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If - endothermic reaction Standard Molar Enthalpies of...

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If H rxn < 0 ( negative ) the reaction is exothermic . If H rxn > 0 ( positive ) the reaction is endothermic . Enthalpy Change H rxn = H products - H reactants H rxn = H substances produced - H substances consumed Notice that this is H rxn = H final – H initial Thermochemical Equations Thermochemical equations are a balanced chemical reaction plus the DH value for the reaction. 1 mol of C 5 H 12 reacts with 8 mol of O 2 to produce 5 mol of CO 2 , 6 mol of H 2 O, and releasing 3523 kJ is referred to as one mole of reactions . Thermochemical Equations This is an equivalent method of writing thermochemical equations. H < 0 ( negative ) designates an exothermic reaction. H > 0 ( positive ) designates an
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Unformatted text preview: endothermic reaction. Standard Molar Enthalpies of Formation, ∆ H f o Example 15-6: Calculate the enthalpy change for the reaction in which 15.0 g of aluminum reacts with oxygen to form Al 2 O 3 at 25 o C and one atmosphere. 4 Al (s) + 3 O 2(g) → 2 Al 2 O 3(s) from Appendix K DH f o for Al 2 O 3 = - 1676 kJ/mol Hess’s Law Hess’s Law of Heat Summation states that the enthalpy change for a reaction is the same whether it occurs by one step or by any (hypothetical) series of steps. Hess’s Law is true because ∆ H is a state function. Hess’s Law If we know the following ∆ H o ’s...
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