KIMIA LINGKUNGAN

BAGIAN 2: TERMODINAMIKA

PREVIEWIn 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.ORA real or imaginary portion of universe whish has distinct boundaries is called system.ORA thermodynamic system is that part of universe which is under thermodynamic study.

1. 1st Law of Thermodynamics

statement of energy conservation for a thermodynamic system internal energy U is a state variable W, Q process dependent

system done work : positivesystem addedheat : positive

byto

WQ

WQU

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

)emperatureconstant tat entropy in (change TQS

2nd Law of Thermodynamics: entropy

2nd Law example

3. Heat Engines and Heat PumpsObjectives 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

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”):

No heat engine operating in a cycle can convert all of its heat input completely to work

Heat Engine

work out 1heat in

out cth

in h

W QeQ Q

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”):

No heat pump operating in a cycle can transfer thermal energy to a low-T reservoir without doing some work

Heat Pump

heat transferwork done

in chp

h c

Q QcopW Q Q

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 low-T 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?

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.

Question

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?