CurtisDean
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Hey everyone,

i'm trying to get my head around Enthalpy change, and the calculations. Okay, let's say we're working out the Enthalpy change using Hess' law cycles - I think this is determining enthalpy change indirectly. This is where I get confused! I have an idea of what to do, I just don't understand why it is that I do it - if you get what I mean.

I wanted to know
1) - how to calculate ΔH combustion using cycle (if possible)
2)- how to calculate ΔH of reaction, using Enthalpy change of combustion values
3)- Why do the arrows point towards the combustion products when calculating ΔH of formation, using combustion values (using Hess' law cycle)
4)- Why do the arrows point from the elements to the reactants when calculating ΔH of reaction, using formation values (using Hess' law cycle)

I think all of my questions are in terms of standard enthalpy change.

Any help would be great, thanks! I'm an A2 Chemistry student, and I got bored, so I thought I'd revisit some AS stuff that I never really understood!
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CurtisDean
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Secret.
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(Original post by CurtisDean)
Hey everyone,

i'm trying to get my head around Enthalpy change, and the calculations. Okay, let's say we're working out the Enthalpy change using Hess' law cycles - I think this is determining enthalpy change indirectly. This is where I get confused! I have an idea of what to do, I just don't understand why it is that I do it - if you get what I mean.

I wanted to know
1) - how to calculate ΔH combustion using cycle (if possible)
2)- how to calculate ΔH of reaction, using Enthalpy change of combustion values
3)- Why do the arrows point towards the combustion products when calculating ΔH of formation, using combustion values (using Hess' law cycle)
4)- Why do the arrows point from the elements to the reactants when calculating ΔH of reaction, using formation values (using Hess' law cycle)

I think all of my questions are in terms of standard enthalpy change.

Any help would be great, thanks! I'm an A2 Chemistry student, and I got bored, so I thought I'd revisit some AS stuff that I never really understood!

A lot of these are quite hard to explain, do you have a textbook/have you looked online for notes/guides to help you on this, try some questions and then ask for help when you get stuck - this will probably be a better way to ensure you understand the actual questions rather than memorising steps
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CurtisDean
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(Original post by Secret.)
A lot of these are quite hard to explain, do you have a textbook/have you looked online for notes/guides to help you on this, try some questions and then ask for help when you get stuck - this will probably be a better way to ensure you understand the actual questions rather than memorising steps
Hey! Thanks for the response! Yep, i've used Chem guide, my AS revision guide, and Chemwiki. I've also done a few practice question, and I've got the answers right on the ones that I have attempted. There have, however, been questions that I was unsure how to answer - such as 'calculate the ΔH of Reaction given the combustion values'

Ahh.. Maybe I should leave it, if I don't need to know it :/
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Secret.
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(Original post by CurtisDean)
Hey! Thanks for the response! Yep, i've used Chem guide, my AS revision guide, and Chemwiki. I've also done a few practice question, and I've got the answers right on the ones that I have attempted. There have, however, been questions that I was unsure how to answer - such as 'calculate the ΔH of Reaction given the combustion values'

Ahh.. Maybe I should leave it, if I don't need to know it :/


No it's fine to want to learn it, I think it may help in unit 5 (unless I'm mistaken but I think one of the AQA unit 5 papers had a enthalpy question in it) but if you give us a specific question we could try and help you

Is it questions like this that you're struggling with? http://www.youtube.com/watch?v=7l_1kIOox_g
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CurtisDean
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(Original post by Secret.)
No it's fine to want to learn it, I think it may help in unit 5 (unless I'm mistaken but I think one of the AQA unit 5 papers had a enthalpy question in it) but if you give us a specific question we could try and help you

Is it questions like this that you're struggling with? http://www.youtube.com/watch?v=7l_1kIOox_g
I'll send a picture of the Questions I am struggling with now. Also, i'm okay with questions such as the one posted in the video.

Thanks again!
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CurtisDean
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(Original post by Secret.)
No it's fine to want to learn it, I think it may help in unit 5 (unless I'm mistaken but I think one of the AQA unit 5 papers had a enthalpy question in it) but if you give us a specific question we could try and help you

Is it questions like this that you're struggling with? http://www.youtube.com/watch?v=7l_1kIOox_g
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Secret.
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(Original post by CurtisDean)
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Ok so did you start of by drawing out the enthalpy diagram? (lets consider a first)
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CurtisDean
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(Original post by Secret.)
Ok so did you start of by drawing out the enthalpy diagram? (lets consider a first)
For the first one, I was unsure how to draw the diagram. Would the arrows be pointing down, or upwards? Because it's enthalpy change of reaction :s
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(Original post by CurtisDean)
For the first one, I was unsure how to draw the diagram. Would the arrows be pointing down, or upwards? Because it's enthalpy change of reaction :s

You're given combustion values so the arrows would point down to the combustion products
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CurtisDean
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(Original post by Secret.)
You're given combustion values so the arrows would point down to the combustion products
okay, and the combustion products would be 2Co2 + (nH2)H20

From here, would I simply substitute the values for the Enthalpy change of combustion, then work out what the Enthalpy change of reaction is?
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(Original post by CurtisDean)
okay, and the combustion products would be 2Co2 + (nH2)H20

From here, would I simply substitute the values for the Enthalpy change of combustion, then work out what the Enthalpy change of reaction is?

Yep but remember to take in to account the moles of each so, for example, remember the multiply the combution value of C by 2 as you have 2C in the equation
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CurtisDean
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(Original post by Secret.)
Yep but remember to take in to account the moles of each so, for example, remember the multiply the combution value of C by 2 as you have 2C in the equation
Awesome!!

and Q E, do you have any idea how to do it?
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(Original post by CurtisDean)
Awesome!!

and Q E, do you have any idea how to do it?
It's practically the same, combustion products are H2O and CO2 and the arrows point downwards
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