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Chemical
Thermodynamics
UNIT 15: THERMODYNAMICS
• ENTHALPY, DH
• ENTROPY, DS
• GIBBS FREE ENERGY, DG
Chemical
Thermodynamics
ENTHALPY, DH
Chemical
Thermodynamics
Heat is the transfer of thermal energy between two bodies that
are at different temperatures.
Energy Changes in Chemical Reactions
Temperature is a measure of the thermal energy.
Temperature = Thermal Energy
900C400C
greater thermal energy
Chemical
Thermodynamics
Exothermic process is any process that gives off heat –
transfers thermal energy from the system to the surroundings.
Endothermic process is any process in which heat has to be
supplied to the system from the surroundings.
2H2 (g) + O2 (g) 2H2O (l) + energy
H2O (g) H2O (l) + energy
energy + 2HgO (s) 2Hg (l) + O2 (g)
energy + H2O (s) H2O (l)
Chemical
Thermodynamics
Enthalpy (H) is used to quantify the heat flow into or out of a
system in a process that occurs at constant pressure.
DH
DH = heat given off or absorbed during a reaction at constant pressure
Hproducts < Hreactants
DH < 0
Hproducts > Hreactants
DH > 0
Chemical
Thermodynamics
Thermochemical Equations
H2O (s) H2O (l) DH = 6.01 kJ
Is DH negative or positive?
System absorbs heat
Endothermic
DH > 0
6.01 kJ are absorbed for every 1 mole of ice that
melts at 00C and 1 atm.
Chemical
Thermodynamics
Thermochemical Equations
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH = -890.4 kJ
Is DH negative or positive?
System gives off heat
Exothermic
DH < 0
890.4 kJ are released for every 1 mole of methane
that is combusted at 250C and 1 atm.
Chemical
Thermodynamics
H2O (s) H2O (l) DH = 6.01 kJ
• The stoichiometric coefficients always refer to the number
of moles of a substance
Thermochemical Equations
• If you reverse a reaction, the sign of DH changes
H2O (l) H2O (s) DH = -6.01 kJ
Chemical
Thermodynamics
ENTROPY, S
Chemical
Thermodynamics
First Law of Thermodynamics
• You will recall that energy cannot be created nor destroyed.
• Therefore, the total energy of the universe is a constant.
• Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.
Chemical
Thermodynamics
Spontaneous Processes
Processes that are
spontaneous in one
direction are
nonspontaneous in the
reverse direction.
Chemical
Thermodynamics
Spontaneous Processes
• Processes that are spontaneous at one
temperature may be nonspontaneous at other
temperatures.
• Above 0C it is spontaneous for ice to melt.
• Below 0C the reverse process is spontaneous.
Chemical
Thermodynamics
Reversible ProcessesIn a reversible process
the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process.
Changes are infinitesimally small in a reversible process.
Chemical
Thermodynamics
Entropy
• Entropy (DS) is a term coined by
Rudolph Clausius in the 19th century.
• Clausius was convinced of the
significance of the ratio of heat
delivered and the temperature at which
it is delivered, q
T
Chemical
Thermodynamics
Entropy
• Entropy can be thought of as a measure
of the randomness of a system.
• It is related to the various modes of
motion in molecules.
Chemical
Thermodynamics
Entropy
• For a process occurring at constant
temperature (an isothermal process):
qrev = the heat that is transferred when the
process is carried out reversibly at a constant
temperature.
T = temperature in Kelvin.
Chemical
Thermodynamics
Entropy on the Molecular ScaleImplications:
• more particles
-> more states -> more entropy
• higher T
-> more energy states -> more entropy
• less structure (gas vs solid)
-> more states -> more entropy
Chemical
Thermodynamics
Entropy on the Molecular Scale
• The entropy tends to increase with
increases in
Temperature.
Volume (gases).
The number of independently moving
molecules.
Chemical
Thermodynamics
Entropy Changes
• In general, entropy
increases when
Gases are formed from
liquids and solids.
Liquids or solutions are
formed from solids.
The number of gas
molecules increases.
The number of moles
increases.
Chemical
Thermodynamics
The Concept of Entropy (S)
Entropy refers to the state of order.
A change in order is a change in the number of ways of arranging the
particles.
solid liquid gas
more order less order
crystal + liquid ions in solution
more order less order
more order less order
crystal + crystal gases + ions in solution
Chemical
Thermodynamics
GIBBS FREE ENERGY
Chemical
Thermodynamics
Gibbs Free Energy
Very key equation:
This equation shows how DG changes with
temperature.
(We assume S° & DH° are independent of T.)
Chemical
Thermodynamics
Gibbs Free Energy
1. If DG is negative, the
forward reaction is
spontaneous.
2. If DG is 0, the system
is at equilibrium.
3. If DG is positive, the
reaction is spontaneous
in the reverse direction.
Chemical
Thermodynamics
Free Energy and Temperature
• There are two parts to the free energy
equation:
DH— the enthalpy term
TDS — the entropy term
• The temperature dependence of free
energy comes from the entropy term.
Chemical
Thermodynamics
Free Energy and Temperature
By knowing the sign (+ or -) of DS and DH,
we can get the sign of DG and determine if a
reaction is spontaneous.
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