Why . . .

1. Why . . . . . . is water a liquid at 25oC but a solid at -5oC? . . . is water a liquid at room temperature while methane (CH4) is a gas?

2. The phase of a substance is determined by three things: • The temperature. • The pressure. • The strength of intermolecular forces. = phase diagram

3. Phase Diagram • A plot of temperature versus pressure for a closed system, with lines to indicate where there is a phase change. Pressure Temperature

4. absorb or release heat? relation to IMFs? positive or negative DH? vaporization condensation sublimation deposition melting freezing

5. Heating and Cooling Curves • A plot showing change in temperature as heat is added or removed at a constant rate. Temperature Temperature Heat Added Heat Removed

6. Which requires more energy: to melt 100. mL of water or to boil 100. mL of water? Why?

7. Calculate the amount of heat needed to convert 25.0 mL of water at 18.8oC to steam at 100.0oC.

8. The heating curve for methanol is shown. Compare the strengths of the IMFs in methanol to those in water. Justify your answer.

9. The partial heating curves below show the effect of adding heat to 0.33 moles of liquid samples of ethane and ethanol. Which heating curve corresponds to ethane and which is ethanol?

10. Let’s study a closed system • When first sealed, molecules slowly escape the surface of the liquid • As the number of gas molecules above the liquid build up some condense back to a liquid.

11. As time goes by the rate of vaporization remains constant but the rate of condensation increases because there are more molecules to condense. • Eventually reaches equilibrium Rate of Vaporization = Rate of Condensation • Amount of liquid and vapor are now constant

12. Vapor pressure • The pressure of vapor present above the liquid at equilibrium.

13. Vapor pressure • Depends on the strength of the IMF’s of the substance • decreases with increasing strength of IMF’s • Liquids with high vapor pressures evaporate easily. They are “volatile”. Diethyl ether, (b) Benzene, (c) Water, (d) Toluene, (e) Aniline.

14. Vapor pressure • Depends on the strength of the IMF’s of the substance • decreases with increasing strength of IMF’s • Depends on temperature. Diethyl ether, (b) Benzene, (c) Water, (d) Toluene, (e) Aniline.

15. Vapor pressure • As temperature increases, so does the number of particles with enough KE to escape the liquid. • As temperature increases, vapor pressure increases.

16. Vapor pressure • A component to understanding the boiling process • Boiling occurs when the vapor pressure of the liquid equals atmospheric pressure

17. Boiling point • - the temperature at which the vapor pressure of a liquid has risen to equal to the current atmospheric pressure. • - the temperature at which the vapor pressure of a liquid is equal to the standard atmospheric pressure. Normal boiling point

19. Phase Diagram

20. What’s a supercritical fluid?

21. Phase Diagram for Water

22. On the left is water; water is unusual because the solid is less dense than the liquid. For most substances, the solid is more dense than the liquid, like for benzene (on the right).

23. Phase Diagram for CO2