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Essentials of General Chemistry,
2nd Edition
Ebbing • Gammon • Ragsdale
Chapter 6
Thermochemistry
Thermochemistry
Thermochemistry
Thermodynamics is the science of the
relationships between heat and other forms of
energy.
Thermochemistry is one area of thermodynamics,
that deals with study of the quantity of heat
absorbed or evolved by chemical reactions.
Energy is defined as the capacity to move matter.
Decomposition of ammonium dichromate
Thermochemistry
Energy and Its Units
The SI unit of energy, kg.m2/s2, is given the name joule (J).
The non-SI unit: The Calorie(cal) is defined as the
amount of energy required to raise the temp. of one gram of
water by one degree Celsius
1cal = 4.184 J
(exact definition)
Energy can be in many forms that can be interconverted
Radiant Energy—Electromagnetic radiation.
Thermal Energy—Associated with random motion of a
molecule or atom.
Chemical Energy—Energy stored within the structural
limits of a molecule or atom.
Thermochemistry
Energy
There are three broad concepts of energy:
Kinetic energy is the energy associated with an
object by virtue of its motion.
Potential energy is the energy an object has by
virtue of its position in a field of force.
Internal energy is the sum of the kinetic and
potential energies of the particles making up a
substance.
We will look at each of these in detail
Thermochemistry
Kinetic Energy
Kinetic Energy: Is energy due to motion of the
object.
An object of mass m and speed or velocity v
has kinetic energy Ek equal to
1 mv 2
2
This shows that the kinetic energy of an object
depends on both its mass and its speed
The SI unit of energy, kg.m2/s2, is given the name
joule (J).
Ek
Thermochemistry
Potential Energy
Potential Energy: This energy depends on the
“position” (such as height) in a “field of force” (such
as gravity).
For example, water of a given mass m at the top of
a dam is at a relatively high “position” h in the
“gravitational field” g of the earth.
Ep
mgh
Thermochemistry
Law of Conservation of Energy
Internal energy (U). The energy of the particles
making up a substance
The total energy of a system is the sum of its
kinetic energy, potential energy, and internal
energy, U.
Etot
Ek
Ep
U
The Law of Conservation of Energy: Energy
may be converted from one form to another, but
the total quantity of energy remains constant.
Thermochemistry
Heat of Reaction
In chemical reactions, heat is often transferred
from the “system” to its “surroundings,” or vice
versa.
Thermodynamic system : The substance or
mixture of substances under study in which a
change occurs is called the system.
Open, closed and isolated system
The surroundings are everything in the vicinity of
the thermodynamic system.
Thermochemistry
Thermodynamic System
Thermochemistry
Heat
Heat (q) is defined as the energy that flows into or
out of a system because of a difference in
temperature between the system and its
surroundings.
Heat flows from a region of higher temperature to
one of lower temperature; once the temperatures
become equal, heat flow stops.
Thermochemistry
Heat; Sign Convention
The sign of q is +ve if heat is absorbed by the
system.
The sign of q is - ve if heat is evolved by the
system
Thermochemistry
Heat of Reaction
Heat of Reaction is the value of q required to
return a system to the given temperature at the
completion of the reaction.
Classification
An exothermic process is a chemical reaction or
physical change in which heat is evolved (q is
negative).
An endothermic process is a chemical reaction
or physical change in which heat is absorbed (q is
positive).
Thermochemistry
Heat of reaction
Exothermicity
“out of” a system
Surroundings
Endothermicity
“into” a system
Surroundings
Energy
Energy
System
System
q<0
q>0
Thermochemistry
Enthalpy and Enthalpy Change
The heat absorbed or evolved by a reaction or
phase change depends on the conditions under
which it occurs.
Usually, a reaction takes place in an open vessel,
and therefore at the constant pressure of the
atmosphere.
The heat of this type of reaction is denoted as qp,
the heat at constant pressure
Thermochemistry
Enthalpy
Enthalpy,(H) , is an extensive property of a
substance that can be used to obtain the heat
absorbed or evolved in a chemical reaction.
(related to qp)
An extensive property is one that depends on the
quantity of substance (mass).
A state function is a property of a system that
depends only on its present state and is
determined by variables such as temperature and
pressure and is independent of any previous
history of the system .
Enthalpy is a state function,
Thermochemistry
An Analogy to Illustrate a State
Function
Thermochemistry
Enthalpy Change
Enthalpy of reaction H:The change in enthalpy for a
reaction (or a phase change) at a given temperature
and pressure is called the enthalpy of reaction and is
obtained by subtracting the enthalpy of the reactants
from the enthalpy of the products.
H H(products) H(reactants)
The enthalpy of reaction is equal to the heat of
H
reaction at constant pressure.
qp
Thermochemistry
Enthalpy change for the Physical
or chemical change
Physical change: Change in state (endothermic)
Melting
solid to liquid,
H2O(s)
H2O(l)
Hfusion = + 6.01 kJ/mol
Vaporization
Liquid to gas
H2O(l)
H2O(g)
Hvap = + 44.0 kJ/mol
Sublimation
solid to gas
H2O(s)
H2O(g)
Hsub = + 46.71 kJ/mol
Chemical change: Formation of water
CH4(g) + 2 O2(g)
2NH3 (g)
CO2(g) + 2H2O(g)
H = - 890 kJ
(exothermic)
N2(g) + 3H2(g) H = + 91.8 kJ
(endothermic)
H=
H(products) -
H (reactants)
Thermochemistry
Enthalpy Diagram
2Na(s) + 2H2O(l)
H=
2NaOH(aq) + H2(g)
H(products) -
H = - 368.6 kJ
H (reactants)
Enthalpy of the system is
decreased by 368.6 kJ.
Thermochemistry
Thermochemical Equations
A thermochemical equation is the chemical
equation for a reaction (including phase labels) in
which the equation is given a molar interpretation,
and the enthalpy of reaction for these molar
amounts is written directly after the equation.
For example
N2 (g) 3H2 (g)
2NH3 (g); H -91.8kJ
Thermochemistry
Thermochemical Equations
In a thermochemical equation it is important to
note phase labels because the enthalpy change,
H, depends on the phase of the substances.
2H 2 ( g ) O 2 ( g )
2H 2O(g ) ; Ho
- 483.7 kJ
2H 2 ( g ) O 2 ( g )
2H 2O(l ) ; Ho
- 571.7 kJ
Thermochemistry
Thermochemical Equations
The two important rules for manipulating
thermochemical equations:
1. When a thermochemical equation is multiplied by
any factor, the value of H for the new equation
is obtained by multiplying the H in the original
equation by that same factor.
2. When a chemical equation is reversed, the value
of H is reversed in sign.
Thermochemistry
Manipulating Thermochemical
Equations
Examples (in class)
Thermochemistry
Calculating the Heat Obtained
from a Reaction
Thermochemistry
Applying Stoichiometry and
Heats of Reactions
Consider the reaction of methane, CH4, burning in
the presence of oxygen at constant pressure.
Given the following equation, how much heat could
be obtained by the combustion of 10.0 grams
CH4?
CH4(g) 2O2(g)
10.0 g CH 4
CO2(g) 2H2O(l); H o
1 mol CH 4
16.0 g
890.3 kJ
1 mol CH 4
-890.3 kJ
556 kJ
Thermochemistry
Sample Problem
• How much heat (in kJ) is absorbed when 4.88g of barium
hydroxide octahydrate react with ammonium chloride in the
following reaction.
Thermochemistry
Measuring Heats of Reaction
Heat capacity (C): The heat required to raise the
temperature of a substance is its heat capacity.
Heat Capacity and Specific Heat
The heat capacity, C, of a sample of
substance is the quantity of heat required to
raise the temperature of the sample of
substance by one degree Celsius.
Changing the temperature of the sample requires
heat equal to:
t = tf-ti
q
C t
Thermochemistry
Specific Heat
The specific heat capacity (or “specific heat”) is
the heat required to raise the temperature of
one gram of a substance by one degree
Celsius.
To find the heat required you must multiply the
specific heat, s, of the substance times its mass
in grams, m, and the temperature change, t.
q s m
t
Thermochemistry
Thermochemistry
Sample Problem
Calculate the heat absorbed when the temperature of 10.0
grams of water is raised from 20.0ºC to 50.0ºC. The specific
heat of water is 4.184 J/(g. ºC).
Thermochemistry
Measuring of Heats of Reaction
Calorimetery
A calorimeter is a device used to measure the
heat absorbed or evolved during a physical or
chemical change. (See Figure 6.10)
The heat absorbed by the calorimeter and its
contents is the negative of the heat of reaction.
qcalorimeter
q rxn
Thermochemistry
Coffee-Cup Calorimeter
Most of the reactions in the
lab. are carried out in open
Containers at constant
pressure
•Coffee-cup is a constant
pressure
calorimeter.
•Heat can be directly related
to enthalpy change H
qp = H
Thermochemistry
Bomb Calorimeter
For combustion reactions , the reactions involving
gases, a bomb calorimeter is used.
Bomb Calorimeter is constant volume calorimeter
and heat of reaction q does not in general equal to
H.
A small correction is usually needed
This correction is negligible
when the reaction does not involve gases
or when
# of moles of reactant gas= # of moles of product gas
Thermochemistry
Bomb Calorimeter
The reaction is started by an
Ignition coil running through
The graphite sample.
qcalorimeter
qrxn
Thermochemistry
Sample Problem
When 23.6 grams of calcium chloride, CaCl2, was dissolved in
water in a calorimeter, the temperature rose from 25.0ºC to
38.7ºC. If the heat capacity of the solution and the calorimeter
is 12.58 kJ/ºC, what is the enthalpy change per mole of
calcium chloride?
Solution:
First, calculate the heat released by the reaction,
using the heat absorbed by the calorimeter.
Calculate moles of Calcium chloride using molar
mass of 111.1 g/mol.
Then calculate the heat per mole of calcium
chloride.
Thermochemistry
Using Heats of Reaction
Hess’s Law of heat summation states that for a
chemical equation that can be written as the sum
of two or more steps, the enthalpy change for the
overall equation is the sum of the enthalpy
changes for the individual steps.
Some times it is not possible to directly determine
the enthalpy change H for a reaction, as reaction
may take place in several steps instead of a single
step.
We can apply Hess’s Law to calculate H for
the reaction e.g.
2C (graphite) + O2 (g) 2CO (g)
H=?
Thermochemistry
Enthalpy Diagram Illustrating
Hess’s Law
Thermochemistry
Hess’s Law
For example, suppose you are given the following
data:
S( s ) O 2 ( g )
2SO3 (g)
SO 2 (g );
Ho
-297 kJ
o
2SO2 (g) O2 (g); H
198kJ
Could you use these data to obtain the enthalpy
change for the following reaction?
2S(s ) 3O 2 (g )
2SO 3 (g );
Ho
?
Thermochemistry
Hess’s Law
If we multiply the first equation by 2 and reverse
the second equation, they will sum together to
become the third
2S(s ) 2O 2 (g )
2SO 2 ( g ) O 2 ( g )
2S(s ) 3O 2 ( g )
2SO 2 (g );
Ho
2SO 3 ( g );
Ho
2SO 3 (g );
Thermochemistry
(-297 kJ) (2)
(198 kJ) (-1)
Ho
-792 kJ
Standard Enthalpies of
Formation
The term standard state refers to the standard
thermodynamic conditions chosen for substances
when listing or comparing thermodynamic data:
1 atm pressure and the specified
temperature (usually 25oC).
The enthalpy change for a reaction in which
reactants are in their standard states is denoted
Hº (“delta H zero” or “delta H naught”).
Standard state is denoted by a superscript degree
sign (º ).
Thermochemistry
Standard Enthalpies of
Formation
The standard enthalpy of formation( Hfo)of
a substance, is the enthalpy change for the
formation of one mole of a substance in its
standard state from its component
elements in their standard state.
Note that the standard enthalpy of
formation for a pure element in its
standard state is zero.
Thermochemistry
Standard Enthalpies of
Formation
An Allotrope is one of two or more distinct
forms of an element in the same physical
state e.g. O2 and O3
The reference form of an element is usually
the stable form (physical state and
allotrope) of the element under standard
thermodynamic conditions.
E.g. O2 is the stable form of oxygen
under standard thermodynamic
conditions
Thermochemistry
Allotropes of Sulfur
Rhombic sulfur
S8
monoclinic sulfur
Rhombic sulfur is stable form at room temperature.
Thermochemistry
Standard Enthalpies of
Formation
The law of summation of heats of formation states
that the enthalpy of a reaction is equal to the total
formation energy of the products minus that of the
reactants.
Ho
n Hof (products)
m Hof (reactants)
means “the sum of,” and m and n are the
coefficients of the substances in the chemical
equation..
Thermochemistry
Table 6.2
Thermochemistry
Table 6.2 (Cont’d)
Thermochemistry
Table 6.2 (Cont’d)
Thermochemistry
A Problem to Consider
Large quantities of ammonia are used to prepare nitric acid
according to the following equation:
4NH3 (g) 5O2 (g)
4NO(g) 6H2O(g)
What is the standard enthalpy change for this reaction? Use
Table 6.2 for data.
Solution
You record the values of Hfo under the formulas in the
equation, multiplying them by the coefficients in the equation:
• You can calculate Hº by subtracting the values for the
reactants from the values for the products.
• Be careful of arithmetic signs as they are a likely source of
mistakes.
Thermochemistry
Enthalpy of formation for Ions
The standard enthalpy of formation of ions
It is not possible to make thermal measurement
on individual ions
We must arbitrarily define the standard enthalpy
of formation of one ion as zero.The enthalpy of
other ions can be deduced from calorimetric
data.
The standard enthalpy of formation for H+ ion (aq)
is taken as zero.
Thermochemistry
Fuels
A fuel is any substance that is burned to provide heat or
other forms of energy.
In this section we will look at:
Foods as fuels
Fossil fuels
Coal gasification and liquefaction
Food fills three needs of the body:
It supplies substances for the growth and repair of tissue.
It supplies substances for the synthesis of compounds
used in the regulation of body processes.
It supplies energy. About 80% of the energy we need is
for heat. The rest is used for muscular action and other
body processes.
Thermochemistry
Fuels
A typical carbohydrate food, glucose (C6H12O6)
undergoes combustion according to the following
equation:
C6 H12O 6 (s ) 6O 2 ( g )
6CO 2 (g ) 6H 2O( l );
Ho
-2803 kJ
One gram of glucose yields 15.6 kJ (3.73 kcal)
when burned.
Thermochemistry
Sources of Energy Consumed in the
United States (1996)
Figure 6.14:
Thermochemistry
Fossil Fuel
Fossil fuels account for nearly 90% of the energy
usage in the United States.
Anthracite, or hard coal, the oldest variety of
coal, contains about 80% carbon.
Bituminous coal, a younger variety of coal,
contains 45% to 65% carbon.
Fuel values of coal are measured in BTUs
(British Thermal Units).
A typical value for coal is 13,200 BTU/lb.
1 BTU = 1054 kJ
Thermochemistry
Natural gas and petroleum
Natural gas and petroleum account for nearly
three-quarters of the fossil fuels consumed per
year.
Purified natural gas is primarily methane, CH4,
but also contains small quantities of ethane,
C2H6, propane, C3H8, and butane, C4H10.
We would expect the fuel value of natural gas to
be close to that for the combustion
CH4 (g) 2O2 (g)
CO2 (g) 2H2O(g) ; Ho
Thermochemistry
802 kJ
Petroleum
Petroleum is a very complicated mixture of
compounds.
Gasoline, obtained from petroleum, contains
many different hydrocarbons, one of which is
octane, C8H18.
C8 H18 (l )
25 O ( g )
2
2
8CO 2 (g ) 9H 2O(g );
This value of
Ho
Ho
-5,074 kJ
is equivalent to 44.4 kJ/gram.
Thermochemistry
Coal
With supplies of petroleum estimated to be 80% depleted by
the year 2030, the gasification of coal has become a possible
alternative.
First, coal is converted to carbon monoxide using steam.
C(s ) H 2O(g )
CO(g ) H 2 (g )
The carbon monoxide can then be used to
produce a variety of other fuels, such as
methane.
CO(g ) 3H 2 (g )
CH 4 (g ) H 2O(g )
Thermochemistry
Operational Skills
Writing thermochemical equations.
Manipulating thermochemical equations.
Calculating the heat of reaction from the stoichiometry.
Relating heat and specific heat.
Calculating H from calorimetric data.
Applying Hess’s law.
Calculating the heat of phase transition from standard
enthalpies of formation.
Calculating the enthalpy of reaction from standard
enthalpies of formation.
Thermochemistry
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