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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: jsino6391 on August 19, 2013, 12:48:43 AM

Title: Thermochemistry help
Post by: jsino6391 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 N_{2 (g)} + 1 ½ H_{2 (g)} → NH_{3 (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.

Title: Re: Thermochemistry help
Post by: Borek on August 19, 2013, 03:02:45 AM

Looks to me like a simple plug and chug, where is the problem?

Title: Re: Thermochemistry help
Post by: rosa 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.