Transcript Energy Changes

Energy Changes
Energy Changes
energy: the ability to do work
thermochemistry: the study of
energy changes
calorimetry:
an experimental technique of
measuring energy changes
Energy Changes
Energy can take a
wide variety of
forms. Although
there are many
forms of energy,
they can all be put
into two categories:
kinetic and potential.
Kinetic and Potential Energy
Kinetic Energy
Energy due to
motion.
q
q
q
q
electrical
light
thermal
sound
Potential Energy
Stored energy and
the energy of
position.
q
chemical
q
nuclear
q
mechanical
Energy Changes


When studying transfers of energy, it is
important to distinguish between the
substances undergoing the change, called
the chemical system and the system’s
environment, called the surroundings.
In an open system, both energy and matter
can move in/out.
 In a closed system, only energy can move
in/out.
 In an isolated system, neither matter nor
energy can move in/out.
Absorbing or Releasing Heat Energy

All chemical reactions occur with either an
absorption or release of heat (energy).
Reactants
(bonds broken)
Products
(bonds made)
energy absorbed
>
energy released
system absorbs energy (endothermic)
energy absorbed
<
energy released
system releases energy (exothermic)
Endothermic vs. Exothermic Reaction


Endothermic Reaction
q Energy is absorbed
by the system.

q More energy is
needed to break bonds
than is released by the
formation of new bonds.

q Temperature of
surroundings
decreases.


Exothermic Reaction
q Energy is released
by the system.

q More energy is
released by the
formation of new bonds
than is needed to break
bonds.

q Temperature of the
surroundings increases.
Heat Change
The following equation can calculate the
heat change of a substance.
q = mcT
Heat
Term
q = mcT
definition
symbol
units
heat
amount of energy transferred
q
J (joules)
mass
amount of matter
m
g (grams)
specific
heat
capacity
quantity of heat required to
raise the temperature of a unit
mass of a substance 1C or 1K
c
J
gC
temp
average kinetic energy of the
particles in a sample of matter
T
C
Practice
What would be the final temperature if
163 kJ of energy is transferred to 625 mL
of water in an electrical kettle at an initial
temperature of 20.0C?
Law of Conservation of Energy
According to the Law of Conservation
of Energy, energy can be converted from
one form to another, or transferred from
one set of molecules to another, but the
total energy of the system and its
surroundings remains the same.
Enthalpy Change
An equation showing this relationship is:
H system = + l q surroundingsl
where H is “enthalpy change” or “heat
of reaction” which is the difference in
energy of a substance during a change.
Types of Enthalpy Changes
Three types of enthalpy changes
observed are during physical changes,
chemical reactions, and nuclear reactions.
Notice how much more energy is
produced in a nuclear change than in a
chemical change, and in a chemical
change than a physical change.
Practice
Handout:
# 1-5, 9-11, 14-15
Together #1-3