Thermochemistry
Chapter 17
Chapter 17: Terms to Know
Thermochemistry
Chemical potential
energy
Heat
System
Surroundings
Law of Conservation of
Energy
Endothermic processes
Exothermic processes
Heat capacity
Specific heat
Calorimetry
Calorimeter
Enthalpy
Thermochemical
equation
Heat of reaction
Heat of combustion
Introduction
• Thermochemistry is the chemistry
associated with heat.
• Heat (q) is a form of energy that flows.
• Heat flow is a measurable quantity.
• Objects have different rates at which they
absorb heat.
• Objects also store and release a specific
amount of heat, that is measurable.
• Calorimetry is the technique used to
measure heat flow.
The Flow of Energy –
Heat and Work
(Section 17.1)
• Energy Transformations
• Exothermic &
Endothermic Processes
• Units for Measuring
Heat Flow
• Heat Capacity and
Specific Heat
I.) Energy Transformations
• Energy = capacity
to do work or
supply heat.
• Energy can be
changed from one
form to another.
• There are different
types of energy
transformations.
Chemical bonds also have potential
energy.
• During a chemical reaction
a substance changes into
another substance with a
different PE in its bonds.
• Thermochemistry is the
study of energy changes
during a chemical reaction
and a change in state.
• Energy changes occur as
either heat transfer or
work or a combination of
both.
What is heat?
• Heat (q): Energy that
transfers from one
object to another
because of
temperature difference
between them.
• Heat always flow from
a warmer object to a
colder one until both
objects are at the
same temperature.
II.) Exothermic & Endothermic
Processes
• Chemical reactions or changes in physical
states involve either the absorption or
release of heat.
• Though energy moves from one place to
another, overall energy does not change.
– Law of Conservation of Energy
• We can describe the movement of heat as
either exothermic or endothermic depending
upon our perspective.
Chemical perspective: System and
surroundings.
• System: The part of the universe on
which you focus your attention.
• Surroundings: Everything else in the
universe.
• System + Surroundings = Universe
Endothermic Processes
• A process in which
the system
absorbs heat from
the surroundings.
• In these processes
the system heats
up while the
surroundings cool
down.
Exothermic processes
• A process in which
the system
releases heat to
the surroundings.
• In these processes
the system loses
heat as the
surroundings heat
up.
Determining direction of heat flow.
• Heat flowing out of a system = -q
• Heat flowing into a system = +q
Sample problem
On a sunny winter day, the snow on a
rooftop begins to melt. As the melt-water
drips from the roof, it refreezes into icicles.
Describe the direction of heat flow as the
water freezes. Is the process endothermic
or exothermic?
III.) Units for Measuring Heat Flow
• Heat flow is measured in two common
units, the calorie and the joule.
• calorie(cal): The quantity of heat needed
to raise the temperature of 1 g. of pure
water 1oC.
• Joule (J): The SI unit of energy.
1 J = 0.2390 cal
4.184 J = 1 cal
IV.) Heat Capacity and Specific
Heat
• Heat capacity: The amount of heat
required to increase the temperature
of an object exactly 1oC.
• Heat capacity is dependent upon the
mass and chemical composition of an
object.
• The greater the mass of an object the
greater its heat capacity.
Specific heat.
Substance
J/(g x oC)
Cal/(g x oC)
Water
4.18
1.00
Aluminum
0.90
0.21
Iron
0.46
0.11
Silver
0.24
0.057
• The specific heat capacity of a substance
is the amount of heat it takes to raise the
temperature of 1 g. of the substance 1oC.
Calculating specific heat.
q
C=
m x ∆T
•
•
•
•
C = specific heat
q = heat (joules or calories)
M = mass (grams)
∆T = Change in temperature (Tf – Ti)
– Tf = Final temperature
– Ti = Initial temperature
Sample problem
The temperature of a 95.4 g. piece of
copper increases from 25oC to 48.0oC
when the copper absorbs 849 J of heat.
What is the specific heat of copper?
Measuring and Expressing
Enthalpy Changes
(Section 17.2)
• Calorimetry
• Thermochemical
Equations
I.) Calorimetry
• The precise
measurement of the
heat flow into and
out of a system for
chemical and
physical processes.
• These
measurements can
be done at constant
pressure or
constant volume.
Constant pressure calorimetry:
coffee cup calorimetry
• Most chemical
reactions and physical
changes are carried
out under constant
pressure.
• The heat content of a
system at constant
pressure is the same
as the enthalpy (H) of
a system.
Calculating enthalpy changes (∆H).
• The heat released or absorbed by a reaction
at constant pressure is the same as the
change in enthalpy.
• Therefore q = ∆H.
• If q = m x C x ∆T,
• And qsystem = -qsurroundings,
• Then qsystem = ∆H = -qsurroundings = -m x C x ∆T
• +∆H = endothermic reactions
• - ∆H = exothermic reactions
Sample problem
When 25 mL of water containing 0.025
mol HCl at 25oC is added to 25.0 mL of
water containing 0.025 mol NaOH at
25.0oC in a foam cup calorimeter, a
reaction occurs. Calculate the enthalpy
change (in kJ) during the reaction if the
highest temperature observed is 32.0oC.
Assume the densities of the solutions are
1.00 g/mL.
Constant volume calorimetry: bomb
calorimetry.
• In this type of calorimetry
a sample is burned at
high pressure.
• The heat released
warms the water
surrounding the
chamber.
• Measuring the
temperature difference
allows for the calculation
of heat released.
Thermochemistry
Chapter 17
The End