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Lect+14+08

# Lect+14+08 - Lecture 14 First law processes Heat capacities...

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Lecture 14  First law processes Heat capacities Phase changes  Chemical reactions Ideal Gas First law processes Ideal gas Reversible-Isothermal   processes

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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?

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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

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Enthalpies of Reaction Since enthalpy is a state function any closed cycle will sum to zero.
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• Spring '08
• SHARP

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