Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: girl101 on March 04, 2015, 09:04:20 PM

Title: hess law help
Post by: girl101 on March 04, 2015, 09:04:20 PM
4.   The enthalpy of formation of naphthalene (C10H8 (g)) is 150.96 kJ mol-1. Using bond enthalpy  given  calculate the enthalpy of formation of naphthalene (g).
Table 1
                  Bond                        D (kJ mol-1)
                  C-C                                      344
                  C=C                                       615
                  C-H                                           415                     
                       
Give your reason for the discrepancy between calculated and experimental values?
Title: Re: hess law help
Post by: Ciubba on March 04, 2015, 09:18:49 PM
ΔH≈amount of energy necessary to break all of the bonds-amount of energy released when new bonds form.

You are given ΔHf and the amount of energy necessary to break each individual bond of a gaseous molecule, called the bond dissociate energy. By Hess' Law, you should be able to find the amount of energy released by the formation of each bond. From there, it's an arithmetic problem.

Does that make sense?
Title: Re: hess law help
Post by: girl101 on March 04, 2015, 09:46:27 PM
no doesnt make sense :-\ but thanks anyway
Title: Re: hess law help
Post by: Ciubba on March 04, 2015, 10:47:15 PM
Basically, you need to find the amount of energy associated with the bonds of napthalene. Then, you need to compare it to the enthalpy of formation of napthalene.

You are given the average amount of energy needed to break certain bonds, and Hess' Law essentially states that state functions, like enthalpy, are additive. Thus, you should be able to find the average amount of energy needed to form those same bonds.
Title: Re: hess law help
Post by: girl101 on March 05, 2015, 02:00:26 PM
i got 150.96- 344*6 +614*5 + 415*8= -8303.04
Title: Re: hess law help
Post by: Borek on March 06, 2015, 02:55:19 AM
Quote from: jenny250 on March 05, 2015, 02:00:26 PM
i got 150.96- 344*6 +614*5 + 415*8= -8303.04

While there are definitely signs that you are on the right track, you have not said what you are calculating, plus, you have ignored parentheses, so the equation is wrong as written.

So, please explain - what have you tried to calculate here?
Title: Re: hess law help
Post by: mjc123 on March 06, 2015, 05:00:38 AM
What is the definition of enthalpy of formation? Can you write the equation for which ΔH is equal to the enthalpy of formation of naphthalene? Can you then see why this is a very bad question?