Presentation on theme: "Thermochemistry Chapter 17:4 Hess’s Law and"— Presentation transcript:
1 Thermochemistry Chapter 17:4 Hess’s Law and Standard Enthalpies of FormationPages 527 – 532
2 A. Hess’s LawHess’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 reaction2S(s) + 3O2(g) 2SO3(g) H = ?
5 If you reverse the reaction, then reverse the sign for ΔH A. Hess’s LawIf you need to multiply your coefficients, then multiply the ΔH as well2S(s) + 3O2(g) 2SO3(g) H = ?-792 kJKnown:S(s) + O2(g) SO2(g) H = -297 kJ2SO3(g) 2SO2(g) + O2(g) H = 198 kJIf you reverse the reaction, then reverse the sign for ΔHBuild your equation:
7 A. Hess’s Law 36 kJ 2B(s) + 3H2(g) B2H6(g) H = ? Known: 2B (s) + 3/2O2(g) B2O3(s) H = kJB2H6(g) + 3O2(g) B2O3 (s) + 3H2O (g) H = kJH2 (g) + ½ O2(g) H2O (l) H = -286 kJH2O (l) H2O (g) H = 44 kJBuild your equation:
8 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 elementsAll substances at standard pressure and 25oCΔHf of free elements (and diatomic molecules) in standard state is set at 0
9 B. Standard Heats of Formation Provides an alternative to Hess’s Law of calculating heat of reaction indirectlyFor 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)
10 B. Standard Heats of Formation Write the thermochemical equation for the formation of sodium oxide.Na(s) + O2(g) Na2O(s)422ΔH=-416 kJ
11 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
12 B. Standard Heats of Formation CaCO3(s) → CaO(s) + CO2 (g) ΔH = ?(ΔHfCaO + ΔHfCO2) – (ΔHfCaCO3) =( ) – (-1207)ΔH =179 kJ/mol
13 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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