THERMOCHEMISTRY
Heating and Cooling Curves, Phase Change Diagrams, and Energy Diagrams
MATTER: Anything that has mass and volume.
STATES OF MATTER:
SOLID
LIQUID
GAS
Tightly packed, in
a regular pattern
Vibrate, but do not
move from place
to place
Close together
with no regular
arrangement.
Vibrate, move
about, and slide
past each other
Well separated
with no regular
arrangement.
Vibrate and move
freely at high
speeds
PLASMA
Has no definite
volume or shape
and is composed
of electrical
charged particles
PHASE CHANGES
Description of
Phase Change
Solid to
liquid
Term for Phase
Change
Melting
Liquid to
Freezing
solid
Heat Movement During
Phase Change
Energy (Heat)
is absorbed by
atoms.
Energy (Heat)
is released by
atoms.
PHASE CHANGES
Description of
Phase Change
Liquid
to Gas
Gas to
Liquid
Term for Phase Change
Evaporation/
Vaporization
Heat Movement During
Phase Change
Energy (Heat)
is absorbed by
atoms.
Energy (Heat)
Condensation is released by
atoms.
PHASE CHANGES
Description of
Phase Change
Solid to
Gas
Gas to
Solid
Term for Phase
Change
Heat Movement During
Phase Change
Energy (Heat)
Sublimation is absorbed by
atoms.
Deposition
Energy (Heat)
is released by
atoms.
Heating and Cooling Curves and
Phase Diagrams
Things to know:
What do all the lines represent?
Where are the phase changes and what
are their names?
Is energy being absorbed or released?
Heating Curve for Water
Gas
Liquid
Solid
• This is what the graph would look like if I took
some ice cubes, put them in a beaker, and heated
them with a bunsen burner.
Heating Curve for Water

What do the horizontal lines indicate?

What do the sloped lines indicate?
Heating Curve for Water

What is happening between point B and C?

What is happening between point D and E?

Is energy being absorbed or released?
Cooling Curve of Water
A
B
C
Blank
• What is the phase change between A and B?
• What is the phase change between C and D?
• Is energy being absorbed or released?
D
Learning Check
A. Water condenses at a temperature of
1)
0°C
2) 50°C
3) 100°C
B. When a gas condenses, energy is
1) released
2) absorbed
C. Melting is
1) endothermic
2) exothermic
Learning Check
Is energy absorbed (1) or released (2) in each of the
following:
____A. Ice to liquid water
____B. Water vapor to rain
____C. Water to ice
Something to think about: Why do drops of liquid water
form on a glass of iced tea ?
Phase Diagrams
•
•
•
The temperature at which a
phase change happens can
change depending on the
pressure.
This is why there are different
cooking directions for “high
altitudes”
A phase diagram shows the
temperatures and pressures
necessary for each state of
matter to exist.
This is the phase diagram for water.
At standard pressure (the pressure
of a regular day for us) what temp
does water melt and boil?
Each black line represents the conditions
when a phase change occurs.
Triple Point – point that exist when all three
states of matter are present at the same time
Critical Point – point on a phase diagram
where the liquid phase has the same density
as the gas phase and are indistinguishable.
No matter what pressure is applied, it cannot
condense into a liquid.
http://www.kentchemistry.com/links/Matter/Phasediagram.htm
1) What
phase would
it be at a
pressure of
0.75 and a
temp of 0?
2) What
phase would
it be at 0.75
atm and
temp of 200?
Thermochemistry

Most simply, it means “chemistry of heat” but we
broaden it to mean energy in general

Every chemical reaction involves some sort of change
in energy.

Every time a bond is made or broken, energy is
required.
 Breaking
 Forming
bonds requires energy
bonds releases energy
Thermochemistry

Chemists and scientists are most interested in the
change of energy in a reaction, not necessarily
how much total energy is left at the end. This is
referred to as ΔH, or enthalpy.
Δ
is the Greek letter “delta” and means “change”
H
refers to “heat” or “energy”
 Measured
in kJ, or kilojoules (pronounced “kilo-
jewels”)

Comparison: Caring more that you earned $120 in
a paycheck than knowing your total bank account
is now $465.
Endothermic and Exothermic

Endothermic means that heat/energy is gained or
absorbed, so the ΔH would be positive

Exothermic means that heat/energy is released or
lost, so the ΔH would be negative
Energy Diagrams

We use energy diagrams to display the energy
changes that need to occur for a reaction to
happen

Would this be an endothermic or exothermic
reaction?
Energy Diagrams

We group the reactants together and add together the
energy from their bonds and make a line on the left side of
the graph at this value

We do the same for the products and make a line on the
right side of the graph

ΔH = enthalpy of products – enthalpy of reactants

Or, it is the difference of energy between the products and
reactants
The Transition State

During a chemical reaction, the atoms of the
reactants recombine and rearrange to form the
products, but this doesn’t happen instantly.

The reactants form a Transition State which is a
temporary and unstable form where all of the
atoms are partially connected as old bonds start
to break and new bonds start to form
Activation Energy

Every reaction needs at least some energy to form
this transition state and get the reaction started
 Sometimes
this is very low and room temperature is
enough to get it started
 Sometimes
this is very high and takes extremely high
temperatures like a furnace to get started

This is called the Activation Energy and is given
the symbol Ea (for energy of activation)
Activation Energy

On the energy diagram, the top of the “hill”
between the products and reactants is the energy
of the activated complex

The Ea is the amount of energy to go from the
reactants to the top of the hill.

Where are the reactants, products, and activated
complex? What are their energies?

What is the Ea and ΔH? Is this endo or exo?
Going in Reverse

The same energy diagram can be used if the
reaction goes in reverse (the products turn back
into reactants), just read the graph backwards.

If the forward reaction was exothermic, what
does the reverse reaction have to be?
Speeding up Reactions
The
following increase the rate of
reactions BECAUSE they increase
collisions!
Increasing
temperature
Increasing surface area
Increasing the concentration of reactants
Increasing the pressure
And
CATALYSTS!
Factors that affect the speed of reaction

Temperature


By adding heat, reactants move faster and have more energy to start with.
This means that collisions are more likely to be successful because they are
closer to reaching the activation energy
Concentration

More chemicals in the reaction means it is more likely they will collide for a
reaction.
Other factors that affect the speed of
reaction

Stirring/Agitation


By stirring a reaction, you are increasing the amount of contact that the
different reactants have with each other, making it more likely to collide
and react
Surface Area

Reactions can only happen on the surface of a solid where two reactants
can collide. By increasing the surface area by using small particles or
powder, there is more surface to react.
Catalysts!
A
catalyst is a substance that speeds up
a reaction
 It
does this by lowering the activation energy of a
chemical reaction.
A
catalyst is never changed, or never used
up.
Think
Each
of it as a tool (like a screwdriver)
catalyst has ONE SPECIFIC JOB
Catalysts!
DEMO:
With
30% hydrogen peroxide + Dish Soap
potassium iodide (KI) catalyst
Elephant Toothpaste!

H2 O2 
H2 O
+
O2
Potassium
Iodide is the catalyst
The iodine (from KI) is not used up. How
can you tell?
The
orange/yellow coloring
Endothermic
or exothermic? Explain why.
Exothermic. Heat is released.
 So
much O2 is produced from the small amount of that
it has to escape the graduated cylinder quickly
 As it escapes, the O2 forms bubbles in the soap, and
turns the soap into foam.
Catalysts

Catalysts DO NOT affect ΔH because it doesn’t change the
energy of the reactants or products

Comparison: You’re paid $20 to mow the lawn. You’re paid
the same regardless of if you cut it with scissors, a pushmower, or a ride-along.
Catalysts

Catalysts work in one of two ways:
 Create
a new pathway that has a
lower activation energy (perhaps in
more than one step)
 Makes
the reactants more likely to
react by putting them in a more
favorable orientation
Other factors that affect the speed of
reaction

Temperature


By adding heat, reactants move faster and have more energy to start with.
This means that collisions are more likely to be successful because they are
closer to reaching the activation energy
Concentration

More chemicals in the reaction means it is more likely they will collide for a
reaction.
Other factors that affect the speed of
reaction

Stirring/Agitation


By stirring a reaction, you are increasing the amount of contact that the
different reactants have with each other, making it more likely to collide
and react
Surface Area

Reactions can only happen on the surface of a solid where two reactants
can collide. By increasing the surface area by using small particles or
powder, there is more surface to react.