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.

Today

Energy Over Time

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 heartuses 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?

Break: 3 O=O

C2H5OH + 3 O2 2 CO2 + 3 H2OCH

H

H

O H

H

H

C

Break: 5 C-H

1 C-C

1 C-O

1 O-H

2 C + 6 H + 7 O

Make: 4 C=O

Make: 6 O-H

Bond Energy Totals

Breaking bonds:

5 C-H 5 (411 kJ)

1 C-C 1 (346 kJ)

1 C-O 1 (358 kJ)

1 O-H 1 (459 kJ)

3 O=O 3 (494 kJ)

4700 kJ

Making bonds:

4 C=O 4 (799 kJ)

6 O-H 6 (459 kJ)

5950 kJ

Reactants C2H5OH + 3 O2

Products 2 CO2 + 3 H2O

Energy difference = 4700 - 5950 = - 1,250 kJ

released as heat and light!

Heat of Combustion

In-class Exercise

Calculate the energy released when burning hydrogen to form water.

2H2 + O2 2H2O + energy

Breaking bonds:

2 H-H 2 (432 kJ)

1 O=O 1 (494 kJ)

1358 kJ

Making bonds:

4 O-H 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.

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.

---------432 KJ ---------459 KJ

--------- 478 KJ

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 methyl-tertiary-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

Fig. 12.16Solar radiation input in cal/cm2-day

Wind Energy• Old Holland used windmills for pumping water.

• Modern equivalent is wind turbine- turning 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.

Geothermal 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.

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 TurbineIn line current turbine

Nuclear Power

Fig. 12.12

Problems with Nuclear Power

1. Mining waste

2. Dangerous maintenance

3. Disposal of nuclear waste

Benefits of Nuclear power

1. High productivity/unit fuel

2. No green house gas

3. Centralized production

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/

Fig. 12.6Coal Reserves

Fig. 12.8Oil Reserves

Fig. 12.3

Renewable Energy Consumer Information

http://www.eere.energy.gov/consumerinfo/