Transcript Lacture №1. Chemical thermodynamics. The first law of

Chemical thermodynamics. The first law of
thermodynamics.
Plan
1 The basic concepts of
thermodynamics
2. First law of thermodynamics.
Heat (Q) and Work ( W)
3. Heat capacity
Assistant Kozachok S.S. prepared
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The branch of science which deals
with energy changes in physical and
chemical processes is called
thermodynamics
Some common terms which are
frequently used in the discussion of
thermodynamics are:
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common terms of
thermodynamics
System
Parameter
Condition(st
ate)
Process
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Classification of the thermodynamics
systems according to a structure
homogeneous
KNO3
heterogeneous
KNO3
PbI2↓
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System is a specified part of the
universe which is under
observation
The remaining portion of the
universe which is not a part of the
system is called the surroundings
The system is separated by real or
imaginary boundaries.
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Types of Systems
ISOLATED
Open
Close
(a
system
which can not
exchange
mass
and energy with
the surroundings )
CLOSE
A system which
can exchange
energy but no
mass with its
surroundings
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Parameters
Extensive
Intensive
(m, V, U, H, G, S, c)
The properties of the
system whose value
depends upon the
amount of
substance present
in the system
(p, T, C, viscosity,
surface tension,
vapour pressure)
The properties of the
system whose value
does not depend upon
the amount of
substance present in
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the system
Process is the change of all or individual
parameters of the system during the length of time
(the period of time) Classification
of a process according to the constant parameter of a
system are:
 Isothermic process – temperature is constant,
T=const
 Isochoric process – volume is constant V = const.
 Isobaric process – pressure of the system is
constant, p = const
 Adiabatic process – the system is completely
isolated from the surroundings. For an adiabatic
(Q=0) system of constant mass, ▲U=W
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Classification
of a process according to the releasing energy
 Exothermic process is a process that releases
energy as heat into its surroundings. We say that in
an exothermic process energy is transferred ‘as heat’
to the surroundings. For example: a reaction of
neutralization (acid + basic).
 Endothermic process is a process in which
energy is acquired from its surroundings as heat.
Energy is transferred ‘as heat’ from the
surroundings into the system. For example: the
vaporization of water
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Classification
of a process according to the direction of
reaction
 Reversible process is a process in
which the direction may be reversed at
any stage by merely a small change in a
variable like temperature, pressure, etc.
 Irreversible process is a process which
is not reversible. All natural process are
irreversible
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State of a system means the condition of the
system , which is described in terms of certain
observable (measurable) properties such as
temperature (T), pressure (p), volume (V)
State function (thermodynamic function)
 Internal energy U [J/mol]
 Enthalpy H [kJ/mol] or [kJ]
 Entropy S [J/mol K] or [J/K]
 Gibbs energy G [J/mol] or [J]
ΔU = Uproducts - Ureactants
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State function depends only upon the
initial and final state of the system
and not on the path by which the
change from initial to final state is
brought about.
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Internal energy U
It is the sum of different types of
energies associated with atoms and
molecules such as electronic
energy, nuclear energy, chemical
bond energy and all type of the
internal energy except potential
and kinetic energies.
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Heat (Q) is a form of energy which the
system can exchange with the
surroundings. If they are at different
temperatures, the heat flows from higher
temperature to lower temperature. Heat is
expressed as Q.
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Work (W) is said to be performed if the point
of application of force is displaced in the
direction of the force. It is equal to the
distance through which the force acts.
There are two main types of work electrical
and mechanical. Electrical work is important
in systems where reaction takes place
between ions.
Mechanical work is important specially in
systems that contains gases. This is also
known as pressure-volume work.
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The work of the expanded
ideal gass
W  Fdh
F ( force)  p( pressure ) S (area )
WORK = Force and Distance
F
s
h
A  pSdh  pdV
2
A
 pdV
1
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If the volume of the system changes by a finite
quantity from volume V1 to V2, then total
work done can be obtained by integrating:
If the gas expands, V2 › V1 and work is done
by the system and W is negative (we will
use sign positive)
V2‹ V1 and the work is done on the system
and W is positive
Note. It may be noted that many books use
the opposite sign convention for work!!!
(according to the IUPAC recommendation)
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Enthalpy H
Chemical reactions are generally
carried out at constant pressure. ΔU
gives the change in internal energy at
constant volume. To express the energy
changes at constant pressure, a new
term called enthalpy was used.
Enthalpy cannot be directly measured,
but changes in it can be.
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Enthalpy H
A thermodynamic function of a
system, equivalent to the sum of
the internal energy of the system
plus the product of its volume
multiplied by the pressure
exerted on it by its surroundings.
▲H = ▲U + p▲V
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The meaning of ▲H and
▲U
1) ▲H = ▲U in solid or
liquid systems
2) ▲H >>▲U in gas system
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Heat absorbed by the system = H positive (Q negative
Heat evolved by the system = H negative (Q positive)
The signs of W or Q are related to the internal energy
change.
The meaning of the state functions in the
thermodynamic processes
Exothermic process
 Qv > 0, ▲U < 0
 Qp > 0, ▲H < 0
2) Endothermic process
 Qv < 0, ▲U > 0
 Qp < 0, ▲H > 0
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
The first law of thermodynamics
Energy can neither be created nor destroyed
although it may be converted from one form to
another.
 The given heat for the system spents on the
change of the internal energy and producing the
work:
Q = ▲U + W
The internal energy of the system can be changed In
two ways:
a) By allowing heat to flow into the system or out of
the system
b) By work done on the system or work done by the
system.
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Determination according to the process:
1) Isochoric process, V-constant
W= p▲V (▲V = V2- V1) = 0
Qv = ▲U;
2) Isobaric process, p-constant
Qp = ▲U + W , Qp = ▲U + p▲V , W=nRT
(according Mendeleyev Klapeyron equation
V=nRT/P)
Qp = ▲ H
3) Isothermic process, T – constant
▲U = n Cv ▲T = 0
QT = W =nRT lnV2/V1 = nRT lnP1/P2
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