Transcript PowerPoint Template
13/14 Semester 2
Physical Chemistry I (TKK-2246)
Instructor: Rama Oktavian Email: [email protected]
Office Hr.: M.13-15, Tu. 13-15, W. 13-15, Th. 13-15, F. 09-11
1. Expansion work 2. Multistage expansion 3. Work of expansion 4. Maximum and minimum work
Outlines
Review
1. Suggest a system, boundary, and surroundings for 10 moles of propane gas in a rigid metal cylinder 2. Suggest a system, boundary, and surroundings for 500 mL of water in an open beaker.
Review
1. Describe three commonplace examples of how work is done on or by a system 2. A plumber of mass 65 kg carries a toolbox of mass 15 kg to a fifth floor walk up apartment 15 m above ground level. Calculate the work required for this process 3. Describe the internal energy change and work performed when a spring is compressed or expanded.
1
st
law of thermodynamics
Mathematical statement for The 1st Law of Thermodyamics
ΔU = q + w
in which w > 0 or q > 0 if energy is transferred to the system as work or heat and w < 0 or q < 0 if energy is lost from the system as work or heat
Heat, work, and energy
Work (W) - any quantity that flows across the boundary of a system during a change in its state Ex: - gas that pushes out a piston and raises a weight - A chemical reaction that drives an electric current through a resistance also does work
the work arising from a change in volume Ex: - the work done by a gas as it expands and drives back the atmosphere The term ‘expansion work’ also includes work associated with negative changes of volume, that is, compression
Expansion work
General expression of expansion work the work required to move an object a distance d
z
against an opposing force of magnitude
F
is
dw
Fdz
Expansion work
Expansion work
General expression of expansion work
dw
Fdz F
p ext A dw
p ext Adz dV
Adz dw
p ext dV
the work arising from a change in volume
General expression of expansion work
dw
p ext dV
Integrating equation from initial to final volume
w
V f
V i p ext dV
Expansion work
Expansion work
Reversible expansion In a reversible process the system is at equilibrium at every stage of the process Reversibility during pressure changes ensures that
p ext
p
the pressure on the inside of the container is always equal to the pressure exerted on the outside of the container
Expansion work
Reversible expansion When we set
p
ex =
p p ext
p
the pressure on the inside of the container is always equal to the pressure exerted on the outside of the container
dw
p ext dV
pdV
The total work of reversible expansion is therefore
w
V V i
f pdV
Expansion work
w
Isothermal reversible expansion Consider the isothermal, reversible expansion of an ideal gas
pV
nRT
the work of reversible isothermal expansion of a perfect gas from
V
i to
V
f at a temperature
T
is
nRT V V i f
dV V
nRT
ln
V f V i
Isothermal reversible expansion
Expansion work
Isothermal reversible expansion
Expansion work
Multistage expansion work
Multistage irreversible isothermal expansion and compression
W
n n
1
W n
where
n
is number of stage
Maximum and minimum work
Reversible processes actually do not occur in nature They are simply idealization of actual Processes Easy to analyze Serve as idealized model
Maximum and minimum work
when Reversible processes are approximated instead of the Actual ones Work-producing devices such as car engine and gas or steam turbine deliver the
maximum
work, and Work-consuming devices such as compressors, fan, and pumps Consume the
minimum
work.
Exercise
A chemical reaction takes place in a container of cross-sectional area 50.0 cm 2 . As a result of the reaction, a piston is pushed out through 15 cm against an external pressure of 121 kPa. Calculate the work done by the system
Exercise
Isothermal compression A sample consisting of 2.00 mol He is expanded isothermally at 22 °C from 22.8 dm3 to 31.7 dm3 (a) reversibly, (b) against a constant external pressure equal to the final pressure of the gas. For the two processes calculate w
Exercise
Isothermal compression, maximum and minimum work Three moles of an ideal gas are compressed isothermally from 60 L to 20 L using a constant pressure of 5 atm. Calculate W.
If that gas is compressed reversibly, calculate W