Transcript Thermochemistry - JH Rose

Chapter 17
Thermochemistry is the study of heat changes
that occur during chemical reactions.
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.
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
Heat
 CANNOT be detected.
 the changes CAUSED by heat however
can be
 ex. Rise in temperature
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.
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)
Endothermic vs. Exothermic
 Exothermic
 Endothermic
Lose Heat
Gain Heat
-q
+q
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 H2O, 0.2390 degrees Celsius.

4.184 J = 1 cal
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
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
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 (ΔH)
 q = ΔH
 Since q = ΔH, then by substitution

ΔH = m x C x ΔT
Heat in Changes of State
 Molar Heat of Fusion (ΔHfus) – Solid to Liquid
 Molar Heat of Vaporization (ΔHvap) – Liquid to
Gas
q = ΔH x mass OR q = ΔH x moles
 Depends on units of ΔH
Phase Change Diagram
Phase Changes
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
can’t 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.
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
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?
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.
Calorimetry Problems:
Changes in Temperature
 How much heat is absorbed by 60.0 g of copper when
its temperature is raised from 20 oC to 80 oC?
 What is the specific heat of a 124 g sample of brass if
3.94 x 103 J raises the temperature of the brass from
12.5 oC to 97.0 oC?
 If 350 J of heat energy are added to 100 g of a metal and
the temperature changes by 25 oC, 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 oC to 60 oC. What is
the specific heat of the metal? Identify the metal.
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.
Calorimetry Problems:
Heat Lost = Heat Gained
 A piece of an unknown metal with mass 23.8 g is
heated to 100.0ºC and dropped into 50.0 cm3 of water
at 24.0ºC. The final temperature of the system is
32.5ºC. What is the specific heat of the metal?
 A blacksmith heated an iron bar to 1445ºC. The
blacksmith then tempered the metal by dropping it
into 42,800 mL of water that had a temperature of
22ºC. The final temperature of the system was 45ºC.
What was the mass of the bar? Note: Specific heat of
iron is 0.4494 J/g·Cº.
How does ENERGY affect REACTIONS
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.
Exothermic and Endothermic Reactions
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
Reaction Path
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
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 can’t do
it by themselves.
 If one factor is favorable and one is unfavorable, then
spontaneity will depend on the values of enthalpy and
entropy.
How fast will the reaction happen?
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.
Collision Theory
 So…anything 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
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
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
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
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
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
 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