Dr. Jie ZouPHY Chapter 20 Heat and the First Law of Thermodynamics (cont.)

Dr. Jie ZouPHY Outline Latent heat (20.3) Phase change Latent heat for different phase changes Example: Convert ice to steam Calorimetry problems involving phase change (20.3) Work and heat in thermodynamic processes (20.4) PV diagram

Dr. Jie ZouPHY Latent heat Phase change: The situation when the physical characteristics of the substance change from one form to another. Common phase changes: Melting (from solids to liquid) and Boiling (from liquid to gas) All phase changes involve a change in internal energy but no change in temperature. (Example: boiling) Latent heat: If a quantity Q of energy transfer is required to change the phase of a mass m of a substance, the ratio L  Q/m is called the latent heat of the substance. L depends on the nature of the phase change and the properties of the substance. (1) Latent heat of fusion L f (from solid to liquid); (2) latent heat of vaporization L v (from liquid to gas; (3) latent heat of condensation (from gas to liquid); (4) latent heat of solidification (from liquid to solid). See Table Q = ±mL; “+” or “-” depends on whether adding or removing energy from the substance.

Dr. Jie ZouPHY Example: Convert ice to steam Find the energy required to convert a 1.00-g cube of ice at -30.0°C to steam at 120.0°C. (See Tables 20.1 and 20.2) Figure 20.2 Experimental results

Dr. Jie ZouPHY Previous example follow-up discussions We can describe phase changes in terms of a rearrangement of molecules when energy is added to or removed from a substance. The latent heat of vaporization L v for a given substance is usually somewhat higher than the latent heat of fusion L f. Explanation: Liquid-type bonds are slightly less strong than the solid-type bonds, while gas-type bonds are much less strong; molecules of the gas essentially are not bonded to each other. Quick Quiz 20.3: Calculate the slopes for the A, C, and E portions of Figure Rank the slopes from least to greatest and explain what this ordering means.

Dr. Jie ZouPHY Calorimetry problems involving phase change Example 20.4 Cooling the steam: What mass of steam initially at 130°C is needed to warm 200 g of water in a 100-g glass container from 20.0°C to 50.0°C? What if the final state of the system is water at 100°C? Would we need more or less steam? How would the analysis above change? See the textbook for problem-solving hints about Calorimetry-type problems

Dr. Jie ZouPHY Work and heat in thermodynamic processes The work done on a gas in a quasi-static process that takes the gas from an initial state to a final state is the negative of the area under the curve on a PV diagram, evaluated between the initial and final states: Quasi-static process: A process during which the system is allowed to remain essentially in thermal equilibrium at all times. PV diagram: If the pressure P and volume V are known at each step of the process, a graph of P v.s. V is called a PV diagram. Path: The curve on a PV diagram is called the path taken between the initial and final states.

Dr. Jie ZouPHY Example: P. 633, Problem #21 A sample of ideal gas is expanded to twice its original volume of 1.00 m 3 in a quasi-static process for which P =  V 2, with  = 5.00 atm/m 6, as shown in the figure. How much work is done on the expanding gas?

Dr. Jie ZouPHY The work done depends on the particular path taken between the initial and final states.

Dr. Jie ZouPHY Heat in thermodynamic processes Energy transfer by heat, like work done, depends on the initial, final, and intermediate states of the system.

Dr. Jie ZouPHY Homework Ch. 20, P.633, Problems: #14, 22, 26.