Temas Básicos de Termodinámica: Conceptos Clave y Origen - Prof. Aguilar Navarro, Apuntes de Química

¡Descarga Temas Básicos de Termodinámica: Conceptos Clave y Origen - Prof. Aguilar Navarro y más Apuntes en PDF de Química solo en Docsity!

Descargado en:

patatabrava .com

QUÍMICA BÀSICA I (UB)

TRANSPARENCIAS TEMA 4

GARGALLO, RAIMON 16-

• Tema 4. Primer principi de la Termodinàmica
  • Sistemes, treball i energia. Calor. Funció d’estat. Energia interna. Primer

principi de la termodinàmica. Entalpia. Capacitats calorífiques. Entalpia dels

canvis físics.

• Entalpia del canvi químic: entalpia de reacció. Estats estàndard. Llei de Hess.

Entalpia de formació. Entalpia d’enllaç. Variació de l’entalpia amb la

temperatura.

What is Thermodynamics?

• A lot of definitions:

“Study of energy transformation” (Atkins)

"Branch of Science that establishes and quantifies the relationship between

thermal and mechanical energy“ (Química, un proyecto de la ACS)

“Thermodynamics is the science of energy conversion involving heat and other

forms of energy, most notably mechanical work.

It studies and interrelates the macroscopic variables, such as T, V and P, which

describe physical, thermodynamic systems” (Wikipedia)

• Thermodynamics can be deceptively simple or exceedingly complex
• Thermodynamics is a macroscopic theory, not molecular
• Thermodynamics is mainly concerned with time-independent systems (equilibrium)

greek: therme + dynamis

Origins of Thermodynamics

"Half an ounce of coal can carry two tons over a mile“ (R.W. Emerson, s. XIX)

Coal ( mass ) + O 2  Chemical products + ability to carry a load over long distances

A question was raised at that time: how to increase the efficiency of steam engines?

Thomas Newcomen’s engine (around 1712)

Stephenson’s rocket (around 1829)

Laws (or principles) of Thermodynamics. A few formulations

• Zeroth law (Fowler, 1931):

“If two thermodynamic systems are separately in thermal equilibrium with a third, they are also in thermal equilibrium with each other”

• First law (Clausius, 1850):

“The energy change, DU, associated with a change in state is: DU = q + w” , or “The energy of Universe is constant”

• Second law (Clausius and Kelvin, ~1850):

“A process involving an isolated system will be spontaneous if the entropy of the system increases over time” or “The entropy of Universe is increasing”

• Third law (Nernst, 1910):

“The entropy ( S ) of perfect crystals of all pure elements and compounds is zero at absolute zero (T = 0 K)”

Química Bàsica II

Química Bàsica I

Thermodynamics: a macroscopic approach

Laws of Thermodynamics were postulated from experimental observations (s. XIX)

prior to any knowledge about atomic and molecular structure (s. XX)

Thermodynamics is a macroscopic theory, not molecular or atomic

Atomic theory

Thermodynamics

Statistical thermodynamics / statistical mechanics

(QF III)

Universe = thermodynamic system + surroundings

• Thermodynamic system: part of Universe in which are concerned
• The thermodynamic system is limited by boundaries
• Exchanges of work, heat, or matter between the system and the surroundings may take place across this boundary

Thermodynamic systems. Definition

Example: dissolution of potassium alum in hot water

What is happening at atomic scale?

What is the source of energy? What is the final drain (“embornal” o “sumidero”)?

If we are interested in following the pathways of energy, it will be necessary to define the part of Universe in which we are concerned

fuel

Thermodynamic systems. Boundaries

• Boundaries can be:
  • Real or imaginary. For closed systems, boundaries are real while for open system boundaries are often imaginary.
  • Fixed (e.g. a constant volume reactor) or moveable (e.g. a piston). Fixed boundaries do not allow transfer of energy as work.
  • Adiabatic or insulating: without heat transfer.
  • Isothermal: the temperature remains constants. Therefore, heat transfer occurs.
  • Permeable or semipermeable: mass transfer occurs
• The characteristics of boundaries may be combined: fixed and permeable / moveable and adiabatic / …
• Anything that passes across the boundary that effects a change in the internal energy needs to be accounted for in the energy balance equation.
• The volume of the system can be:
  • A single atom resonating energy (Planck)
  • Air in a steam engine (Carnot, 1824);
  • A tropical cyclone (Kerry Emanuel, 1986) in the field of atmospheric thermodynamics

Thermodynamic systems. Properties

• A thermodynamic system is characterized by means of a series of measurable properties:
  • extensive properties: they are proportional to the mass of the system
    • Mass
    • Volume
    • …
  • intensive properties: they are independent to the mass of the system
    • Temperature
    • Density
    • Color
    • …
  • extensive  intensive properties
    • mass / volume = density
    • Heat capacity / mass = specific heat capacity

• Temperature (T):
  • Wikipedia: “It is a numerical measure of hot or cold”
  • TERMCAT: “ Magnitud termodinàmica que descriu la possibilitat de transferir calor a un altre

cos o de rebre'n. ”

• Kinetic theory of gases:
• Thermodynamics requires a unique T scale: Kelvin scale
• The lowest T is 0 K (-273.15oC)… what happens there to atoms? And to electrons? Charles’s law?
• Thermal equilibrium: when two systems are at the same temperature, they are at thermal

equilibrium

Temperature and thermal equilibrium

• Zeroth law (Fowler, 1931): “If two thermodynamic systems are separately in thermal equilibrium with a third, they are

also in thermal equilibrium with each other”

Surroundings

Energy transfer: heat and thermodynamic systems

System

75 oC

Time

Surroundings

System

40 oC

If heat is transferred from surroundings to system  qsystem > 0

Endothermic process

Examples: melting, vaporization, …

If heat is transferred from system to surroundings  qsystem < 0

Exothermic process

Examples: dissolution of hydrated ammonium nitrate

Units for heat: Joule (J) or calorie (cal)

1 cal = 1000 cal = 1 kcal 1 J = 0’24 cal

NH 4 SCN(s) + Ba(OH) 2 ·8H 2 O(s) http://www.youtube.com/watch?v=5RJLvQXce4A

Heat capacity of a system (C)

C: relationship between heat transferred to a system and the observed increase of T:

C = q / DT

Units : J (oC)-

How qsystem is measured?: calorimeter

qsystem = -qsurroundings = -qcalorimeter

If we know Ccalorimeter and DT  qcalorimeter

Then: qsystem = -qcalorimeter

vas Dewar

sonda de temperatura

termòmetre digital

Resum

• L’energia interna d’un sistema termodinàmic té dos components: energia potencial i energia

cinètica

• Un gas que es comporta idealment només té energia cinètica i, per tant, la seva energia

interna només depèn de la temperatura

• Un gas real sí té energia potencial i, per això, es pot observar l’efecte Joule-Thomson
• La calor és una forma de transferència d’energia
• La calor passa d’un sistema a un altre, sempre que el primer tingui una temperatura major
• Els mecanismes per transferir energia en forma de calor són: conducció, convecció i radiació
• La capacitat calorífica relaciona la calor transferida a un sistema i l’augment observat de T
• Temperatura: T (no Tª)