Mean bond enthalpies

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Karina Kong 2H
Posts: 98
Joined: Thu Jul 25, 2019 12:16 am

Mean bond enthalpies

Postby Karina Kong 2H » Thu Feb 13, 2020 5:12 pm

Why do we do reactants minus products for mean bond enthalpies rather than vice versa like the other methods of calculating enthalpy?

Ruby Tang 2J
Posts: 102
Joined: Fri Aug 30, 2019 12:15 am

Re: Mean bond enthalpies

Postby Ruby Tang 2J » Thu Feb 13, 2020 6:28 pm

When we are given mean bond enthalpy values, those values do not directly represent a change in energy coming about as a result of a specific reaction, unlike when we use the standard enthalpy of formation of a substance, which is by definition the change in energy resulting from the formation of a substance from its constituent elements in their standard states. For bond enthalpy, we use reactants - products, because we say that breaking the bonds of the reactants requires energy (therefore change in energy is positive, since there must be some form of energy input), whereas forming the bonds of the producst releases energy (therefore change in energy is negative, since there must be some form of energy output). Hope this helps!

Aarja Pavade 1H
Posts: 103
Joined: Sat Aug 17, 2019 12:18 am

Re: Mean bond enthalpies

Postby Aarja Pavade 1H » Thu Feb 13, 2020 8:45 pm

Instead of thinking of them as being products and reactants, it is much easier if you think of it as bonds broken (reactants) - bonds formed (products). Since all bond enthalpies are positive because heat must be supplied to break a bond, you will need to taken into consideration that bond breaking is always endothermic (positive) and bond formation is always exothermic (negative). Thus, it makes sense it it would be reactants-products.

Harry Zhang 1B
Posts: 101
Joined: Sat Sep 14, 2019 12:16 am

Re: Mean bond enthalpies

Postby Harry Zhang 1B » Thu Feb 13, 2020 11:17 pm

This is so because bond enthalpy measures the change in energy when breaking bonds. On the reactant side, bonds are broken but on the product side bonds are formed; therefore, we use reactants-products to give the bond enthalpy on the product side a negative sign, which makes sense because forming bonds is exothermic.

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