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Hess Cycles

Hello,

Everytime I attempt one fo these I get confused with directiona dn signs and end up getting it wrong so could someone explain this please.

For example in in enthalpy of formation of ethane from enthalpies of combustion

Also i was wondering when giving a description of endothermic reaction do you say differnce in enthalpies or different in energies, are energy and enthalpy interchangeable??

Thanks
Reply 1
Avatar for sparklet
sparklet
2
If your given a reaction for enthalpy of formations, with values of enthalpy of combustion, arrows point down. Id its a reaction for comb, with formation values...arrow points up.
Reply 2
Avatar for happyheart
happyheart
OP
3
ok thanks what about the overall enthalpy change with values of formation?

and do i go left to right or right to left along it or does it depends on quesiton?
Reply 3
Avatar for No Future
No Future
17
DRAWING THE CYCLE

1. Write the equation you are trying to find the value for across the top.
2. Write delta H R on the arrow.
3. Look at the given data.
4. Think, how can you slot in the given data?
5. After adding the given data, what routes to the reaction are shown in the cycle? Find an alternative route using the given data to the unknown
6. If your arrows and data point the 'wrong' way, simply make the value negative

CALCULATION:

According to Hess's Law:
Delta H for a reaction is independent of the route taken.

SO, the value of the unknown will be equal to the total of all the other steps of an alternative route.

Add up the values of each step of the alternative route.

Units should be in kJmol-1 normally.

Hope that helps.
Reply 4
Avatar for happyheart
happyheart
OP
3
Thanks alot, i just did one and for the first time it was right! thanks both of you!
Reply 5
Avatar for No Future
No Future
17
:smile: No problem!

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