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Thermochemistry
Thermodynamics
Energy
Heat
calorie/Calorie
Joule
Energy transfers occur btwn the system and
its surroundings
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calorie is the amount of heat energy needed
to raise the temperature of 1 g of water 1oC
Calorie is 1000 calories
Food energy is measured in Calories
 Fats are 9 Calories/g
 Carbs and proteins are 4 Calories/g
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aka Law of conservation of energy
Energy in the universe is constant, cannot be
created or destroyed
Energy can be converted to different forms
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System transfers heat to its surroundings
Temperature of the surroundings increases
“feels hot”
Potential energy of the system is converted
to heat energy that is released (system does
not “cool off”)
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System absorbs heat from the surroundings
Temperature of the surroundings decreases
“feels cool”
Heat energy from the surroundings is
converted into potential energy in the system
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2.
Imagine an ice cube melting in your hand. Is
the melting of ice endothermic or
exothermic? Explain using the terms system
and surroundings
Imagine warming your hands near a
campfire. Is a campfire an endothermic or
exothermic process? Explain.
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Property of a system that explains heat flow
between the system and its surroundings
(constant P)
State function (middle steps don’t matter,
just the beginning and end of a reaction)
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Defined as heat absorbed by the system
during a physical or chemical change
DH is positive for endothermic rxns (because
heat is absorbed by the system)
DH is negative for exothermic rxns (because
heat is released by the system)
Expressed in kJ or kJ/mol
Magnitude of DH is directly proportional to
moles of reactants and products
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DHrxn = heat of reaction of any chemical
reaction
DHcomb = heat of combustion for combustion
reactions (rxns with O2) only
DHfus = heat of fusion when a solid melts
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DHvap = heat of vaporization when a liquid
vaporizes
DHBDE = bond dissociation energy or the heat
required to break a bond
DHf = heat of formation or the heat change
when a compound is formed from its
elements
DHsols = heat of solution or the heat change
when a solute dissolves in a solvent
How many kJ of heat are released when 25.0 g
of methane burn in air? Methane has a
DHcomb of -802 kJ/mol.
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If DH is positive for the forward reaction, then
it will be equal in size but opposite in sign for
the reverse reaction
H2O (s)  H2O (l)
DH = +6.0 kJ/mol
H2O (l)  H2O (s)
DH = -6.0 kJ/mol
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The sign for DH is positive or negative
depending on the direction of energy flow
The sign does NOT indicate a positive or
negative value for energy
What scientific law requires that the
magnitude of the heat change for a forward
and reverse reactions be the same with
opposite signs? Explain.
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DH for phase changes from solid to liquid and
liquid to gas are ALWAYS positive
(endothermic/absorb heat)
DH for phase changes from gas to liquid and
liquid to solid are ALWAYS negative
(exothermic/release heat)
Why is this? What is the sign of DH for the
process of sublimation? Deposition?
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aka. measurement of heat flow
Calorimeter measure heat flow
Heat capacity (C) = amount of heat required
to raise the temperature of any object 1oC.
Expressed in J/K or J/oC
We will more often use specific heat capacity
(Cp) which is the capacity of 1 g of a substance
Water has a Cp of 1 cal/gK or 4.184 J/gK
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Why is the Cp of water 1 cal/gK? What is the
conversion between calories and joules?
What is the molar heat of combustion of liquid
ethanol if the combustion of 9.03 grams of
ethanol causes a calorimeter to increase in
temperature by 3.54 K? The heat capacity of
the calorimeter is 75.8 kJ/K.
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If a reaction is carried out in a series of steps
the DH of the overall reaction is equal to the
sum of the DH’s for each individual step.
Useful for determining DH for reactions that
are difficult to measure directly, like sulfur
trioxide…
The overall reaction is
2S (s) + 3O2 (g)  2SO3 (g)
The reaction occurs in 2 measurable steps
S (s) + O2 (g)  SO2 (g) DHrxn = -269.9 kJ
2SO2 (g) + O2 (g)  2SO3 (g)
DHrxn = -196.6
kJ
To get the total DH for the reaction, manipulate
the equation steps like an algebraic equation.
Whatever you do to the reaction, you must also
do to DH.
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Reaction that produces 1 mole of a substance
from its constituent elements in their most
stable thermodynamic state
To form 1 mole of HI, the equation looks like
this:
½ H2 + ½ I2  HI
DH = +25.94 kJ
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Heat absorbed when 1 mole of a substance is
formed from its elements in their standard
states at 25oC and 1 atm.
There is a BIG table in the back of your book
listing standard heats of formation.
Write the thermochemical equation associated
with the standard heat of formation of AlCl3.
What is the DHof for this equation?
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The table of standard heats of formation
includes elements, ions, and compounds.
The DHof of pure elements is always zero.
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Hess’s law allows us to calculate the DHorxn for
just about any reaction.
Breaking the overall reaction into the
formation reactions for both products and
reactants and then putting them all together
like this:
DHorxn = SnDHof products – SmDHof reactants
Calculate the standard enthalpy change for the
combustion of 1 mole of liquid ethanol. Tip:
carefully watch the signs of DHof.
What is:
1. The enthalpy of sublimation of solid calcium?
2. The heat of solution of gaseous ammonia?
3. The bond dissociation energy of hydrogen gas?
4. The heat change when gaseous bromine
condenses to a liquid?
Write chemical equations to illustrate your answers.