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Name: Mzkittyb2001
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Around 1999 


Question:
How do we calculate delta H for the reaction C2H5Cl(g) plus Cl2(g) yields to C2H4Cl2(g) plus HCl(g)


Replies:
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.

So,

(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, and HCl.

Best Regards,

Prof. Topper
Dept of Chemistry
The Cooper Union
New York, NY



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