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hess' law triangles/cycle?

Can anyone guide me on how i'm meant to construct the cycles please? Am having a lot of trouble with it. Seems basic enough but I keep getting the wrong answers and cant work out how i know which way the arrows are meant to go. I thought in delta H combustion the arrows went downwards and in delta H f the arrows went upwards? All the examples from class seem to have the arrows going in different ways :confused:

Are there basic rules or something I'm missing? 4 text books and a power-point later and i'm so confused.
Any help would be very much appreciated.

Reply 1

charco

Study Forum Helper

Original post by studentgirlbex

Are there basic rules or something I'm missing? 4 text books and a power-point later and i'm so confused.
Any help would be very much appreciated.

Hess's law allows you to go via any suitable route to get from reactants to products. In practise there are three common ways to use the concept.

1. Going via elements at standard state
2. Going via combustion products
3. Going via separated gaseous atoms

1. Requires the use of formation enthalpies
2. Requires combustion enthalpies
3. Requires bond enthalpies

My advice is always to draw the arrows in the direction corresponding to the specific enthalpy definition and then when substituting the numbers make sure you change the sign when you are going 'against' the arrow.

Example (2)
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