1. Energy Relationships
in Chemical Reactions

2. Energy is the capacity to do work
• Thermal energy is the energy associated with
the random motion of atoms and molecules
• Chemical energy is the energy stored within the
bonds of chemical substances
• Nuclear energy is the energy stored within the
collection of neutrons and protons in the atom
• Electrical energy is the energy associated with
the flow of electrons
• Potential energy is the energy available by virtue
of an object’s position

3. Thermochemical Definitions
System : That part of the Universe whose change we are
going to measure.
Surroundings : Every thing else that is relevant to the change
is defined as the “surroundings”.
Internal Energy : The sum of the kinetic and potential energy
of all the particles in a system.
Heat (q) : Is the energy transferred between a system
and it’s surroundings as result in the differences in
their temperatures only!
Work (w) : The energy transferred when an object is moved by
a force.
Therefore: E=q+w

4. Energy Changes in Chemical Reactions
Heat is the transfer of thermal energy between two bodies that
are at different temperatures.
Temperature is a measure of the thermal energy.
Temperature = Thermal Energy
900C
400C
greater thermal energy
6.2

5. Change in Enthalpy = H
Enthalpy is defined as the system’s internal energy
plus the product of its pressure and volume.
H = E + PV
For a change in enthalpy:
H = E+ PV
Exothermic and Endothermic Reactions:
H = H final - H initial = H products - H reactants
Exothermic : H final H initial H 0
Endothermic : H final H initial H 0

6. For processes occurring at constant
pressure the enthalpy change equals
the heat gained or lost.
H = qp
enthalpy of reaction or heat of
reaction. (Energy change + small
correction factor.)

7. Constant-Pressure Calorimetry
qsys = qwater + qcal + qrxn
qsys = 0
qrxn = - (qwater + qcal)
qwater = ms t
qcal = Ccal t
Reaction at Constant P
H = qrxn
No heat enters or leaves!
6.4

8. The specific heat (s) of a substance is the amount of heat (q)
required to raise the temperature of one gram of the
substance by one degree Celsius.
The heat capacity (C) of a substance is the amount of heat
(q) required to raise the temperature of a given quantity (m)
of the substance by one degree Celsius.
C = ms
Heat (q) absorbed or released:
q = ms t
q=C t
t = tfinal - tinitial
6.4

9. How much heat is given off when an 869 g iron bar cools
from 940C to 50C?
s of Fe = 0.444 J/g • 0C
t = tfinal – tinitial = 50C – 940C = -890C
q = ms t = 869 g x 0.444 J/g • 0C x –890C = -34,000 J

10. 0
The standard enthalpy of reaction ( Hrxn ) is the enthalpy of
a reaction carried out at 1 atm.
aA + bB cC + dD
Ho = [ c Hof (C) + d Hof (D) ] - [ a Ho (A) + b Ho (B) ]
rxn f f
Ho =
rxn n Hof (products) - m Hfo (reactants)
Hess’s Law: When reactants are converted to products, the
change in enthalpy is the same whether the reaction takes
place in one step or in a series of steps.
(Enthalpy is a state function. It doesn’t matter how you get
there, only where you start and end.)

11. Hess’s Law of Heat Summation
The enthalpy change of an overall process is the sum of
the enthalpy changes of its individual steps.
Example:
Problem: Calculate the energy involved in the oxidation of elemental
sulfur to sulfur trioxide from reactions:
1) S (s) + O2 (g) SO2 (g) H1 = -296.0 kJ
2) 2 SO2 (g) + O2 (g) 2 SO3 (g) H2 = -198.2 kJ
3) S (s) + 3/2 O2 (g) SO3 (g) H3 = ?

12. Hess’s Law of Heat Summation
The enthalpy change of an overall process is the sum of
the enthalpy changes of its individual steps.
Example:
Problem: Calculate the energy involved in the oxidation of elemental
sulfur to sulfur trioxide from reactions:
2 X 1) S (s) + O2 (g) SO2 (g) 2H1 = -296.0 kJ X2
+
2) 2 SO2 (g) + O2 (g) 2 SO3 (g) H2 = -198.2 kJ
3) S (s) + 3/2 O2 (g) SO3 (g) H3 = ?
H3 = 2H1 + H2