Practice question · Put in order
Arrange the steps of a Hess-cycle calculation in the order you would carry them out.
- Adjust the sign and magnitude of each enthalpy change accordingly
- Reverse or scale each known equation to match the target
- Write the target equation with its correct coefficients
- Add the adjusted enthalpy changes to obtain the target value
Hints
- You cannot manipulate anything until you know what you are aiming at.
- Each equation must be adjusted before its enthalpy can be added.
Show the answer
- Write the target equation with its correct coefficients
- Reverse or scale each known equation to match the target
- Adjust the sign and magnitude of each enthalpy change accordingly
- Add the adjusted enthalpy changes to obtain the target value
Why
Define the target first, then manipulate the known equations to build it, then apply exactly the same manipulations to their enthalpies, and only then add. Adjusting the equations without adjusting the enthalpies is the classic failure.
Practise Enthalpy and Thermochemistry
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