Chapter 3: Energy and Its Conservation
3.1 Types of Energy
3.2 Thermodynamics
3.3 Energy Changes in
Chemical Reactions
3.4 Measuring Energy
Changes: Calorimetry
3.5 Enthalpy
3.6 Energy Sources
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.1 Types of Energy
Learning objective:
Recognize the types of energy of interest to chemists.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.1 Types of Energy
Energy – the ability to do work
Kinetic Energy
Potential Energy:
1.
2.
a.
b.
c.
3.
4.
Electrical
Chemical
Mass
Thermal Energy
Radiant Energy
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Kinetic Energy
 Every moving object has kinetic energy which is
dependent on the velocity (v) and mass (m) of an
object:
1
E kinetic  mv 2
2
 Joule (J): the SI unit of energy
kg m 2
1 joule  1 J  1 2
s
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Potential Energy
Electrical Energy - energy from positive and negative
ions held a small distance apart.
Eelectrical
q1q2
=k
r
q1 and q2 are the charges of two ions
r is the distance between the ions in pm
k = 2.31 x 10-16 J pm
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Potential Energy
Chemical Energy: energy
resulting from attraction of
the electrons and nuclei in
molecules (bond energy)
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Potential Energy
 Mass: transformation of mass into energy
(E = mc2)
 Thermal Energy: the total energy of random
movements of molecules
 Radiant Energy: energy as a result of electromagnetic
radiation.
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Energy Transfer and
Transformations
 Energy can be transferred
from one type to another.
 Energy transformations
accompany chemical
reactions.
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3.2 Thermodynamics
Learning objective:
Understand the first law of thermodynamics and
the concepts of heat and work.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.2 Thermodynamics
 The study of energy transfers
and transformations
 “how much energy goes where”
 Terms:
 System: whatever we want to
describe and study by itself.
 Surroundings: everything else but
the system
 Boundary: - what separates the
system from its surroundings
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Conservation of Energy
Energy is neither created nor destroyed in any process,
although it may be transferred from one body to
another or transformed from one form into another
or, restated
Energy may be transferred as work or heat, but no
energy can be lost, nor can heat or work be obtained
from nothing
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Heat
Thermal energy that is exchanged with its surroundings is
referred to as heat (q) and is measured in joules (J).
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Heat Flows and Temperature
1.
2.
•
•
3.
4.
T depends on q, the amount of heat transferred.
T depends on the direction of heat flow:
If a substance absorbs heat, T > 0
If a substance releases heat, T < 0
T depends inversely on the amount of material.
T depends on the identity of the material.
Molar heat capacity (C) – amount of heat needed
to raise the temperature of 1 mol of substance by
1° C.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Temperature Change
q
ΔT =
nC
Where q is the amount of heat transferred, n is the
number of moles of material and C is the molar heat
capacity of the substance in J mol-1 °C-1.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
So you can also say that:
H eat transferred  q  m C  T
Energy transfer is directional, so we must keep track of
the signs associated with heat flows
qsurroundings = – qsystem
Note: T = final temperature - initial temperature
Remember to use the correct algebraic sign!
If Tf < Ti, then the T should be negative.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Example 3 - 1
Calculate the temperature change that results from
adding 250 J of thermal energy to each of the following
(a) 0.75 mol of Hg; (b) 0.35 mol of Hg; (c) 0.35 mol of
H2O.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Example 3 - 2
An aluminum frying pan that weighs 745 g is heated on a
stove from 25°C to 205°C. What is q for the frying pan?
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Work
 Work (w): energy used to move an object against an
opposing force
 The amount of work depends on the magnitude of the
force
wsurroundings = – wsystem
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
First Law of Thermodynamics
Simply a restatement of the law
of conservation of energy
E = q + w
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State and Path Functions
 State Functions: properties that depend only on the
conditions that describe the system. Energy is a state
function.
 Path Functions: properties that depend on how the
change occurs. Distance travelled is a path function.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Distance Travelled is a Path Function…
…but distance between
two cities is
a state function
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Thermodynamic Path Functions
 Energy is a state function, but heat and work are path
functions
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.3 Energy Changes in Chemical Reactions
Learning objective:
Understand the origins of energy changes in chemical
reactions
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3.3 Energy Changes in Chemical Reactions
 Bond breakage requires energy
 Bond formation releases energy.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Example 3 - 3
The Haber reaction for the formation of ammonia
releases energy:
N2 + 3 H2  2 NH3
E =  40.9 kJ
How much energy is released in the production of 1.00 kg
of ammonia?
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Path Independence
A change in any state
function is independent
of path.
Thus, the energy change
in a chemical reaction is
independent of the
manner in which the
reaction takes place.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Bond Energies
 Bond energy (BE): energy required to break a bond,
always positive
 Usually expressed in kJ/mol
H2 (g)  H (g) + H (g) Ebond breaking = BE = + 435 kJ/mol
And the reverse process:
H (g) + H (g)  H2 (g) Ebond making =  BE =  435 kJ/mol
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Table 3 – 2: Average Bond Energies
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Reaction Energy
 Bond energies can be used to estimate the energy
change that occurs in a chemical reaction
 E rxn   BE
(bonds broken)
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-  BE
(bonds form ed)
Example 3 - 4
The world produces tens of billions of vinyl chloride annually. Most
is converted to the polymer poly(vinyl) chloride (PVC), which is
used to make piping, siding, gutters, floor tiles, clothing and toys.
Vinyl chloride is made in a two-step process. The balanced
overall equation is as follows:
Based on average bond energies, what energy change accompanies
the formation of one mole of vinyl chloride? Does the synthesis
require an input of energy?
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.4 Measuring Energy Changes: Calorimetry
Learning objective:
Apply the principles of calorimetry to determine energy
changes in a chemical reaction
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.4 Measuring Energy Changes: Calorimetry
 Calorimeter: a device used to measure heat flows that
accompany chemical reactions.

A Styrofoam cup is a simple calorimeter.
 Exothermic: if the reaction releases heat.
 Endothermic: if the reaction absorbs heat.
qreaction = - qcalorimeter
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
qcalorimeter = CcalT
Example 3 - 5
A calorimeter is calibrated with an electrical heater.
Before the heater is turned on, the calorimeter
temperature is 23.6 °C. The addition of 2.02 x 103 J of
electrical energy from the heater raises the
temperature to 27.6 °C. Determine the total heat
capacity of this calorimeter.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Example 3 - 6
Ammonium nitrate (NH4NO3, M = 80.05 g/mol) is used in
cold packs to “ice” injuries. When 20.0 g of this
compound dissolves in 125 g of water in a coffee-cup
calorimeter, the temperature falls from 23.5 °C to
13.4 °C. Determine the q for the dissolving of the
compound. Is the process exothermic or endothermic?
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Molar Energy Changes
 Energy change is an extensive quantity – it is
dependent on the amount of substance.
ΔEmolar
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
ΔE
=
n
Example 3 - 7
A 0.125 g sample of octane (C8H18, M = 114.2 g/mol) is
burned in excess O2 in the constant-volume calorimeter
described in Example 3 – 5. The temperature of the
calorimeter rises from 21.1 to 32.9 °C. What is Emolar
for the combustion of octane?
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.5 Enthalpy
Learning objective:
Understand and calculate enthalpy and internal energy
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.5 Enthalpy
 Enthalpy (H) is a thermodynamic state function that
describes heat flow at constant pressure.
H = E + pV
 Enthalpy change, H: heat transferred into or out of a
system at constant pressure. For a reaction, it can be
calculated according to:
Hreaction ≈ Ereaction + RTngas
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Example 3 - 8
Find the difference between molar H and E for the
combustion of octane at 298 K.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Energy and Enthalpy of Vapourization
 Changes of state always take place at a constant
temperature
 Heat of vapourization Hvap: heat required to convert
liquid to gas
Hvap ≈ Evap + RTvap
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Enthalpies of Formation
 A formation reaction produces 1 mol of a chemical
substance from the elements in their most stable forms
 There is a single product with a stoichiometric
coefficient of 1.
 All the starting materials are elements, and each is in
its most stable form.
 Enthalpies of reactions involving gases vary with
pressure, so pressures must be specified.
 Enthalpies of reactions occurring in solution vary with
concentration, so concentrations must be specified.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Remember State Functions
 State function - a quantity whose value is determined
only by the state of the system. It does not depend on
the path taken to get there.
 Standard state - is the most stable form of a substance
at T = 25 °C and p = 1 bar, and 1 M if it is in solution
 The superscript ° indicates standard conditions.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Therefore...
 The standard enthalpy change of a reaction for the
formation of 1 mole of a compound directly from its
elements is called the standard molar enthalpy of
formation, Hf°
Mn (s) + O2 (g)  MnO2 (g) Hfo = -520.0 kJ/mol
Br2 (l)  Br2 (g)
Hfo = 30.9 kJ/mol
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Enthalpy Changes for Chemical Reactions
 Hess’s Law: the enthalpy change for any overall
process is equal to the sum of enthalpy changes for any
set of steps that leads from the starting materials to the
products.
 To calculate the total enthalpy change for a reaction,
H°rxn:
H°reaction = Sp H°f,p- S r H°f,r
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Enthalpy Changes Under Nonstandard Conditions
 Energies and enthalpy change as temperature,
concentration, and pressure change.
 Therefore, H depends on these variables, too.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.6 Energy Sources
Learning objective:
Be familiar with our sources of energy
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
3.6 Energy Sources
 Energy and Civilization: advances of civilization can be
viewed as the results of people figuring how to increase
the availability of energy
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Ultimate Energy Sources
 The vast majority of our energy sources originate in
solar energy.
 Photosynthesis converts some solar energy into more
concentrated forms:
6 CO2 (g) + 6 H2O (l) + 2880 kJ  C6H12O6 (s) + 6 O2 (g)
 One nonsolar source is nuclear energy
 Another is the Earth’s hot interior (geothermal) energy
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Future Resources
 Economically, we want energy sources to be high
intensity, and readily extracted and transported.
 Environmentally, they would be renewable and
environmentally benign.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Chapter 3 Visual Summary
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Chapter 3 Visual Summary
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Chapter 3 Visual Summary
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Chapter 3 Visual Summary
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Chapter 3 Visual Summary
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Chapter 3 Visual Summary
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.