CH 301 Chapter 7 notes

CH 301 Chapter 7 notes - CH301 Chapter 7: A crash course in...

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CH301 Chapter 7: A crash course in entropy, free energy and the 2 nd and 3 rd laws of TM: The Second Law of Thermodynamics In spontaneous changes the universe tends towards a state of greater entropy .” A mirror shatters when dropped and does not reform. It is easy to scramble an egg and difficult to unscramble it. Food dye when dropped into water disperses Spontaneous processes are those which occur by themselves. Think of a spontaneous process as being ‘feasable’ . Spontaneity (unlike our usual interpretation) has nothing to do with speed. What is Entropy? Entropy is disorder, but on a molecular level : A jumbled sock drawer is a metaphor for disorder; it is not really in a higher state of entropy than a tidy one. The translational, vibrational and rotational energy levels possessed by the molecules themselves would be similar in both cases. Entropy consists of thermal and positional disorder: Heating a sample causes its molecules to have more thermal disorder ; they can occupy a more varied combination of translational, vibrational and rotational energy levels. Allowing a sample to expand into a greater volume or mix with another material will increase its positional disorder ; the molecules have more physical locations they can occupy. Quantitative definition of Entropy Entropy has the symbol S. Its a state function, so Δ S is path independent. For any process where heat, q, is transferred reversibly* at constant temperature: Δ S = q rev / T Units of S are J/K (for a specific amount) or, J/mol.K If Δ S > 0 disorder increases If Δ S < 0 disorder decreases *ie done slowly: surroundings stay at almost the same temperature Entropy Changes in Changes of State Two possibilities: melting (fusion) or boiling (vaporization) Use Δ S = q rev / T and note q = Δ H fus or Δ H vap . T will be either the melting point or boiling point (in K): Δ S fus = Δ H fus / T m.p. Δ S vap = Δ H vap / T b.p.
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This note was uploaded on 09/02/2009 for the course CHE 301 taught by Professor Fatimafakhreddine during the Spring '08 term at University of Texas.

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CH 301 Chapter 7 notes - CH301 Chapter 7: A crash course in...

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