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A2 enthalpy question

20160415_140521.jpg when they were trying to work out the enthalpy change using mean bond enthalpies they got answer of +42 but when they drew the hess's law diagram they got answer of - 42. Am I missing something?
Hi BiologyA2Student

If you look at the book carefully, they did work it out with the mean bond enthalpies to be 42 but they also recognised the fact that deltaH is negative (it is in the writing below it) because more energy was given out than taken in.

Hope that helps

Alex
Don't draw the diagrams (they usually confuse people plus you have the consider the negative stuff).
Learn the formula: Mean bond enthalpy = bonds broken -bonds made = 3192 - 3234 = -42 kJ mol^-1
Original post by ajsullivan
Hi BiologyA2Student

If you look at the book carefully, they did work it out with the mean bond enthalpies to be 42 but they also recognised the fact that deltaH is negative (it is in the writing below it) because more energy was given out than taken in.

Hope that helps

Alex


So It's always the enthalpy of reactants - enthalpy of products when using mean bond enthalpies
Original post by BiologyA2Student
So It's always the enthalpy of reactants - enthalpy of products when using mean bond enthalpies


Yes bonds broken-bonds made for mean bond enthalpy.

If you need the other formulas (might be for AS chemistry):

crp: combustion = reactants-products
fpr: formation = products-reactants
Original post by BiologyA2Student
So It's always the enthalpy of reactants - enthalpy of products when using mean bond enthalpies


Yep! :smile:
Original post by Math12345
Don't draw the diagrams (they usually confuse people plus you have the consider the negative stuff).
Learn the formula: Mean bond enthalpy = bonds broken -bonds made = 3192 - 3234 = -42 kJ mol^-1


For most specs you need to know how to construct and use hess cycles anyway so not a lot of point in learning general formulas if you can easily use a hess cycle
Original post by samb1234
For most specs you need to know how to construct and use hess cycles anyway so not a lot of point in learning general formulas if you can easily use a hess cycle


But most of the time (especially in the exam) using the formula is quicker. You do not have to waste time drawing/worrying about the cycle.

I did AQA chemistry and I never used the cycle in any past paper I did.
Original post by Math12345
But most of the time (especially in the exam) using the formula is quicker. You do not have to waste time drawing/worrying about the cycle.

I did AQA chemistry and I never used the cycle in any past paper I did.


If you properly understand hess cycles drawing a hess cycle for one of the situation you have described (e.g. combustion formation etc) takes maybe 20s, and a hess cycle is much more versatile in that yiu can adapt it to any situation as long as you know the definitions of the enthalpy changes involved (which you need to know anyway) e.g. enthalpies of solution, hydration etc so imo its better to just learn how to do hess cycles (which arent very difficult anyway) than to remember formulas for certain situations.
I have to disagree with Math12345, a good grasp of the hess cycle is key. This comes up in both AS and A2 chemistry, and will help understand what is happening. Mean bond enthalpies is a separate topic.

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