1. Water and Its Phase Changes Wednesday, February 20, 2008 Homework: Due Thursday Page 514, #9, 19, 21, 22, 23

2. The heating/cooling curve for water heated or cooled at a constant rate.

3. Heating Water is heated until it reaches 100 o C At 100 o C, bubbles develop on the interior of the liquid, rise to top, and burst! Temperature STAYS at 100 o C until all of the water has changed to vapor THEN the temperature rises as we heat the vapor

4. Heating At 100 o C, both liquid and gas coexist. At 1atm, liquid always changes from a liquid to a gas Called the normal boiling point

5. Cooling Cooling the water, temperature decreases until 0 o C All the liquid must change to ice before the temperature cools anymore At 0 o both liquid and solid coexist. Above 0 o C ice melts, below 0 o C water freezes Called the normal freezing point

6. Ice Density Recall, water expands as it freezes Same mass, greater volume, makes ice less dense Therefore, ice floats on water

7. Energy Requirements Changing phase is a physical change No chemical bonds are changed- still water molecules When water begins to form steam, the molecules move away from one another, but still intact

8. Energy Requirements Moving to the right on the curve requires energy; energy added (heating) Moving to the left on the curve, energy is removed (cooling)

9. Energy Requirements It takes energy to melt ice and vaporize water because intermolecular forces have to be overcome Takes MUCH more energy to overcome the bonds for a chemical change

10. Energy Requirements Energy required to melt one mol of a substance is called the molar heat of fusion Energy required to change 1 mol of liquid to its vapor is called molar heat of vaporization

11. Heating/Cooling Curve Which requires more Energy, melting or boiling? Look at length of horizontal line.

12. The heating/cooling curve for water heated or cooled at a constant rate.

13. Heating/Cooling Curve Which requires more Energy, melting or boiling? Look at length of horizontal line. Hvaporization is 7x greater than the Hfusion. Solid and liquid much more similar than gas and liquid.

14. Energy for Changes in State  H fus is 6.02 kJ/mol  H vap is 40.6 kJ/mol Calculate the energy needed to boil 35.0 g H 2 O at 100 o C.

15. Energy for Changes in State What would happen if this 35.0 g H 2 O started at 25.0 o C. How much energy is now required to boil? Q=s x m x  T 4.184 J/g o C x 35.0 g x 75.0 o C = 10983 J 10.983 kJ + 78.9 kJ = 89.9 kJ Do practice problem 14.2 on pg. 497