Calculation Delta H
Date: Around 1999
How do we calculate delta H for the reaction C2H5Cl(g) plus
Cl2(g) yields to C2H4Cl2(g) plus HCl(g)
Dear Mzkittyb2001 (interesting name!),
delta H for any reaction is equal to the chemically balanced
sum of the heats of formation of products minus reactants.
(1) Balance the chemical reaction. For H2O and graphite reacting to form
CO vapor and H2 gas, we'd have H2O + C -> CO + H2.
(2) Look up the heats of formation, remembering that the heat of
formation of a pure element in its standard state is zero. The reaction
above happens to involve 2 elements (C and H2) so we need only look
up numbers for H2O and CO. Note that H2O vapor and H2O liquid have
different heats of formation, so the state of matter is important.
(3) delta H = Hf (CO) + Hf (H2) - Hf (H2O) - Hf(C) = Hf (CO) - Hf (H2O)
If you have other stoichiometric coefficients you have to use them
consistently with the balanced chemical reaction. In your case
C2H5Cl + Cl2 --> C2H4Cl2 + HCl is already balanced, so you don't
have to worry about that. So just look up data for C2H5Cl, C2H4Cl2,
Dept of Chemistry
The Cooper Union
New York, NY
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Update: June 2012