Thermochemistry
branch of chem dealing with the relationship between chemical action
and heat.
Applications of Heat and Energy
Energy
• All energy can be classified as either potential or
kinetic.
• Potential energy is any type of energy that is stored.
• Examples: batteries (stored chemical
converts to electrical)
• Top of a roller coaster hill (gravitational)
• Candy Bar (stored energy in its chemical bonds)
• Kinetic energy – any energy from the movement of
matter.
Which has more heat - Lake Erie in
December of a drop of boiling oil?
1. Frozen Lake Erie
2. Drop of boiling oil
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Which has a higher temperature- Lake
Erie in December of a drop of boiling
oil?
1. Frozen Lake Erie
2. Drop of boiling oil
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Heat vs. Temperature
• Heat is the amount of energy that flows from
a hotter object to a colder one.
– It is the sum energy of all of the molecules in a
substance
• Temperature is the average kinetic energy of
an object.
Heat Transfer
• Heat always flows from an object with more
heat (hotter) to an object with less heat.
• Note: Something that is cold just lacks heat.
There is no unit for “cold”.
• When you feel cold, it is because you are
losing heat (not gaining “cold”)
True or False: When you place ice
cream in the freezer, heat is
transferred from the ice cream to the
freezer.
1. True
2. False
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True or False: When we open the
window, heat is transferred from your
body to the air outside.
1. True
2. False
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Units of Heat
• Heat is measure in Joules (J).
• (The joule is the SI unit for all types of energy.)
• Example: when you heat a cup of tea, you use
about 75,000J (or 75 kilojoules) of heat.
Joule is pronounced jewel
Other units of heat
• Heat is also measured in calories (cal).
• 1000 calories equals a kilocalorie (Cal).
• A calorie is the amount of heat needed to raise
the temperature of 1 gram of water by 1 degree
Celsius.
• Also, 1 calorie = 4.184 Joules
• And … 1 Cal = 4.184 kJ
What is the boiling point of water?
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100oF
273K
32oF
212oC
100oC
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What is the freezing point of
water?
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0oF
32K
100oF
273K
32oC
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Units of Temperature
Units of Temperature
TF = 1.8TC + 32
TK = TC + 273
A solid becoming a liquid is called:
Freezing
Melting
Evaporation
Sublimation
Condensation
Deposition
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A gas becoming a liquid is called:
Freezing
Melting
Evaporation
Sublimation
Condensation
Deposition
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A solid becoming a gas is called:
Freezing
Melting
Evaporation
Sublimation
Condensation
Deposition
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A liquid becoming a gas is called:
Freezing
Melting
Evaporation
Sublimation
Condensation
Deposition
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Attractive Forces
• In gases, these attractive forces are minimal.
• In solids and liquids, the forces are strong
enough to keep the materials from scattering
everywhere.
• These attractive forces also determine the
melting point and boiling point of different
compounds. (ex. NaCl melts at 801o C)
Changes in State
• Materials experience a change in state when
enough heat energy is applied to break apart
(or form) the attractions between molecules.
• When intermolecular bonds are broken, heat
is absorbed from the surroundings; when the
bonds are formed, heat is taken from the
system and released to the surroundings.
Changes in State
• Different states of matter (solids, liquids, and
gases) have very different properties due to
attractive forces that exist between atoms.
• To change from a solid to a liquid, for example,
these attractive forces in solids must be
broken so that the liquid molecules have more
freedom to move. In gases, the molecules
have even more mobility.
When a solid melts, heat is
removed from the surrounding
environment to break those
intermolecular forces.
1. True
2. False
se
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When a gas condenses, heat is
removed from the surrounding
environment?
1. True
2. False
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What happens at B?
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Liq
1. Solid starts to melt
2. Liquid starts to
freeze
3. Gas starts to
condense
4. Liquid starts to
evaporate
What happens at D?
1. Solid starts to melt
2. Liquid starts to
freeze
3. Gas starts to
condense
4. Liquid starts to boil
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What happens at E moving from
right to left in the graph?
1. Solid starts to melt
2. Liquid starts to
freeze
3. Gas starts to
condense
4. Liquid starts to
evaporate
64%
18%
18%
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What happens at C moving from
right to left in the graph?
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Liq
1. Solid starts to melt
2. Liquid starts to
freeze
3. Gas starts to
condense
4. Liquid starts to
evaporate
Heating Curves
Melting Point / Freezing Point
• The melting point (same temperature as
freezing point) is the temperature at which a
solid turns to a liquid.
• Latent Heat of Fusion – the amount of heat
needed to freeze one gram of a substance (or
the heat released when one gram of a
substance melts.)
• Note: units in J/g
Latent Heat of Fusion
Q = m x Hfus
Heat = mass x Heat of Fusion
****For ice to water: Hfus = 334 J/g
(every substance has a difference heat of fusion)
Boiling Point / Condensation Point
• The boiling point (same temperature as
condensation point) is the temperature at
which a liquid turns to a gas.
• Latent Heat of Vaporization – the amount of
heat needed to vaporize one gram of a
substance (or the heat release when one gram
of a substance condenses.)
• Note: units in J/g
Latent Heat of Vaporization
Q = m x Hvap
Heat = mass x Heat of Vaporization
****For water to steam: Hvap = 2260 J/g
(every substance has a difference heat of
vaporization)
Heat Graph calculations
• When the graph is flat, use latent heat
equations because of change of state.
• For melting: Q = m x Hfus
• For evaporating: Q = m x Hvap
Specific Heat
• Different substances absorb (and lose) heat at
different rates.
• Specific heat (SH) is the amount of heat (q)
needed to raise the temperature of 1 gram of
a substance by 1 degree Celsius.
Heat Graph calculations
• When the graph is sloped, use specific heat
equations because of change of temperature
• Q = SH x m x Temp. Change or
Measuring Heat Changes
• A calorimeter is an instrument
used to measure heat changes.
By placing an object in an insulated
container, the heat loss can be
measured by the temperature gain
of the water.
Heat lost by object = Heat gained by water
-qobject = qwater
Why do chemical reactions occur
between some substances and not in
others?
• Chemical reactions occur so
that the atoms in the
elements involved attain a
more stable state of being.
Collision Theory
• Collision theory – molecules must collide with
the proper orientation and sufficient energy to
react.
Activation Energy
• The activation energy is the amount of energy
required to break the bonds between the
atoms of the reactants.
Condition Necessary for Reactions to
Occur
1) Collision: Reactants must collide.
2) Orientation: The reactants must align
properly to react.
3) Energy: The activation energy must be
attained to react.
Energy in Chemical Reactions
• Many chemical reactions also produce energy
changes.
• Definitions:
• System – the reactants and products in the
reaction
• Surroundings – everything else around the
reaction (eg air in the room, reaction flask)
Heat of Reaction
• Heat of Reaction (ΔH) – the amount of heat
lost or gained in a reaction
• Heat of Reaction: ΔH = Hproducts – Hreactants
Hair products
Exothermic Reactions
• Exothermic Reactions – energy is produced by
a reaction; energy flows from the system to
the surroundings
• ΔH is negative because the reaction loses
heat.
Exothermic Reaction Graph
Endothermic Reactions
• Endothermic Reactions – energy is gained by
a reaction; energy flows from the
surroundings into the system
• ΔH is positive because the reaction gains heat.
Endothermic Reaction Graph
Classify the reaction:
2H2O + 572kJ -> 2H2+ O2
67%
2
33%
1
1. Exothermic
2. Endothermic
ΔH of the reaction = -560kJ
73%
2
27%
1
1. Exothermic
2. Endothermic
After the reaction, your hand gets
burnt from the heat. The reaction
must be:
73%
2
27%
1
1. Exothermic
2. Endothermic
Heat Values in Chemical Reactions
• Heat of Reaction is a stoichiometric value and
is proportional to the coefficients of the
reactants and products.
• 2H2O + 572kJ -> 2H2+ O2
• Therefore, for every 2 moles of water that
react, 572kJ of energy are required.
Presence of a Catalyst – a substance that
increases the rate without being
permanently changed
- lowers activation energy
Also used:
inhibitors – “tie up” a reaction so that it does
not occur (opposite of a catalyst)
- preservatives
- anti-rust agents