Transcript CH04 - Chemistry at Winthrop University

Chapter 4: Energy from Combustion
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.
Complete Combustion – produces more
energy:
fuel + oxygen → CO2 + H2O + energy
Incomplete Combustion – not as efficient:
fuel + oxygen → CO + H2O + energy
Some energy is still stored in CO, so:
CO + oxygen → CO2 + H2O + energy
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.
4.1
There are many different forms of Energy:
Potential – stored energy
Gravitational, electrical, coiled springs
Chemical – energy stored in chemical bonds
and vibration/rotation of atoms
Nuclear – energy stored as matter or in
binding subatomic particles together
Electrical – energy in motion and electrical
potential of electrons

Kinetic – energy of motion: Anything with
mass that is moving, vibrating, or spinning
Mechanical energy – moving machinery
Thermal energy – motion of atoms
Temperature measures average kinetic
energy of matter: at absolute zero there
is no motion

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
Conservation of energy
(First Law of Thermodynamics)
Energy may be converted from one form to
another, but the total amount of energy
remains constant.
Energy is often measured in:
Joules (J)
calories (cal): 1 cal = 4.18 J
Calories (Cal) = food calories
1Cal = 1000 cal

Coal is used to create electricity in a power plant:
1. Combustion
2. Boil water in a closed, high pressure system
3. Turn turbine to create electricity
4.1
Coal Power Plants
Convert energy to various forms:
Potential (chemical energy in coal)
Thermal (burning coal, heating water)
Kinetic (moving steam through pipes)
Mechanical (steam turns turbines)
Electrical (turbines produce electricity)
??? : homes and businesses turn
electrical energy back into many other
forms (light, heat, mechanical...)
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.
Kinetic energy
gas turbine
Taking random, thermal energy and transforming it
into ordered work goes against the
Second Law of Thermodynamics.
Mechanical energy
generator
Electrical energy
The Second Law of Thermodynamics
The entropy of the universe is
increasing.
There is no free lunch!
4.1
The stored chemical energy in fossil fuels
(coal, oil, gas,...) and wood comes from
photosynthesis.
Photosynthesis is basically the reverse of a
combustion reaction:
6CO2 + 6H2O + energy → C6H12O6 + 6O2
versus combustion:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
CO2 and H2O have very little potential
chemical energy.
The suns energy is stored in the products:
saccharides (sugars) and oxygen.
Sugar molecules link together to form other
sugars, starches and cellulose.
With time, pressure, and heat, dead plant
material is converted into fossil fuels.
Coal
• Coal is a complex mixture of substances.
• Although not a single compound, coal can
be approximated by the chemical formula
C135H96O9NS.
4.3
History of U.S. Energy Consumption by Source 1800–2008
1 EJ = 1018 J
4.3
Coal use around the world:
Green = 1999
Gold = 2009
The unit is million metric tons oil equivalent, the
approximate energy released in burning a million
metric tons of oil.
4.3
Efficiency of Energy Conversion
We are not able to completely convert energy
from one form to another
During the conversion, energy is often also
changed into other forms
Most if this energy is 'lost', since it is not in a
useful form.
Some examples of this are friction, heat,
light, and kinetic energy lost to the
environment
For coal power plants:
electrical energy produced
Efficiency=
∗ 100%
heat from fuel
Each energy conversion step may be 5-90%
efficient, depending on the technology used.
Overall, most coal power plants are 35-50%
efficient, with most energy lost as heat.
Higher temperature steam  more efficient
For coal power plants:
electrical energy produced
Efficiency=
∗ 100%
heat from fuel
Each energy conversion step may be 5-90%
efficient, depending on the technology used.
Overall, most coal power plants are 35-50%
efficient, with most energy lost as heat.
Higher temperature steam  more efficient
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!
4.6
4.6
Consider: 2 H2 + O2
2 H2O
Bonds breaking
2 H–H + O=O
Bonds forming
4 O–H
(2 H–O–H)
4.6
Usually, reactions occur to lower the stored
chemical (potential) energy.
For these reactions, the potential energy lost
is usually released as heat and light.
Some reactions actually absorb thermal
energy from the surroundings.
For this to happen, the overall Entropy must
increase.
Entropy – measure of disorder or chaos:
more chaos → more entropy.
Second Law of Thermodynamics – the
entropy of the universe is increasing.
The entropy and energy change both
determine whether or not a reaction occurs.
Examples of increasing entropy
(increasing disorder):
solid → liquid
liquid → gas
cold object → warm object
one compound → 2 compounds

Coal – a major driving force behind the
industrial revolution.
Much more energy per gram than wood
More abundant – not enough wood to meet
energy needs
There are different grades of coal
Some produce more energy per gram
Some are more pure carbon, while most
contain many impurities, such as sulfur.
Not all coal is created equal:
4.3
Some problems with coal fuel:
Mine safety
mine collapse
dangerous gases
black lung disease from breathing coal
dust
Environmental damage
strip mining

Pollution from combustion:
particulates
SO – lung irritant, leads to acid rain
x
CO production – in recent years coal has
2
accounted for up to 40% of anthropogenic
CO2 production
Some ways to reduce pollution:
Coal washing – remove sulfur from coal
before burning
Gasification – process used to turn coal into
CO and H2 gases, which is a much cleaner
fuel, though some potential energy is lost
Wet scrubbing – scrubs the exhaust gases to
remove SOx.
This waste can be used for commercial
products, such as gypsum drywall.
Hydrocarbons (HCs) – any compound
containing only hydrogen and carbon
Crude oil – a mixture of HCs, such as
methane, ethane, propane, octane, kerosene,
waxes...
This mixture is separated by distillation:
Smaller molecules turn into gases at lower
temperatures, so each compound is boiled off
at a different temperature.
An Oil Refinery
A Port
Arthur, TX
Oil Refinery
4.4
Distillation Tower:
One of the
drawbacks to
petroleum is
that it must
be refined
before use.
4.4
How do we use each barrel (42 gal) of petroleum?
This 7.3 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.
4.4
Heat of Combustion – energy released per
mole or per gram of fuel.
Different HCs have different Heats of
combustion, so some produce more energy
per gram than others.
This energy comes from making/breaking
chemical bonds.
The products have lower potential energy,
and the lost energy is released as heat/light
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.
4.5
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.
4.5
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(l)
Exothermic
reaction
The products are lower in energy than the reactants.
Exothermic reaction: E is a negative value.
4.5
Are all fuels created equal?
4.5
Gasoline
Compared to combustion of octane, C H
8 18
(Octane rating)
Not a pure HC, but a mixture of HCs,
isomers, and additives
Isomers – same formula, different
structure
Crude oil does not have enough octane to
be used for fuel, and with mixtures, the
combustive properties can be adjusted
Gasoline Additives

Additives improve properties
combustion rate (knocking)
combustion efficiency
residue left in engine
pollution
Gasoline Additives
Tetra ethyl lead (TEL)
Used to be a common additive
Potential problem for lead poisoning
can cause sterility, insanity, loss of
teeth...
children more susceptible than adults
Absorbed by inhalation of exhaust fumes
Data shows a significant drop in lead
concentrations in children after TEL was
banned
Gasoline Additives
MTBE used to replace TEL
methyl tertiarybutyl ether (H, C, and O)
improves octane rating
provides oxygen for combustion
possible carcinogen
Ethanol often used to replace MTBE
Also contains only H, C and O
Renewable energy source
But attracts water – can cause rusting if
left for prolonged periods
Gasoline Additives
Elimination of octane enhancing tetraethyl lead (TEL) created a
need to find substitutes.
MTBE, methyl
tertiary-butyl 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.
4.7
Fuel Alternatives
Biodiesel fuel use is on the rise.
Made from natural, renewable
sources (vegetable oils, animal
fats), it can be used as pure fuel
or blended with petroleum.
Ethanol is 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.
4.9
Additives may be used to raise/lower the
activation energy of the combustion.
Activation energy – an energy barrier that
needs to be overcome for the reaction to
proceed.
The activation energy controls how easily the
fuel ignites, and how quickly it burns.
Catalysts – something that lowers the
activation energy (speeds up the reaction),
but is not consumed.
The catalytic converter in automobiles is a
hot metal (Pt) surface that:
catalyzes reactions in the exhaust gases
CO + O → CO
2
2
NO → N + O
x
2
2
Energy released in this converter is 'lost'
as heat
Historic and projected energy consumption worldwide:
Note: All three projections slope upward.
4.11