Download Chapter 5 Thermochemistry

Have you ever wondered how your body
regulates its own temperature?
The human body has the ability to maintain a
constant temperature within the range 96.599°F (35.8-37.2°C)
This is controlled by your own thermostat, the
hypothalamus gland.
This gland controls the rate at which the body
metabolizes glucose and other heat related
functions
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) ΔH = -2803 kJ/mol
Thermochemistry
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) ΔH = -2803 kJ/mol
• For every mole of glucose you burn, you are
burning _____ calories!
• This process releases energy, therefore it is
exothermic.
• The hypothalamus gland controls other
mechanisms in the body to help retain or
release heat.
• Thermochemistry at work!!!
Thermochemistry
Chemistry, The Central Science, 10th edition
Theodore L. Brown; H. Eugene LeMay, Jr.;
and Bruce E. Bursten
Hmwk pg. 188-196, # 7, 12, 15, 19, 21, 23, 29, 33,
41, 45, 51, 53, 59, 61, 67, 71, 77, 83, 97, 103
Chapter 5
Thermochemistry
John D. Bookstaver
St. Charles Community College
St. Peters, MO
 2006, Prentice Hall, Inc.
Thermochemistry
Energy
• The ability to do work or transfer heat.
Work: Energy used to cause an object that
has mass to move.
Heat: Energy used to cause the
temperature of an object to rise.
Thermochemistry
Potential Energy
Energy an object possesses by virtue of its
position or chemical composition.
(Chemicals have potential within the bonds)
Thermochemistry
Kinetic Energy
Energy an object possesses by virtue of its
motion.
1
KE =  mv2
2
Thermochemistry
Units of Energy
• The SI unit of energy is the joule (J).
kg m2
1 J = 1 
s2
• An older, non-SI unit is still in widespread use: The
calorie (cal).
1 cal = 4.184 J
• Calories that we eat are actually kilocalories are
abbreviated as Cal
Thermochemistry
System and Surroundings
• The system includes
the molecules we want
to study (here, the
hydrogen and oxygen
molecules).
• The surroundings are
everything else (here,
the cylinder and
piston).
Thermochemistry
Work
• Energy used to
move an object over
some distance.
• w = F  d,
where w is work, F
is the force, and d is
the distance over
which the force is
exerted.
Thermochemistry
Heat
• Energy can also be
transferred as heat.
• Heat flows from
warmer objects to
cooler objects.
• Recall, cold is
simply a lack of
heat.
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of
clay is increased when it is moved
from the ground to the top of the wall.
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of
clay is increased when it is moved
from the ground to the top of the wall.
b) As the ball falls, its potential energy is
converted to kinetic energy.
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of
clay is increased when it is moved
from the ground to the top of the wall.
b) As the ball falls, its potential energy is
converted to kinetic energy.
c) When it hits the ground, its kinetic
energy falls to zero (since it is no
longer moving); some of the energy
does work on the ball, the rest is
dissipated as heat.
Thermochemistry
First Law of Thermodynamics
• Energy is neither created nor destroyed.
• In other words, the total energy of the universe is
a constant; if the system loses energy, it must be
gained by the surroundings, and vice versa.
Use Fig. 5.5
Thermochemistry
Internal Energy
The internal energy of a system is the sum of all
kinetic and potential energies of all components
of the system; we call it E.
Use Fig. 5.5
Thermochemistry
Internal Energy
By definition, the change in internal energy, E,
is the final energy of the system minus the initial
energy of the system:
E = Efinal − Einitial
Use Fig. 5.5
Thermochemistry
Changes in Internal Energy
• If E > 0, Efinal > Einitial
Therefore, the system
absorbed energy from
the surroundings.
This energy change is
called endergonic.
Thermochemistry
Changes in Internal Energy
• If E < 0, Efinal < Einitial
Therefore, the system
released energy to the
surroundings.
This energy change is
called exergonic.
Thermochemistry
Changes in Internal Energy
• When energy is
exchanged between
the system and the
surroundings, it is
exchanged as either
heat (q) or work (w).
• That is, E = q + w.
Thermochemistry
E, q, w, and Their Signs
Thermochemistry
Exchange of Heat between
System and Surroundings
• When heat is
absorbed by the
system from the
surroundings, the
process is
endothermic.
Thermochemistry
Exchange of Heat between
System and Surroundings
When heat is released
by the system to the
surroundings, the
process is
exothermic.
Thermochemistry
State Functions
Usually we have no way of knowing the
internal energy of a system; finding that value
is simply too complex a problem.
Thermochemistry
State Functions
• However, we do know that the internal energy
of a system is independent of the path by
which the system achieved that state.
 In the system below, the water could have reached
room temperature from either direction.
Thermochemistry
State Functions
• Therefore, internal energy is a state function.
• It depends only on the present state of the
system, not on the path by which the system
arrived at that state.
• And so, E depends only on Einitial and Efinal.
Thermochemistry
State Functions
• However, q and w are
not state functions.
• Whether the battery is
shorted out or is
discharged by running
the fan, its E is the
same.
 But q and w are different
in the two cases.
Thermochemistry
State Functions
Yes!
Your bank account
(the balance is $25
whether you simply
deposited $25 or
deposited $100 and
withdrew $75)
No!
The distance traveled
from Penncrest to
K o P mall. The
distance will depend
on the route taken.
Thermochemistry
Work
When a process
occurs in an open
container, commonly
the only work done is a
change in volume of a
gas pushing on the
surroundings (or being
pushed on by the
surroundings).
Thermochemistry
Work
We can measure the work done by the gas if
the reaction is done in a vessel that has been
fitted with a piston.
w = −PV
Thermochemistry
Enthalpy
• If a process takes place at constant
pressure (as the majority of processes we
study do) and the only work done is this
pressure-volume work, we can account for
heat flow during the process by measuring
the enthalpy of the system.
• Enthalpy is the internal energy plus the
product of pressure and volume:
H = E + PV
Thermochemistry
Enthalpy
• When the system changes at constant
pressure, the change in enthalpy, H, is
H = (E + PV)
• This can be written
H = E + PV
Thermochemistry
Enthalpy
• Since E = q + w and w = −PV, we
can substitute these into the enthalpy
expression:
H = E + PV
H = (q+w) − w
H = q
• So, at constant pressure the change in
enthalpy is the heat gained or lost.
Thermochemistry
Endothermicity and
Exothermicity
• A process is
endothermic, then,
when H is
positive.
Thermochemistry
Endothermicity and
Exothermicity
• A process is
endothermic when
H is positive.
• A process is
exothermic when
H is negative.
Thermochemistry
Endo or Exo? State whether ΔH
is positive or negative
•
•
•
•
•
The combustion of fuel in a car engine
Salting winter roads
Making snow on the ski trails
Skiing down the trails
Snow-covered trails melt in the spring
time.
• Placing an ice pack on an injury
Thermochemistry
• The combustion of fuel in a car engine
Heat is leaving, ΔH is negative, exothermic
• Salting winter roads
Energy between NaCl molecules will leave, ΔH is negative,
exothermic
• Making snow on the ski trails
Heat is leaving, ΔH is negative, exothermic
• Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form
of heat, ΔH is negative, exothermic
• Snow covered trails melt in the spring time
Heat is absorbed, ΔH is positive, endothermic
• Placing an ice pack on an injury
Heat released by injury is absorbed, ΔH is positive, endothermic
Thermochemistry
Enthalpies of Reaction
The change in
enthalpy, H, is the
enthalpy of the
products minus the
enthalpy of the
reactants:
H = Hproducts − Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity, H, is called the enthalpy of
reaction, or the heat of reaction.
Thermochemistry
The Truth about Enthalpy
1. Enthalpy is an extensive property.
2. H for a reaction in the forward
direction is equal in size, but opposite
in sign, to H for the reverse reaction.
3. H for a reaction depends on the state
of the products and the state of the
reactants.
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and
water. The enthalpy change can be
measured in the laboratory as shown
C12H22O11 + 12O2 → 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the
oxidation of 5.00 g of sugar?
q = -82.5 kJ
Thermochemistry
Calorimetry
Since we cannot
know the exact
enthalpy of the
reactants and
products, we
measure H through
calorimetry, the
measurement of
heat flow.
Thermochemistry
Heat Capacity and Specific Heat
• The amount of energy required to raise
the temperature of a substance by 1 K
(1C) is its heat capacity.
• We define specific heat capacity (or
simply specific heat) as the amount of
energy required to raise the temperature
of 1 g of a substance by 1 K.
Thermochemistry
Heat Capacity and Specific Heat
Specific heat, then, is
heat transferred
Specific heat =
mass  temperature change
s=
q
m  T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a
reaction in aqueous
solution in a simple
calorimeter such as this
one, one can indirectly
measure the heat
change for the system
by measuring the heat
change for the water in
the calorimeter.
Thermochemistry
Constant Pressure Calorimetry
Because the specific
heat for water is well
known (4.184 J/g-K), we
can measure H for the
reaction with this
equation:
q = m  s  T
Thermochemistry
Using Specific Heat Capacity
A 88.5-g piece of iron has a temperature of
78.8°C is placed in a beaker containing 244
g of water at 18.8°C. When thermal
equilibrium is reached, what is the final
temperature? (Assume no heat is lost to
warm the beaker or the surroundings.) The
specific heat of iron is 0.45 J/g-K.
Tfinal = 289 K = 16.2°C
Thermochemistry
Another Example
A 15.5-g piece of chromium, heated to
100.0°C, is dropped into 55.5 g of
water at 16.5°C. The final temperature
of the metal and water is 18.9°C.
What is the specific heat capacity of
chromium? (Assume no heat is lost to
warm the container or the
surroundings.)
Thermochemistry
smetal = 0.44 J/g·K
Bomb Calorimetry
Reactions can be
carried out in a sealed
“bomb,” such as this
one, and measure the
heat absorbed by the
water.
The bomb and its
contents are defined as
the system:
qrxn = -Ccal x ΔT
Ccal is the heat capacity
of the bomb calorimeter.
Thermochemistry
Bomb Calorimetry
• Because the volume in
the bomb calorimeter is
constant, work cannot
occur. What is measured
is really the change in
internal energy, E, not
H.
• For most reactions, the
difference is very small.
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid
rocket fuel. The combustion of methylhydrazine is as
follows:
2CH6N2 + 5O2 → 2N2 + 2CO2 + 6H2O
When 4.00 g of methylhydrazine is combusted in a
bomb calorimeter, the temperature of the calorimeter
increases from 25.00°C to 39.50°C. In a separate
experiment the heat capacity of the calorimeter is
measured to be 7.794 kJ/°C. What is the heat of
reaction for the combustion of a mole of CH6N2 in this
calorimeter? Was the reaction endo- or exothermic?
-1.30 x103 kJ/mol CH6N2
Thermochemistry
Hess’s Law
 H is well known for many reactions,
and it is inconvenient to measure H
for every reaction in which we are
interested.
• However, we can estimate H using
H values that are published and the
properties of enthalpy.
Thermochemistry
Recall…The Truth about Enthalpy
1. Enthalpy is an extensive property.
2. H for a reaction in the forward
direction is equal in size, but opposite
in sign, to H for the reverse reaction.
3. H for a reaction depends on the state
of the products and the state of the
reactants.
Thermochemistry
Hess’s Law
Hess’s law states that
“If a reaction is carried
out in a series of
steps, H for the
overall reaction will be
equal to the sum of
the enthalpy changes
for the individual
steps.”
Thermochemistry
Hess’s Law
Because H is a state
function, the total
enthalpy change
depends only on the
initial state of the
reactants and the final
state of the products.
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) → C2H2 (g)
Given the following reactions and their
respective enthalpy changes
C2H2 (g) + 5/2O2 (g) → 2CO2 (g) + H2O (l) ΔH = -1299.6 kJ
C (s) + O2 (g) → CO2 (g) ΔH = -393.5 kJ
H2 (g) + 1/2O2 (g) → H2O (l) ΔH = -285.8 kJ
226.8 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change
for the formation of methane, CH4, from solid
carbon (as graphite) and hydrogen gas:
C(s) + 2H2 (g) → CH4 (g)
ΔH = ?
The enthalpy change for this reaction cannot be
measured in the laboratory because the
reaction is very slow. We can, however,
measure the enthalpy changes for the
combustion of carbon, hydrogen and
methane.
C (s) + O2 (g) → CO2 (g)
ΔH = -393.5 kJ
H2 (g) + ½ O2 (g) → H2O (l) ΔH = -285.8 kJ
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) ΔH = -890.3 kJ
Use these energies to obtain ΔH for the
Thermochemistry
formation of methane.
ΔH = -74.8 kJ
Calculation of H
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (l)
• Imagine this as occurring
in 3 steps:
C3H8 (g)  3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g)  3 CO2 (g)
4 H2 (g) + 2 O2 (g)  4 H2O (l)
Thermochemistry
Calculation of H
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (l)
• Imagine this as occurring
in 3 steps:
C3H8 (g)  3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g)  3 CO2 (g)
4 H2 (g) + 2 O2 (g)  4 H2O (l)
Thermochemistry
Calculation of H
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (l)
• Imagine this as occurring
in 3 steps:
C3H8 (g)  3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g)  3 CO2 (g)
4 H2 (g) + 2 O2 (g)  4 H2O (l)
Thermochemistry
Calculation of H
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (l)
• The sum of these
equations is:
C3H8 (g)  3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g)  3 CO2 (g)
4 H2 (g) + 2 O2 (g)  4 H2O (l)
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (l)
Thermochemistry
Enthalpies of Formation
An enthalpy of formation, Hf, is defined
as the enthalpy change for the reaction
in which a compound is made from its
constituent elements in their elemental
forms.
Thermochemistry
Standard Enthalpies of Formation
Standard enthalpies of formation, Hf, are measured under
standard conditions (25°C and 1.00 atm pressure) and

represent the change in enthalpy associated with the reaction
that forms 1 mole of the compound from its elements, with all
substances in their standard states. If the element exists in
more than one form, the most stable form of the element is used
for the formation reaction.
Thermochemistry
Calculation of H
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (l)
H =
=
=
=
[3(-393.5 kJ) + 4(-285.8 kJ)] - [1(-103.85 kJ) + 5(0 kJ)]
[(-1180.5 kJ) + (-1143.2 kJ)] - [(-103.85 kJ) + (0 kJ)]
(-2323.7 kJ) - (-103.85 kJ)
Notice, that by
-2219.9 kJ
definition, the
standard enthalpy of
formation of the
most stable form of
any element is zero,
and therefore, the
standard enthalpy Thermochemistry
of
O2 is zero.
Calculation of H
We can use Hess’s law in this way:


H = nHf(products)
- mHf(reactants)
where n and m are the stoichiometric
coefficients.
Thermochemistry
Standard Enthalpies of Formation
Example: Write the equation
corresponding to the standard enthalpy
of formation of acetylene (C2H2) gas.
C(graphite) + H2(g) → C2H2 (g)
Write the equation corresponding to the
standard enthalpy of formation of solid
silver chloride.
Ag(s) + ½Cl2(g) → AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of
reaction, use the standard enthalpies of
formation in the table to calculate the
standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) → Cu(s) + H2O(l)
ΔH° = -129.7 kJ
Thermochemistry
Energy in Foods
Most of the fuel in the
food we eat comes
from carbohydrates
and fats.
The energy content in
foods in reported in
Calories and is
determined with the
use of a bomb
calorimeter.
Thermochemistry
Fuels
The vast majority
of the energy
consumed in this
country comes
from fossil fuels.
Thermochemistry
Thermochemistry is Clearly
Important to Our Everyday Lives
• Personal Nutrition – nutritional value
labels
• Creation of alternative fuel sources
• Addressing Global Warming
Thermochemistry
Chapter Summary
•
•
•
•
•
•
Thermodynamics is the study of energy and its transformations.
All chemical changes involve a transfer of energy, be it into the reaction
or out of the reaction.
Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat and/or
work.
When a gas is produced or consumed in a chemical reaction at
constant pressure, we call the energy change enthalpy. All substances
have a characteristic enthalpy.
Calorimeters and bomb calorimeters are devices that allow us to
measure the enthalpy change of a chemical reaction by measuring the
temperature change associated with such a chemical reaction.
Because enthalpy is a state function, the enthalpy of a reaction
depends only on the final and initial states of the system and therefore,
the enthalpy of a process is the same whether a reaction is carried out
in one or in multi-steps. Hess’s Law allows us the calculate the
unknown enthalpy of a reaction from a series of know enthalpies of
reactions.
Thermochemistry