Energy, Chemistry, and Society
GISAT 112
Calculate
How many gallons of gasoline are consumed by the typical
American annually?
UGH!!!
10,000 miles/(person-yr) * 1 gal/25 miles
= 400 gallons/(person-yr)
15,000 miles/(person-yr) * 1 gal/15 miles
= 1,000 gallons/(person-yr)
Objectives
• Apply the terms exothermic, endothermic, and activation
energy to chemical systems
• Interpret chemical equations to calculate heats of reaction
• Use bond energies to describe the energy content of
materials
• Evaluate risks and benefits associated with various energy
sources
• Describe how energy flows in ecosystems and the
implications for populations at various trophic levels
Energy Conservation and Consumption
1 mton =
.04 quads =
4 x 1013 Btu
Consumption in U.S.
Energy Over Time
Today
Energy: Hard Work and Hot Stuff
• Energy (work): the capacity to create movement against a
restraining force, equal to force times the distance over
which it is applied
• Heat: thermal energy, characterized by the random motion
of molecules; thermal energy always flows from hotter to
colder bodies
• Temperature: property that determines the direction of heat
flow
Energy Units
• Joule (J): One beat of the human heart
uses about one Joule of energy
• calorie (cal): metric measure, defined as amount of energy
needed to raise 1 g of water 1 oC.
• 1 cal = 4.184 J (note: food Calories are 1 Cal = 1 kcal)
• Other units: BTU, ergs, foot-pounds
( 1 BTU = 1055 J )
• See Your Turn 4.5 for conversion practice
Energy: Where From and How Much?
•
•
•
•
•
Making and breaking bonds
Breaking bonds in reactants REQUIRES energy
Making bonds in products RELEASES energy
Heat of reaction = NET change in bond energies
Exothermic vs. endothermic
Thermodynamics
• Combustion: the combination of fuel with oxygen to form
product compounds where the potential energy of the
reactants is greater than that of the products
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + energy
• First Law of Thermodynamics: Energy is neither created
nor destroyed.
– Energy may be transformed, but the energy of the
universe is constant.
Energy and Heat of Combustion
• During this reaction, potential energy is released in the
form of heat.
Calculating Energy Changes
• For calculations, we need to determine the amount of
energy used to break every bond in the reactants versus the
amount of energy used to create every bond in the products
• Bond energy: the amount of energy that must be absorbed
to break a specific chemical bond (kJ/mole of bonds)
Calculating Heat of Combustion
• Energetic bookkeeping - keeping track of the energy
changes involved in each step and whether the energy is
absorbed or released
STEPS
1. Determine the balanced chemical equation
2. Write Lewis structures for all reactants and products
3. Count up the bonds in reactants and products
4. Total the bond energies for reactants (use Table 4.1)
5. Total the bond energies for the products
6. Heat of reaction = reactants’ energy - products’ energy
Example: Combustion of Ethanol
C2H5OH + 3 O2
2 CO2 + 3 H2O
How many molecules are on each side?
How many chemical bonds are on each side?
2C + 6H + 7O
Break: 5 C-H
1 C-C
1 C-O
1 O-H
Make: 4 C=O
Break: 3 O=O
H H
H C CC2HO5OH
H + 3 O2
H H
Make: 6 O-H
2 CO2 + 3 H2O
Bond Energy Totals
Breaking bonds:
Making bonds:
5 C-H
5 (411 kJ)
4 C=O
4 (799 kJ)
1 C-C
1 (346 kJ)
6 O-H
6 (459 kJ)
1 C-O
1 (358 kJ)
1 O-H
1 (459 kJ)
3 O=O
3 (494 kJ)
4700 kJ
5950 kJ
Heat of Combustion
Reactants
C2H5OH + 3 O2
Energy difference = 4700 - 5950
= - 1,250 kJ
released as heat and light!
Products
2 CO2 + 3 H2O
In-class Exercise
Calculate the energy released when burning hydrogen to form
water.
2H2 + O2  2H2O + energy
Breaking bonds:
Making bonds:
2 H-H
2 (432 kJ)
4 O-H
1 O=O
1 (494 kJ)
1358 kJ
4 (459 kJ)
1836 kJ
So energy released is 1358 kJ – 1836 kJ = -478 kJ
…per 2 moles of H2!
Released energy carries
a negative sign,
absorbed energy carries
a positive sign.
---------459 KJ
---------432 KJ
---------
478 KJ
The release of heat
corresponds to a
decrease in the energy
of the chemical system,
which is why the
energy change is
negative in this
exothermic reaction.
Other Issues
• In reality, not all bonds are actually broken and then
formed—but the energy difference between the reactants
and products is what we need and our approach provides a
useful mental construct for these calculations.
• Results are typically close to actual values, but not always,
for the following reasons:
– Bond energies in the table only apply to gases
– Bond energies are really average values—bond energy
is actually dependent on other atoms in the molecule
Getting Started: Activation Energy
• Activation energy: often energy is necessary to initiate a
reaction
• What is the source of activation energy in your car?
Fossil Fuels
• Fossil fuels are compounds formed by the remains of
animal and plant matter millions of years old
– Coal, petroleum oil, natural gas (methane)
• Burning fossil fuels recaptures the solar energy that the
plant matter originally captured via photosynthesis:
2800 kJ + 6CO2(g) + 6H2O(l)  C6H12O6(s) + 6O2(g)
Coal
• Coal: a mixture, although we can use the chemical
formula: C135H96O9NS.
• Usually, the higher C content, the more energy is released.
• Energy content: 30 kJ/g
• Try Your Turn 4.14
Coal: Advantages and Disadvantages
• Advantages
– Ample supplies
– High net energy yield
– Low cost
• Disadvantages
– High land use
– SO2, NOx, PM emissions
– High fossil CO2 emissions
– May release mercury and
radioactive particles into the
air
Petroleum
• Complex mixture of hydrocarbons
• Liquid form and higher energy content (48 kJ/g v. 30 kJ/g)
gave petroleum dominance over coal in many applications
• Petroleum that we use must be
refined from crude oil through
a distillation process
Manipulating Molecules
• Demand for gasoline would never be met with distillation
alone
• We need to employ cracking to increase the amount of
gasoline produced at the refinery
• Cracking involves the breaking down of large molecules to
smaller ones, e.g.:
C16H34  C8H18 + C8H16
• Isomers--different compounds with the same formula (e.g.,
n-octane v. iso-octane)
Newer Fuels: Oxygenated and Reformulated
Gasoline
• Oxygenated gasoline contains compounds, such as methyltertiary-butyl ether (MTBE) or ethanol (C2H5OH) that
have higher oxygen content
• Reformulated gasoline (RFG) is oxygenated gasoline that
also contains lower percentage of volatile hydrocarbons
(such as benzene)
Transforming Energy
• To obtain useful energy (that which does work for us), we
must transform it from one state to another (e.g., from
chemical potential energy to kinetic or thermal energy)
• We lose energy
each step of the
way, usually in
the form of heat.
• Efficiency can
be defined as:
Eff = Eout/Ein.
Order versus Entropy
• Heat (thermal energy) is characterized by the random
motion of molecules
• Entropy is defined as this randomness
• To get all heat energy transferred to work, then all this
randomness would have to be controlled and focused,
without adding additional energy—impossible!
• Second Law of Thermodynamics--the entropy of the
universe is increasing. Therefore, it is impossible to
completely convert heat into work without making some
other changes in the universe.
Renewable Energy
Includes those forms of energy that we cannot deplete or that are
quick to regenerate:
• Biofuels: ethanol, biogas, biodiesel, wood
• Solar: passive/active hot water, photovoltaic
• Wind: mechanical, generators
• Geothermal: home use, underground
• Hydropower: low head, large scale
• Ocean: tidal, currents
Renewables account for approximately 10% of total domestic
electricity generation.
New Fuels and Energy Substitutes
• Electricity: Solar, biomass, wind, fuel cells, solid waste
• Process Heat: Solar, biomass, solid waste, biogas
• Transportation: Natural gas, propane, methanol (CH3OH),
ethanol (C2H5OH), electricity, hydrogen, gasoline from
coal
• Energy conservation and efficient technologies
Fig. 12.2
Worldwide commercial
energy production
Fig. 12.23
Types of biofuels
•
•
•
•
•
Wood
Grass
Manure
Ethanol – from grain or sugar
Biodiesels – from oilseeds
Sugar Cane Field and bagasse
Sugar cane to ethanol and bagasse
Biodiesel
•
•
•
•
Derived from cooking oil
Recycled or used directly
Limited processing
Easily available
– Even in Harrisonburg!
Solar Energy
Passive Solar – uses natural materials to absorb heat energy.
e.g. adobe dwellings, greenhouses
Active Solar – involves pumping heat absorbing fluids through a collector.
In Greece, Italy and Israel 70% of domestic hot water comes from solar
collectors.
High Temp Solar – parabolic mirrors collect light and focus it on one
concentrated point.
Photovoltaics – direct solar to electrical energy conversion.
Fig. 12.18
Solar Applications
Sunlight—through photovoltaic technology—provides this
building at Oberlin College with electricity.
Credit: Robb Williamson
Solar
radiation
input in
cal/cm2-day
Fig. 12.16
• Old Holland used windmills for
pumping water.
• Modern equivalent is wind turbineturning shaft spins a generator to make
electricity.
• Stand-alone or grid connected.
• UK and Denmark expect to produce
20% of their energy from wind farms.
• Germany leads world in wind power.
Wind Energy
• In most power plants steam from fossil
fuel burning rotates a turbine that
activates a generator, which produces
electricity.
• Geothermal power plants, however, use
steam produced from reservoirs of hot
water found a couple of miles or more
below the Earth's surface.
• Three types of geothermal power
plants: dry steam, flash steam, and
binary cycle.
Geothermal Energy
This geothermal power plant
generates electricity for the
Imperial Valley in California.
Credit: Warren Gretz
Geothermal Energy
• The upper 10 feet of the Earth,
maintains a nearly constant
temperature between 50° and
60°F (10°–16°C).
• Like a cave, this ground
temperature is warmer than the
air above it in the winter and
cooler than the air in the
summer.
• Geothermal heat pumps take
advantage of this resource to
heat and cool buildings.
The West Philadelphia Enterprise Center uses
a geothermal heat pump system for more than
31,000 square feet of space.
Credit: Geothermal Heat Pump Consortium
Hydropower
•
Falling water used as an energy source
since ancient times.
•
Norway, New Zealand, Switzerland get
most of their electricity from falling
water.
•
Canada has 400 hydroelectric power
stations.
•
Create comparatively less air pollution,
but still have environmental
consequences.
Hydroelectric power generates
about 10% of the nation's energy.
Credit: US Army Corps of Engineers
Oceans: Thermal and Tidal Energy
•
•
•
•
Oceans cover 70% of Earth making them largest solar collectors.
Tidal energy conversion – as above
Wave energy conversion: channel systems that funnel the waves into reservoirs; float systems
that drive hydraulic pumps; and oscillating water column systems that use the waves to
compress air within a container.
Thermal energy from the ocean: closed-cycle, open-cycle, and hybrid.
Ocean Energy
Wells Turbine
In line current turbine
Nuclear Power
Fig. 12.12
Benefits of Nuclear power
1. High productivity/unit fuel
2. No green house gas
3. Centralized production
Problems with Nuclear Power
1. Mining waste
2. Dangerous maintenance
3. Disposal of nuclear waste
The problem of energy storage
• Electricity cannot be stored in the form of free electrons.
• Transforming one form of energy to another involves a
loss predicted by the second law of thermodynamics.
• Transformation of chemical energy to electrical energy via
a heat engine is the least efficient mechanism.
• Batteries are the weak point of solar systems.
• What is the role of hydrogen?
Fig. 12.22
Fig. 12.28
Notes from DOE and NREL
http://www.eere.energy.gov/
Coal
Reserves
Fig. 12.6
Oil
Reserves
Fig. 12.8
Fig. 12.3
Renewable Energy Consumer Information
http://www.eere.energy.gov/consumerinfo/