Chemistry 104
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Two Key Questions
1. Will a chemical reaction
go?
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Two Key Questions
1. Will a chemical reaction
go?
2. If a reaction goes, how
fast?
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Two Key Questions
1. Will a chemical reaction
go? (thermodynamics)
2. If a reaction goes, how
fast?
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Two Key Questions
1. Will a chemical reaction
go? (thermodynamics)
2. If a reaction goes, how
fast? (kinetics)
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Thermochemistry
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Thermochemistry
Thermochemistry: Deals with energy changes in
chemical reactions.
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Thermochemistry
Thermochemistry: Deals with energy changes in
chemical reactions.
Energy: Capacity to do work or transfer heat.
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Thermochemistry
Thermochemistry: Deals with energy changes in
chemical reactions.
Energy: Capacity to do work or transfer heat.
Work:
work  force x distance
W  Fd
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Thermochemistry
Thermochemistry: Deals with energy changes in
chemical reactions.
Energy: Capacity to do work or transfer heat.
Work:
work  force x distance
W  Fd
Kinetic energy: Kinetic energy  1 mass x velocity2
2
EKE  1 m v2
2
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Potential energy:
Potential energy  mass x (accel. due to gravity)x height
EPE  m g h
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Potential energy:
Potential energy  mass x (accel. due to gravity)x height
EPE  m g h
Energy units: In the SI system of units (International
System of units, from the French Système
International d’Unités) the unit of energy is the
joule.
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A common non SI unit of energy is the calorie:
1 cal = 4.184 J
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A common non SI unit of energy is the calorie:
1 cal = 4.184 J
The energy unit used for food is the Calorie:
1 Cal = 1000 cal
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Some definitions
Thermodynamics : The scientific discipline that
deals with the interconversion of heat and other
forms of energy.
Thermochemistry is a sub-branch of
thermodynamics.
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Some definitions
Thermodynamics : The scientific discipline that
deals with the interconversion of heat and other
forms of energy.
Thermochemistry is a sub-branch of
thermodynamics.
System: Any predefined part of the universe that is
of interest to us.
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Some definitions
Thermodynamics : The scientific discipline that
deals with the interconversion of heat and other
forms of energy.
Thermochemistry is a sub-branch of
thermodynamics.
System: Any predefined part of the universe that is
of interest to us.
Surroundings: Everything in the universe except for
the part that has been defined as the system.
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Open System: A system that can exchange both
mass and energy with its surroundings.
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Open System: A system that can exchange both
mass and energy with its surroundings.
Closed system: A system that allows energy but not
mass transfer with its surroundings.
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Open System: A system that can exchange both
mass and energy with its surroundings.
Closed system: A system that allows energy but not
mass transfer with its surroundings.
Isolated system: A system that does not allow
energy or mass transfer with its surroundings.
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Open System: A system that can exchange both
mass and energy with its surroundings.
Closed system: A system that allows energy but not
mass transfer with its surroundings.
Isolated system: A system that does not allow
energy or mass transfer with its surroundings.
State of a system: The macroscopic variables such
as composition, volume, pressure, temperature,
etc., that define a particular system.
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State Function: Any property of a system that
is fixed by the state the system is in. A change
in a state function is independent of the path
followed – it depends only on the initial and
final states of the system.
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Heats of reaction
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Heats of reaction
Most physical and chemical processes take place
under constant pressure conditions.
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Heats of reaction
Most physical and chemical processes take place
under constant pressure conditions.
The heat change (absorbed or evolved) of any
reaction carried out at a constant pressure is called
the enthalpy change.
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Heats of reaction
Most physical and chemical processes take place
under constant pressure conditions.
The heat change (absorbed or evolved) of any
reaction carried out at a constant pressure is called
the enthalpy change.
Enthalpy is represented by the symbol H.
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Heats of reaction
Most physical and chemical processes take place
under constant pressure conditions.
The heat change (absorbed or evolved) of any
reaction carried out at a constant pressure is called
the enthalpy change.
Enthalpy is represented by the symbol H.
Change in enthalpy is represented by the symbol
H .
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The symbol delta  is used to denote a change.
So
X  Xfinal - Xinitial
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The symbol delta  is used to denote a change.
So
X  Xfinal - Xinitial
Example: For a phase change, such as
liquid water
water vapor
that is,
H2O(l)
H2O(g)
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The symbol delta  is used to denote a change.
So
X  Xfinal - Xinitial
Example: For a phase change, such as
liquid water
water vapor
that is,
H2O(l)
H2O(g)
H enthalpy (water vapor) - enthalpy (liquid water)
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The symbol delta  is used to denote a change.
So
X  Xfinal - Xinitial
Example: For a phase change, such as
liquid water
water vapor
that is,
H2O(l)
H2O(g)
H enthalpy (water vapor) - enthalpy (liquid water)
H  44.0kJ (at 25 oC)
(think about a steam burn)
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For the combustion of methane:
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l)
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For the combustion of methane:
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l)
H  enthalpy of CO2(g) + 2 enthalpy of H2O(l)
- enthalpy of CH4(g) - 2 enthalpy of O2(g)
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For the combustion of methane:
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l)
H  enthalpy of CO2(g) + 2 enthalpy of H2O(l)
- enthalpy of CH4(g) - 2 enthalpy of O2(g)
that is, H is the enthalpy of the products enthalpy of reactants, with the stoichiometric
factors included.
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For the combustion of methane:
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l)
H  enthalpy of CO2(g) + 2 enthalpy of H2O(l)
- enthalpy of CH4(g) - 2 enthalpy of O2(g)
that is, H is the enthalpy of the products enthalpy of reactants, with the stoichiometric
factors included.
Note: that it is important to specify the state of
the reactant, i.e. is it solid, liquid, or gas.
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Keep the following points in mind:
1. The stoichiometric coefficients always refer to the
number of moles.
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Keep the following points in mind:
1. The stoichiometric coefficients always refer to the
number of moles.
2. We must always specify the physical state of the
reactants and products.
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Keep the following points in mind:
1. The stoichiometric coefficients always refer to the
number of moles.
2. We must always specify the physical state of the
reactants and products.
3. The enthalpy of a substance increases with
temperature. It follows that the enthalpy change
for a reaction must also depend on the
temperature.
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Enthalpy changes are most commonly tabulated at a
temperature of 25 oC.
Temperature changes during a reaction present no
problem. If the products form at a temperature
higher than 25 oC, they will eventually cool down to
25 oC, and the heat evolved on cooling will become
part of the enthalpy change for the reaction.
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Endothermic process: If a reaction (or other
process) is accompanied by absorption of heat
from the surroundings, H is positive, and the
reaction (or process) is said to be endothermic.
Example:
H2O(s)
H2O(l)
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Exothermic process: If a reaction (or other
process) produces heat, H is negative, then the
reaction (or process) is said to be exothermic.
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CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l) + heat
We can think of the heat very loosely as a
“reactant” or “product”. In the case of the reaction
above, heat is produced in the reaction.
Hence, the reaction is exothermic.
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Whenever a reaction is reversed, the magnitude of
H remains the same, but its sign is reversed.
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Whenever a reaction is reversed, the magnitude of
H remains the same, but its sign is reversed.
Example: H2O(l)
H2O(g)
H2O(g) H  44.0 kJ
H2O(l)
H  -44.0 kJ
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If we multiply both sides of a chemical equation (or
more general process relationship, e.g. a phase
transition) by a factor n, the H must also change
by the same factor.
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If we multiply both sides of a chemical equation (or
more general process relationship, e.g. a phase
transition) by a factor n, the H must also change
by the same factor.
Example: H2O(l)
H2O(g) H  44.0 kJ
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If we multiply both sides of a chemical equation (or
more general process relationship, e.g. a phase
transition) by a factor n, the H must also change
by the same factor.
Example: H2O(l)
2 H2O(l)
H2O(g) H  44.0 kJ
2 H2O(g) H  88.0 kJ
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If we multiply both sides of a chemical equation (or
more general process relationship, e.g. a phase
transition) by a factor n, the H must also change
by the same factor.
Example: H2O(l)
H2O(g) H  44.0 kJ
2 H2O(l)
2 H2O(g) H  88.0 kJ
½ H2O(l)
½ H2O(g) H  22.0 kJ
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Standard enthalpies of formation
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Standard enthalpies of formation
The enthalpy of formation of a compound is the
heat change that occurs when one mole of a
compound is synthesized from its elements under
constant pressure conditions.
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Standard enthalpies of formation
The enthalpy of formation of a compound is the
heat change that occurs when one mole of a
compound is synthesized from its elements under
constant pressure conditions.
This quantity varies with changes in experimental
conditions, so the following definition is adopted.
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Standard enthalpy of formation: The standard
enthalpy of formation of a compound is the heat
change when one mole of the compound is formed
from its component elements in their standard
states under constant pressure conditions.
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Standard enthalpy of formation: The standard
enthalpy of formation of a compound is the heat
change when one mole of the compound is formed
from its component elements in their standard
states under constant pressure conditions.
The standard state of a substance is the most stable
form at 1 bar. Formally, this was 1 atm.
(1 atm = 1.01325 bar, so the new and old
definitions are very close.)
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Usually a reference temperature of 25oC is
selected (for the purposes of data tabulation).
Symbol employed: H0f (common usage)
 f H0 (IUPAC recommended)
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Usually a reference temperature of 25oC is
selected (for the purposes of data tabulation).
Symbol employed: H0f (common usage)
 f H0 (IUPAC recommended)
The superscript refers to standard state. The
subscript refers to the formation of 1 mole of the
compound.
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Usually a reference temperature of 25oC is
selected (for the purposes of data tabulation).
Symbol employed: H0f (common usage)
 f H0 (IUPAC recommended)
The superscript refers to standard state. The
subscript refers to the formation of 1 mole of the
compound.
Examples: C(graphite) + O2(g)
CO2(g)
H0f = -393.5 kJ/mol
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Usually a reference temperature of 25oC is
selected (for the purposes of data tabulation).
Symbol employed: H0f (common usage)
 f H0 (IUPAC recommended)
The superscript refers to standard state. The
subscript refers to the formation of 1 mole of the
compound.
Examples: C(graphite) + O2(g)
CO2(g)
H0f = -393.5 kJ/mol
C(graphite) + ½ O2(g)
CO(g)
H0f = -110.5 kJ/mol
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General reaction: Enthalpy change
Consider the reaction
aA + bB
cC + dD
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