L21ThermoII145s09 - Lecture 21 Equilibrium Spontaneous...

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Lecture 21, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Reading assignment : OGC §12.5, 13.1, 13.5-13.7 Learning objectives: Understand the meaning of standard states Understand the concept of spontaneity of a process Understand the meaning of entropy (S) Understand the meaning of Gibbs free energy (G) Be able to compute ∆H, ∆S, and ∆G On heating or cooling a phase During phase transformations For reactions from handbook data Use Gibbs free energy to predict nature of reactions: Spontaneous Reversible Non-spontaneous Lecture 21: Equilibrium & Spontaneous Change 1
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Lecture 21, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Standard-State Enthalpy H ° T [OGC §12.5] Absolute values of H (and U ) cannot be measured or computed Only ∆H (and ∆U) are measurable Must define a zero point for the enthalpy scale (analogy: sea level serves as zero point for elevation) Convention: Elements in standard states at 298.15 K have H ° 298.15 = 0 What is meant by “standard state”? For solids and liquids: the thermodynamically stable form at 1 atm pressure For gases: gas at 1 atm pressure Standard enthalpy of formation of a compound, ∆H° f : enthalpy change on forming a compound from its elements in their stable states at 25 °C and 10 5 Pa (~1 atm) 2
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H rxn = H prods - H reacts = H f , CO 2 o - H f , CO o + 1 2 H f , O 2 o = - 393.51 - - 110.52 + 0 ( 29 = - 283.0 kJ Ê mol - 1 Example: carbon monoxide and oxygen reacting in a pressure vessel Reaction gives off energy — exothermic per mole of CO: Reverse reaction consumes energy — endothermic per mole of CO 2 : Lecture 21, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University ( 29 ( 29 ( 29 2 2 1 2 CO g O g CO g + H = q p = - 282.99 kJ (
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