Thermochemistry 4 Boon Chemistry February 4 & 5, 2013

Catalyst According to the heat curve for water shown below:  Does the system absorb or release heat as ice melts?  Does the temperature of water increase when it is melting?  What phase change happens at #4? According to the heat curve for water shown below:  Does the system absorb or release heat as ice melts?  Does the temperature of water increase when it is melting?  What phase change happens at #4?  Objectives I can calculate the amount of energy transferred as heat during a phase change.  Objectives I can calculate the amount of energy transferred as heat during a phase change.  Agenda  Catalyst  Exit Slip Review  Notes: Latent Heat of Phase Change  Practice Calculations and heat curve analysis  Exit Slip: Calculations  Agenda  Catalyst  Exit Slip Review  Notes: Latent Heat of Phase Change  Practice Calculations and heat curve analysis  Exit Slip: Calculations  Take out your homework please.

Phase Diagram Review (1) Ice; temperature is increasing (2) Ice and water; temperature is constant (3) water; temperature is increasing (4) Water & steam; temperature is constant (5) steam; temperature is increasing #2: The temperature remains constant when the water is a mixture of ice and water because the energy is being used to melt the ice. #4: When water boils the kinetic energy of molecules increases. The molecules move faster. This is endothermic because it requires energy. #5: When water freezes the kinetic energy of molecules decreases. The molecules move slower. This is exothermic because energy is released.

Exit Slip Review  7. Which of the following statements best describes the relationship between the transfer of heat and temperature?  (a) The transfer of heat always raises temperature.  (b) The transfer of heat never causes a change in temperature.  (c) The transfer of heat always decreases temperature.  (d) The transfer of heat does not always cause a change in temperature.   8. What happens to the temperature of matter as energy is added during a change of state?  (a) temperature increases  (b) temperature decreases  (c) temperature does not change  (d) temperature changes depending on the material

 9. List the names two phase changes that result in a release of energy to the surroundings:  Freezing, condensation, deposition  10. List the names of two phase changes that result in an absorption of energy from the surroundings:  Vaporization, fusion, sublimation

Exit Slip Review  Exothermic  2NaHCO 3 + energy → Na 2 Co 3 + H 2 O + CO 2  Endothermic

Exit Slip Review  3. In an endothermic process, energy is (absorbed/released).  4. In an exothermic process, energy is (absorbed/released).  5. Which of the following is an example of energy:  (a) color (c) copper  (b) heat (d) temperature  Why? Heat is one way that energy is transferred. Temperature is a measure of the average kinetic energy of particles in an object. But it is not energy itself.  6. When energy is added to liquid water, what happens?  (a) The molecules move and vibrate less.  (b) There is no change in the motion of the molecules.  (c) The molecules move and vibrate more.  (d) The water becomes ice.

Specific Heat Review:  Use your notes to do the following specific heat problem.  3000 g of water are heated from 50 °C to 99°C. The specific heat of water is 4.18 J/g*K. How much heat was absorbed?

Notes: Heat Transfer During Phase Changes  We discussed the fact that temperature does not increase during a phase change. Instead, all the energy that is released or absorbed is used to power the phase change. On the phase change diagram this portion of the heating curve has a slope of zero. 

 Today we will calculate how much heat is absorbed or released during a phase change. We will use water as our example, but a similar calculation can be done for other substances. What do you think? Which requires more energy to melt ?

 The amount of heat absorbed or released during a phase change is generally called the latent heat of transformation. Different substances have different latent heats of transformation. Also, different phase changes absorb or release different amounts of heat. For example, less energy is absorbed to melt ice than is absorbed when water is vaporized. Table 1 contains the heats of transformation for water.

Analyzing Data.  How do the heats of transformation for water compare to the specific heat of water?   How many joules are released when 1 gram of water freezes?   Which requires more energy: melting or vaporization?   Why is the heat of fusion the same as the heat of crystallization?

Latent Heat of Transformation Calculations  If we know the latent heat of transformation of a substance, we can calculate the amount of heat energy absorbed or released during a phase change.   The formula:Q = m × Δ H tranformation

Phase Change Calculations  Example 1: How much heat is required to melt 1000 g of ice at 0°C?

Phase Change Calculations  Example 2: How much heat is needed to vaporize 200 g of water at 100°C?

Heat Transfer Calculations  Example 3: How much ice, at 0°C, can be melted by 100 J of heat?

Work Time  1. Complete today’s notes worksheet  2. Get a stamp when you are done and check your answers.  3. Work on the phase change diagram worksheet (passed out today). This is due Friday.  4. Work on any worksheets that you have not completed from last week. These are due Friday.

Exit Slip  Expectations:  You will work silently and independently.  When you are done, cover your exit slip with your handouts.  Tools:  You may use all your notes, worksheets, and flash cards.  You may use your own calculator.  What do I turn in?  You will turn in your exit slip only.  Homework:  Complete any worksheets or article questions that you have not finished.

Homework  Due Thurs/Fri (Feb. 7 & 8): All Thermochemistry notes and handouts.