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Oswald Hills

Feb 20, 2025

Hess’ Law Question, Calculating Enthalpy Change?

So the question is:Calculate the standard enthalpy change for the following reaction:2HCl (g) + F2 (g) --> 2HF (l) + Cl2 (g)Given:4HCl (g) + O2 (g) --> 2H2O (l) + 2Cl2 (g) , H = -202.4 kJ/mol rxn(1/2)H2 (g) + (1/2)F2 (g) --> HF (l) , H = -600 kJ/mol rxnH2 (g) + (1/2)O2 --> H2O (l) = -285.8 kJ/mol rxnSo I multiplied the top given by coefficient (1/2) and the middle given by coefficient 2.That gives:2HCl (g) + (1/2)O2 (g) --> H2O (l) + Cl2 (g) , H = -101.2 kJ/mol rxnH2 (g) + F2 (g) --> 2HF (l) , H = -1200 kJ/mol rxnH2 (g) + (1/2)O2 (g) --> H2O (l) , H = -285.8 kJ/mol rxnThat gives me an answer of -1587 kJ/mol reaction, but it's wrong.Can someone point out my mistake and please help me correct itThanks!
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3 Answers

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you end up subtracting the last line of reactions because if you add that, your result ends up having H2, O2, and H2O so your answer should be -1015.4 kJ/mol... Show More
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I thing in equation 3 you need to divide the answer to 1/2?... Show More
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I don't see your F2(g) canceling out, maybe thats why? Idk I only got a 75% on this unit last week so I could be wrong... Show More
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