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kidd101

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need help on enthalpy question

« on: November 09, 2008, 11:06:20 am » 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/molthis 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= 0kJ3H2O(l) --> 3H2 + 3/2O2 == 3*286 kJcan't think of a next step. any help would be appreciated « Last Edit: November 09, 2008, 11:24:12 am by kidd101 » Logged

Astrokel

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Re: need help on enthalpy question

« Reply #1 on: November 09, 2008, 11:42:32 am » 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. Logged No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

kidd101

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Re: need help on enthalpy question

« Reply #2 on: November 09, 2008, 12:03:03 pm » Quote from: Astrokel on November 09, 2008, 11:42:32 am

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) « Last Edit: November 09, 2008, 12:13:30 pm by kidd101 » Logged

Astrokel

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Re: need help on enthalpy question

« Reply #3 on: November 09, 2008, 12:30:09 pm » Quote

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? Logged No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

kidd101

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Re: need help on enthalpy question

« Reply #4 on: November 09, 2008, 12:55:59 pm » here are some formulas i was thinking:H2(g) + 1/2O2(g) H2O(l) DHf = -285.5 kJ/molC(grap.) + O2 (g) CO2 DHf = -393.5 kJ/moldo 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 « Last Edit: November 09, 2008, 01:13:27 pm by kidd101 » Logged

Astrokel

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Re: need help on enthalpy question

« Reply #5 on: November 09, 2008, 01:12:55 pm » You have picked the right data but wrong approach. Do you know  :delta: Hr =  :delta: Hf(products) -  :delta: Hf(reactants) ? You just have to simply put in your values and solve for standard enthalpy change of formation of ethanol. Logged No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

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