[hope35]

So I'm given the internal energy  :delta:U, for benzoic acid and I'm suppose to find the enthalpy of benzoic acid  :delta:H, using the following equations:

 :delta: H= :delta: U+ :delta: (PV)
 :delta:(PV)=( :delta: n)RT.

The balance equation for benzoic acid is: C₆H₅CO₂H + 15/2O₂  7CO₂ +3H₂O.

Here's what I did:

From the balance equation I found  :delta:n = -0.5.
Then  :delta:(PV)= -0.5mole*8.3145J/mol*K*298.15K=-1239.48 J

But I'm given  :delta:H=-26 434 J/g.

The units won't work out. What am I suppose to do?
Do I simply times  :delta:(PV) I got by the molar mass of benzoic acid.

The question gives me a hint saying to treat  :delta:n as a unitless quantity.

[tamim83]

If you treat delta n as a unitless quantity, your answer comes out in J/mol.  You can use the molar mass to get J/g.

[zeoblade]

so how would you find the molar heat of formation and combustion?
2C₇H₆O₂ + 15O₂ --> 14CO₂ + 6H₂O
molar heat of combustion = dU right?
So 26.44kJ/g x 122.12g/mol = 3228.9kJ/mol
Then dH = dU + (dn)RT
So dH = 3228.9kJ/mol + (-1mol x 8.31447kJK^-1mol^-1 x 298.15K) = 749.9kJ/mol
And d_formationH = (14*-393.3kJ/mol+6*-286.2kJ/mol) - 2*749.9 = -8723.2kJ/mol??
In CRC handbook the value is -385.2kJ/mol so I'm really way off....

[zeoblade]

can i say dU = heat of combustion constant V = heat of reaction or dH = heat of combustion constant P = heat of reaction?

if either one is heat of reaction, then can i deduce the heat of formation by rearranging the equation:
heat of reaction = heat of formation products - heat of formation reactants
heat of formation reactants = heat of formation products - heat of reaction?

in a steel bomb, i have a feeling the Cv of the steel bomb needs to be considered. but how can i consider it when it rapidly gains and releases heat relative to water? would i not need to integrate over the duration it increases to peak T and falls back to initial T? therefore i would have needed to couple it to software that could record and give me an area count under the curve, right?

also would i need to separately calculate the heat absorbed by the water produced by mass of water x Cv x 100K?

i wish atkins 8e would teach you how to solve these problems or just even relate terms together instead of differentiating all these terms when integration is needed. how heat of combustion at constant V or P relates to an overall heat of combustion or if heat of combustion at constant P is the overall term that is the heat of reaction would be helpful. just staring at the words and trying to associate when the textbook has no common term linking them together. you would have figured that after 9 editions, they would already cater for the areas where students have trouble associating components of knowledge together