Clear my confusion about Average bond enthalpy question?

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abitpissy
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So if a reaction is endothermic then it would take energy in from its surroundings. Correct?

Say for example the total reactants equate to 400jkmol and the products are 800kjmol. Then this would clearly be an endothermic reaction as the products gain energy.

My question is, when you find out the enthalpy change it is a negative. ( reactants- products. Eg 400-800=-400) This indicates that it is an exothermic reaction. Hence my confusion. Please help, I know I must of misunderstood something. Thanks
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username986184
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(Original post by Questioness)
So if a reaction is endothermic then it would take energy in from its surroundings. Correct?

Say for example the total reactants equate to 400jkmol and the products are 800kjmol. Then this would clearly be an endothermic reaction as the products gain energy.

My question is, when you fidnd out the enthalpy change it is a negative. ( reactants- products. Eg 400-800=-400) This indicates that it is an exothermic reaction. Hence my confusion. Please help, I know I must of misunderstood something. Thanks
Using your numbers.

Energy required to break bonds of reactants = 400
Energy released in forming bonds of products = 800

Therefore, nett energy change is a release of 400. Exothermic.
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abitpissy
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Thank you.

I have one more question about Hess's law. Question 2b.
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I keep getting -154 as the answer, however the answer is +154kj.
Attachment 487053487057487040
^^This is my working out so you can see how I got it
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abitpissy
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I realise that some of the molecules aren't correctly put, please forgive me (it's hardly relevant to my answer)
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username986184
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(Original post by Questioness)
I realise that some of the molecules aren't correctly put, please forgive me (it's hardly relevant to my answer)
See below
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Mayhem™
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(Original post by TeachChemistry)
See below
At what level of education is this taught? I'm only in year 11 but it looks like something I could have a go at
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abitpissy
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(Original post by Mayhem™)
At what level of education is this taught? I'm only in year 11 but it looks like something I could have a go at
AS chemistry.
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username986184
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(Original post by Mayhem™)
At what level of education is this taught? I'm only in year 11 but it looks like something I could have a go at
Hess cycles are year 12
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Mayhem™
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(Original post by Questioness)
AS chemistry.
(Original post by TeachChemistry)
Hess cycles are year 12
Thanks!
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abitpissy
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(Original post by TeachChemistry)
See below
Does it matter which way the arrows are pointing? my book shows it pointing down for combustion and arrows point up for formation.
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rxns_00
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(Original post by Questioness)
So if a reaction is endothermic then it would take energy in from its surroundings. Correct?

Say for example the total reactants equate to 400jkmol and the products are 800kjmol. Then this would clearly be an endothermic reaction as the products gain energy.

My question is, when you find out the enthalpy change it is a negative. ( reactants- products. Eg 400-800=-400) This indicates that it is an exothermic reaction. Hence my confusion. Please help, I know I must of misunderstood something. Thanks
Hun its product-reactants not the other way round.

So 800-400=+400

Therefore deltaH + hence endothermic reaction
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username986184
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(Original post by Questioness)
Does it matter which way the arrows are pointing? my book shows it pointing down for combustion and arrows point up for formation.
The values in your book are formation.
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abitpissy
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(Original post by RonnieRJ)
Hun its product-reactants not the other way round.

So 800-400=+400

Therefore deltaH + hence endothermic reaction
I was talking about calculating enthalpy change in average bond enthalpies.

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abitpissy
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(Original post by TeachChemistry)
The values in your book are formation.
Ahah right. That makes a whole lot more sense now. Thank you so much.
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rxns_00
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(Original post by Questioness)
I was talking about calculating enthalpy change in average bond enthalpies.

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Yeah still. No offence that book is ****

Ok when you work out enthalpy change using bond enthalpies follow these:

- work out bond enthalpies of products
- work out bond enthalpies of reactants
- find the difference between the two (take whichever is larger away from the smaller)
- if dH products>reactants, stick a + to the enthalpy change
- if dH reactants>products, stick a - to the enthalpy change.

That is it
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abitpissy
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(Original post by RonnieRJ)
Yeah still. No offence that book is ****

Ok when you work out enthalpy change using bond enthalpies follow these:

- work out bond enthalpies of products
- work out bond enthalpies of reactants
- find the difference between the two (take whichever is larger away from the smaller)
- if dH products>reactants, stick a + to the enthalpy change
- if dH reactants>products, stick a - to the enthalpy change.

That is it
Okay, sounds simpler. Thanks :P
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