basics of thermodynamics: easy learning by dr. anjana sen€¦ · basics of thermodynamics: easy...
TRANSCRIPT
Basics of Thermodynamics:
Easy learning
by Dr. Anjana Sen
Part 1: Theory and concept
Part 2: Definitions and equations
Part 3: Laws of Thermodynamics
Part 1: theory and concept
Thermodynamics means conversion of thermal energy (heat) to mechanical energy (work).
Thermodynamics deals with properties of systems where temperature is an essential
coordinate.
Attention!
What is possibility of reaction? “Thermodynamics”.
What is rate of reaction? “Kinetics”.
Why do we perform evaluation of thermodynamics parameters?
in order to elucidate molecular mechanisms of interactions.
in order to facilitate rational design of interactions.
Attention!
3 important “E”s in thermodynamics.
(1) Energy
(2) Equilibrium
(3) Entropy
Thermodynamic variables
(1) Temperature
(2) Pressure
(3) Volume
Part 2: definitions and equations
isolated system
A system that is not subject to outside influence.
State Function
A function relating to the state variables that depends only on the current equilibrium state
of the system.
State Function does not depend on the path taken to arrive at the present state or specific
value.
Heat
The flow of energy which changes the temperature in a system.
Extent of temperature change depends on
(1) The amount of heat
(2) The identity of the object
heat of reaction
gaining heat
(melting) (vaporization)
solid state liquid state gaseous state
(freezing) (condensation)
losing heat
Internal Energy is defined as the sum of the kinetic and potential energies of the particles
that form the system. Change in Internal Energy of a system is equal to the sum of the heat
gained / lost by the system + the work done by / on the system.
heat transferred work done
Enthalpy is an extensive thermodynamic quantity and a state function. It is defined as the
total heat content of the system under certain very specific conditions. Enthalpy is
evaluated as the sum of the internal energy + pressure multiplied volume.
Enthalpy reflects capacity to do non-mechanical work.
Simple description of Enthalpy
change in Enthalpy = heat evolved (released/absorbed) at constant pressure.
At constant pressure, ΔH = qp
At constant volume, ΔE = qv
Effect of temperature on Enthalpy
Temperature increases
molecular interaction increases
internal energy rises
ΔH increases
Endothermic heat absorbed
Exothermic heat emitted
Please note that…………
“absolute value of Enthalpy” does not exist
total Enthalpy of a system cannot be measured
only the difference in Enthalpy ΔH is measured
Why do we use the concept of Enthalpy?
Because it’s the preferred expression of changes in the system energy.
Because it simplifies the description of energy transfer.
common examples
Enthalpy of hydrogenation
Enthalpy of neutralization
Enthalpy of denaturation
Enthalpy of vaporization
Enthalpy of sublimation
Enthalpy of combustion
Enthalpy of atomization
Enthalpy of formation
Enthalpy of hydration
Enthalpy of reaction
Enthalpy of solution
Enthalpy of fusion
Discussing now…………
Entropy
definition of Entropy
Entropy is a thermodynamic quantity and a state function.
Entropy is a measure of:
the amount of energy, which is unavailable to do useful work.
the multiplicity of a system.
how far the equilibrium has progressed.
Thermodynamic Definition of Entropy
It’s the transfer of heat energy at uniform absolute temperature of a closed system in a
reversible process.
Physical meaning of Entropy
Entropy is a measure of spreading and sharing of thermal energy within a system.
Entropy applies only to molecular-level systems exchanging thermal energy with the
surroundings.
What is Isentropic Process ?
The process where Entropy of the system remains constant, ΔS = 0
What are bulk properties?
composition, volume, energy, pressure, temperature,……..
There can be trillions of different microscopic states, at constant and defined bulk
properties.
role of Entropy
order & chaos
Entropy defines existing number of different microscopic states in a system.
Role of Entropy is sorting of energy into different states in a system.
Entropy describes the course of a process whether it’s a spontaneous process with a
probability of occurring in a defined direction.
Entropy increases………
due to external interaction: exchange of heat with surroundings.
due to internal changes: disturbance inside the system.
Entropy is a measure of microscopic disorder of a system.
Entropy is a measure of disorder or randomness (different possible arrangements) of a
system.
larger Entropy
more possible microscopic states
more disorder
Increasing Entropy provides the basic reason for the events happening in the universe.
more microscopic states
greater molecular disorder
enhanced flexibility
greater molecular mobility
higher degrees of freedom
larger Entropy
ΔS = change in entropy
can be + or
increasing disorder = positive
decreasing disorder = negative
not a naturally occurring process
Discussing now…………
Gibbs Free Energy
definition of Gibbs free energy
ΔG determines:
direction of the chemical reaction
extent of the chemical reaction
feasibility of the chemical reaction
Energy is partitioned into
the maximum number
of states possible
ad
ditio
na
l
Entropy increases here
Entropy decreases here
Discussing now…………
Heat Capacity
definition of Heat Capacity
In a defined system, it’s the amount of heat needed to raise the temperature by 1°
Discussing now…………
Equilibrium
Equilibrium of a reaction:
The reaction has stopped progressing
The converted amount of reactants remains constant
The amount of leftover reactants remains constant
definition of equilibrium constant
Discussing now…………
Exothermic / Endothermic
reaction progress
Po
ten
tia
l E
nerg
y
products
reactants
Acti
vati
on
En
erg
y
en
erg
y r
ele
ased
Exothermic Reaction
products
reaction progress
Po
ten
tia
l E
nerg
y
reactants
Acti
vati
on
En
erg
y
en
erg
y a
bso
rbed
Endothermic Reaction
exothermic endothermic
more energy is released
by formation of new bonds
less energy is released
by formation of new bonds
less energy is consumed
by breaking old bonds
more energy is consumed
by breaking old bonds
weaker bonds are
replaced by stronger bonds
stronger bonds are
replaced by weaker bonds
Discussing now…………
“favourable” / “spontaneous”
+ΔS favors spontaneous change !
favorable occurrence
of the chemical processunfavorable conditions
ΔH0 < 0 ΔH0 > 0
ΔS0 > 0 ΔS0 < 0
spontaneous non-spontaneous
ΔG0 < 0
Keq > 1
ΔG0 > 0
Keq < 1
equilibrium
ΔG0 = 0
Keq = 1
is the reaction spontaneous ?
ΔG = ΔH − TΔS
if ΔG is negative, the reaction is spontaneous
if ΔH is negative (exothermic), the reaction is spontaneous
if ΔS is positive, the reaction is spontaneous
favorable enthalpy –ΔH
always negative
ΔG = ΔH TΔS
favorable entropy +ΔS +
reactants
products
unfavorable enthalpy +ΔH
always positive
ΔG = ΔH TΔS
unfavorable entropy –ΔS
+
reactants
products
favorable enthalpy –ΔH
ΔG = ΔH TΔS
unfavorable entropy –ΔS
+ –
reactants
products
ΔG = ΔH TΔS
+–
unfavorable enthalpy +ΔH
ΔG = ΔH TΔS
favorable entropy +ΔS
+ –
products
ΔG = ΔH TΔS+–
reactants
Enthalpy Entropy Free Energy
exothermic, ΔH < 0
endothermic, ΔH > 0
increased disorder, ΔS > 0
increased order, ΔS < 0
spontaneous, ΔG < 0
not spontaneous, ΔG > 0
Part 3: Laws of Thermodynamics
Zeroth law of thermodynamics
If 2 systems are in thermal equilibrium with a 3rd system, they are in
thermal equilibrium with each other.
This law defines the notion of temperature.
1st law of thermodynamics
This is the law of conservation of energy.
Energy cannot be created or destroyed in an isolated system.
Energy only changes form.
net heat net work
2nd law of thermodynamics
Heat always flows from hot to cold.
Entropy of any isolated system always increases.
Entropy approaches a maximum value at equilibrium.
3rd law of thermodynamics
Entropy approaches a constant value, when temperature approaches
absolute zero.
Entropy at absolute zero = logarithm of the product of the quantum
ground states.
The perfect crystal has only one minimum energy state and absolutely
no molecular motion.