Heat, Work, Internal Energy, Enthalpy, and the First Law of Thermodynamics. Internal Energy and the First Law of Thermodynamics

Transcription

1 CHAPTER 2 Heat, Work, Internal Energy, Enthalpy, and the First Law of Thermodynamics Internal Energy and the First Law of Thermodynamics Internal Energy (U) Translational energy of molecules Potential energy of the constituents of the system Internal energy stored in the forms of molecular vibrations and rotations Internal energy stored in the forms of chemical bonds that can be released through a chemical reactions Potential energy of interactions between molecules First Law of Thermodynamics 1

2 Separate the isolated system into two subsystems: the system and the surroundings First Law of Thermodynamics Work (w) Any quantity of energy that flows across the boundary between the system and the surroundings as a result of a force acting through a distance Characteristics of work (w) Work is transitory in that it only appears during a change instate of the system and the surroundings. The net effect of w is to change U of the system and the surroundings in accordance with the first law A system is shown in which compression work is done on a gas. The walls are adiabatic. 2

3 Work (w) If w > 0, U > 0 for an adiabatic process. That is, work is done on the system by the surroundings If w < 0, U < 0 for an adiabatic process. Work is done to the surroundings by the system PV work Other Types of Work 3

4 Heat (q) Any quantity of energy that flows across the boundary between the system and the surroundings because of a temperature difference between the system and the surroundings Characteristics of heat (q) Heat is transitory in that it only appears during a change instate of the system and the surroundings. The net effect of q is to change U of the system and the surroundings in accordance with the first law For q > 0, the temperature of the system increases; for q < 0, the temperature of the system decreases Exothermal reaction Rigid diathermal wall w = 0, q > 0, and U > 0 w surroundings = 0, q surroundings < 0, U surroundings < 0 w = 0, q > 0, and U > 0 q = -w surroundings = I t Two subsystems, I and II, are enclosed in a rigid adiabatic enclosures. System I consists solely of the liquid in the beaker for each case. System II consists of everything else in the enclosure and is the surroundings of system I. 4

5 Example 2.2 Doing Work on the System and Changing the System Energy from a Molecular Level Perspective Distribution of molecules at different temperatures: (a) 0.20 K and (b) 0.40 K 5

6 Heat Capacity Extensive variable C m, molar heat capacity, intensive variable 6

7 Quantum Connection: The more complex the molecule (higher degrees of freedom), the higher the heat capacity (energy storage capability) Heat flow between the system and surroundings under constant pressure For water within the range of 0 to 100 C, C p = 4.18 J g -1 K -1, so when the temperature of 1.5 kg of water increases by 14.2 C under constant pressure, q p = C p T = 1.5 kg J g -1 K K = 89.1 J 7

8 State Functions and Path Functions Change in the kinetic energy depends only on the initial and final states 8

9 State Function (U) exact differential For three dimensions, a differential is an exact differential in a simply-connected region R of the xyz-coordinate system if between the functions A, B and C there exist the relations: Path Functions (w and q) independent of path varies with block mass (path) 9

10 Equilibrium, Change, and Reversibility Thermodynamics can only applied to systems in internal equilibrium Equilibrium surface of ideal gas PV = nrt Reversible: infinitesimal change of states along the way Comparing Work for Reversible and Irreversible Processes Irreversible transition (P 1, V 1, T) (P 2, V 2, T) (P 1, V 1, T) Sudden expansion from V 1 to V 2 with pressure dropped to P 2 Sudden compression from V 2 to V 1 with pressure increased to P 1 Indicator diagram 10

11 Reversible (Isothermal) Path Indicator diagram Example

12 expansion compression Determining U and Introducing Enthalpy, H, a New State Function At constant volume, w = 0 At constant pressure 12

13 Calculating q, w, U, and H for Processes Involving Ideal Gases Constant P ext Reversible expansion Reversible isothermal expansion Example