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**15.3 Thermochemical Equations**

Pages 529 – 533

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**A. Thermochemical Equations**

A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products, and energy change In a thermochemical equation, the enthalpy of change for the reaction can be written as either a reactant or a product

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**A. Thermochemical Equations**

How to write thermochemical equations: Endothermic (positive ΔH) 2NaHCO3(s)+ 129kJ Na2CO3(s) + H2O(g) + CO2(g) Exothermic (negative ΔH) CaO(s) + H2O(l) Ca(OH)2(s) kJ

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**B. Thermochemical Equation Practice**

Write the thermochemical equation for the addition of oxygen gas to solid iron (III) if the ΔHrxn= kJ Fe(s) O2(g)→ Fe2O3(s) kJ Write the thermochemical equation for the decomposition of sulfur trioxide gas into sulfur dioxide gas and oxygen gas if the ΔHrxn= 198 kJ SO3(g) kJ → SO2 (g) + O2 (g) Exothermic 4 3 2 Endothermic 2 2

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15.4 Hess’s Law Pages 534 – 540

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A. Hess’s Law Hess’s law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction 2S(s) + 3O2(g) 2SO3(g) H = ?

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A. Hess’s Law

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**B. Hess’s Law Practice N2(s) + 2O2(g) 2NO2(g) H = ? Known:**

N2 (g) + O2(g) 2NO(g) H = -180 kJ 2NO(g) + O2(g) 2NO2(g) H = -112 kJ Build your equation:

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**B. Hess’s Law Practice 2H2O2(l) 2H2O(l) + O2(g) H = ?**

Known: 2H2(g) + O2(g) 2H2O(l) H = -572 kJ H2(g) + O2(g) H2O2(l) H = -188 kJ Build your equation:

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**B. Hess’s Law Practice 2S(s) + 3O2(g) 2SO3(g) H = ? Known:**

S(s) + O2(g) SO2(g) H = -297 kJ 2SO3(g) 2SO2(g) + O2(g) H = 198 kJ Build your equation:

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**B. Hess’s Law Practice 2B(s) + 3H2(g) B2H6(g) H = ?**

Known: 2B (s) + 3/2O2(g) B2O3(s) H = kJ B2H6(g) + 3O2(g) B2O3 (s) + 3H2O (g) H = kJ H2 (g) + ½ O2(g) H2O (l) H = -286 kJ H2O (l) H2O (g) H = 44 kJ Build your equation:

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