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# AS Chemistry- helping each other out! watch

1. (Original post by Jmedi)
I'm pretty sure it's meant to be H2O2 -> H2O + 0.5O2
(I'm pretty sure as the one in the picture doesn't make sense unless I'm missing something?)

Therefore, using sum of bonds broken - sum of bonds formed

(2*463+146) - (2*463 + 496/2)
496/2 because the oxygen molecule is O=O
[ this can be simplified to
146-(496/2) ]
So, answer = -102 kJmol^-1
Does that equation always show enthalpy change? Regardless if it is formation/ combustion or...?
2. (Original post by haemo)
When given mean bond enthalpies:

Enthalpy change = reactants - products

When given formation data:

Enthalpy change = products - reactants

When given combustion data:

Enthalpy change = reactants - products
Is there a specific reason why some of them are the other way round?
3. Need some help with an enthalpy change question

Calculate the enthalpy changes of reaction for each of the following reaction

a). C2H4(g) + H2(g) ----> C2H6(g)

Right so it gives you a ton of values. So since they are in the gas state it is Sum of bonds broken - Sum of bonds made.

There is a C=C double bond which is 612Kjmol^-1
But it says there is one H-H bond that gives the 436Kjmol^-1
I don't really understand that bit isn't there 4 C-H bonds? Can someone explain this. I get the rest of the question apart from this bit.

Thanks
4. (Original post by Super199)
Need some help with an enthalpy change question

Calculate the enthalpy changes of reaction for each of the following reaction

a). C2H4(g) + H2(g) ----> C2H6(g)

Right so it gives you a ton of values. So since they are in the gas state it is Sum of bonds broken - Sum of bonds made.

There is a C=C double bond which is 612Kjmol^-1
But it says there is one H-H bond that gives the 436Kjmol^-1
I don't really understand that bit isn't there 4 C-H bonds? Can someone explain this. I get the rest of the question apart from this bit.

Thanks
Did they not give the bond enthalpy for C-H & C-C bonds??
p.s. What is the answer they obtained?
5. (Original post by Elhamm)
Did they not give the bond enthalpy for C-H & C-C bonds??
p.s. What is the answer they obtained?
Na they gave all of it. It's just the ethene bit I don't understand.
C-H = 413
C-C= 347
C=C = 612
H-H= 436

The answer is -125KJmol^-1

I don't understand the bonds in the ethene.

It says answer: [612+436]-[(2x413)+347) =-125KJ mol^-1
6. (Original post by Super199)
Na they gave all of it. It's just the ethene bit I don't understand.
C-H = 413
C-C= 347
C=C = 612
H-H= 436

The answer is -125KJmol^-1

I don't understand the bonds in the ethene.

It says answer: [612+436]-[(2x413)+347) =-125KJ mol^-1
Best way to do these question is to draw out the structures

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7. (Original post by Super199)
Na they gave all of it. It's just the ethene bit I don't understand.
C-H = 413
C-C= 347
C=C = 612
H-H= 436

The answer is -125KJmol^-1

I don't understand the bonds in the ethene.

It says answer: [612+436]-[(2x413)+347) =-125KJ mol^-1
Bonds broken:
1 C=C (612)
4 C-H (4 x 413 = 1652)
1 H-H (436)
The total is when you add them together = 2700

Bonds made:
1 C-C (347)
6 C-H (6 x 413 = 2478)
Total = 2825

When finding the enthalpy change you use like you said 'Bond broken - Bonds made'
Therefore in this case we have to do the following: 2700 - 2825 = -125kJmol^-1

(every value is in kJmol^-1)
Hope that helped
8. (Original post by Elhamm)
Bonds broken:
1 C=C (612)
4 C-H (4 x 413 = 1652)
1 H-H (436)
The total is when you add them together = 2700

Bonds made:
1 C-C (347)
6 C-H (6 x 413 = 2478)
Total = 2825

When finding the enthalpy change you use like you said 'Bond broken - Bonds made'
Therefore in this case we have to do the following: 2700 - 2825 = -125kJmol^-1

(every value is in kJmol^-1)
Hope that helped
oooh I've been such an idiot. Thank you very much
9. hey everyone
need help with this question: what oxidation number does nitrogen have in HCN?
I kind of understand that it is probably -5 if the ON of C is +4. However, there are different answers in the internet
10. (Original post by aigerimk)
hey everyone
need help with this question: what oxidation number does nitrogen have in HCN?
I kind of understand that it is probably -5 if the ON of C is +4. However, there are different answers in the internet
When you're looking at carbon in terms of oxidation state, you add +1 for each bond that the carbon atom has to the more electronegative atom (in this case Nitrogen) and -1 for each bond with the least electronegative atom (in this case Hydrogen) the HCN shows that carbon has three bonds to Nitrogen (so +3) and 1 bond to hydrogen (-1) for carbon to have a total of +2 so nitrogen has to be -3 as the molecule is neutral overall.

Sorry for the long explanation.

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11. Ahh I see! Thank you so much for the explanation, this question finally makes sense to me
12. Helloooo, I wonder if somebody can help with something I came across in a textbook and am really confused.
So the question was to draw the structure of 1,1-dimethyl-3-ethylcyclohexane, which I thought was simple enough, except this is the answer they put in the book ... ( see attached picture below)

I just don't see how the ethyl group can be on the carbon which I have labelled 2 ( the labels in red were done by me, they weren't in the book) when it says "3-ethyl"... the book even says, "Add two methyl groups to one carbon atom, which becomes carbon atom 1. Then add an ethyl group to carbon atom 3". But to me that ethyl group is on carbon 2! Am I missing something?

I hope my question makes sense I know its a bit long winded haha... I just don't get it!!!!!!!!!
Attached Images

13. (Original post by Georgiam247)
Helloooo, I wonder if somebody can help with something I came across in a textbook and am really confused.
So the question was to draw the structure of 1,1-dimethyl-3-ethylcyclohexane, which I thought was simple enough, except this is the answer they put in the book ... ( see attached picture below)

I just don't see how the ethyl group can be on the carbon which I have labelled 2 ( the labels in red were done by me, they weren't in the book) when it says "3-ethyl"... the book even says, "Add two methyl groups to one carbon atom, which becomes carbon atom 1. Then add an ethyl group to carbon atom 3". But to me that ethyl group is on carbon 2! Am I missing something?

I hope my question makes sense I know its a bit long winded haha... I just don't get it!!!!!!!!!
You've added two methyl groups on carbon-1 where it says 3-ethyl, you can count clockwise or anticlockwise, so you put the ethyl group on the third carbon from carbon-1 clock or anticlockwise.
14. (Original post by ThatMadClown)
You've added two methyl groups on carbon-1 where it says 3-ethyl, you can count clockwise or anticlockwise, so you put the ethyl group on the third carbon from carbon-1 clock or anticlockwise.
But in the picture the two groups are on carbons next to each other whatever way you count the carbons...
15. (Original post by Georgiam247)
Helloooo, I wonder if somebody can help with something I came across in a textbook and am really confused.
So the question was to draw the structure of 1,1-dimethyl-3-ethylcyclohexane, which I thought was simple enough, except this is the answer they put in the book ... ( see attached picture below)

I just don't see how the ethyl group can be on the carbon which I have labelled 2 ( the labels in red were done by me, they weren't in the book) when it says "3-ethyl"... the book even says, "Add two methyl groups to one carbon atom, which becomes carbon atom 1. Then add an ethyl group to carbon atom 3". But to me that ethyl group is on carbon 2! Am I missing something?

I hope my question makes sense I know its a bit long winded haha... I just don't get it!!!!!!!!!
Sounds like the textbook's made a mistake...
16. (Original post by Georgiam247)
But in the picture the two groups are on carbons next to each other whatever way you count the carbons...
They've probably made a mistake, I wouldn't read too much into it.

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17. (Original post by ThatMadClown)
They've probably made a mistake, I wouldn't read too much into it.

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Do you think so? That crossed my mind but I just didn't think it was possible, I've got every other question right /understood my mistake, so yeah you're probably right about over thinking it...I tend to do that haha. Thank you!!
18. (Original post by BWV1007)
Sounds like the textbook's made a mistake...
Gosh I really hope so! Haha thank you
19. (Original post by Georgiam247)
Do you think so? That crossed my mind but I just didn't think it was possible, I've got every other question right /understood my mistake, so yeah you're probably right about over thinking it...I tend to do that haha. Thank you!!
No problem

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20. Hey guys,
I do OCR A.
I was just wondering why my teacher stressed on the importance of the 5 ions (Carbonate, Nitrate, Sulphate, Hydroxide and Ammonium)?
Why do we NEED to learn their formula and names?

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