explain terms

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explain terms

Postby aphung1E » Wed Jan 29, 2020 9:25 pm

I'm having trouble understanding the differences between these:
Can someone explain the concepts of:
1) bond enthalpy
2) standard enthalpy of formation
3) standard reaction enthalpy

Ariel Davydov 1C
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Re: explain terms

Postby Ariel Davydov 1C » Wed Jan 29, 2020 9:58 pm

1) Bond enthalpy is the amount of energy needed to break a singular bond. For example, the energy needed to break a C-C bond (single bond between two carbon atoms) is +346 kJ/mol. If a bond is broken, it will release the same amount of energy and will be a negative term.

2) Standard enthalpy of formation is the amount of energy released when a molecule is formed. It can be found in a table provided or calculated using bond enthalpies and looking at the Lewis structure of a molecule. It will be zero for elements and molecules in their “standard” states (lowest energy) - ex: diatomic molecules (H2, O2, N2) and elements in their natural state (C (s)).

3) Standard reaction enthalpy is the overall enthalpy of a reaction. If it is positive, the reaction absorbs heat and is endothermic. If it is negative, it releases heat and is exothermic. It can be calculated using a number of ways: Hess’s Law (adding up the reaction enthalpies of sub-reactions), subtracting the standard enthalpies of formation of the reactants from the products, or subtracting the bond enthalpies of the products from the reactants.

Hope this helps!

Angela Prince 1J
Posts: 102
Joined: Sat Aug 24, 2019 12:17 am

Re: explain terms

Postby Angela Prince 1J » Thu Jan 30, 2020 11:43 am

I am confused about how finding the standard enthalpy using the standard enthalpies of formation works. How and why does it work? Are the values of the standard enthalpy of formation negative because the molecule is losing energy in creating a bond?

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