Meggggggggggg
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When working out enthalpy change is it products - reactants or the other way? I have seen both and so I have no idea which to use
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Chowderzzz
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It’s always: reactants - products.
This is how you get a negative value for enthalpy change where exothermic reactions are concerned. Look at a photo of the different graphs and it makes sense.
In exothermic reactions more energy is required to make the bonds than break the bonds. Hence the “right” of the graph is lower than the “left”.
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Kian Stevens
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It's always \sum\Delta H_{products} - \sum\Delta H_{reactants}, and it always takes this form for the entropy change and the Gibbs free energy change

If you saw it the other way around, then it's incorrect and you shouldn't use it
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Kian Stevens
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(Original post by Chowderzzz)
In exothermic reactions more energy is required to make the bonds than break the bonds. Hence the “right” of the graph is lower than the “left”.
Technically speaking, there's no energy requirement in making bonds as it is inherently an energy-liberating process

If energy was required to make bonds and break them, then theoretically all reactions would be endothermic
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Chowderzzz
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(Original post by Kian Stevens)
Technically speaking, there's no energy requirement in making bonds as it is inherently an energy-liberating process

If energy was required to make bonds and break them, then theoretically all reactions would be endothermic
I mean that breaking bonds is endothermic and making bonds is exothermic. More energy required to make bonds -> exothermic reaction.
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Meggggggggggg
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(Original post by Chowderzzz)
It’s always: reactants - products.
This is how you get a negative value for enthalpy change where exothermic reactions are concerned. Look at a photo of the different graphs and it makes sense.
In exothermic reactions more energy is required to make the bonds than break the bonds. Hence the “right” of the graph is lower than the “left”.
Name:  Capture1.PNG
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Size:  88.6 KBthis was in my textbookName:  Capture.PNG
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Size:  78.7 KB why are they the same but not
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Kian Stevens
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(Original post by Meggggggggggg)
Name:  Capture1.PNG
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Size:  88.6 KBthis was in my textbookName:  Capture.PNG
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Size:  78.7 KB why are they the same but not
That is incorrect, and would give opposite signs to any literature enthalpy change value

The enthalpy change is defined as the difference in enthalpies between a system's final state and a system's initial state, as it is a state function, and so opposes whatever is in your textbook

A simple Hess diagram would disprove the last screenshot about average bond enthalpies, anyway
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Kian Stevens
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I would like to apologise, as well as make it clear that I answered this question with the enthalpy changes of reaction and formation in mind, in which yes, these are \sum\Delta H_{products} - \sum\Delta H_{reactants} (and so the last screenshot on your comment is incorrect)

However, for the enthalpy change of combustion I have to confess that it is \sum\Delta H_{reactants} - \sum\Delta H_{products}, and this too is verifiable via a Hess diagram... But the textbook hasn't clearly distinguished this, so I understand your confusion (I was as well; I don't like thermodynamics that much!)
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Meggggggggggg
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(Original post by Kian Stevens)
I would like to apologise, as well as make it clear that I answered this question with the enthalpy changes of reaction and formation in mind, in which yes, these are \sum\Delta H_{products} - \sum\Delta H_{reactants} (and so the last screenshot on your comment is incorrect)

However, for the enthalpy change of combustion I have to confess that it is \sum\Delta H_{reactants} - \sum\Delta H_{products}, and this too is verifiable via a Hess diagram... But the textbook hasn't clearly distinguished this, so I understand your confusion (I was as well; I don't like thermodynamics that much!)
Thank you! this makes a lot more sense. which way round is it for enthalpy change of neutralisation, or is this not something that would come up?
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Kian Stevens
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(Original post by Meggggggggggg)
Thank you! this makes a lot more sense. which way round is it for enthalpy change of neutralisation, or is this not something that would come up?
From my own experience, these things aren't asked in neutralisation reaction questions

I always used to get questions which stated a temperature change and some amounts of acid and base, and made you calculate \Delta H by using \frac{mc\Delta T}{n}, but your mileage may vary so I guess it's best to look around at different past paper questions regarding this
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