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Thermochemistry Chapter 17:4 Hesss Law and Standard Enthalpies of Formation Pages 527 – 532.

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Presentation on theme: "Thermochemistry Chapter 17:4 Hesss Law and Standard Enthalpies of Formation Pages 527 – 532."— Presentation transcript:

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2 Thermochemistry Chapter 17:4 Hesss Law and Standard Enthalpies of Formation Pages 527 – 532

3 A. Hesss Law Hesss 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 Hesss law 2S(s) + 3O 2 (g) 2SO 3 (g) H = ?

4 A. Hesss Law

5 Known: N 2 (g) + O 2 (g) 2NO(g) H = -180 kJ 2NO(g) + O 2 (g) 2NO 2 (g) H = -112 kJ N 2 (g) + 2O 2 (g) 2NO 2 (g) H = ? Build your equation: -292 kJ

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

7 A. Hesss Law Known: 2H 2 (g) + O 2 (g) 2H 2 O(l) H = -572 kJ H 2 (g) + O 2 (g) H 2 O 2 (l) H = -188 kJ Build your equation: 2H 2 O 2 (l) 2H 2 O(l) + O 2 (g) H = ? -196 kJ

8 A. Hesss Law Known: 2B (s) + 3/2O 2 (g) B 2 O 3 (s) H = kJ B 2 H 6 (g) + 3O 2 (g) B 2 O 3 (s) + 3H 2 O (g) H = kJ H 2 (g) + ½ O 2 (g) H 2 O (l) H = -286 kJ H 2 O (l) H 2 O (g) H = 44 kJ 2B(s) + 3H 2 (g) B 2 H 6 (g) H = ? Build your equation: 36 kJ

9 B. Standard Heats of Formation Standard heat of formation, Δ H f, 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 25 o C Δ H f of free elements (and diatomic molecules) in standard state is set at 0

10 B. Standard Heats of Formation Provides an alternative to Hesss Law of calculating heat of reaction indirectly For reactions occurring at standard conditions, calculate heat of reaction, Δ H o, using standard heats of formation (given to you on appendix copies) Δ H o = Δ H f (products) – Δ H f (reactants)

11 Write the thermochemical equation for the formation of sodium oxide. Na(s) + O 2 (g) Na 2 O(s) B. Standard Heats of Formation 4 2 ΔH= 2 ½ -416 kJ

12 B. Standard Heats of Formation What is the standard heat of reaction of CO(g) with O 2 (g) to form CO 2 (g)? 2CO(g) + O 2 (g) 2CO 2 (g) ΔH = ? 2( ΔH f CO2 ) – (2( ΔH f CO ) + ΔH f O2 ) = 2(-393.5) – (2(-110.5) + 0) ΔH = kJ/mol

13 B. Standard Heats of Formation CaCO 3 (s) CaO(s) + CO 2 (g) ΔH = ? ( ΔH f CaO + ΔH fC O2 ) – ( ΔH f CaCO3 ) = ( ) – (-1207) ΔH = 179 kJ/mol

14 B. Standard Heats of Formation What is the standard heat of reaction for the combustion of methane? CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) ΔH = ? (2( ΔH f H2O ) + ΔH fC O2 ) – ( ΔH f CH4 + 2( ΔH f O2 )) = (2(-242) ) – ( (0)) ΔH = -803 kJ/mol


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