UNIT 5
THERMOCHEMISTRY 1
A study of the energy
produced by
Chemical Reactions
HEAT





Energy: The ability to do work
HEAT: form of energy transferred from a body
at high temperature to one at lower temperature
TEMPERATURE: average kinetic energy of the
molecules of a substance; measures heat flow
JOULES (J) or (kJ): unit of measure of heat (or
any form of energy)
PHASE: state of matter and depends on
temperature and pressure
Heat



System: part of the universe on which
attention is focused
Surrounding- exchanges energy with the
system, make up the rest of the universe
Example: a beaker with water sitting on a
hot plate warming.


System: water
Surroundings: beaker, hot plate, air
surrounding the beaker
HEAT is represented by q

ENDOTHERMIC: heat flows from surroundings
into system; heat absorbed; +q


50 g of water is being warmed on the hot plate
EXOTHERMIC: heat flows from system into
surroundings; heat escapes; -q

The hot plate is turned off and the water starts
heating its surroundings. The temp of water
decreases and the temp of the surroundings increase
Measuring Heat


HEAT CAPACITY: amount of heat to raise the
temperature 1oC; C = J/ D oC (will sometimes be
given in KJ
SPECIFIC HEAT: amount of heat to raise 1 g of
a substance 1oC; c = J/g D oC
 Specific heat is an intensive property.
***Note that Heat Capacity is a Big C and
Specific Heat is a little c
Calorimeter


A device to measure heat flow. Walls are
insulated do no exchange with outside air.
Two-types


Coffee-cup calorimeter
Bomb calorimeter (used for gases)
q reaction = - q calorimeter
Calorimeter




q = m c Dt
q = -C Dt
(q in J, m in g, t in Celsius)
Exothermic when q reaction is < 0, q
calorimeter is +
Endothermic when q reaction is > 0, q
calorimeter is -
Example: Coffee-Cup
Calorimeter

When 1 gram of calcium chloride is added
to 50 grams of water in a coffee-cup
calorimeter, it dissolves. The temperature
rises from 25oC to 28.51oC. Assuming that
all the heat given off by the reaction is
transferred to the water, calculate q for
the reaction system. Is this exothermic or
endothermic?
Example: Bomb Calorimeter

The reaction between hydrogen and
chlorine can be studied in a bomb
calorimeter. It is found that when a 1.00
gram of hydrogen completely reacts, the
temperature rises from 20oC to 29.82oC.
Taking the heat capacity of the
calorimeter to be 9.33 kJ/oC, calculate the
amount of heat evolved in the reaction.
Enthalpy



The is the measure of heat from at
constant pressure between reactants and
products.
DH rxn = DHproducts - DH reactants
q= DH rxn
Exothermic vs Endothermic
Energy Diagram
Thermochemical equations





An equation will be given and it will tell
you the overall DH rxn
The sign of DH rxn indicates whether the
reaction is endothermic and exothermic.
The coefficients represent the number of
moles
The phase symbols must be used
Again, this is at constant pressure and at
25oC
Rules of Thermochemistry
1.
the magnitude of DH is directly
proportional to the amount of reactant
and product



Heat of fusion: solid to liquid
Heat of vaporization: liquid to gas
If going from in opposite direction (liquid to
solid) Heat of fusion is same but opposite
Calorimetry



A 2.200 g sample of quinone, C6H4O2, is
burned in a bomb calorimeter whose heat
capacity is 7.854 kJ/oDC. The temperature
increases from 23.44oC to 30.57oC.
What is the heat of combustion per gram?
Per mole?
Calorimetry


The heat of combustion of glucose,
C6H12O6, is 15.57 kJ/g. A 2.500 g sample
burned in a bomb calorimeter raises the
temperature from 20.55oC to 23.25oC.
What is the heat capacity of the
calorimeter?
Calorimetry


A 1.200 g sample of benzoic acid,
HC7H5O2, is burned in a calorimeter with a
heat capacity of 2.423 kJ/oDC. When the
calorimeter contains 1.500 kg of water,
the temperature rises from 22.45oC to
26.10oC.
What is the heat of combustion of benzoic
acid in kJ/mol?
Phase Changes



The phase of a material changes based on
temperature and pressure.
The phase is dependent on the motion of
the molecules of the substance.
When a substance melts(freezes) or
boils(condenses), there is no temperature
change, but there is a change of heat
involved.
Phase Diagrams
PHASE
DIAGRAM





Normal melting (freezing) and boiling (condensing) points at
standard pressure
Triple point: 3 phases exist at same T&P
Critical temperature: above this T, only exists as a gas
Critical pressure: the pressure to cause condensation at critical
temperature
Sublimation: from solid to vapor directly
Example

How much heat is needed to melt 25 g of
ice?
Example

How much heat is evolved when one mole
of water cools from 100oC to 5oC?
Rules of Thermochemistry
2. DH for a reaction is equal in magnitude by
opposite in sign to DH for the reverse
reaction.
3. The value of DH for a reaction is the
same whether it occurs in one step or in a
series

AKA: Hess’s Law
 DHtotal = DH1 + DH2, etc..
How much heat?


Calculate DH for the decomposition of
liquid sulfuric acid to steam, oxygen and
sulfur dioxide gas.
How much heat will be generated when
25 g of sulfuric acid decomposes?
What is the Hf?


Chlorine trifluoride reacts with ammonia to
form nitrogen, chlorine and hydrogen
fluoride gases. When two moles of
chlorine trifluoride reacts, 1196 kJ of heat
is evolved.
Find Hf for chlorine trifluoride.
Enthalpy Change and Hf



Acetylene, C2H2, and benzene, C6H6, have the
same empirical formula. Benzene can be made
from acetylene: 3C2H2C6H6.
The heats of combustion for C2H2 and C6H6 are 1299.4 kJ/mol and -3267.4 kJ/mol, respectively.
Calculate the heat of formation for each and the
heat of reaction of the formation of benzene
from acetylene.