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CHAPTER 2Heat, 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

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Separate the isolated system into two subsystems: the system and the surroundings

First Law of Thermodynamics

Work (w)

A system is shown in which compression work is done on a gas. The walls are adiabatic.

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

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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

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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

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.

w = 0, q > 0, and U > 0

wsurroundings = 0, qsurroundings < 0, Usurroundings < 0

w = 0, q > 0, and U > 0

q = -wsurroundings = It

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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

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Heat Capacity

Extensive variable

Cm, molar heat capacity, intensive variable

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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, Cp = 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, qp = CpT = 1.5 kg ×4.18 4.18 J g-1 K-1 ×14.2 K = 89.1 J

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State Functions and Path Functions

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3 4

Change in the kinetic energy depends only

on the initial and final states

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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)

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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

• Irreversible transition (P1, V1, T) (P2, V2, T) (P1, V1, T) • Sudden expansion from V1 to V2 with pressure

dropped to P2

• Sudden compression from V2 to V1 with pressure increased to P1

Comparing Work for Reversible and Irreversible Processes

Indicator diagram

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Reversible (Isothermal) Path

Indicator diagram

Example 2.4

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expansion

compression

Determining U and Introducing Enthalpy, H, a New State Function• At constant volume, w = 0

• At constant pressure

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Calculating q, w, U, and H for Processes Involving Ideal Gases

Constant Pext

Reversible expansion

Reversible isothermal expansion

Example 2.5