Chapter 5 Thermochemistry

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Chapter 5 Thermochemistry
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Types of Systems
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Energy
Energy (E) The capacity to do work or transfer
heat. Kinetic Energy The energy of
motion. Potential Energy Energy due to condition,
position, or composition. Internal Energy The
sum of all kinetic potential energy contained
in a system
First Law of thermodynamics The energy of the
universe is constant (i.e. energy is neither
created nor destroyed)
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Energy Units
Calorie (cal) The quantity of heat required to
change the temperature of one gram of water by
one degree Celsius. Joule (J) SI unit for heat
1 cal 4.184 J (exactly)
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Internal Energy Changes
?E q w
q Heat The transfer of energy due to
temperature changes Heat and temperature are not
the same w Work Force acting through a
distance.
Energy can be transferred between a system and
its surroundings as a result of a temperature
difference (q) and/or work being done (w).
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Internal Energy Changes
The sign of q and w are from the systems point
of view
In an endothermic process, heat flows into a
system - ?E is positive In an exothermic process,
heat flows out of a system - ?E is negative
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Calculate ?E for a system undergoing an
endothermic process in which 15.6 kJ of heat
flows and where 1.4 kJ of work is done on the
system.
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Pressure Volume Work

• The expansion/compression of the system against
  the external atmosphere is pressurevolume work.

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Calculating Energy Changes (?E)
When a reaction is run, the internal energy
(kinetic, potential) can be transferred as heat
and/or pressure-volume work
?E q w q P?V
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How much work is associated with the expansion of
a gas from 46L to 64L at a constant pressure of
15 atm?
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Filling (and heating) a hot air balloon takes
1.3X108 J of heat. At the same time, the volume
changes from 4.00X106 L to 4.5X106 L. Assuming a
constant pressure of 1 atm, what is the energy
change for the process?
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Heat Capacities

• Heat capacity, C
• Amount of heat needed to raise the temperature of
  the system by one degree Kelvin

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Heat Capacities

• Heat capacity, C
• Amount of heat needed to raise the temperature of
  the system by one degree Kelvin
• Molar heat capacity, C
• System one mole of substance.
• Specific heat capacity, c
• System one gram of substance

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If we add 500 J of heat to 25g of water initially
at 25 C, what is the final temperature of the
water? (the specific heat of water 4.184 J/gC)
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Heat Capacity and Conservation of Energy
If substances at two different temperatures are
mixed and allowed to come to a constant
temperature
?a qb
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What is the final temperature when 125 g of iron
at 92.3 C is dropped into 50.0 g of water at
27.7 C? The specific heat of iron is 0.444
J/gC and the specific heat of water is 4.184
J/gC. (Assume an isolated system)
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A 150.0 gram sample of metal at 75.0?C is added
to 150.0g of water at 15.0 ?C . The temperature
of the water rises to 23.0 ?C. What is the
specific heat of the metal? (Assume an isolated
system)
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Reaction (Bomb) Calorimetry
If we run a reaction in an isolated system (a
calorimeter), we can very accurately measure the
heat transferred as a result of the reaction.
Note that this is a constant volume process.
qrxn qcal C?T
Well insulated considered isolated
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Reactions at Constant Volume
?E q P?V
In a system at constant volume, no
pressure-volume work is done
P?V P(0) 0
Therefore, at constant volume, the internal
energy change is equal to the heat of reaction
?E q 0 qv
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A Coffee Cup (Simple) Calorimeter
Note that this is a constant pressure process.
qrxn -qcal
Well insulated considered isolated
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50.0 mL each of 1.0M HCl and 1.0M NaOH at 25 C
at mixed in a calorimeter. After reaction, the
temperature of the calorimeter is 31.9 C. What
is the heat generated for the reaction? (We will
estimate that the specific heat of the system
(the solution the calorimeter) is the same as
that of water 4.184 J/gC)
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State Functions

• Any property that has a unique value for a
  specified state of a system is said to be a State
  Function.
• Water at 293 K and 1.00 atm is in a specified
  state.
• In this state, the density of water is 0.99820
  g/mL
• This density is a unique function of the state.
• It does not matter how the state was established.
• Capitalized letters are used to identify State
  functions

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Internal Energy A State Function
?E has a unique value between two states
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Reactions at Constant Volume
In a system at constant volume, no
pressure-volume work is done
P?V P(0) 0
Therefore, at constant volume, the internal
energy change is equal to the heat of reaction
?E qv
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Reactions at Constant Pressure
Normally, reactions are run at constant pressure
(and changing volume).
At constant pressure, both heat and
pressure-volume work results from energy changes
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Enthalpy Change
Because we are usually only interested in the
heat of reaction at constant pressure, we will
define a new state function
?E qP - P?V
qP ?E P?V
Let H E PV
qP ?H ?E P?V
?H, the enthalpy change, is the measurement we
will generally use to describe thermal changes in
a chemical system.
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Exothermic and Endothermic Reactions
Negative ?H an exothermic reaction Positive ?H
an endothermic reaction
Note Enthalpy change is an extensive property
it is directly proportional to the amount of
substances in the system
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How Does a Hand Warmer Work?
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How much energy is needed to heat the water used
in a 5 minute shower on Colbys campus? Colby
showers are set to 2 gal/min. We will assume
municipal water is at 15 C (about 60 F) and
hot water is 43 C (about 110 F).
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Heat (Enthalpy) of Reaction
All reactions will have an accompanying enthalpy
change
?Hrxn Hfinal - Hinitial
?Hrxn is called heat of reaction because it is
the amount of heat transferred due to the
chemical process.
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Why Do We Use These Energy Sources?
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How much natural gas must we burn to produce the
heat (4435 kJ) needed for a single 5 minute hot
shower on Colbys campus?
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Changes in States of Matter
Why doesnt a pot of (boiling) water at 100 C
all become steam at once? Why do we have to
continually apply heat?
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Changes in States of Matter
Any change of state will cause an enthalpy change

Unless stated otherwise, ?H values are assumed to
be per mole
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Changes in States of Matter
Any change of state will cause an enthalpy change

How much energy is required to convert 5 g of ice
at 0 C to water at 50 C? To steam at 100 C?
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Changes in States of Matter
Any change of state will cause an enthalpy change

Which will cause a more damaging burn skin
exposed to 1 g of water at 100 C or skin exposed
to 1 g of steam at 100 C?
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Manipulating Reaction Enthalpies
The reverse of any reaction will have an equal
enthalpy change of opposite sign.
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Heat (Enthalpy) of Reaction
All reactions will have an accompanying enthalpy
change
?Hrxn Hfinal - Hinitial
?Hrxn ?Hproducts- ?Hreactants
For any reaction, the enthalpy change is the sum
of the product enthalpies minus the sum of the
starting material enthalpies.
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Standard States and Standard Enthalpy Changes

• First we must define a particular state as a
  standard state
• ?Hrxn is the standard enthalpy of reaction
• The enthalpy change of a reaction in which all
  reactants and products are in their standard
  states
• The Standard States are defined as
• The pure element or compound at a pressure of 1
  bar (approximately 1 atm) and at the temperature
  of interest (usually 25 C).

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Standard Enthalpies of Formation

• ?Hf , the standard enthalpy of formation, is the
  enthalpy change that occurs in the formation of
  one mole of a substance in the standard state
  from the reference (most common) forms of the
  elements in their standard states.
• The standard enthalpy of formation of a pure
  element in its standard state is 0.

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Standard Enthalpies of Formation
?Hf for CH2O 108.6 kJ/mol. ?Hf for Al2O3
1670 kJ/mol. ?Hf for Fe2O3 822 kJ/mol.
What reactions do these heats of formation
represent?
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Standard Enthalpies of Formation
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Standard Enthalpies of Formation
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Reaction Summation Hesss Law
Hesss law If a process occurs in stages or
steps (even hypothetically), then the enthalpy
change for an overall process is the sum of the
enthalpy changes for the individual steps.
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Manipulating DH Hesss Law

• The enthalpy change of a chemical transformation
  is directly proportional to the amounts of
  substances
• The reverse of a chemical reaction has an equal
  but opposite DH

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What is the standard enthalpy of reaction for the
thermite reaction? How can we use a table of
standard heats of formation to determine this?
?Hf for Al2O3 1670 kJ/mol. ?Hf for Fe2O3
822 kJ/mol.
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What is the standard enthalpy of reaction for the
reaction below? How can we use a table of
standard heats of formation to determine this?
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What is the standard enthalpy of reaction for the
reaction below? How can we use a table of
standard heats of formation to determine this?
?Hrxn S ?Hfproducts- S ?Hfreactants
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What is the standard enthalpy of reaction for the
formation of N2O5 as shown below?