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Topic: Thermochemistry help  (Read 1550 times)

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Offline jsino6391

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Thermochemistry help
« on: August 19, 2013, 12:48:43 AM »
There is a question in one of my Chemistry text books to do with bond enthalpy and I'm having trouble making sense of how to do one of the questions:

Calculate the bond enthalpy for the N≡N bond using the bond enthalpy information and the following equation:

½ N2 (g) + 1 ½ H2 (g) → NH3 (g)                  ΔfH = -46.5 kJ mol-1


H-H bond enthalpy = 436 kJ mol-1
N-H bond enthalpy = 391 kJ mol-1


How would I find the bond enthalpy for N≡N?

I understand how to do ∑(bonds broken) - ∑(bonds formed) but can't solve this one.

Please help.

Offline Borek

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Re: Thermochemistry help
« Reply #1 on: August 19, 2013, 03:02:45 AM »
Looks to me like a simple plug and chug, where is the problem?
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Offline rosa

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Re: Thermochemistry help
« Reply #2 on: August 20, 2013, 12:12:05 PM »
You need to remember that bond breaking is an endothermic process and bond formation is an exothermic process. The overall enthalpy change is the sum of energy used in both processes.

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