5.3 Hess's Law and Enthalpies of Formation

[@heisyahweh7148]

This is life changing! I've always done Hess's law the long way and never knew of this short way! Thank you Professor Chad for making chemistry bearable!

[@ChadsPrep]

My pleasure - glad you found the channel!

[@hutshutrules123]

Thank you so much . for . this! couldnt figure . out an mcat question and this helped tremendously!

[@ChadsPrep]

You're very welcome!🙂

[@user-sv2en5cp2i]

Hi What happens if we don't use an equation? E.g. if we have 4 equations and 1 of the equations doesn't have any of the products/reactants that we were using in the original formula. My notes say to just add it but i was confused as to why

[@ChadsPrep]

You are typically going to be provided with just the equations you need. Remember that the goal is so that when you add them all up, it adds up to exactly the reaction of interest. Sometimes when you add the reactions required to get the correct reactants and products, you have some extra reactants and/or products, and you'll be provided with a reaction that, when added, will eliminate them from the overall. These are the most difficult examples for Hess' Law, and I think this is most likely the sort of example you're referencing. Hope this helps!

[@priyankiis6207]

Hello Chad, For formation reaction of H @15:30 ish minutes, how come the O2 and N2 are not included when the sum of delta H is calculated? Thank you in advance!

[@ChadsPrep]

Hello Priyankiis! An element in its standard state has an enthalpy of formation of zero. The standard states for both oxygen and nitrogen are diatomic gases so they both had values of zero in this example. This is something you're definitely expected to remember fyi. Hope this helps and happy studying!

[@destinycross7977]

why did you subtract 100? and not doing 95.4?

[@ilovekitkat7857]

@ 13:30 H2O should be labled liquid not gas Anyway thank you for your very helpful videos:)

[@ChadsPrep]

Actually it could have been liquid or gas depending on reaction conditions. What you may be confusing this with is that the reactants in a properly written formation reaction must be in their standard states. But this doesn't apply to the products of a formation reaction or to any random reaction for which we are calculating delta H using enthalpies of formation as is the case here. If I've misunderstood your point by all means let me know. Happy studying!

[@devpatel2842]

Is iodine's elemental form not a gas? You did not include it with the other gases but if it is not, can you explain, please? Thank you!