Chapter 4: Energy, Chemistry, and Society
What do you think of when you hear the word
ENERGY?
Like the energy of a crowd,
You can’t see it,
Can’t measure it,
But you know it is there.
Terms that need to be defined:
Energy is the capacity to do work.
Work is movement against
a force (w = f x d).
Heat is energy that flows from a
hotter to a colder object.
Temperature determines the direction
of heat flow.
Heat is a consequence of motion at the molecular level;
temperature is a measure of the average speed of that motion.
4.2
What are units of heat?
The joule (J): 1 J is the amount of energy required to
raise a 1-kg object 10 cm against the force of gravity.
The calorie (cal): 1 cal is the amount of heat required to
raise the temperature of 1 g of water by 1 oC.
1 calorie = 4.184 J
1 kcal = 1000 cal = 1 Cal (1 dietary cal)
So that 450 Cal doughnut is really 450,000 calories!
4.2
Diagram of a Power Plant
work
electricity
heat
4.2
Power plants are inevitably inefficient
Potential energy
(chemical bonds)
burner
The First Law of Thermodynamics
Energy is neither created nor
destroyed, but may be transformed
from one form to another.
Heat energy
gas turbine
Mechanical energy
generator
Taking random, thermal energy and transforming
it into ordered work goes against the
Second Law of Thermodynamics.
Second Law of Thermodynamics –
the entropy of the universe is
increasing
Electrical energy
4.2
Bomb calorimeters can be used to determine
the heat of combustion
If you test a
reaction that
releases heat, the
temperature of
the water will
increase.
4.3
Hydrocarbon fuels like methane (CH4) burn in the presence of
oxygen to produce carbon dioxide and water.
Energy is released in this process called combustion.
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l) + ENERGY
When energy is released during the course of a chemical
reaction, it is said to be an EXOTHERMIC reaction.
The combustion of methane gas releases 50.1 kJ/g of CH4
This is the equivalent of 802.3 kJ/mol CH4.
4.3
CH4(g) + 2 O2(g)
Exothermic
reaction
CO2(g) + 2 H2O(l)
Net energy
change is
- 802.3 kJ/mol CH4
The products are lower in energy than the reactants
Exothermic reaction: DE is a negative value.
4.3
Activation energy –the energy required to initiate the reaction
Exothermic
reaction
Molecules must
possess enough
energy so that
their collisions
will be
productiveleading to
products.
ENDOTHERMIC reactions will have products higher in energy
(DE will be positive) than the reactants; there will still be a required
activation energy.
4.5
Energy Changes at the Molecular Level
The energy changes are due to the rearrangement of the
atoms of the reactants and products;
it is the breaking and forming of bonds that dictates if a
reaction will be endothermic or exothermic.
Bond energy is the amount of energy that must be absorbed
to break a chemical bond.
energy
Breaking bonds ALWAYS requires energy!
4.4
4.4
Consider: 2 H2 + O2
2 H2O
Bonds breaking
2 H-H + O=O
Bonds forming
4 O-H
(2 H-O-H)
4.4
History of U.S. Energy Consumption by Source 1800-2000
1 EJ = 1018 J
4.6
How do Different Fuels Stack Up?
High E content and the
only product of its
combustion is water!
4.6
Petroleum – from where does the U.S. import it?
2003: U.S.
consumption was
180 billion gallons.
Over 57% of it was
imported.
The U.S. has 5% of the
world’s population, yet
consumes 25% of the oil
produced worldwide.
4.8
One of the
drawbacks to
petroleum is
that it must
be refined
before use.
4.8
How do we use each barrel (42 gal) of petroleum?
This 1.25 gal
includes
nonrenewable
feedstocks for
all plastics,
pharmaceuticals,
fabrics and other
carbon-based
products.
Over 87% of each barrel is used for transportation and heating.
4.8
Gasoline Additives
Elimination of octane enhancing tetraethyl lead (TEL) created a
need to find substitutes.
MTBE –methyl tertiarybutyl ether
O
H3C
CH3
C
H
H
H
H
CH3
CH3
Ethanol (ethyl alcohol)
C
C
H
O
H
Human health effects of exposure to MTBE are not known.
4.10
Fuel Alternatives
Biodiesel fuel use is on the rise.
•Made from natural, renewable
sources (veg oils, animal fats).
•Can be used as pure fuel or
blended with petroleum
Ethanol-renewable, but more
expensive than gasoline.
•Some believe it takes more energy to
produce a gallon of ethanol than you
will obtain from burning it.
•Controversy: National Corn Growers
vs. American Petroleum Institute