Energy and Chemical
Reactions
Ms. Grobsky
What is Energy?
•
Energy is the capacity to do work
•
S.I. unit of energy is the joule (J)
1 kJ (kilojoule) = 103 J
•
•
Another energy unit is the calorie (cal) – more on this later!
1 cal = 4.184 J
•

There are many different forms of energy:
•
Thermal energy is the energy associated with the random motion of atoms
and molecules
•
Kinetic energy is the energy due to motion
•
Potential energy is the energy available by virtue of an object’s position
•
Chemical energy is the energy stored within the bonds of chemical substances
•
•
Type of potential energy
All forms of energy are able to interconvert!
Example of Energy Conversion –
Kinetic Energy and Potential Energy
Total Internal Energy =
E
Ball thrown
upwards slows &
loses kinetic
energy but gains
potential energy
Kinetic Energy
=
EK
+
+
Potential Energy
EP
The reverse
happens as it
falls back to the
ground
Why Can Energy Interconvert?
 Because of the Law of Conservation of Energy!
 The total energy of the universe is constant and can neither
be created nor destroyed
 It can only be transformed
What Does Energy Have to Do
with Chemical Reactions?
Introducing Thermochemistry!
Energy Changes in Chemical Reactions - HEAT
• Thermochemistry is the study of heat change in chemical
reactions
• Well, what is heat?
• Heat (represented by “q”) is the thermal energy that
transfers from one object to another when the two
things are at different temperatures and in some kind of
contact
• Kettle heats on a gas flame
• Cup of tea cools down (loses energy as heat)
• Thermal motion or random molecular motion (temperature)
is increased by heat energy
• Heat stimulates thermal motion
Temperature = Heat
• When studying heat changes, the world is divided into two parts:
• The system
• The surroundings
SURROUNDINGS
SYSTEM
Exchange:
Open
Closed
Isolated
Mass & Energy
Energy
Nothing
6.2
Heat Exchange between System and the
Surroundings
•
All chemical reactions have energy changes associated with it
•
•
Energy is stored in bonds between atoms
An exothermic process is any process in which energy (heat) flows out of the
system into the surroundings
•
“q” is negative
•
Energy of reactants is greater than that of the products
•
Energy can be thought of as a product in a chemical reaction because it is
“given off”
H2O (g)
•
H2O (l) + Energy
An endothermic process is any process in which energy (heat) flows into the
system from the surroundings
•
“q” is positive
•
Energy of products is greater than that of the reactants
•
Energy can be thought of as a reactant in a chemical reaction because it is
being absorbed
Energy + 2HgO (s)
2Hg (l) + O2 (g)
Endothermic vs. Exothermic
Reactions