Objective
 To
understand the forms of energy
 kinetic, potential, chemical
 To
and thermal
understand the law of
conservation of energy and how
energy is measured
 To understand the process of heat
transfer
Thermochemistry –
Heat and Chemical Change
The flow of energy
 Thermochemistry is concerned with the heat changes
that occur during chemical reactions.
Four Types of Energy
Kinetic Energy – energy of motion
1.


Depends on the mass and speed of the object
Example: car vs. bicycle
Potential Energy – stored energy from the attraction and
repulsion an object experiences
2.


Based on gravity or electrostatic attractions
Example: bicycle at the top of a hill
Chemical Energy – potential energy stored in the bonds of
atoms in a chemical
3.

Released during chemical reactions
Thermal Energy – kinetic energy of a substance related to
the temperature of that substance
4.


Caused by vibrating or moving molecules
NOT the same thing as heat!
The Flow of Energy
 Energy
is the capacity for doing work.
 The law of conservation of energy says:
Law of Conservation of Energy
Energy is never created or destroyed in any
chemical or physical change.
You can ONLY change energy from one form
to another…
Heat (Q)


Heat (Q) is energy that transfers from one object to
another because of a temperature difference between
them.
Heat cannot be detected by instruments or our senses,
only the changes that are caused by heat can be detected
What causes heat to transfer from an
object?
A.
B.
C.
Only objects at equilibrium can transfer heat
A difference in temperature between objects
A difference in mass between objects
Heat (Q)


Heat (Q) always flows from a warmer object to a cooler
object until the objects are at equilibrium.
Which direction is heat flowing in each case?
Room A
Room B
Exothermic and Endothermic processes
 Chemical
Potential Energy is energy stored
in chemical substances

some examples: gasoline, AA battery, firefly
Exothermic and Endothermic processes
All chemical reactions either absorb or release
heat to their surroundings
 The law of conservation of energy is always
true
 We can account for all the energy in a
chemical reaction as either work, stored
energy, or heat.

Exothermic and Endothermic processes

A process that releases heat to the
surroundings is called an exothermic process.

A process that absorbs heat from the
surroundings is called an endothermic
process.
Energy Diagrams – complete the diagrams
in your notes


Energy Diagrams show the change in potential energy in the reactants and
products of a reaction.
Energy Diagrams can be used to determine if a chemical reaction is exothermic or
endothermic
If potential energy in the reactants is
released as heat, the reaction is
exothermic and feels hot.
If potential energy in the products is
increased (absorbed) the reaction is
endothermic and feels cold.
Energy Diagrams – complete the diagrams
in your notes


Energy Diagrams show the change in potential energy in the reactants and
products of a reaction.
Energy Diagrams can be used to determine if a chemical reaction is exothermic or
endothermic
If potential energy in the reactants is
released as heat, the reaction is
exothermic and feels hot.
If potential energy in the products is
increased (absorbed) the reaction is
endothermic and feels cold.
Endothermic Reaction Pathway
Exothermic Reaction Pathway
final energy in products
Potential Energy
DH
DH is
negative
final energy in products
Course of reaction (time)
Potential Energy
initial energy in reactants
DH
DH is
positive
initial energy in reactants
Course of reaction (time)
Example: Nitrogen gas reacts with hydrogen gas to
form ammonia gas according to the energy diagram
below:
This energy diagram represents a(n) ______ reaction.
A.
B.
C.
D.
Exothermic
Endothermic
Ectoplasmic
Cannot be determined
The energy diagram below represents a(n)
______ reaction.
A.
B.
C.
D.
[Default]
[MC Any]
[MC All]
exothermic
endothermic
echolothermic
cannot be
determined
Measuring Heat (Q)


Heat is measured in calories, Calories or joules.
Whether you are breaking down sugar in your body to
create heat or burning sugar in a fire to create heat, the
chemical reaction produces the same number of calories.
Measuring Heat (Q)
What is a calorie?
 1 calorie ( cal ) = the quantity of heat needed to raise the
temperature of 1 g of pure water 1 oC

the food Calorie is spelled ‘CAPITOL C’

1 Calorie = 1 kilocalorie = 1000 calories
We will use the SI unit for energy, the Joule
1 calorie = 4.184 J

Quiz Question #1: How many joules of
energy does it take to raise the temperature
of 1 g of pure water by 1oC? _______
A.
B.
C.
D.
1 joule
22.4 joule
4.184 joule
1000 joule
Measuring Heat (Q)
What is a calorie?
 1 calorie ( cal ) = the quantity of heat needed to raise the
temperature of 1 g of pure water 1 oC

the food Calorie is spelled ‘CAPITOL C’

1 Calorie = 1 kilocalorie = 1000 calories
We will use the SI unit for energy, the Joule
1 calorie = 4.184 J

Key terms and concepts to know











Thermochemistry
Energy and its forms
Heat (Q) and how does it flow
Chemical Potential Energy
Law of Conservation of Energy
Exothermic reaction
Endothermic reaction
Energy Diagram: recognize endothermic vs. exothermic
calorie
Calorie
joule