Thermochemistry is the study of heat changes that occur during
chemical reactions.
Slide 3
Energy is the ability to do work or cause change. Work is force
applied over a distance Potential Energy is stored energy Kinetic
Energy is energy of motion Chemical Potential Energy is stored
within the structural units of chemical substances.
Slide 4
Heat Heat (q) is Energy that is transferred from one object to
another Transfer is due to temperature difference between the two
substances Heat ALWAYS travels from hot to cold Remember,
temperature is a measure of the average kinetic energy of a
substance
Slide 5
Heat CANNOT be detected. the changes CAUSED by heat however can
be ex. Rise in temperature
Slide 6
Law of Conservation of Energy In any chemical or physical
process, energy is neither created nor destroyed. All of the energy
involved can be accounted for as Work, Potential Energy, or
Heat.
Slide 7
Terms for studying heat: System what you are focusing on
Surroundings everything else Universe = System + Surroundings
Direction of heat flow is always described relative to the system.
Endothermic process Energy is absorbed by the system from the
surroundings (Heats up) Exothermic process Energy is released by
the system into the surroundings (Cools down)
Slide 8
Endothermic vs. Exothermic Exothermic Lose Heat- q
EndothermicGain Heat+ q
Slide 9
Units of Heat calorie- the quantity of heat needed to raise the
temperature of 1 g of pure H2O 1 degree Celsius. Calorie = 1000
calories = 1 kilocalorie => Dietary Calorie Joule the quantity
of heat needed to raise the temperature of 1 g of pure H 2 O,
0.2390 degrees Celsius. 4.184 J = 1 cal
Slide 10
Heat Capacity vs. Specific Heat Capacity Heat capacity the
amount of heat needed to raise the temperature 1 degree Celsius,
for any substance Depends on mass and composition Specific heat
capacity (specific heat) the amount of heat it takes to raise the
temperature of 1 g of a substance 1 degree Celsius. Depends on
composition only
Slide 11
Calculation Specific heat = Heat______________ Mass x Change in
Temperature Specific heat (C or Cp) Heat (q) Mass (m) Change in
temperature ( T) q = m C T
Slide 12
Measuring and Expressing Heat Changes (change in temperature)
Calorimetry- Accurate and precise measurement of heat change
Instrument Calorimeter For systems at constant pressure, the heat
content is the same as enthalpy (H) Heat changes are the same as
changes in enthalpy (H) q = H Since q = H, then by substitution H =
m x C x T
Slide 13
Heat in Changes of State Molar Heat of Fusion (H fus ) Solid to
Liquid Molar Heat of Vaporization (H vap ) Liquid to Gas q = H x
mass OR q = H x moles Depends on units of H
Slide 14
Phase Change Diagram
Slide 15
Phase Changes
Slide 16
Group Concept Questions Your text defines energy as the ability
to do work or to cause change. Another definition of energy is the
ability to resist a natural tendency. Explain this definition and
provide an example. A friend of yours reads that the process of
water freezing is exothermic. This friend tells you that this cant
be true because exothermic implies hot, and ice is cold. Is the
process of water freezing exothermic? If so, explain it so your
friend can understand it. If not, explain why not.
Slide 17
Group Concept Questions You place hot metal into a beaker of
cold water. Eventually what is true about the temperature of the
metal compared to that of the water? Explain why this is true.
Label this process as endothermic or exothermic if we consider the
system to be: the metal. Explain the water. Explain
Slide 18
Group Concept Questions The text describes the law of
conservation of energy. Is there a law of conservation of heat?
Explain why or why not. What does it mean when the heat for a
process is reported with a negative sign? You place 100.0g of a hot
metal in 100.0g of cold water. Which substance (metal or water)
undergoes a larger temperature change? Why is this?
Slide 19
Group Concept Questions A desert is very hot during day but
quite cold at night. In the Midwest of the United States, the
temperature is more constant between day and night in the summer.
Why is this? Explain why aluminum cans make good storage containers
for soft drinks.
Slide 20
Calorimetry Problems: Changes in Temperature How much heat is
absorbed by 60.0 g of copper when its temperature is raised from 20
o C to 80 o C? What is the specific heat of a 124 g sample of brass
if 3.94 x 10 3 J raises the temperature of the brass from 12.5 o C
to 97.0 o C? If 350 J of heat energy are added to 100 g of a metal
and the temperature changes by 25 o C, what is the specific heat of
the metal? What is the identity of the metal? 10g of an unknown
metal requires 39J of energy to increase its temperature from 50 o
C to 60 o C. What is the specific heat of the metal? Identify the
metal.
Slide 21
Calorimetry Problems: Heat Lost = Heat Gained When 80.0 grams
of a certain metal at 90.0 C was mixed with 100.0 grams of water at
30.0 C, the final equilibrium temperature of the mixture was 36.0
C. What is the specific heat of the metal? Calculate the specific
heat of a metal if a 55.0 g sample of an unknown metal at 99.0 C
causes a 1.7 C temperature rise when added to 225.0 g of water at
22.0 C.
Slide 22
Calorimetry Problems: Heat Lost = Heat Gained A piece of an
unknown metal with mass 23.8 g is heated to 100.0C and dropped into
50.0 cm 3 of water at 24.0C. The final temperature of the system is
32.5C. What is the specific heat of the metal? A blacksmith heated
an iron bar to 1445C. The blacksmith then tempered the metal by
dropping it into 42,800 mL of water that had a temperature of 22C.
The final temperature of the system was 45C. What was the mass of
the bar? Note: Specific heat of iron is 0.4494 J/gC.
Slide 23
How does ENERGY affect REACTIONS
Slide 24
Enthalpy The amount of energy gained or released in a reaction
is the ENTHALPY (H). ALL reactions require energy to occur. The
amount of energy needed to occur is called the ACTIVATION ENERGY.
The more energy required for the reaction to occur, the less likely
the reaction will happen. Because of the energy released in
exothermic reactions, exothermic reactions are more likely to occur
than endothermic reactions. In other words, they are more
spontaneous.
Slide 25
Exothermic and Endothermic Reactions
Slide 26
Catalysts Catalysts speed up reactions by lowering the
activation energy required for the reaction to occur. Enzymes are
biological catalysts. Most catalysts work by helping ions and
molecules to line up the right way so they can react.
http://www.dlt.ncssm.edu/tiger/Flash/kinetics/EnzymeCatalyst.html
Slide 27
Reaction Path
Slide 28
Entropy Whether or not a reaction will occur depends on both
ENTHALPY and ENTROPY. Entropy is a measure of disorder. Increasing
disorder is a spontaneous process. Entropy of States: solid <
liquid < gas Kinetic..Energy Gas Kinetic Energy Liquid Kinetic
Energy Solid Mixing substances, combining or separating elements,
and changing the temperature can affect the level of disorder as
well. Mixing gases
Slide 29
Spontaneous? If energy is released and disorder is increased,
the reaction will be spontaneous. In other words, it will happen on
its own. If energy is absorbed and disorder decreases, the reaction
will be non-spontaneous. These reactions will need help to occur.
They cant do it by themselves. If one factor is favorable and one
is unfavorable, then spontaneity will depend on the values of
enthalpy and entropy.
Slide 30
How fast will the reaction happen?
Slide 31
Rates of Reaction A rate is the measure of change over time.
Reaction rates measure the change in reactants over time. The rate
of a reaction is governed by collision theory. Collision theory
states that in order for a reaction to occur, the reactants must
collide and collide with enough energy to overcome the activation
energy barrier to form products.
Slide 32
Collision Theory Soanything that affects the number of
collisions that occur in a reaction or the amount of energy the
reactants have will affect the rate of reaction. The rate of a
reaction is dependent on several factors. Properties of the
elements and compounds in the reaction Temperature of the reaction
Amount of reactants present Size of the particles Presence of
catalysts or inhibitors
Slide 33
Properties The combination of elements and compounds in a
reaction will affect the rate of reaction. Some substances are more
reactive than others. Ex. Metals rusting Also, some substances are
more reactive when in the presence of other substances. Ex. Baking
soda in water vs. baking soda in vinegar
Slide 34
Temperature Measures the average kinetic energy of a substance
Raising the temperature, raises the kinetic energy of the substance
Increasing kinetic energy makes it MORE likely a reaction will
occur, thereby INCREASING the reaction rate Raising the temperature
also increases the number of collisions that occur Increasing the
collisions makes it MORE likely a reaction will occur, thereby
INCREASING the reaction rate
Slide 35
Concentration Measures the amount of reactants that are present
Moles / Liter (molarity) Increasing the concentration, increases
the number of collisions that occur Increasing the collisions makes
it MORE likely a reaction will occur, thereby INCREASING the
reaction rate
Slide 36
Particle Size Measures the size of the particles of the
substance Ex. powder vs. crystal vs. chunks Decreasing the particle
size, increases the surface area of the substance. The increased
surface area increases the number of collisions that occur
Increasing the collisions makes it MORE likely a reaction will
occur, thereby INCREASING the reaction rate
Slide 37
Catalysts and Inhibitors Alter the activation energy or the
ability of substances to collide Catalysts affect the activation
energy Inhibitors affect the substances ability to interact
Catalysts lower the activation energy making it easier for the
reactions to happen Catalysts Lowering the activation energy makes
it MORE likely a reaction will occur, thereby INCREASING the
reaction rate Inhibitors make it difficult for reactants to
interact with each other Decreasing the collisions makes it LESS
likely a reaction will occur, thereby DECREASING the reaction
rate