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**Thermochemistry Chapter 17:4 Hess’s Law and**

Standard Enthalpies of Formation Pages 527 – 532

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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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**A. Hess’s Law -292 kJ N2(g) + 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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**If you reverse the reaction, then reverse the sign for ΔH**

A. Hess’s Law If you need to multiply your coefficients, then multiply the ΔH as well 2S(s) + 3O2(g) 2SO3(g) H = ? -792 kJ Known: S(s) + O2(g) SO2(g) H = -297 kJ 2SO3(g) 2SO2(g) + O2(g) H = 198 kJ If you reverse the reaction, then reverse the sign for ΔH Build your equation:

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**A. Hess’s Law 2H2O2(l) 2H2O(l) + O2(g) H = ? -196 kJ**

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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**A. Hess’s Law 36 kJ 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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**B. Standard Heats of Formation**

Standard heat of formation, ΔHf, of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements All substances at standard pressure and 25oC ΔHf of free elements (and diatomic molecules) in standard state is set at 0

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**B. Standard Heats of Formation**

Provides an alternative to Hess’s Law of calculating heat of reaction indirectly For reactions occurring at standard conditions, calculate heat of reaction, ΔHo , using standard heats of formation (given to you on appendix copies) ΔHo = ΔHf (products) – ΔHf (reactants)

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**B. Standard Heats of Formation**

Write the thermochemical equation for the formation of sodium oxide. Na(s) + O2(g) Na2O(s) 4 2 2 ΔH= -416 kJ

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**B. Standard Heats of Formation**

What is the standard heat of reaction of CO(g) with O2(g) to form CO2 (g)? 2CO(g) + O2(g) → 2CO2(g) ΔH = ? 2(ΔHfCO2) – (2(ΔHfCO) + ΔHfO2) = 2(-393.5) – (2(-110.5) + 0) ΔH = kJ/mol

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**B. Standard Heats of Formation**

CaCO3(s) → CaO(s) + CO2 (g) ΔH = ? (ΔHfCaO + ΔHfCO2) – (ΔHfCaCO3) = ( ) – (-1207) ΔH =179 kJ/mol

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**B. Standard Heats of Formation**

What is the standard heat of reaction for the combustion of methane? CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔH = ? (2(ΔHfH2O) + ΔHfCO2) – (ΔHfCH4 + 2(ΔHfO2)) = (2(-242) ) – ( (0)) ΔH =-803 kJ/mol

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It has been suggested that hydrogen gas obtained by the decomposition of water might be a substitute for natural gas (principally methane). To compare.

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