February 27, 2015

Homework Help: thermochemistry

Posted by Sienna on Tuesday, January 23, 2007 at 3:17pm.

what is the enthalpy change in kilojoules for the combustion of 1 mol of ethanol, C2H5OH (l), to form gaseous carbon dioxide and gaseous water? use bond energies to calculate your answer. (hint: there is a double bond in carbon dioxide and oxygen)

the answer is supposed to be -1100 but i have no idea how to get it.

Look in your text for a table listing bond energies.
C2H5OH + 3O2 ==> 2CO2 + 3H2O

delta H = (sum B.E. reactants)-(sum B.E. products).

break 5 C-H bonds
break 1 C-C bond
break 1 C-O bond
break 1 O-H bond
break 1 O-O bond (and I used O=O)although it isn't exactly a double bond and that x 3 for 3 mols.

break 2 C=O bonds x 2 mols
break 2 O-H bonds x 3 mols

Using the table in my text at home I came up with -1022 kJ/mol C2H5OH but I have looked on the internet for numbers, too, and these numbers go all over the map. Use the above procedure but the numbers in your text or your notes and you should get close to -1100 kJ. Check my thinking. Check my work. I suggest you draw out the structures of C2H4OH, O2, CO2, and H2O and count the bonds.

That should be C2H5OH in the last line.

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