Chem_208_Lecture_5_color - Hesss Law Procedure: - If a...

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Hess’s Law Procedure: - If a reaction is reversed, change the sign of Δ H o . - If a reaction is multiplied by a const., multiply Δ H o by the const. - Handle a chemical equation like a math equation; it remains valid if the same quantities are added or subtracted from both sides. What is the standard enthalpy change for producing ethane from the elements? 2C (s) + 3H 2 (g) C 2 H 6 (g) Δ H o = ? Carry out the following combustion reactions: (1) C 2 H 6 (g) + 7 / 2 O 2 (g) 2CO 2 (g) + 3H 2 O(l) Δ H o 1 = -1559.7 kJ (2) C(s) + O 2 (g) CO 2 (g) Δ H o 2 = -393.5 kJ (3) H 2 (g) + ½ O 2 (g) H 2 O (l) Δ H o 3 = -285.8 kJ
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Calorimetry measures enthalpy changes , not absolute enthalpies. Absolute values require a reference point, or zero of enthalpy . Elements: Set the standard enthalpy of formation, Δ H o f = 0 Δ h=1816’ h=243’ above sea level h=2059’ above sea level CN Tower, Toronto
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Standard enthalpies of formation, Δ H o f 0 200 100 -100 -200 -300 Δ H o f (kJ/mol) C 2 H 2 (g) + 5 / 2 O 2 (g) 2CO 2 (g)+ H 2 O (g) Δ H 0 < 0
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Calculations Using Standard Enthalpies of Formation, Δ H o f
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This note was uploaded on 03/29/2009 for the course CHEM 2080 taught by Professor Davis,f during the Spring '07 term at Cornell University (Engineering School).

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Chem_208_Lecture_5_color - Hesss Law Procedure: - If a...

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