How to Draw Energy Cycle involving Enthalpy Change of Combustion - Energetics

In this video we want to learn how to draw the energy cycle given enthalpy changes of combustion of compounds and calculate the enthalpy change of a reaction using Hess' Law.
Let's take a look at this exercise.
1. Draw Energy Cycle
First we have to write out the equation that represents enthalpy change of formation for propanol.
Remember formation is forming 1 mole of a compound from its elements in their standard states.
Since the products of combustion of carbon, H2 gas and propanol are all CO2 and H2O, we can put them together at the bottom of the energy cycle and balance the equations.
For energy cycle involving combustion, we need to add O2 gas to force out the combustion reactions so that we can make use of the enthalpy change of combustion for calculation.
Now we can add arrows for each combustion reaction.
First let's account for combustion of C3H8O. Enthalpy change of combustion is with respect to per mole of substance reacted with excess oxygen. Since coefficient of C3H8O is 1, enthalpy change of combustion for C3H8O remains as it is.
Next for combustion of C. Since coefficient of C is 3, enthalpy change of combustion for C has to be multiplied by 3.
Finally for combustion of H2. Since coefficient of H2 is 4, enthalpy change of combustion for H2 has to be multiplied by 4.
2. Apply Hess' Law
Once the energy cycle is complete, we can determine which of these arrows are clockwise and counter-clockwise.
Finally we can apply Hess' Law by putting all the clockwise arrows on one side of the equation and all the counter-clockwise arrows on the other side of the equation to solve for the unknown enthalpy change.
For the detailed step-by-step discussion on how to draw this energy cycle, check out this video!
Topic: Energetics, Physical Chemistry, A Level Chemistry, Singapore
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[@anarosca2291]

You explain everything so well and patiently! Thank you so much.

[@abdiamin1264]

Thank you very much chemistry guru you made me to understand this hess law topic very easily

[@lee3951]

Thank you! you really helped a lot. Many videos were complicated and confusing, but yours were easy to follow and understand!

[@ChemistryGuru]

Hi Lee thank you so much! Glad you found the videos useful 😄😄

[@qiaoyilim8572]

sir you are a lifesaver

[@naveedhasan6738]

You explained it so simply TYSM

[@nasibrahman7490]

made my life so much easier

[@judith____]

This really helped me

[@margitrezacova3941]

Thanks so much for the videos on how to draw the energy cycles. I know to do the math of it but didn't know how to draw the cycles. I understand it now

[@ChemistryGuru]

Hi Margit thank you for the compliment. Glad you found the video useful. Energy cycle needs constant practice to get the hang of it. All the best!

[@Slhtte24]

Thank you, I have a test tomorrow and this vid helps me a lot !

[@firas7308]

wau, hheheheh aku baru tgok balik vid ni huhu nyesal baru jumpe skrg

[@Slhtte24]

@@firas7308 Nicee, okayla at least kau jumpe before final 😂

[@nawalabdul6456]

Thank you now it’s easier 👏

[@amiiranorbani]

It's really helpful ! Thank you sir !

[@klausaceline7027]

Thank you!

[@ayanadezoysa9639]

thank you so much

[@reifuruya228]

thank you very much

[@sweet_glimpses]

Thanks for helping out

[@meralabotte6838]

Thanks

[@nurulaida242]

can i know in the full reaction should combine together with the added oxsigen or not??

[@strictlyedits_123]

In the box of elements ( the bottom box), do we always have to put carbon dioxide and water?

[@karling8955]

What do you mean by clockwise or counterclockwise ? How do you determine which one is which

[@jaank8982]

Why do we not include calculation of O2 enthalpy change in the cycle?

[@ChemistryGuru]

Enthalpy change of combustion is the heat change when one mole of substance is burnt in excess oxygen. So there is no such thing as enthalpy change of combustion of oxygen, as this is the nonexistent reaction where oxygen is burnt in excess oxygen.

[@BladerINCThe_Hope]

I don’t get it

[@ChemistryGuru]

You can start with writing down equations for enthalpy change of combustion first.
eg combustion of carbon: C + O2 = CO2
combustion of hydrogen gas: H2 + 1/2 O2 = H2O
we need to be able to write down the equation for each enthalpy change before we can combine them together into an energy cycle.