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Topic: Bond enthalpy  (Read 1450 times)

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Offline Ter

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Bond enthalpy
« on: January 12, 2013, 01:28:23 AM »
Hi I dont get why HCl(g) -> (1/2)H2(g) + (1/2)Cl2(g) will lose out to HCl(g) -> H(g) + Cl(g) in representing the bond enthalpy for the H-Cl bond!
I would have chosen the first option since hydrogen and chlorine exist as diatomic molecules, but why isn't that the answer?

'The reaction is exothermic overall as the bonds which are formed are stronger than the bonds which are broken.' (From IB Chemistry HL)
I don't understand, how do you know which will be stronger? I remember my teacher saying that to determine exo or endo, there is a need to count the number of bonds, to see which quantity outweigh the other. However, this is talking about comparing strength, how to compare? Please help me here, I'd appreciate examples!

Thank you so much!

Offline fledarmus

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Re: Bond enthalpy
« Reply #1 on: January 12, 2013, 06:04:02 PM »
Are you talking about bond enthalpy or enthalpy of a reaction?

If you are trying to describe the strength of a bond between two atoms, what you are physically describing is the difference in energy between the two atoms locked together at a specific distance (the bond length) and the same two atoms at infinite separation. Chemically, you could write this process as:

HCl(g)  :rarrow: H·(g) + Cl·(g)

or you could say that the H-Cl bond is homolytically cleaved to give a hydrogen radical and a chlorine radical. Then the strength of the bond is the energy given off by that reaction.

You could also be considering heterolytic bond cleavage, which would be represented as:

HCl(g)  :rarrow: H+(g) + Cl-(g)


The other reaction you drew:

HCl(g) :rarrow: (1/2)H2(g) + (1/2)Cl2(g)

is actually concerned with the strengths of three different bonds. The H-Cl bond is broken and H-H and Cl-Cl bonds are formed. The energy of the reaction in this case doesn't represent just the strength of the HCl bond, but the comparative strengths of HCl, H-H, and Cl-Cl bonds.

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