c161l16_10_13

# c161l16_10_13 - Hess's Law, the 1st Law again Most of what...

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Hess Hess s Law, the 1 s Law, the 1 st st Law again Law again W Most of what we need to know about enthalpy is a consequence of 2 properties It’s extensive It’s a state function W Let’s see what that means Extensive A B B Δ H=a H=a 2A 2A 2B 2B Δ H=2a H=2a State function AB C Δ H 1 Δ H 2 Δ H 3 = Δ H 1 + Δ H 2

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Other statements of Hess Other statements of Hess s s Law Law W The enthalpy of a reaction is the same whether the reaction occurs in one step or 100 steps. W The enthalpy of a reaction is equal to the sum of the enthalpies of the steps into which a reaction can be divided W The sum of enthalpies around any cycle is exactly zero. W These are all statements of the first law: conservation of energy
Example Example W A sample of 23.0g of CH 3 CH 2 OH required 22 kJ to vaporize. What is its enthalpy of vaporization? molsCH 3 CH 2 OH = 23.0 g×1mol/46.07g = 0.499 mol Δ H vap = 22kJ/ 0.499 mol = 44.1 kJ/mol

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Example Example W Given the following thermodynamic data, find
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## c161l16_10_13 - Hess's Law, the 1st Law again Most of what...

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