Download Bagian 2 termodinamika

KIMIA LINGKUNGAN
BAGIAN 2: TERMODINAMIKA
PREVIEW
In this third part of the course we:
 define and apply a number of thermodynamic ideas and
concepts
 become familiar with and apply the 1st and 2nd law of
thermodynamics
 discuss several thermodynamic processes with the aid of
pressure-volume (pV) diagrams
 define and discuss the concept of entropy
 apply the laws of thermodynamics to discuss heat engines,
refrigerators and heat pumps
 introduce parameters to quantify the efficiency at which
thermodynamic devices operate
 learn about the Carnot cycle and its relation to the concept of
an 'ideal' engine
Thermodynamic Systems, States and Processes
Objectives are to:
 define thermodynamics systems and states of systems
 explain how processes affect such systems
 apply the above thermodynamic terms and ideas to the
laws of thermodynamics
Thermodynamic Systems

A thermodynamic system is a
collection of matter which has
distinct boundaries.
OR
A real or imaginary portion of
universe whish has distinct
boundaries is called system.
OR
A thermodynamic system is that
part of universe which is under
thermodynamic study.
1. 1st Law of Thermodynamics
U  Q  W
positive Q : heat added to system
positive W : work done by system



statement of energy conservation for a thermodynamic system
internal energy U is a state variable
W, Q process dependent
Isoprocesses



apply 1st law of thermodynamics to closed system
of an ideal gas
isoprocess is one in which one of the thermodynamic
(state) variables are kept constant
use pV diagram to visualise process
Isobaric Process

process in which pressure is kept constant
Isochoric Process

process in which volume is kept constant
Isothermal Process

process in which temperature is held constant
Adiabatic Process

process in which no heat transfer takes place
2. Second Law of Thermodynamics and Entropy
Objectives are to:
 state and explain the second law of thermodynamics
 explain the concept of entropy
2nd Law of Thermodynamics

states in which direction a process can take place
 heat
does not flow spontaneously from a cold to a
hot body
 heat cannot be transformed completely into
mechanical work
 it is impossible to construct an operational perpetual
motion machine

introduces concept of entropy
Entropy

property that indicates the direction of a process
 entropy
is a measure of disorder
 entropy is a measure of a system’s ability to do
useful work
 entropy determines direction of time

the entropy of an isolated system increases
Q
S 
(change in entropy at constant t emperature )
T
2nd Law of Thermodynamics: entropy
2nd Law example
3. Heat Engines and Heat Pumps
Objectives are to:
 explain what a heat engine is, and compute its thermal
efficiency
 explain what a heat pump is, and compute its coefficient of
performance
Diagram of a Heat Engine
Heat Engine
Heat Engine




A heat engine is a cyclic device that converts thermal
energy into work output
It is a device that takes heat from a high-T reservoir,
converts some of to (useful) work, and transfers the rest
to the surroundings (a low-T reservoir)
Examples: steam engines; internal combustion engines
(car engines)
Thermal efficiency (“what you get out/what you put in”):
Qc
work out Wout
eth 

 1
heat in
Qin
Qh

No heat engine operating in a cycle can convert all of
its heat input completely to work
Heat Pump
Heat Pump



A heat pump is a (cyclic) device that transfers heat
energy from a low-T reservoir to a high-T reservoir
Examples: air conditioner; refrigerator
Coefficient of performance (“what you get out/what
you put in”):
cophp 

Qc
heat transfer Qin


work done
W Qh  Qc
No heat pump operating in a cycle can transfer
thermal energy to a low-T reservoir without doing
some work
Refrigerator (1)
Refrigerator
(2)
4. The Second Law Revisited
•it is impossible to produce a cyclic engine that generates work by extracting heat from a
reservoir without expelling some waste heat
•it is impossible to produce a heat pump in which the sole result is the transfer of heat from a lowT to a high-T body
5. Third Law of Thermodynamics

The 3rd law states that:
 It
is impossible to reach a temperature of absolute zero
 It is impossible to have a (Carnot) efficiency equal to
100% (this would imply Tc = 0).
Isobaric Expansion: Change in Internal Energy


A quantity of an ideal gas has a volume of 22.4
litres at STP (standard temperature and pressure).
While absorbing 315 cal of heat from the
surroundings, the gas expands isobarically to 32.4
litres. What is the change in internal energy of the
gas?
What is the equilibrium temperature (in degrees
Celsius) of the gas after expansion?
Question
Three different experiments are run, in which a gas expands from point A to
point D along the three paths shown below. Calculate the amount of work done
for paths 1, 2 and 3.
Questions


Free Loader: Consider the following idea. A ship heats its boilers
and propels itself without the use of coal or oil in the following
way. It pumps in warm sea water, extracts heat from that sea
water, concentrates the extracted heat in its boilers, and
discharges the cooled seawater back into the ocean. The
discharged water may be ice if enough heat has been taken from
it. Could this idea be made to work?
Gulf of Mexico: Another free loader idea is to generate power as
follows. Water on top of the Gulf of Mexico is quite warm but
deep down the water is cold. The plan is to heat some gas with
warm water from the top so it will expand, and then cool the gas
with water from the bottom so it will contract. The gas is
alternately expanded and contracted so it drives a piston back
and forth. The moving piston is attached by conventional means to
an electric generator to make electricity. Can this idea be made to
work?