1. Energy Changes

2. Energy Changes energy: the ability to do work
thermochemistry: the study of
energy changes
calorimetry:
an experimental technique of
measuring energy changes

3. 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.

4. Kinetic and Potential Energy
Kinetic Energy
Potential Energy
Energy due to
motion.
q       electrical
q       light
q       thermal
q       sound
Stored energy and
the energy of
position.
q       chemical
q       nuclear
q       mechanical

5. 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.

6. Absorbing or Releasing Heat Energy
All chemical reactions occur with either an absorption or release of heat (energy).
Reactants Products
(bonds broken) (bonds made)
energy absorbed > energy released system absorbs energy (endothermic)
energy absorbed < energy released
system releases energy (exothermic)

7. 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.

8. Heat Change The following equation can calculate the
heat change of a substance.
q = mcT

9. Heat q = mcT Term definition symbol units
heat amount of energy transferred q J (joules)
mass amount of matter m g (grams)
specific quantity of heat required to c J
heat raise the temperature of a unit gC
capacity mass of a substance 1C or 1K
temp average kinetic energy of the T C
particles in a sample of matter

10. 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?

11. 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.

12. H system = + l q surroundingsl
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.

13. 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.

14. Practice
Handout:
# 1-5, 9-11, 14-15
Together #1-3