Week 2-2 Lecture Notes.docx - Chem 125 Lecture Notes Week 2-2 Learning Objectives Explain how entropy and enthalpy changes affect the magnitude and sign

Week 2-2 Lecture Notes.docx - Chem 125 Lecture Notes Week...

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Chem 125 Lecture Notes Week 2-2 Learning Objectives: Explain how entropy and enthalpy changes affect the magnitude and sign of the change in Gibbs Free Energy for chemical reactions and other chemical processes We learned thermochemistry, learned a lot about Enthalpy (which is correlated to the heat of reaction). Thermodynamics Thermodynamics, the study of relationships between the energy and work associated with chemical and physical processes, provides predictive ability (example: predicting if a reaction happens by itself). 12.1 Spontaneity A spontaneous process is one that occurs naturally under certain conditions. A nonspontaneous process will not take place unless it is “driven” by the continual input of energy from an external source. At room temperature: Ice melting => spontaneous At room temperature: Water freezing (reverse reaction of melting!)=> nonspontaneous PQ1: Which, if any, of the following processes is spontaneous under the specified conditions? a) H 2 O( l ) H 2 O( s ) at 25°C b) CO 2 ( s ) CO 2 ( g ) at 0°C c) 2H 2 O( g ) 2H 2 ( g ) + O 2 ( g ) d) C(graphite) C(diamond) at 25°C and 1 atm pressure 12.2 Entropy Entropy is understood as a measure of disorder within a macroscopic system. It’s said that universe likes to be more disordered! But why? It’s all statistics! A microstate ( W ) is a specific configuration of the locations and energies of the atoms or molecules that comprise a system. S = k ln W Here k is the Boltzmann constant and has a value of 1.38 × 10 −23 J/K. 1
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Chem 125 Lecture Notes Week 2-2 As for other state functions , the change in entropy for a process is the difference between its final ( S f ) and initial ( S i ) values. Δ S = S f S i = k ln( W f )− k ln( W i )= k ln( W f / W i ) OR PQ2: Elemental boron can be formed by reaction of boron trichloride with hydrogen. If  S ° = 80.3 J/K for the reaction above , what is S ° for BCl 3 ( g )? BCl 3 ( g ) + 1.5H 2 ( g ) B( s ) + 3HCl( g ) a) –18.2 J/K.mol b) 18.2 J/K.mol c) 290.1 J/K.mol d) 355.4 J/K.mol Predicting the Sign of Δ S The entropy of a substance is influenced by structure of the particles (atoms or molecules) that comprise the substance.
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