A-level chemistry

[x-samia-x]

Salaam,

is anybody doing/has done a-level chemistry nuffield?

cos i'm finding it quite difficult, and wondered if any1 had any tips or anything?

jazaka'Allah khairun

walaikumasalam
samia

[Muhammad]

Wa Alaykum Assalaam,

I'm sure we have some science threads in this section, and that there are quite a few people who've done A-level chemistry.

If there are any specific questions or problems, feel free to post them and Insha'Allaah someone can try and help.

I think it goes for most subjects that they initially seem daunting, but if you put effort into it - ask the teacher, friends and read books available to you, you will find that you can do well with the help of Allaah (swt). And remember to place your trust in Allaah (swt) and constantly ask for His help.

[The Ruler]

I'm doing AS Chemistry at the moment. But what's nuffield?

[x-samia-x]

I'm doing AS Chemistry at the moment. But what's nuffield?

nuffield is bleugh :enough!:

its experiment based.. no decent txt books about it. so many calculations.. all over my head, woah :|

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[al_islam]

I did Nuffield :)

What would you like to know ?

[x-samia-x]

I did Nuffield :)

What would you like to know ?

dont get calculations with Hess' law :hmm:

[al_islam]

OOh....

Hess's law.

Just do the triangles.

Remeber not to forget the negative values for the equations going in the other direction.

Other than that...its quiet simple really. Just practise

[x-samia-x]

hmm :(

enthalpy change.. and delta H? im lost...

thank u for such a quick reply

jazaka'Allah khair

[al_islam]

Its ok...

Right:

Hess's law is about energy conservation.

Enthalpy change is jus a posh word for loss or gain of heat.

Delta H is is the change in Temp.

When you understand the calculations and the numbers are inserted into the triangle (like they show in the book , well i think), Its just maths.

[Starseeker]

I did AS chem and sat in every lesson not understanding anythins.

If your doing salters-nuffield which is the one I did theres this really good revision guide that explain each and every equation step by step. Its costs about £10 but its worth every penny as it covers all of the practical aspects of the syllabus.

Im not doing it for A2 and everything I did for AS has just flown right over my head so I cant be much help. Sorry.

[x-samia-x]

Its ok...

Right:

Hess's law is about energy conservation.

Enthalpy change is jus a posh word for loss or gain of heat.

Delta H is is the change in Temp.

When you understand the calculations and the numbers are inserted into the triangle (like they show in the book , well i think), Its just maths.

remind myself of the triangle i think.. even tho i spent ages 2day trying to figure it out.. :) thank you. jazaka'Allah khairun

Starseeker

i is doing Edexcel nuffield. cud use a book explaining equations, would be reli helpful! wots it called? thank you for your reply :) Jazka'Allah khairun

[H4RUN]

I know exactly what you guys are talking about :D :okay:
Please please, dont all PM me at once...

Aslamualaikum

[x-samia-x]

reli? :-|

[Mikayeel]

salam sister samia, i had the same problems with chemistry!, to me it was like reading japanese upside down!, my first year i failed, i am currently resitin it agian :P, but now am using CGP revision guides RELE RELE helpful, am doin WJEC btw, which is also quite easy, soo if i was you go to the nearest book shop and get your self the coolest book!!! ( no i dnt get paid to do this:P)

[x-samia-x]

salam sister samia, i had the same problems with chemistry!, to me it was like reading japanese upside down!, my first year i failed, i am currently resitin it agian :P, but now am using CGP revision guides RELE RELE helpful, am doin WJEC btw, which is also quite easy, soo if i was you go to the nearest book shop and get your self the coolest book!!! ( no i dnt get paid to do this:P)

japanese upside down.. i can so relate! wots wjec?
for edexcel nuffield.. thers one text book (which my sixth form has invested in.. but i dont find helpful) and one revision guide which isnt detailed.

but the people r reli helpful on here, masha'Allah :)

[Mikayeel]

Wjec is the board they tend to use in Wales, similar to OCR, but the CGP revision books covers all the major syllabusses including nuffield its very clear written and with alot of picture(i love the pictures) and sum cracking jokes to i bought mine for uhmm 9.99 to be precise! salam

[al_islam]

Ok,

the triangle is jus where the compounds go and where each of the enthalpies are listed.

just makes it easier to calculate.

it has 3 compounds and their enthalpies....bt u got to know if its positive or negative.

when u do...u jus use maths to solve it.

ps remember to use a negative sign for the reaction going in the oppostie direction.

[x-samia-x]

Ok,

the triangle is jus where the compounds go and where each of the enthalpies are listed.

just makes it easier to calculate.

it has 3 compounds and their enthalpies....bt u got to know if its positive or negative.

when u do...u jus use maths to solve it.

ps remember to use a negative sign for the reaction going in the oppostie direction.

sorry for late reply
ive been revising!

ahhh yeh, i get it now. sort of. thank u!!!!
well, sort of get it.. more practise questions i guess =/

jazaka'Allah khairun!!!

Samia

[Whatsthepoint]

ps remember to use a negative sign for the reaction going in the oppostie direction.

I thought a negative enthalpy refers to exothermic reactions...
I am not familiar with the anglosaxon chemistry levels though. I have no idea what triangles are supposed to be, I suspect they're some sort of tables with data?

Delta H is is the change in Temp.

Delta H is a change in enthalpy - energy. Delta T is a change in temperature.

[x-samia-x]

ok im lost again.
isnt negative... if its going in the opposite direction.. orrr if thers an increase in temp?
and delta H energy change/mole?

[Whatsthepoint]

ok im lost again.
isnt negative... if its going in the opposite direction.. orrr if thers an increase in temp?
and delta H energy change/mole?

If a system releases heat(exothermic reaction) the energy of the system logically decreases so the enthalpy is negative.
If a system absorbs energy, it's energy level increases, hence the positive enthalpy.
Delta H - change in enthalpy
Delta T - change in temperature
Delta n -change in the number of moles...
That's how I was taught..:skeleton:

[Whatsthepoint]

Let's see if we're talking about the same thing...

Here's a simple chemical reaction, the burning of methane.

CH4(g)+2O2(g) --> CO2(g) + 2H2O(l, just to make it easier)

the enthalpy of a recation is defined as
the enthalpy of the products minus the enthalpy of the reactants(reagents)

1 mol × (-393 kJ/mol) + 2 mol × (-286 kJ/mol)
minus
1 mol × ( -74,8 kJ/mol) + 2 mol × 0 (because the enthalpy of an elemnet (oxygen) equals zero)
equals
-890 kJ

890kJ is the amount of energy released from burning 1 mole of methane. As you know burning produces heat - releases energy.

Are we talking about the same thing then?:muddlehea

[x-samia-x]

woahhh :S
uv reli lost me. sorry, i must sound reli unintelligent but i dont get it.

i reli appreciate your responses.

isnt it:
Q= change in temp * mass * specific heat capacity ~4.18

thenn
Delta H = Q/moles

and if its exothermic, delta H is negative?

[Whatsthepoint]

woahhh :S
uv reli lost me. sorry, i must sound reli unintelligent but i dont get it.

i reli appreciate your responses.

isnt it:
Q= change in temp * mass * specific heat capacity ~4.18

thenn
Delta H = Q/moles

and if its exothermic, delta H is negative?

That sounds more like physics to me.
Yes, Delta Q (change in energy) = Delta T (chnage in temperature) × c (specific heat capacity) × m (mass).
letters my be different in the UK, I think they're different (some of them) in the US.
i'm not sure what 4.18 signifies...
I've been taught that Delta H signifies a change in enthalpy, wikipedia says the same.
Delta H is not Q/moles. Standard enthalpy(enthalpy per one mole) is Q/moles. You get an enthalpy(or change in it) by multiplying the standard one with the number of moles, as shown in the reaction above.
Yes, enthalpy of an exothermic reaction is negative.

sorry if I'm confusing you.:uuh:

[Whatsthepoint]

I figured out what al_islam has been saying

ps remember to use a negative sign for the reaction going in the oppostie direction.

When a reaction goes in the opposite direction all enthalpies change +/- signs.
It's logical, when you think about it, samia. If a reaction going in one direction released energy, the reversal of the reaction will have to absorb it, exactly the same amount of it. And vise versa. therefore, the enthalpy changes its sign (+,-).

[Whatsthepoint]

Now, if you have to claculate an enthalpy for a reaction you just have to calculate the entalpies of every compund and sum it up as in the reaction above.
If the reaction goes in the opposite direction and the data you have (enthalpies fo every compound) refer to the original (left to right) direction, you need to change signs (+/-). As easy as that.

I'm still not sure what the triangles are though...:playing:

[al_islam]

I'm still not sure what the triangles are though...:playing:

The triangle is just a way of expressing the equations you have written in another form.

this makes it easier to see the equation and the -/+ values, and which way is negative and which is positive.

CH4(g)+2O2(g) --> CO2(g) + 2H2O(l, just to make it easier)

the enthalpy of a recation is defined as
the enthalpy of the products minus the enthalpy of the reactants(reagents)

1 mol × (-393 kJ/mol) + 2 mol × (-286 kJ/mol)
minus
1 mol × ( -74,8 kJ/mol) + 2 mol × 0 (because the enthalpy of an elemnet (oxygen) equals zero)
equals
-890 kJ

890kJ is the amount of energy released from burning 1 mole of methane. As you know burning produces heat - releases energy.

CH4 + 2O2





CO2 2H2O




Imagine arrows to the CO2 and 2H2O, I cant seem to get them here.

The enthalpy change that you need to work out can be clearly seen.

If you need to go in the oppostie direction, then you must use the OPPOSITE temperature sign. So if its positive then you use negative...and the other way around.

Also remember to use the equation.

Delta H is a change in enthalpy - energy. Delta T is a change in temperature

.

OOPs sorry, i got it the other way around....mustve been tired that day, thanks for the correction 'whatsthe point'.

if u dont get it, PM me the question...ill hav a crack at it for you....i was jus tellin the theory behind it.

[al_islam]

I'm still not sure what the triangles are though...:playing:

The triangle is just a way of expressing the equations you have written in another form.

this makes it easier to see the equation and the -/+ values, and which way is negative and which is positive.

CH4(g)+2O2(g) --> CO2(g) + 2H2O(l, just to make it easier)

the enthalpy of a recation is defined as
the enthalpy of the products minus the enthalpy of the reactants(reagents)

1 mol × (-393 kJ/mol) + 2 mol × (-286 kJ/mol)
minus
1 mol × ( -74,8 kJ/mol) + 2 mol × 0 (because the enthalpy of an elemnet (oxygen) equals zero)
equals
-890 kJ

890kJ is the amount of energy released from burning 1 mole of methane. As you know burning produces heat - releases energy.

CH4 + 2O2





CO2 2H2O




Imagine arrows to the (CO2) and (2H2O) coming from (CH4 + 202) - 2 separate ones: 1 to CO2 and one to 2H2O.

AND ALSO from (CO2) to (2H2O) - I cant seem to get them here.

The enthalpy change that you need to work out can be clearly seen.

Insert the temperature values given for each reaction, and work out the one you need to find.

If you need to go in the oppostie direction, then you must use the OPPOSITE temperature sign. So if its positive then you use negative...and the other way around.

Also remember to use the equation.

Delta H is a change in enthalpy - energy. Delta T is a change in temperature

.

OOPs sorry, i got it the other way around....mustve been tired that day, thanks for the correction 'whatsthe point'.

if u dont get it, PM me the question...ill hav a crack at it for you....i was jus tellin the theory behind it.

[x-samia-x]

i think

i finally get it

thank u so much for ur help!

[al_islam]

i think

i finally get it

thank u so much for ur help!

Its ok.

Just show me the question and ur answer.

Ill check it for you.

But its not so bad.

Any more questions just ask.....Nuffield needs work....good to hear ur revising.

Good luck.

[x-samia-x]

Its ok.

Just show me the question and ur answer.

Ill check it for you.

But its not so bad.

Any more questions just ask.....Nuffield needs work....good to hear ur revising.

Good luck.

i read that just now ^^

its driving me round the bend.
thanks for the offer :)

[Believer_786]

:sl:
I did AS Salters OCR Chemistry, I hated every lesson of it, I'm so glad I dropped it at the end. Now Computing all the way for me :D