# Hess cycles. Watch

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Could someone please explain the differences between the two Hess cycles and when to use each one.

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Are the two standard exam questions:

- calculate the enthalpy change of the reaction

- calculate the enthalpy change of formation

any help is appreciated.

- calculate the enthalpy change of the reaction

- calculate the enthalpy change of formation

any help is appreciated.

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#3

(Original post by

Are the two standard exam questions:

- calculate the enthalpy change of the reaction

- calculate the enthalpy change of formation

any help is appreciated.

**OL350**)Are the two standard exam questions:

- calculate the enthalpy change of the reaction

- calculate the enthalpy change of formation

any help is appreciated.

I think another question AQA liked was to explain that they're theoretical and can differ in practice due to covalent bonding and not just ionic bonding.

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#4

There aren't two kinds of Hess cycles? I don't think you have understood it correctly. Lets say for example we have a reaction where A+B goes to C. In order to find the enthalpy of this reaction we will use the cycle. If we knew how to go directly from A and B to C then we would not need to apply the Hess Cycle, however we don't. However we do know that A and B break down into D and E, and the reaction of D and E also makes C. We know the enthalpy of formation for D and E, we also know the enthalpy of formation for C from D and E. Therefore, we find the enthalpy of A+B-->C by using the enthalpy of A+B--->D+E with the addition of enthalpy from D+E--->C.

This may be a confusing reply, but I hope it helps

This may be a confusing reply, but I hope it helps

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(Original post by

There aren't two kinds of Hess cycles? I don't think you have understood it correctly. Lets say for example we have a reaction where A+B goes to C. In order to find the enthalpy of this reaction we will use the cycle. If we knew how to go directly from A and B to C then we would not need to apply the Hess Cycle, however we don't. However we do know that A and B break down into D and E, and the reaction of D and E also makes C. We know the enthalpy of formation for D and E, we also know the enthalpy of formation for C from D and E. Therefore, we find the enthalpy of A+B-->C by using the enthalpy of A+B--->D+E with the addition of enthalpy from D+E--->C.

This may be a confusing reply, but I hope it helps

**ElChapo**)There aren't two kinds of Hess cycles? I don't think you have understood it correctly. Lets say for example we have a reaction where A+B goes to C. In order to find the enthalpy of this reaction we will use the cycle. If we knew how to go directly from A and B to C then we would not need to apply the Hess Cycle, however we don't. However we do know that A and B break down into D and E, and the reaction of D and E also makes C. We know the enthalpy of formation for D and E, we also know the enthalpy of formation for C from D and E. Therefore, we find the enthalpy of A+B-->C by using the enthalpy of A+B--->D+E with the addition of enthalpy from D+E--->C.

This may be a confusing reply, but I hope it helps

- Calculating the enthalpy change of combustion (both arrows point up)?

- Calculating the enthalpy change of formation (both arrows point down)?

Is that correct?

I seem to think there is another one...'calculate the enthalpy change of the reaction?'

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#6

(Original post by

Hmm. It's the direction of the arrows I'm struggling on and when to use what I each situation.

- Calculating the enthalpy change of combustion (both arrows point up)?

- Calculating the enthalpy change of formation (both arrows point down)?

Is that correct?

I seem to think there is another one...'calculate the enthalpy change of the reaction?'

**OL350**)Hmm. It's the direction of the arrows I'm struggling on and when to use what I each situation.

- Calculating the enthalpy change of combustion (both arrows point up)?

- Calculating the enthalpy change of formation (both arrows point down)?

Is that correct?

I seem to think there is another one...'calculate the enthalpy change of the reaction?'

Calculating the enthalpy change of the reaction is the end result.

Its a bit hard to say without seeing example questions, I suggest you try some past papers, and see how that goes. Good luck.

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(Original post by

Okay yes, the direction can be confusing to some from what I remember. I like to think of it similar to vectors in maths, If your enthalpy change going from the reactants (A and B) to the intermediates (D and E) is in terms of an enthalpy of combustion then the arrow will go down, however if it was an enthalpy of formation it would point up as you form A and B from D and E so the direction shows you the route. In calculations if you follow the arrow direction, you should make any enthalpy where you go against the arrow direction negative, and if you go with the direction of the arrow it is positive. Going from D and E to C will be a formation so your arrow will point downwards if you were to use another enthalpy of combustion, as C combusts to form D and E. It all depends on the data you get in a question, it will either be formation enthalpies or combustion enthalpies.

Calculating the enthalpy change of the reaction is the end result.

Its a bit hard to say without seeing example questions, I suggest you try some past papers, and see how that goes. Good luck.

**ElChapo**)Okay yes, the direction can be confusing to some from what I remember. I like to think of it similar to vectors in maths, If your enthalpy change going from the reactants (A and B) to the intermediates (D and E) is in terms of an enthalpy of combustion then the arrow will go down, however if it was an enthalpy of formation it would point up as you form A and B from D and E so the direction shows you the route. In calculations if you follow the arrow direction, you should make any enthalpy where you go against the arrow direction negative, and if you go with the direction of the arrow it is positive. Going from D and E to C will be a formation so your arrow will point downwards if you were to use another enthalpy of combustion, as C combusts to form D and E. It all depends on the data you get in a question, it will either be formation enthalpies or combustion enthalpies.

Calculating the enthalpy change of the reaction is the end result.

Its a bit hard to say without seeing example questions, I suggest you try some past papers, and see how that goes. Good luck.

- if formation figures are given then both arrows point up

if combustion figures are given then both arrows point down

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#8

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To simplify this would I be correct in saying:

- if formation figures are given then both arrows point up

if combustion figures are given then both arrows point down

**OL350**)To simplify this would I be correct in saying:

- if formation figures are given then both arrows point up

if combustion figures are given then both arrows point down

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#9

Both arrows downwards for Combustion (bottom should have co2 and water), both arrows upwards for formation (bottom should have elements, yus

When calculating, make the arrow that has been flipped' delta H value negative.

When calculating, make the arrow that has been flipped' delta H value negative.

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(Original post by

Both arrows downwards for Combustion (bottom should have co2 and water), both arrows upwards for formation (bottom should have elements, yus

When calculating, make the arrow that has been flipped' delta H value negative.

**AnnekaChan173**)Both arrows downwards for Combustion (bottom should have co2 and water), both arrows upwards for formation (bottom should have elements, yus

When calculating, make the arrow that has been flipped' delta H value negative.

going by what you've said the top answer must be correct? How would I go about answering the second one though?

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(Original post by

Yes I believe thats correct.

**ElChapo**)Yes I believe thats correct.

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#12

(Original post by

Any help on the above question^?

**OL350**)Any help on the above question^?

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(Original post by

Work your way backwards in the cycle

**ElChapo**)Work your way backwards in the cycle

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#14

(Original post by

Am I right in saying the answer is (-718) - (-10) = +708?

**OL350**)Am I right in saying the answer is (-718) - (-10) = +708?

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(Original post by

I don't know, check the mark scheme?

**ElChapo**)I don't know, check the mark scheme?

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#16

(Original post by

I don't have one ynfortunately...they were questions made up by my teacher

**OL350**)I don't have one ynfortunately...they were questions made up by my teacher

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(Original post by

Yeah I think you're right

**ElChapo**)Yeah I think you're right

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#18

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(Original post by

You also form 2 moles of PbO so you have to take the enthalpy change associated with those 2 moles into account.

**ElChapo**)You also form 2 moles of PbO so you have to take the enthalpy change associated with those 2 moles into account.

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#20

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