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Bond enthalpies vs Standard Enthalpies of Formation

Posted: Wed Jan 24, 2018 9:55 pm
by Alyssa Parry Disc 1H
Why when calculating bond enthalpies are reactants positive and products negative but then when calculating standard enthalpies of formation are reactants negative and products positive?

Re: Bond enthalpies vs Standard Enthalpies of Formation

Posted: Wed Jan 24, 2018 9:59 pm
by Sungyoon_Baek_1A
for bond enthalpies we are calculating the energy of bonds broken minus the energy of the bonds formed (therefore the products are negative and reactants are positive). However, for the standard enthalpies of formation we use the equation of the sum of the moles of each product multiplied by the standard formation enthalpy minus the sum of the moles of each reactant multiplied by the standard formation enthalpy.

Re: Bond enthalpies vs Standard Enthalpies of Formation

Posted: Wed Jan 24, 2018 10:00 pm
by Jeremiah Samaniego 2C
When calculating bond enthalpies, reactants appear positive and products appear negative because by definition, breaking bonds requires energy whereas forming bonds release energy.

Standard enthalpies of formation are experimentally found. By nature of the equation H° = ∑nHf°(products) - ∑nHf°(reactants), if the reactants have a negative Hf° value, you are essentially adding the number because you are subtracting a negative number.