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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: G Campbell on May 12, 2005, 02:31:32 PM

Title: Use the standard enthalpies of formation ?Hof in table 1 to calculate the heat
Post by: G Campbell on May 12, 2005, 02:31:32 PM

Use the standard enthalpies of formation ?Hof  in table 1 to calculate the heat of combustion of propane in kJ/mole.

C3H8(g) + 5O2(g) ----à 3CO2(g) + 4H2O(g)

-103.85        0                  -393.51       -241.82

Table.1
Compound               Stanard enthalpy

The compound is c3h8 etc. and the standard entalphy is the numbers.

Can someone help me out please? ???

Title: Re:Please *delete me*
Post by: jdurg on May 12, 2005, 03:08:16 PM

Remember that the heat of combustion (Also known as the reaction enthalpy in this case) is the difference between the total enthalpy of the formation of the products and the total enthalpy of formation of the reactants.  Remember that when dealing with thermodynamic situations, it doesn't matter how you get there just as long as you get there.  So when calculating the enthalpy of a reaction, the enthalpy of formation can be used.  (As this makes the assumption that all the reactants break down into their elements, and all the products are formed from their elements).  Hence the enthalpy of formation of a pure element is zero since all the values are determined based upon formation from pure elements.  A pure element is a pure element, so it has no heat of formation.  

So with your products, you are trying to determine how much energy is given off when the products form directly from their elements.  With the reactant, you have to determine how much energy is needed to break it down into the elements it's made of.  When you get the difference between these two values, you'll have your heat of combustion of one mole of propane.  ;D

Title: Re: Use the standard enthalpies of formation ?Hof in table 1 to calculate the
Post by: GCT on May 13, 2005, 12:06:27 PM

What's the problem?  It's seems that you have everything needed to solve the problem, simply find the net change in standard enthalpy, remember to account for the moles for individual enthalpies of formation (that is use the correct correlation).

Title: Re: Use the standard enthalpies of formation ?Hof in table 1 to calculate the
Post by: G Campbell on May 14, 2005, 03:38:18 PM

I have never seen a sum like that before. I am used to seeing the main equation and then 2 others which you have to reverse or multiply by 2 and so on. ??? So could some give the solution if it's not to much bother so I can see what had to be done and where the figures come from?