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Unformatted text preview: H: H rxn = H f (products) - H f (reactants) where H f (elements in normal room temperature form) = 0 Predicting the sign of H: Some things that H cant explain ! NH 4 NO 3 (s) NH 4 + (aq) + NO 3-(aq), H rxn = NaCl (s) Na + (aq) + Cl-(aq), H rxn = B. Entropy: S Calculating S: S rxn = S (products) - S (reactants) Predicting the sign of S: S(g) > S(l) S(aq) > S(s) 2 C(s) + O 2 (g) 2 CO(g), S = + 179 J/mol K C(s) + O 2 (g) CO 2 (g), S = 3 J/mol K 2 CO(g) + O 2 (g) 2 CO 2 (g), S = - 173 J/mol K 2 Na(s) + Cl 2 (g) 2 NaCl(s), S = - 129 J/mol K A reaction tends to be spontaneous if H = - and/or S = +. At low T , H is more important. At high T , S is more important....
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