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CH301 - formationrxns - Standard Reactions of Formation...

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That entire reaction has the following values H = -277.69 kJ/mol G = -174.78 kJ/mol Standard Reactions of Formation H ° rxn = n H ° f (products) - n H ° f (reactants) McCord © 11/2005 Now we can just use H ° f values for any reaction now. We can account for the energies lost and gained when reactants are consumed and products are formed. This leads to the following version of Hess’s Law. Realize that the reactions that you are summing up are just formation reactions and all the elements will cancel out of the net overall equation. You will get these values from Tables 2 C ( graphite ) + O 2 ( g ) + 3 H 2 ( g ) C 2 H 5 OH (liq) 1 2 Since we can’t really measure absolute enthalpies ( H ) and free energies ( G ), we use an alternative which utilizes state functions that are called standard molar enthalpy (or free energy) of formation. The “of formation” means that the substance (formula) is formed from only the elements in their standard forms. Standard state is 1 bar (or the older standard, 1 atm) and 25 ° C. Do realize that you
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