Ch20 - CHAPTER 20 Thermodynamics Thermodynamics...

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Unformatted text preview: CHAPTER 20 Thermodynamics Thermodynamics Thermodynamics Thermodynamics Why do we want to know this material ? Is it because CM 103 is a requirement for our major ? OF COURSE NOT !! ITS BECAUSE. Energy is part of all chemical change, and thermodynamics (TD)-- the study of energy transformations--is central to chemistry TD explains why rxns occur at all. TD lets us predict heat released or required by chemical reactions Heat output allows us to determine if we can use compounds as fuels and foods, and the first law of thermodynamics allows us to discuss these topics in a systematic way SPONTANEITY of Physical & Chemical Changes Spontaneous & Nonspontaneous Changes Spontaneous & Nonspontaneous Changes Spontaneous Change - a natural change. Given enough time to occur , one that occurs by itself without outside assistance or after given initial outside assistance. E.g.s : (i) cooling of a hot piece of metal to temp. of its surroundings (ii) water flowing downhill (iii) burning (of a match) (iii) expansion of a gas into a vacuum (iv) chemical rxn moving toward equilibrium can be fast ( Na reacting with air ) or slow ( diamonds turning into graphite, aging ) Spontaneous & Nonspontaneous Changes Spontaneous & Nonspontaneous Changes Nonspontaneous Change (the reverse process) - an unnatural change. It CANNOT occur by itself. work must be done (on it) for it to occur . Some e.g.s : (i) heating up a piece of metal (ii) water moving uphill (iii) compressing a gas NOTE NOTE : A thermodynamic change ONLY means the LIKELINESS or TENDENCY of a process to occur. Whether the process actually occurs depends on its rate (i.e., its kinetics) Spontaneous Spontaneous (cooling of (cooling of a hot metal block) a hot metal block) & & Nonspontaneous Nonspontaneous (block (block does not heat up on its own) does not heat up on its own) Changes Changes Spontaneous Spontaneous (gas expands into both containers) (gas expands into both containers) & & Nonspontaneous Nonspontaneous (gas does not contract to upper (gas does not contract to upper container on its own) container on its own) Changes Changes Entropy, S Entropy, S Entropy, S a multifaceted concept; no simple definition OLD DEFINITION : a measure of the amount of disorder in a system CURRENT DEFINITION : the natural tendency of a systems energy to become more dispersed as it is distributed among various motions of molecules in the system. {Microstates (quantized energy states) = # of ways energy can be dispersed} Low S = ( little disorder ) low energy dispersal (few microstates) High S = ( great disorder ) high energy dispersal (high microstates) Entropy & the 2 Entropy & the 2 nd nd & 3 & 3 rd rd Laws of Laws of Thermodynamics Thermodynamics 2 2 nd nd Law : Law : Entropy of the universe remains constant in a reversible process, whereas it increases in an irreversible (spontaneous) process ....
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Ch20 - CHAPTER 20 Thermodynamics Thermodynamics...

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