 kg 20% H2SO4 solution?
2. To 7.36 g H2SO4 solution was put Zn cutting excess. 0.336 l of hydrogen separated. Find H2SO4 concentration in the solution.