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A2 enthalpy question
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#2
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
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
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#3
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
Learn the formula: Mean bond enthalpy = bonds broken -bonds made = 3192 - 3234 = -42 kJ mol^-1
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(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
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
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#5
(Original post by BiologyA2Student)
So It's always the enthalpy of reactants - enthalpy of products when using mean bond enthalpies
So It's always the enthalpy of reactants - enthalpy of products when using mean bond enthalpies
If you need the other formulas (might be for AS chemistry):
crp: combustion = reactants-products
fpr: formation = products-reactants
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#6
(Original post by BiologyA2Student)
So It's always the enthalpy of reactants - enthalpy of products when using mean bond enthalpies
So It's always the enthalpy of reactants - enthalpy of products when using mean bond enthalpies
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#7
(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
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
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#8
(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
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
I did AQA chemistry and I never used the cycle in any past paper I did.
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#9
(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.
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.
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#10
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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