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Thermochemistry
Thermochemistry
Thermochemistry is the study of the heat
released (-DH) or absorbed (+DH) by chemical
and physical changes.
Some terminology
Temperature is defined as the degree of hotness/centigrade
scale, °C; kelvin, K/
Heat is a form of energy
Heat capacity is the quantity of heat required to raise the
temperature of a substance by 1°C
C = mass x specific heat
Thermochemistry
Specific heat is the heat capacity of one gram of a
substance.
Specific heat of water: 4.184 J/(g°C)
Calorimeter is a device used to measure the heat
transferred in chemical reactions.
Thermodynamic Systems - Definitions
Isolated System: No matter
or energy cross system
boundaries. No work can be
done on the system.
Open System: Free exchange
across system boundaries.
Closed System: Energy can be
exchanged but matter cannot.
Adiabatic System: Special case
where no heat can be exchanged
but work can be done on the
system (e.g. PV work).
Thermodynamic Systems - Definitions
Surroundings: is the portion of the universe with
which a system interacts
Exothermic reaction: is a chemical reaction in
which heat is liberated (-DH)
Endothermic reaction: is a chemical reaction in
which heat is absorbed (+DH)
Thermochemical equations
e.g.
C6H6(l) + 15,O’(g) = 6CO2(g) + 3H2O(l) -DH
DH = Hproducts – Hreactants
DH = heat of reaction
H = enthalpy (heat content P=k)
DH is given at 25°C and standard atmospheric pressure
First Law of Thermodynamics
Basic concepts:
1.
2.
3.
Work and heat are both forms of energy
One form of energy can be converted into another
form
Energy cannot be created or destroyed
First law of thermodynamics is the law of conservation of
energy
The total energy of the univers is a constant
Internal energy: E
In an isolated system E = constant
not known, cannot be
calculated
DE = Ef – Ei (f = final) (i= initial)
can be measured
DE = q - w
q = heat
W = work
q=+
q=w=+
w=w = PDV
heat absorbed by the system
heat evolved by the system
work done by the system
work done on the system
(on constant temperature)
DE = q-PDV
q = DE + PDV
Definition of Enthalpy
We can define a new state variable (one where the path to
its current state does not affect its value) called enthalpy:
H = Ei + PV
Enthalpy = Internal Energy + PV
Heat content
q = DH
Second Law of Thermodynamics
 One statement defining the second law is that a
spontaneous natural processes tend to even out the
energy gradients in a isolated system.
 Can be quantified based on the entropy of the system,
S, such that S is at a maximum when energy is most
uniform. Can also be viewed as a measure of disorder.
DS = Sfinal - Sinitial > 0
The law states that a system will always undergo a
spontaneous change in such a way as to increase the
entropy
DS total = DS system + DS surroundings
Entropy change: DS is
DS =
q
t
If the reaction is exothermic:
DH
DStotal = DS T
TDS – DH > 0
The reaction is spontaneous if
DH < TDS
Change in Entropy
Relative Entropy Example:
Ssteam > Sliquid water > Sice
ISOLATED SYSTEM
Third Law Entropies:
All crystals become
increasingly ordered
as absolute zero is
approached (0 K =
-273.15°C) and at
0 K all atoms are fixed
in space so that entropy
is zero.
Spontaneous Reaction Direction
Law of Hess
law of constant heat summation
The change in enthalpy for any chemical reaction is
constant, whether the reaction occurs in one step or in
several steps.