09 - Chapter 9 9/3/09 IF the previous reaction were carried...

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Chapter 9 9/3/09 IF the previous reaction were carried out at Pconstant, the surroundings would have done work on the system -11 kJ = ∆nRT = P∆V Only very few reactions are suitable for calorimetry Ex: What is the enthalpy change when diamond is converted to graphite? C diamond C graphite ∆H Hess’s Law of Heat Summation: the change in Enthalpy for any chemical reaction is constant, whether that reaction occurs in one step or several. We need to find a set of appropriate reactions where ∆H is known, and then sum these in a manner that gives ∆H for the reaction The bottom line: Thermodynamic data can be treated algebraically C graphite + O 2 CO 2 (g) ∆H = -393.0 kJ/mol C diamond + O 2 CO 2 ( g) ∆H = -395.3 kJ/mol Endothermic A + B + heat C Exothermic A + B C + heat Tabulated Data: a means to compute ∆H; These values that are tabulated are referred to as Standard Enthalpies of Formation (∆H° f ) ∆H° f = change in enthalpy for the reaction in which one mole of a compound in its
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09 - Chapter 9 9/3/09 IF the previous reaction were carried...

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