Wednesday, Dec. 5 th : “A” Day Thursday, Dec. 6 th : “B” Day Agenda  Homework Questions/Problems?

 Sec. 11.2 Quiz: “Intermolecular Forces”  Section 11.3: “Energy of State Changes”  Enthalpy, Entropy, and Changes of Sate, Gibbs Energy and Changes of State, Calculating Melting and Boiling Points  Lab write-up: “Energy and Entropy: Phase Changes”  Homework  Sec 11.3 review, pg. 398: #1-3, 6-9  “What Would Life Be Like?” Paper  Concept Review: “Energy of State Changes”

 Problems?

Homework pg. 392: #6-12  Questions?

Section 11.2 Quiz “Intermolecular Forces”  You may use your notes and your book to complete the quiz on your own…

Good Luck!

Enthalpy, Entropy, and Changes of 

Enthalpy

State is the total energy of a system.



Entropy

measures a system’s disorder.

 The energy added during melting or removed during freezing is called the

enthalpy of fusion.

 Particle motion is more random in the liquid state, so as a solid melts, the entropy of its particles increases. This increase is called the

entropy of fusion.

Enthalpy, Entropy, and Changes of State  As a liquid evaporates, a lot of energy is needed to separate the particles. This energy is called the

enthalpy of vaporization.

 Particle motion is much more random in a gas than in a liquid. A substance’s

entropy of vaporization

fusion.

is much larger than its entropy of

Enthalpy and Entropy Changes for Melting and Evaporation  Enthalpy and entropy both change as energy (in the form of heat) is added to a substance.

 The energy added as ice melts at its melting point is the

molar enthalpy of fusion (∆H fus ).



∆H fus

is the difference in enthalpy between solid and liquid water at 273 K (O˚C).

∆H fus = H (liquid at melting point)



H (solid at melting point)

Enthalpy and Entropy Changes for Melting and Evaporation

Enthalpy and Entropy Changes for Melting and Evaporation  The energy added as liquid evaporates at its boiling point is the

molar enthalpy of

vaporization (∆H

vap

).



∆H vap

is the difference in enthalpy between liquid and gaseous water at 373 K (100˚C).

∆H vap = H (vapor at boiling point)



H (liquid at boiling point)

Enthalpy and Entropy Changes for Melting and Evaporation  Because intermolecular forces are not significant in the gaseous state, most substances have similar values for

molar entropy of vaporization, ∆S vap .

Gibbs Energy and State Changes  The relative values of H and S determine whether any process, including a state change, will take place.

 A change in Gibbs energy is defined as:

∆G = ∆H



T∆S

 A process is spontaneous if ∆G is negative OR if ∆S is positive.

 If ∆G is positive, then a process will not take place unless there is an outside source of energy.

Gibbs Energy and State Changes  If ∆G is zero, then the system is at

equilibrium.

 At equilibrium, the forward and reverse processes are happening at the same rate.

 For example, when solid ice and liquid water are at equilibrium, ice melts at the same rate that water freezes.

Determining Melting and Boiling Points  For a system at the melting point, a solid and a liquid are in equilibrium, so ∆G is zero.

 Remember, ∆G = ∆H - T∆S and if ∆G is zero, then ∆H = T∆S.

 Rearranging the equation, you get the following equation for the melting point of a substance:

Determining Melting and Boiling Points  For a system at the boiling point, a gas and a liquid are in equilibrium.

 Rearranging the equation as before, you get the following equation for boiling point : 

When ∆H vap



When ∆H vap > T∆S vap , the liquid state is favored.

< T∆S vap , the gaseous state is favored.

Pressure Can Affect Change-of-State Processes  Boiling points are pressure dependent because pressure has a large effect on the entropy of a gas.

 When a gas expands (pressure is decreased) its entropy increases because the degree of disorder of the molecules increases.

 Thus, boiling points change as atmospheric pressure changes due to changes in elevation.

Pressure Can Affect Change-of-State Processes  Liquids and solids are almost incompressible.

 Therefore, changes in atmospheric pressure have little effect on the entropy of substances in liquid or solid states.

 Also, ordinary changes in pressure have essentially no effect on melting and freezing.

 Thus, melting and freezing points hardly change at all with changes in elevation.

Sample Problem A Pg. 397 The enthalpy of fusion of mercury is 11.42 J/g, and the molar entropy of fusion is 9.79 J/K∙mol. The enthalpy of vaporization at the boiling point is 294.7 J/g, and the molar entropy of vaporization is 93.8 J/K∙mol. Use the molar mass of mercury, 200.59 g/mol to calculate the melting point and the boiling point.

Practice Problem #1 Pg. 397 For ethyl alcohol, C 2 H 5 OH, the enthalpy of fusion is 108.9 J/g, and the entropy of fusion is 31.6 J/K∙mol. The enthalpy of vaporization at the boiling point is 837 J/g, and the molar entropy of vaporization is 109.9 J/K∙mol. Calculate the freezing and boiling points for each substance. (Hint: you need the molar mass of ethyl alcohol)

“Energy and Entropy: Phase Changes” Lab Discussion  As heat energy flows from a liquid, its temperature drops.

 The entropy (degree of disorder) also decreases.

 The continuous flow of energy from a liquid will cause the liquid to eventually undergo a phase change to a solid.

“Energy and Entropy: Phase Changes” Lab Discussion  During the time needed for the phase change to be completed, the temperature of a pure substance will not change.

 Entropy, however, will continue to decrease.

 Once the phase change is complete, the temperature of the solid will decrease as energy continues to be removed.

“Energy and Entropy: Phase Changes” Purpose  In this lab, you will monitor the temperature of sodium thiosulfate pentahydrate (Na 2 S 2 O 3 ∙ 5 H 2 O) as it is cooled to several degrees below its freezing temperature and then warmed to several degrees above its melting temperature.

 The results of the lab will allow you to determine the freezing and melting temperatures of sodium thiosulfate pentahydrate and to interpret changes in energy and entropy.

“Energy and Entropy: Phase Changes” Materials  The materials for this lab are as listed under “Apparatus” and “Materials” section of the “Experiment 5” handout.

 You will be using a bunsen burner, NOT a hot plate.

 Listen for other changes….

“Energy and Entropy: Phase Changes” Procedure  For this lab, you will follow the procedure from the “Experiment 5” handout but complete the data table, analysis, conclusion, and extension questions from the “Construction a Heating/Cooling Curve” handout.

Homework

 Section 11.3 review, pg. 398: #1-3, 6-9  Finish lab write-up: “Energy and Entropy: Phase Changes”  Concept Review: “Energy of State Changes”

*Remember* Your “What Would Life Be Like?” paper is due next week!