17.1 17.2 lecture slides

Chapter 17: Thermochemistry
Section 1: The Flow of Energy—Heat
and Work

Energy Transformations
• Heat, represented by q, is energy that
transfers from one object to another because
of a temperature difference between them
– Heat always flows from a warmer object to a
cooler object
• Thermochemistry is the study of energy
changes that occur during chemical reactions
and changes in state

Chemical Potential Energy
• The energy stored in the chemical bonds of a
substance
– When fuel is burned in a car engine, chemical potential
energy is released and is used to do work

Exothermic and
Endothermic Processes
• In an endothermic process, the system gains heat as
the surroundings cool down
• In an exothermic process, the system loses heat as the
surroundings heat up
– In studying energy changes, you can define a system as the
part of the universe on which you focus your attention
– The surroundings include everything else in the universe
• The law of conservation of energy states that in any
chemical or physical process, energy is neither created
nor destroyed

Endothermic Processes
• One that absorbs heat from the surroundings

Exothermic Processes
• One that releases heat to its surroundings

On a sunny winter day, the snow
on a rooftop begins to melt. As
the melt—water drips from the
roof, it freezes into icicles.
Describe the direction of heat
flow as the water freezes. Is this
process endothermic or
exothermic?
Practice Problem #1

On a sunny winter day, the snow on a rooftop begins
to melt. As the melt—water drips from the roof, it
freezes into icicles. Describe the direction of heat
flow as the water freezes. Is this process endothermic
or exothermic?
Solution

Practice Problem #2
A container of melted paraffin wax is allowed to
stand at room temperature until the wax
solidifies. What is the direction of heat flow as
the liquid wax solidifies? Is the process
exothermic or endothermic?

Solution
A container of melted paraffin wax is allowed to
stand at room temperature until the wax
solidifies. What is the direction of heat flow as
the liquid wax solidifies? Is the process
exothermic or endothermic?

Units for Measuring
Heat Flow
• Heat flow is measured in two common units,
the calorie and the joule
– The energy in food is usually expressed in Calories

Heat Capacity and
Specific Heat
• The heat capacity of an object depends on both
its mass and its chemical composition
– The amount of heat needed to increase the
temperature of an object exactly 1°C is the heat
capacity of that object
– The specific heat capacity, or simply the specific heat,
of a substance is the amount of heat it takes to raise
the temperature of 1 g of the substance 1°C

Heat Capacity and
Specific Heat
• Water releases a lot of heat as it cools
• During freezing weather, farmers protect citrus
crops by spraying them with water

Heat Capacity and
Specific Heat
• Because it is mostly water, the filling of a hot
apple pie is much more likely to burn your
tongue than the crust

Practice Problem #3
The temperature of a 95.4 g piece of copper
increases from 25 °C to 48.0 °C when the copper
absorbs 849 J of heat. What is the specific heat
of copper?

Solution
The temperature of a 95.4 g piece of copper
increases from 25 °C to 48.0 °C when the copper
absorbs 849 J of heat. What is the specific heat
of copper?

Practice Problem #4
How much heat is required to raise the
temperature of 250.0 g of mercury 52 °C?

Solution
How much heat is required to raise the
temperature of 250.0 g of mercury 52 °C?
Q = m x C x ΔT
Q = (250.0 g) (0.14 J/g °C) (52 °C)
Q = 1820 J

The energy released when a piece of wood is
burned has been stored in the wood as
a. sunlight.
b. heat.
c. calories.
d. chemical potential energy.
Practice Problem #5

The energy released when a piece of wood is
burned has been stored in the wood as
a. sunlight.
b. heat.
c. calories.
d. chemical potential energy.
Solution

Which of the following statements about heat is
false?
a. Heat is the same as temperature.
b. Heat always flows from warmer objects to cooler
objects.
c. Adding heat can cause an increase in the
temperature of an object.
d. Heat cannot be specifically detected by senses or
instruments.
Practice Problem #6

Which of the following statements about heat is
false?
a. Heat is the same as temperature.
b. Heat always flows from warmer objects to cooler
objects.
c. Adding heat can cause an increase in the
temperature of an object.
d. Heat cannot be specifically detected by senses or
instruments.
Solution

Choose the correct words for the spaces: In an
endothermic process, the system ________ heat
when heat is ________ its surroundings, so the
surroundings _____________.
a. gains, absorbed from, cool down.
b. loses, released to, heat up.
c. gains, absorbed from, heat up.
d. loses, released to, cool down.
Practice Problem #7

Choose the correct words for the spaces: In an
endothermic process, the system ________ heat
when heat is ________ its surroundings, so the
surroundings _____________.
a. gains, absorbed from, cool down.
b. loses, released to, heat up.
c. gains, absorbed from, heat up.
d. loses, released to, cool down.
Solution

Which of the relationships listed below can be
used to convert between the two units used to
measure heat transfer?
a. 1 g = 1ºC
b. 1 J = 0.2390 cal
c. 1ºC = 1 cal
d. 1 g = 4.184 J
Practice Problem #8

Which of the relationships listed below can be
used to convert between the two units used to
measure heat transfer?
a. 1 g = 1ºC
b. 1 J = 0.2390 cal
c. 1ºC = 1 cal
d. 1 g = 4.184 J
Solution

Assuming that two samples of different
materials have equal mass, the one that
becomes hotter from a given amount of heat is
the one that
a. has the higher specific heat capacity.
b. has the higher molecular mass.
c. has the lower specific heat capacity.
d. has the higher density.
Practice Problem #9

Assuming that two samples of different
materials have equal mass, the one that
becomes hotter from a given amount of heat is
the one that
a. has the higher specific heat capacity.
b. has the higher molecular mass.
c. has the lower specific heat capacity.
d. has the higher density.
Solution

Chapter 17: Thermochemistry
Section 2: Measuring and Expressing
Enthalpy Changes

Calorimetry
• The precise measurement of the heat flow
into or out of a system for chemical and
physical processes
– In calorimetry, the heat released by the system is
equal to the heat absorbed by its surroundings
– Conversely, the heat absorbed by a system is
equal to the heat released by its surroundings

Calorimeter
• The insulated device used to measure the
absorption or release of heat in chemical or physical
processes

Constant-Pressure Calorimeters
• The heat content of a system at constant
pressure is the same as a property called the
enthalpy (H) of the system

Constant-Volume Calorimeters
• Calorimetry experiments can be performed at
a constant volume using a bomb calorimeter

Practice Problem #1
When 25.0 mL of water containing 0.025
mol HCl at 25.0 °C is added to 25.0 mL of
water containing 0.025 mol NaOH at 25.0 °C
in a foam cup calorimeter, a reaction occurs.
Calculate the enthalpy change (in kJ) during
this reaction if the highest temperature
observed is 32.0 °C. Assume the densities of
the solutions are 1.00 g/mL.

Solution
When 25.0 mL of water containing 0.025 mol HCl at 25.0 °C is added
to 25.0 mL of water containing 0.025 mol NaOH at 25.0 °C in a foam
cup calorimeter, a reaction occurs. Calculate the enthalpy change (in
kJ) during this reaction if the highest temperature observed is 32.0
°C. Assume the densities of the solutions are 1.00 g/mL.

Practice Problem #2
A small pebble is heated and placed in a foam
cup calorimeter containing 25.0 mL of water at
25.0 °C. The water reaches a maximum
temperature of 26.4 °C. How many joules of
heat were released by the pebble?

Solution
A small pebble is heated and placed in a foam
cup calorimeter containing 25.0 mL of water at
25.0 °C. The water reaches a maximum
temperature of 26.4 °C. How many joules of
heat were released by the pebble?
146 kJ

Thermochemical Equations
• A chemical equation that includes the
enthalpy change
– In a chemical equation, the enthalpy change for
the reaction can be written as either a reactant or
a product

Thermochemical Equations
• The heat of reaction is the enthalpy change
for the chemical equation exactly as it is
written

Practice Problem #3
Using the
thermochemical
equation below,
calculate the amount
of heat (in kJ) required
to decompose 2.24
mol NaHCO3 (s).
Endothermic Reaction

Solution
Using the thermochemical equation below, calculate
the amount of heat (in kJ) required to decompose
2.24 mol NaHCO3 (s).

Practice Problem #4
The production of iron and carbon dioxide from
iron(III) oxide and carbon monoxide is an
exothermic reaction. How many kilojoules of
heat are produced when 3.40 mol Fe2O3 reacts
with an excess of CO?
Fe2O3 (s) + 3 CO (g)  2 Fe (s) + 3 CO2 (g) + 26.3 kJ

Solution
The production of iron and carbon dioxide from
iron(III) oxide and carbon monoxide is an
exothermic reaction. How many kilojoules of
heat are produced when 3.40 mol Fe2O3 reacts
with an excess of CO?
Fe2O3 (s) + 3 CO (g)  2 Fe (s) + 3 CO2 (g) + 26.3 kJ
89.4 kJ

Heat of Combustion
• The heat of reaction for the
complete burning of one mole
of a substance
– Combustion of natural gas is an
exothermic reaction

The change in temperature recorded by the
thermometer in a calorimeter is a measurement
of
a. the enthalpy change of the reaction in the
calorimeter.
b. the specific heat of each compound in a
calorimeter.
c. the physical states of the reactants in a
colorimeter.
d. the heat of combustion for one substance in a
calorimeter.
Practice Problem #5

The change in temperature recorded by the
thermometer in a calorimeter is a measurement
of
a. the enthalpy change of the reaction in the
calorimeter.
b. the specific heat of each compound in a
calorimeter.
c. the physical states of the reactants in a
colorimeter.
d. the heat of combustion for one substance in a
calorimeter.
Solution

Practice Problem #6
For the reaction CaO(s) + H2O(l) Ca(OH)2(s), 
H = 65.2 kJ. This means that 65.2 kJ of heat is
__________ during the process.
a. absorbed
b. destroyed
c. changed to mass
d. released

Solution
For the reaction CaO(s) + H2O(l) Ca(OH)2(s), 
H = 65.2 kJ. This means that 65.2 kJ of heat is
__________ during the process.
a. absorbed
b. destroyed
c. changed to mass
d. released

How much heat is absorbed by 325 g of water if
its temperature changes from 17.0°C to 43.5°C?
The specific heat of water is 4.18 J/g°C.
a. 2.00 kJ
b. 3.60 kJ
c. 36.0 kJ
d. 360 kJ
Practice Problem #7

How much heat is absorbed by 325 g of water if
its temperature changes from 17.0°C to 43.5°C?
The specific heat of water is 4.18 J/g°C.
a. 2.00 kJ
b. 3.60 kJ
c. 36.0 kJ
d. 360 kJ
Solution

Which of the following is a thermochemical
equation for an endothermic reaction?
a. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 890 kJ
b. 241.8 kJ + 2H2O(l) 2H2(g) + O2(g)
c. CaO(s) + H2O(l)  Ca(OH)2(s) 65.2 kJ
d. 2NaHCO3(s) 129 kJ Na2CO3(s) + H2O(g) + CO2(g)
Practice Problem #8

Which of the following is a thermochemical
equation for an endothermic reaction?
a. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 890 kJ
b. 241.8 kJ + 2H2O(l) 2H2(g) + O2(g)
c. CaO(s) + H2O(l)  Ca(OH)2(s) 65.2 kJ
d. 2NaHCO3(s) 129 kJ Na2CO3(s) + H2O(g) + CO2(g)
Solution

Oxygen is necessary for releasing energy from
glucose in organisms. How many kJ of heat are
produced when 2.24 mol glucose reacts with an
excess of oxygen?
C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(g) + 2808 kJ/mol
a. 4.66 kJ
b. 9.31 kJ
c. 1048 kJ
d. 6290 kJ
Practice Problem #9

Oxygen is necessary for releasing energy from
glucose in organisms. How many kJ of heat are
produced when 2.24 mol glucose reacts with an
excess of oxygen?
C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(g) + 2808 kJ/mol
a. 4.66 kJ
b. 9.31 kJ
c. 1048 kJ
d. 6290 kJ
Solution

17.1 17.2 lecture slides

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17.1 17.2 lecture slides