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

Info iconThis preview shows pages 1–7. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Lecture 14 - First law processes Heat capacities Phase changes Chemical reactions Ideal Gas First law processes Ideal gas Reversible-Isothermal 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....
View Full Document

{[ snackBarMessage ]}

Page1 / 27

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

This preview shows document pages 1 - 7. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online