CN2121 Basic Concepts_Chap 1

Basic Thermodynamic Concepts

Chapter 1

H, W, E

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Outline 1. What is Thermodynamics

Applications and Various Scales

2. Energy, Heat, and Work Types of Energy and Work

3. Thermodynamic Systems Isolated, Close, and Open Systems

4. Basic Thermodynamic Properties Intensive and Extensive Properties

5. State and State Functions State and Path Functions

6. Equilibrium

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“Thermodynamics is the only physical theory of general nature of which I am convinced that it will never be overthrown.”

— Albert Einstein

“Nothing in life is certain except death, taxes, and the laws of thermodynamics.”

— Seth Lloyd

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• Determine the state (PVT) at a given condition.

• Estimate thermophysical properties (CP, H, ∆H, …)

• Define equilibria and driving forces for physical processes/chemical reactions

• ⋅ ⋅ ⋅ ⋅ ⋅ ⋅

• Initially, study the effects of heat, work, and energy

1. Thermodynamics

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• Transition/transfer rates of physical processes

(heating/cooling, expansion/compression, melting/freezing, vaporization/condensation, …)

• Reaction rates of chemical processes

(combustion, ammonia synthesis, hydrocarbon cracking and conversion, …)

Cannot Do

2 2 3N + 3H 2NH⇔ 6 5 4-C 2-methyl-C 2,3-dimethyl-Cn ⇔ ⇔

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Applications

Thermodynamics

Thermodynamics is one of the scientific cornerstones in science & engineering.

Process Design

Drug Design

Gas Separation

“nano-thermodynamics” “bio-thermodynamics”

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• Classical scale (macroscopic)

• Molecular scale (mesoscopic)

• Electronic scale (microscopic)

At Various Scales

H2, 1 mol

6.022 x 1023 H2 molecules

www.fzu.cz/~kolorenc/qmc/

electronic structure of one molecule

a batch of molecules

no molecular information

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2. Energy, Heat, and Work

Energy (E)

Heat (H) Work (W)

E, H, W

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Energy (E)

• Energy = “the ability to do work”

• Forms of Energy – Potential

– Kinetic (translational, rotational, vibrational)

– Internal (what is this?)

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• Potential Energy (EP)

EP = mgh

The gravitational potential energy is given by:

Potential energy is defined as the energy resulting solely from the position in an external field. The filed may be gravitational, electric or magnetic.

m is mass g is acceleration of gravity h is height

h

m

reference

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• Kinetic Energy (EK)

Kinetic energy is the energy of motion.

• Translational motion:

EK = m v2 / 2

• Rotational motion:

EK = I ω2 / 2

• Vibrational motion:

EK = k0 (r − r0)2 / 2

k0 is force constant, r is position, r0 is equilibrium position.

I is inertia moment, ω is angular velocity.

m is mass, v is translational velocity.

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• Internal Energy (U)

• Energy of substance associated with the motions, interactions and chemical bonding of molecules.

• The translational, rotational and vibrational kinetic energy of molecules.

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Heat (Q)

Heat is a form of energy transferred solely as a result of temperature difference.

Zeroth law of thermodynamics: Objects in thermal equilibrium have the same temperature.

• Heat added to system is defined as positive. • Heat released by system is defined as negative.

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Work (W)

Work is the transfer of energy. • Work done on system is defined as positive. • Work done by system is defined as negative.

Types of work:

• Electrical • Shaft • Elastic • Mechanical • PV (Pressure-Volume) • ……

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• Electrical Work

Electrical work is expressed as kilowatt hour (kWh).

1 kWh = 3.6 million Joules

Watt: W = J/s = N⋅m /s = kg⋅m2/s3

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• Shaft Work

• work from a stirrer

• work from a crank

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• Elastic Work

This type of work refers to the deformation of an elastic object such as spring and flexible metal. Hooke’s law relates stress to strain (of force to deflection) Elastic work for a displacement from r1 to r2 is: 2 2

2 1( ) ( ) / 2= = − = − −∫ ∫d dW F r Kr r K r r

( ) = −F r K rr is the displacement of the end of the spring from its equilibrium position.

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• Mechanical Work

Basic definition of mechanical work: work = force × displacement Integrating

2

1

( )h

hW F h hd= ∫

W F hd d=F F

h

By convention, the mechanical work is positive when the displacement is in the same direction of the force and negative when they are in opposite directions.

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PV Work

For piston-cylinder process, under Pext

= −∫W P Vd

d d d dextextVW F h P A VA

P = = − = −

For a reversible process (Pext = Psys = P): (e.g. very slow change along a continuous PV path)

Integrate

then

W P Vd d= −

Pext

Psys Why “−”? By definition.

path dependent

Pext

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Assumption: – Frictionless piston-cylinder – Balanced driving force – Very slow

Process:

– Infinitesimal mass is removed from (or added to) the piston in succession

– Only Infinitesimal displacement of piston with equilibrium maintained

– Infinite time required for a finite change

Reversible Process

How to know a reversible process? Look for keyword!

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P1 > P2

P1 P2

P1 = P+dP P2 = P

P1 P2

P1 = P2

P1 P2

Finite difference in P, the mass transfer is irreversible

Infinitesimal difference in P, the mass transfer is reversible

P1 = P2, the mass transfer is reversible (in equilibrium)

T1 > T2

T1 T2

T1 = T+dT T2 = T

T1 T2

T1 = T2

T1 T2

Finite difference in T, the heat transfer is irreversible

Infinitesimal difference in T, the heat transfer is reversible

T1=T2, the heat transfer is reversible (in equilibrium)

Example: heat or mass transfer

permeable membrane

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System: The portion (stationary or moving) chosen for thermodynamic analysis

Surroundings: The portion outside the system

3. Thermodynamic Systems

System

Surroundings

Boundary

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Systems

Three types of systems

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Example:

(a) (b) (c)

Determine open, close or isolated system:

hot casting

300 °C

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4. Basic Thermodynamic Properties

• Properties of a system chosen for thermodynamic analysis

T, P, V, U, H, Cp, Cv ….

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• Independent of the amount of material – T – P – Specific properties (property per unit mass/mole) Vm, specific volume (m3/kg, m3/kmol) Um, specific internal energy (kJ/kg, kJ/kmol) Hm, specific enthalpy (kJ/kg, kJ/kmol) Cp, heat capacity at constant pressure (kJ/kg/K, kJ/kmol/K) Cv, heat capacity at constant volume (kJ/kg/K, kJ/kmol/K) …

Intensive Properties

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• Directly proportional to the amount of material

Vt = mVm, total volume (m3) Ut = mUm, total internal energy (kJ) Ht = mHm, total enthalpy (kJ) …

Extensive Properties

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• Measurable thermodynamic properties (T, P, V, ρ, Cp, Cv) • Changes in conceptual properties can be

determined from measurable properties

U Q W∆ = +

Measurable & Conceptual Properties

2

1

T

VT

U C dT∆ = ∫

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5. State & State Functions

Thermodynamic state is the total condition of a given system as reflected by its thermodynamic properties. T, P, V, U, …

1 mol H2O at 400 K & 1 bar

• State functions: thermodynamic properties whose values do not depend on the path or past history.

• Path functions: thermodynamic properties whose values depend on the path or past history.

P

V 1

2 b

a

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State Function Path Function T, P, V, U, H, S, etc. Q, W

Independent of path or past history Dependent of path or past history

Represented by point graphically Represented by area graphically

No change in property for a cyclic process

Change in property for a cyclic process

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State functions which describe a system:

energy E volume V # of particles N entropy S pressure P chemical potential µ

How many state functions are necessary to describe a unique system? — The phase rule answers this question.

electric potential dipole momentum viscosity refractive index …

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A macroscopic state, at which the state properties do not change and have no tendency to change.

• An equilibrium state is static in the macroscopic scale but dynamic in the microscopic scale.

• Thermodynamic state properties are defined and measurable only at equilibrium.

• We can approximate equilibrium when state properties change very slowly.

6. Equilibrium

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• Thermal equilibrium: equality of temperature. (zeroth law of thermodynamics)

• Mechanical equilibrium: equality of pressure.

• Phase equilibrium: equality of chemical potential.

• Chemical equilibrium: equality of chemical potential.

Vapor-Liquid phase equilibrium Chemical reaction equilibrium

H2O(V) 100°C, 1 bar

H2O(L) 100°C, 1 bar

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(for system at equilibrium only)

F: degree of freedom π: number of phases N: number of chemical species

F = number of thermodynamic properties defined → system is well defined, all other intensive thermodynamic properties are defined and can be determined.

2 F N

Phase Rule (Gibbs)

longitude & latitude

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Example: (a) Pure liquid H2O → F = 2-1+1 = 2. If specify T, P (298 K, 1 bar) → F = 0.

(b) Saturated vapor H2O in equilibrium with liquid H2O → F = 2-2+1 = 1. If specify T (100 °C) → F = 0.

H2O(L) @ 298 K, 1 bar

H2O(V) 100°C, 1 bar

H2O(L) 100°C, 1 bar

36 Josiah Willard Gibbs

(1839-1903) American mathematical physicist who contributed much of the theoretical foundation for chemical thermodynamics.

Between 1876 and 1878, Gibbs wrote a series of papers collectively titled On the Equilibrium of Heterogeneous Substances.

Engineering began at Yale in 1852 with a course on civil engineering. Yale awarded the first engineering Ph.D. in the U.S. The recipient was J. Willard Gibbs (1858 B.S., 1863 Ph.D.) who would become one of this country's greatest scientists. He was one of the earliest theoretical physicists in U.S. and perhaps one of the earliest theoretical chemists. The U.S. Postal Service honored Gibbs with a stamp in 2005.

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Summary 1. What is Thermodynamics

Applications, Various Scales

2. Energy, Heat, and Work Types of Energy and Work

3. Thermodynamic Systems Isolated, Close, and Open Systems

4. Basic Thermodynamic Properties Intensive and Extensive Properties

5. State and State Functions State and Path Functions

6. Equilibrium