[kidd101]

Determine the standard enthalpy of formation (deltaH*f) of ethanol (C3H5OH) from its standard enthalpy of combustion?
C2H5OH(l) + 7/2O2(g) --> 2CO2(g) + 3H2O(g) deltaH*f = --1367.4 kJ/mol

this is what i have done so far:
c2H5OH + 7/2O2 --> 2CO2(g) + 3H2O(g)
7[H20(l) --> H2 + 1/2O2] == (7)*286kJ
---
3O2  --> 3O2(g)  DHf= 0kJ
3H2O(l) --> 3H2 + 3/2O2  == 3*286 kJ

can't think of a next step.
any help would be appreciated

[Astrokel]

Hi kidd101, did your question only gives you value for standard enthalpy of combustion or are there any other values like standard enthalpy of formation of water which you have stated.

[kidd101]

Hi kidd101, did your question only gives you value for standard enthalpy of combustion or are there any other values like standard enthalpy of formation of water which you have stated.

The question only gave me DHf of the equation. nothing else. so, i'm assuming i look at back of the book values or something...

edit:
is the reaction balanced? there are 8 Oxygen on the reactant side and 7 on the product side.
isn't the rxn suppose to be something like this: C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)

[Astrokel]

isn't the rxn suppose to be something like this: C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)

Yes you are right on that, good catch. so probably your question is not balanced correctly.

Anyway if you have values from the back, then you could work it out using formula. What formula involving formation do you know?

[kidd101]

here are some formulas i was thinking:
H2(g) + 1/2O2(g)  H2O(l)   DHf = -285.5 kJ/mol

C(grap.) + O2 (g)  CO2  DHf = -393.5 kJ/mol

do you just add them with the original equation?
such as:

2CO2(g) + 3H2O(g)   C2H5OH(l) + 3O2(g)  :delta:Hf = +1367.4 kJ/mol

3[H2(g) + 1/2O2(g)  H2O(l)    :delta:Hf = -285.5 kJ/mol] (-857.7 kJ/mol)

2[C(grap.) + O2 (g)  CO2   :delta:Hf = -393.5 kJ/mol]   (-787.0 kJ/mol)

-------------------------------------------------------------
2C(graphite)  + 3H2 + O2  C2H5OH (l)   DHf = 277.3 kJ/mol

[Astrokel]

You have picked the right data but wrong approach. Do you know  :delta: H_r =  :delta: H_f(products) -  :delta: H_f(reactants) ? You just have to simply put in your values and solve for standard enthalpy change of formation of ethanol.