05 Fuels - Application: Use Hess's Law to determine the...

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1 1 Application: Use Hess’s Law to determine the enthalpy change for reaction 3 from the enthalpy changes for reactions 1 and 2. R1) N 2 + 2O 2 2NO 2 H 1 = 68 kJ R2) NO + ½O 2 NO 2 H 2 = -56 kJ R3) ½N 2 + ½O 2 NO H 3 = ? A) 90 kJ B) – 90 kJ C) 12 kJ D) 124 kJ E) – 124 kJ 2 How can R1 & R2 be combined to produce R3? If we just add R1 & R2, we don’t get the right product. To get NO as a product we need to reverse R2. R4) NO 2 NO + ½O 2 H 4 = 56 kJ If we add R1 and R4, we have N 2 as a reactant, but we need ½N 2 . So multiply R1 by ½. R5) ½N 2 + O 2 NO 2 H 5 = 34 kJ R1) N 2 + 2O 2 2NO 2 H 1 = 68 kJ R2) NO + ½O 2 NO 2 H 2 = -56 kJ R3) ½N 2 + ½O 2 NO H 3 = ?
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2 3 R4) NO 2 NO + ½O 2 H 4 = 56 kJ R5) ½N 2 + O 2 NO 2 H 5 = 34 kJ Now add R4 and R5 to get R6, R6) NO 2 + ½N 2 + O 2 NO + ½ O 2 + NO 2 NO 2 appears both as a reactant and product so it is removed, as is ½O 2 from each side, and we have R3. R3) ½ N 2 + ½O 2 NO R3 = R4 + R5 = -R2 + ½R1 H 6 = H 4 + H 5 = - H 2 H 1 = 90 kJ R1) N 2 + 2O 2 2NO 2 H 1 = 68 kJ R2) NO + ½O 2 NO 2 H 2 = -56 kJ R3) ½N 2 + ½O 2 NO H 3 = ? 4 A diagram illustrates that Hess’s Law works because enthalpy is a state function. NO 2 NO + ½ O 2 ½ N 2 + O 2 90 kJ 56 kJ 34 kJ R4) NO 2 NO + ½ O 2 H 1 = 56 kJ R5) ½ N 2 + O 2 NO 2 H 2 = 34 kJ R3) ½N 2 + ½O 2 NO H 3 = 90 kJ Two paths for the formation of NO, the enthalpy change is independent of the path. R5 R4 R3 with an extra ½O 2 The extra ½O 2 has no effect on H because it is present both as a reactant and as a product.
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3 5 An Everyman Perspective H = E + P V You eat and take in energy, that is H You store energy as fat, that is E You exercise or work, that is P V Conservation of energy requires that H = E + P V Actually your exercise/work is not PV work. This analogy is not so good when H, E, and/or V are negative. 6 Standard Molar Enthalpies of Formation, H f o H f o is the enthalpy change for the formation of one mole of a compound from its elements in their standard states.
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This note was uploaded on 06/29/2008 for the course CHE 132 taught by Professor Hanson during the Spring '08 term at SUNY Stony Brook.

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05 Fuels - Application: Use Hess's Law to determine the...

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