ch302 notes ch 16 and 10

ch302 notes ch 16 and 10 - CH302: Chapter 10 Review. and...

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CH302: Chapter 10 Review. . and More. . The Second Law of Thermodynamics In spontaneous changes the universe tends towards a state of greater entropy .” Spontaneous processes occur by themselves. Spontaneity = ‘feasable’ i.e, has nothing to do with speed . What is Entropy? Entropy (S) is disorder, but on a molecular level . thermal disorder: add heat energy: molecules can occupy a more varied combination of translational, vibrational and rotational energy levels. positional disorder: expansion or mixing molecules can occupy more physical locations. In general, S solid <S liquid <S gas with solutions somewhere between liquid and gas. Can use this to qualitatively analyze a chemical reaction or physical process to predict the sign of Δ S. Expansion Work against CHANGING External Pressure FOR A REVERSIBLE, ISOTHERMAL expansion: Gradually reduce P ext from P initial to P final but keep system at constant T (e.g., water bath) since P = (nRT) / V: Substitute into w = - P ex Δ V d w = - (nRT/V) dV Integrate over all values of V (V 1 to V 2 ) : w = - nRT ln (V 2 / V 1 ) REVERSIBLE vs. IRREVERSIBLE Reversible : w = - nRT ln(V 2 /V 1 ) Irreversible: w = - P ext Δ V Free expansion: w = 0 Work done by the system is LARGER for a REVERSIBLE process!! BUT in reality, THERE are NO truly reversible processes!! Example An ideal gas is allowed to undergo a reversible, isothermal; expansion. It begins at STP and occupies a volume of 15.5L. It ends up at 26.0L. Find the work done. FOR A REVERSIBLE, ISOTHERMAL expansion: For any isothermal process, q = - w, so: q = nRT ln(V 2 /V 1 ) Quantitative definition of Entropy From statistical arguments, when heat is transferred reversibly * and isothermally Δ S = q rev / T Units of S are J/K 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
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Entropy Change in Ideal Gas due to Isothermal Volume Change Δ S = nR ln (V 2 /V 1 ) _or_ Δ S = nR ln (P 1 /P 2 ) R is 8.314 J/mol.K (Boyle’s Law links these two version together) Example: Allow 30g of steam at 135 °C to expand isothermally from 30L to 40L. What is Δ S?
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ch302 notes ch 16 and 10 - CH302: Chapter 10 Review. and...

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