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Unformatted text preview: Lecture 14  First law processes Heat capacities Phase changes Chemical reactions Ideal Gas First law processes Ideal gas ReversibleIsothermal processes Heat Capacity of Ideal Gases • The heat capacity of an ideal gas is 1/2 R per degree of freedom (in the classical limit). The classical limit is approached as the spacing between energy levels is small with respect to the thermal energy available • At constant volume an ideal monatomic gas has 3 degrees of freedom (x,y, and z) so Cv = 3/2R • At constant pressure an ideal monatomic gas has 5 degrees of freedom (x,y, z, P, and V) so Cp = 5/2R • Cp = Cv + 2/2 R for any ideal gas • At constant volume an ideal diatomic gas has 5 degrees of freedom if the rotations are closely spaced relative to the temperature (x,y, and z and two rotational degrees of freedom) so Cv = 5/2R • Heat Capacity increases with increasing number of degrees of freedom available to store energy (1/2 R per degree of freedom) • R = 8.31 J/mol K • For Ideal gas Cp = 5/2 R/mol =20.8 J/mol x, y, z, P, V. 1 mol of O 2 weighs 32 grams • Cp would be 0.65 J/g for an ideal model with 3 degrees of freedom. That is about .22 J/g for each degree of freedom • But O 2 (g) at 25 ° C has a heat capactity Cp of 0.917 J/g how many degrees of freedom? • Three? Four? Five? Thermochemistry • During chemical reactions  energy is converted from electronic energy into heat, work … • For exothermic reactions the products generally have stronger bonds than the reactants so the electrons involved in bonding are more tightly bound after the reaction • Since chemical reactions are often studied at constant pressure enthalpies are often used to keep track of energies Enthalpies of Reaction • Exothermic  gives off heat • Endothermic  absorbs heat • LINEAR SCALING APPLIES FOR ENTHALPIES • Reversing a reaction reverses the sign of the enthalpy change, • Multiplying the amount reacted by a factor of two increases the heat by a factor of two Enthalpies of Reaction • Since enthalpy is a state function any closed cycle will sum to zero....
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 Winter '08
 SHARP
 Thermodynamics, pH, Reaction

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