Physical Chemistry for the Chemical and Biological Sciences

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Outline of the Course 1) Review and Definitions 2) Molecules and their Energies 3) 1 st Law of Thermodynamics 4) 2 nd Law of Thermodynamics 5) Gibbs Free Energy 6) Phase Diagrams and REAL Phenomena 7) Non-Electrolyte Solutions 8) Chemical Equilibrium 9) Chemical Kinetics and Rates of Processes
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Section 3.0. The 1 st Law of Thermodynamics (CHANG text – Chapter 4 – almost ALL ! ) 3.1. Revisiting Heat Capacities 3.2. Definitions and Concepts 3.3. The First Law of THERMODYNAMICS 3.4. Enthalpy 3.5. Adiabatic Expansion of an Ideal Gas 3.6. Thermochemistry 3.7. Bond Energies and Bond Enthalpies
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3.1. A Closer Look at Heat Capacities Goals: 1. To review definition of heat capacity and molar heat capacity 2. To compare and understand difference between Cv and Cp 3. To review difference between C for gases and substances in the condensed states (i.e. liquids and solids)
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3.1. A Closer Look at 3.1. A Closer Look at Heat Capacities Heat Capacities ……. .when heat is added to a substance its temperature will rise …… How much the temperature will rise depends on: (1) the amount of heat delivered (2) the amount of substance present (3) the chemical nature and physical state of the substance (4) conditions under which heat is added to substance
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Heat Capacity (C) C = U/ T = q/ T measured in J/K where U is the amount of energy required to raise the T of the substance by T or q is heat required to raise the T of substance by T Molar Heat Capacity ( C ) = heat capacity of one mole of a substance C = C/n = q /(n T) measured in J mol -1 K -1 • heat capacity is a quantity that can be measured directly • can be measured at constant volume or constant pressure
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heat capacity can be measured at constant P or constant V At constant volume : C v = U/ T = q v / T • at constant V the heat absorbed by the system is equal to the change in internal energy At constant pressure : Cp = H/ T = q p / T • at constant P the heat absorbed is equal to the change in enthalpy ( H) **** Remember that C = C/n = q /(n T) so we can rearrange to calculate values for U and H for processes at constant V and P, respectively. U = Cv T = n Cv T H = Cp T = n Cp T
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U = Cv T = n Cv T H = Cp T = n Cp T for these equations we have assumed that Cv and Cp are independent of temperature ………………only true at T 300K • at T less than 300K only translational and rotational motions make major contribution to heat capacity as we T vibrational motions makes contribution and C • Cp = a + BT ……where a and B are constants for a given substance (equation helps to predict temperature dependence of C for a substance)
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In general Cv and Cp for a substance are not equal ……. . Why Why ? ? because in a constant pressure process volume is changing (expansion !) so work must be done on the surroundings…….
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3a - Outline of the Course 1) Review and Definitions 2)...

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