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c161l18_10_14b

# c161l18_10_14b - Hesss Law the 1st Law again Hess Most of...

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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 Given the following thermodynamic data, find the enthalpy of the reaction 2CH 4 (g) +O 2 (g) CH 3 OH(l) CH 4 (g) + H 2 O(g) CO(g) + 3H 2 (g) Δ H°=+206kJ 2H 2 (g) + CO(g) CH 3 OH(l) Δ H°=-128.33kJ 2H 2 (g) + O 2 (g) 2H 2 O(l) Δ H°=-483.64kJ

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Solution Solution W Step 1:
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c161l18_10_14b - Hesss Law the 1st Law again Hess Most of...

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