Thermodynamics
Chapter 5
Thermochemistry
Thermochemistry
•Thermodynamics –
study of energy and
its transformation.
•Thermochemistry –
relationship of energy
changes in chemical
reactions.
The Nature of Energy
Force
Work
Heat
Energy
is the “push” or “pull”
exerted on an object.
is the energy required to
overcome a force. w =
force * distance
the transfer of energy
from one object to another
because of a difference in
temperature
capacity to due work or to
transfer heat.
Energy
•Kinetic – energy due
to an objects motion
(Ke = ½ mv^2)
represents the
performance of work
•Potential – energy
due to an objects
position relative to
another object –
represents the
capacity to due work
•Usage of Energy
Energy Units
• Joule (S.I. Unit) energy of a 2 kg obj
traveling at a velocity of 1 m/s
1 J = 1kg –m^2/s^2
• Calorie (cal) – energy req to raise the
temp of 1 g of water 1 deg C.
• 1 cal = 4.184 J
• In foods 1 Cal = 1000 cal = 1kcal
Thermo Environment’s
System
A. Open
B.Closed
part of the universe considered
(reactants, products, etc)
both energy and matter can be
transferred into or out of
energy can be trans into or out
but not matter
C.Isolated
neither energy or matter can be
trans into or out of the system
Surrounds
all other parts of the universe
including the vessel
The First Law of Thermodynamics
• "Energy can neither be created nor
destroyed". Another approach is to
say that the total energy of an isolated
system remains constant.
• Law of conservation of energy = 1st
law of thermo
• Internal Energy (U or E) consist of all
the energy of a system – sum of
potential and kinetic energies
First Law Cont.
• Internal Energy can not be measured
however changes in energy can be
• Delta U or E is defined as the change
in internal energy
• DU = E final – E initial
• + DU energy is gained by system or
(endothermic)
• - DU energy is lost by the system of
(exothermic)
First Law cont.
• Delta U = q + w is another way of
expressing the first law
q = heat = integral c delta t
c = heat capacity delta t = change in
tempt
q = + heat absorbed
q = - heat released
w = work = neg integral p delta v
p = external pressure delta v =
change in volume
w = + done to system
w = - done by system
State Functions
• Depend on the current state of the
system and are independent on the
pathway leading to the current state
• Internal Energy U is an example of a
state function
• q and w are not state functions
• Delta H enthalpy = q ( pressure
constant)
Thermochemical Equations
• Balanced chemical equations that
show the associated enthalpy
change
• ex. 2Na(s) + 2H2O(l)  2NaOH(aq) +
H2(g) (delta H = -367.5kj)
* note – the enthalpy change
depends on the phase of the
substances
Thermochem eq
• Ex 2H2(g) + O2(g)  2H2O(g) DH -483.7kj
• 2H2(g) + O2(g)  2H2O(l) DH -571.7kj
• Molar Interpretation
2 moles of H2 and 1 mole of O2 react give
liquid water and 572kj of heat evolve
How much heat would evolve if one mole of
H2 reacted
1H2(g) +1/2 O2(g)  1H2O(l) DH -286kj
1H2O(l)  1H2(g) +1/2 O2(g) DH 286kj
Heat and Enthalpy Change
• The amount of energy in an open
system is best described by the
amount of heat produced ( little work
is done because the atmosphere
produces little resistance)
• Enthalpy- is the amount of heat
contained by substance (can’t
measure - can only measure change in
a system at cons. pressure)
• DH = q (p) (constant pressure)
Delta H enthalpy
•Is a state function
•DH = H final – H
initial
products – reactants
DH = (-) exothermic
DH = (+) endothermic
Calculating the heat of reactions
• How much heat is evolved when 907
kg of ammonia is produced according
to the following equation (-2.45 x
10^6kj)
N2(g) + 3H2(g)  2NH(g) DH = -91.8kj
Thermo Problems Cont.
• Ammonia burns in the presence of
copper catalyst to form nitrogen gas.
What is the enthalpy change to burn
25.6g of ammonia?
4NH3(g) + 3O2(g)  2N2 + 6H2O(g)
DH= -1267kj
Calorimetry (measurement of heat flow)
•Heat Capacity of an obj
(C) is the amt of heat
necessary to raise tits
tempt by 1 deg C
•Is directly port to the
amt of the sub.
• q = Cdt C = q/dt
•So molar heat capacity
is often used- the amt of
heat necessary to raise
the temp of one mole 1
deg C
Specific Heat
• heat capacity of 1g of a substance is
called its specific heat capacity of
specific heat
• s = q (quantity of heat trans)/ m(grams
of sub x dt (temp change)
• Calc the amt of heat a substance has
gained or lost
q = s x grams sub x dt
Hess’s Law
for a chem eq that
can be written as the
sum of two or more
steps, the enthalpy
change for the overall
eq equals the sum of
the enthalpy changes
for the individual
parts