first-law-thermodynamics in science chemistry

Thermodynamics M. D. Eastin
Outline:
 Forms of Energy
 Energy Conservation
 Concept of Work
 PV Diagrams
 Concept of Internal Energy
 Joules Law
 Thermal Capacities (Specific Heats)
 Concept of Enthalpy
 Various Forms of the First Law
 Types of Processes
First Law of Thermodynamics

Forms of Energy
Energy comes in a variety of forms…
Potential
Mechanical Chemical Electrical
Internal Kinetic
Heat

Energy Conservation
The First Law of Thermodynamics states that total energy is conserved for any
thermodynamic system → energy can not be created nor destroyed
→ energy can only change from one form to another
constant
)
( 
E
Energy
constant
electrical
chemical
heat
mechanical
potential
kinetic
internal







E
E
E
E
E
E
E
Our main concern in meteorology…

The Concept of Work
Work is a Mechanical form of Energy:
Distance
Force
Work 

x
F
dW 


Force
Distance
x

The Concept of Work
Work is a Mechanical form of Energy:
Recall the definition of pressure:
We can thus define work as:
Distance
Force
Work 

x
F
dW 


 2
Area
Force
p
x
F



pdV
dW 

The Concept of Work
Changes in Volume Cause Work:
• Work is performed when air expands
Work of Expansion:
• Occurs when a system performs work
(or exerts a force) on its environment
• Is negative:
• Rising air parcels (or balloons) undergo expansion work
• Since the environmental pressure decreases with height,
with height a rising parcel must expand
to maintain an equivalent pressure
F
ve
dw 

0
dW 

The Concept of Work
• Similar to a piston in a car engine
F
F

The Concept of Work
• Work is performed when air contracts
Work of Contraction:
• Occurs when an environment performs work
(or exerts a force) on a system
• Is positive:
• Sinking air parcels (or balloons) undergo contraction work
• Since the environmental pressure decreases with height,
with height a sinking parcel must contract
to maintain an equivalent pressure
F
F
ve
dw 

0
dW 

Sign convention for work and heat
-------------------------------------------------------------------
process sign
Work done by the system on the surroundings _
Work done on the system by the surroundings +
Heat absorbed by the system from the surroundings +
(endothermic process)
Heat absorbed by the surroundings from the system _
(exothermic process)

Pressure-Volume (PV) Diagrams
Another Way of Depicting Thermodynamic Processes:
• Consider the transformation: i → f
p
V
Vf
Vi
pi
pf
i
f

Another Way of Depicting Work:
• Consider the transformation: i → f
p
V
pdV
dW 


f
i
pdV
W
Vf
Vi
pi
pf
i
f The work done is the area
under the i → f curve
(or gray area)
Pressure-Volume (PV) Diagrams

Internal Energy = Kinetic Energy + Potential Energy
(of the molecules in the system)
• Depends only on the current system state (p,V,T)
• Does not depend on past states
• Does not depend on how state changes occur
• Changes are the result of external forcing
on the system (in the form of work or heat)
First Law of Thermodynamics
t
environmen
t
environmen
internal Heat
Work
E 


dQ
dW
dU 


dQ
pdV
dU 



Internal Energy
The evolution or absorption of energy in different
processes clearly shows that every substance must be
associated with some definite amount of energy, the
actual value of which depends upon the nature of the
substance (i. e. arrangement of atoms and electrons
within the molecules)and the conditions of temperature,
pressure, volume and composition. The energy
associated with a substance is called its internal energy
and is usually denoted by the symbol E or U.
Change of internal
R
P E
E
E
E
E
U
or
E






 1
2
)
(
E
 is (–ve) ---- energy is evolved
Is (+ve) ---- energy is absorbed
E


H,U,A,S, and G = state function
W = path function

Joules Law
Valve
Closed
Air
Vacuum
Thermally Insulated System

Joules Law
Thermally Insulated System
Valve
Open
Air
Air

Joules Law
dQ
pdV
dU 


Valve
Open
Air
Air
• Air expanded to fill the container
• Change in volume
• Change in pressure
• No external work was done
• Air expanded into a vacuum
within the system
• No heat was added or subtract
• Thermally insulated system
• No change in internal energy
• No change in temperature
What does this mean?
0
dU 

Joules Law
dQ
pdV
dU 


Valve
Open
Air
Air
• Air expanded to fill the container
• Change in volume
• Change in pressure
• No external work was done
• Air expanded into a vacuum
within the system
• No heat was added or subtract
• Thermally insulated system
• No change in internal energy
• No change in temperature
Internal Energy is only a function of
temperature
0
dU 
U(T)
U 

Thermal Capacities (Specific Heats)
Assume: A small quantity of heat (dQ) is given to a parcel
The parcel responds by experiencing a small temperature increase (dT)
Specific Heat (c):
Two Types of Specific Heats:
• Depends on how the material changes as it receives the heat
Constant Volume:
Constant Pressure:
volume
constant
v
dT
dQ
c 





 Parcel experiences no
change in volume
Parcel experiences no
change in pressure
pressure
constant
p
dT
dQ
c 






dT
dQ

c

Specific Heat at Constant Volume:
• Starting with:
• If the volume is constant (dV = 0), we can re-write the first law as:
• As w is negative if work is done by the system
• And substitute this into our specific heat equation as
volume
constant
v
dT
dQ
c 






dQ
pdV
dU 

 dQ
dU 
→







dT
dU
cv or dT
c
dU v


Specific Heat at Constant Volume:
• Since the internal energy is a state variable and does not depend on past states
or how state changes occur, we can define changes in internal energy as:
• Also, if we substitute our specific heat equation into the first law:
We can obtain an alternative form of the First Law of Thermodynamics:



2
1
dT
c
U v
T
T
pdV
dT
c
dQ v 

dQ
pdV
dU 


→
dT
c
dU v


Specific Heat at Constant Pressure:
• Starting with
we can obtain another alternative form of the First Law of Thermodynamics:
Also,
pressure
constant
p
dT
dQ
c 






pdV
dT
c
dQ v 

*
v
p nR
c
c 
 T
nR
pV *


Concept of Enthalpy
Assume: Heat (dQ) is added to a system at constant pressure
Impact: 1) The system’s volume increases (V1→V2) and work is done
2) The system’s internal energy increases (U1→U2)
Using the First Law:
We can then define Enthalpy (H) as:
)
V
-
p(V
dW 1
2

1
2 U
-
U
dU 
   
1
2
1
2 V
V
p
U
U
dQ 



pV
U
H 


Concept of Enthalpy
Enthalpy:
If we differentiate the definition of enthalpy and use prior relationships, we can
obtain the following relation:
We shall see that Enthalpy will be a useful concept since most sources and
sinks of heating in the atmosphere occur at roughly constant pressure
   
1
2
1
2 V
V
p
U
U
dQ 



pV
U
H 

dT
c
dH
dQ p



Forms of the First Law of Thermodynamics
For a gas of mass m
dW
dU
dQ 

pdV
dU
dQ 

pdV
dT
c
dQ v 

Vdp
dT
c
dQ p 

where: p = pressure U = internal energy
V = volume W = work
T = temperature Q = heat energy
n = number of moles
cv = specific heat at constant volume (717 J kg-1 K-1)
cp = specific heat at constant pressure (1004 J kg-1 K-1)
Rd = gas constant for dry air (287 J kg-1 K-1)
R* = universal gas constant (8.3143 J K-1 mol-1)
nR
c
c *
v
p 


Types of Processes
Isobaric Processes:
• Transformations at constant pressure
• dp = 0
Isochoric Processes:
• Transformations at constant volume
• dV = 0
p
V
i f
p
V
i
f

Types of Processes
Isothermal Processes:
• Transformations at constant temperature
• dT = 0
Adiabatic Processes:
• Transformations without the exchange of heat
between the environment and the system
• dQ = 0
p
V
i
f

first-law-thermodynamics in science chemistry

Recommended

Recommended

More Related Content

Similar to first-law-thermodynamics in science chemistry

Similar to first-law-thermodynamics in science chemistry (20)

Recently uploaded

Recently uploaded (20)

first-law-thermodynamics in science chemistry