Chapter 15: Energy – Chemistry for Changing Times (Introductory Chemistry Study Notes)

Energy in Chemistry

Definition and Forms of Energy

Energy is a fundamental concept in chemistry, defined as the ability to do work or transfer heat. It is essential for all chemical and physical processes.

• SI unit of energy: Joule (J). 1 J = 0.2388 cal.

• SI unit of power: Watt (W). 1 W = 1 J/s.

• Potential energy: Energy stored due to position or composition.

• Kinetic energy: Energy of motion.

Earth’s Energy Sources

Nearly all energy on Earth originates from the sun. The distribution and utilization of this energy are crucial for sustaining life and driving chemical processes.

• Solar radiation accounts for 99.97% of Earth's energy input.

• Other sources include internal heat and tidal energy, but these are minor by comparison.

Source	Approximate Percentage
Solar radiation	99.97%
Internal heat	0.025%
Tides	0.002%

Additional info: Most energy leaving Earth is in the form of reflected sunlight or infrared radiation (heat).

Energy and the Life-Support System

The Biosphere and Photosynthesis

The biosphere is the thin layer of air, water, and soil where life exists. Only a small fraction of incoming solar energy supports life, primarily through photosynthesis.

• 30% of solar radiation is reflected back to space.

• 23% powers the water cycle.

• Less than 0.02% is used by green plants for photosynthesis.

• Photosynthesis produces oxygen and stores energy in glucose for all animals.

Energy and Chemical Reactions

Factors Affecting Reaction Rates

The rate of a chemical reaction depends on several factors:

• Temperature: Higher temperatures increase reaction rates.

• Concentration of reactants: Higher concentrations lead to faster reactions.

• Catalysts: Catalysts speed up reactions by lowering activation energy.

Exothermic and Endothermic Reactions

Chemical reactions involve energy changes, classified as exothermic or endothermic.

• Exothermic reactions: Release heat energy to the surroundings.

• Endothermic reactions: Absorb heat energy from the surroundings.

Exothermic Processes	Endothermic Processes
Freezing of water	Melting of ice
Condensation of water vapor	Evaporation of water
Metabolism in animals	Photosynthesis in plants
Forming chemical bonds	Breaking chemical bonds
Discharging a battery	Charging a battery

Example: Energy Release in Chemical Reactions

Burning 1.00 mol (16.0 g) methane releases 803 kJ of energy as heat. For 2.00 mol methane, the heat released is:

$2.00 \times 803\ \mathrm{kJ} = 1606\ \mathrm{kJ}$

Thermodynamics in Chemistry

The First Law of Thermodynamics

Energy can be neither created nor destroyed; it can only be transformed from one form to another. This is known as the law of conservation of energy.

The Second Law of Thermodynamics and Entropy

Energy flows spontaneously from hot objects to cooler objects. Entropy is a measure of the dispersal of energy, and for all spontaneous processes, the entropy of the universe increases.

Historical and Modern Uses of Energy

Early Uses of Energy

Early humans relied on plant materials and animal products for energy. Waterwheels and windmills were among the first machines to convert kinetic energy into mechanical energy.

Fossil Fuels

Fossil fuels (coal, oil, natural gas) provide over 90% of the energy consumed in modern society. Fuels are substances that release significant energy when burned (exothermic oxidation).

Coal

Coal is a complex mixture, primarily carbon. Its combustion is represented as:

• Coal is abundant but hazardous to obtain and inconvenient to use.

• Coal often contains sulfur, producing SO2 and contributing to acid rain.

• Heating coal without air produces coke (used in steel production) and coal tar (source of organic compounds).

Natural Gas

Natural gas is mainly methane (CH4) and is one of the cleanest burning fossil fuels. It is also a raw material for many industrial chemicals.

Petroleum

Petroleum is a complex mixture of hydrocarbons, separated into fractions by distillation. Its combustion can be represented as:

$2\ \mathrm{C}_8\mathrm{H}_{18} + 25\ \mathrm{O}_2 \rightarrow 16\ \mathrm{CO}_2 + 18\ \mathrm{H}_2\mathrm{O}$

Fraction	Hydrocarbon Size	Boiling Point (°C)	Uses
Gas	CH4 to C4H10	<40	Fuel, plastics
Gasoline	C5H12 to C12H26	40–200	Fuel, solvents
Kerosene	C12H26 to C16H34	175–275	Diesel, jet fuel
Heating oil	C15H32 to C16H34	250–400	Heating, gasoline production
Lubricants	C17H36 and up	>300	Lubricants
Residue	C20H42 and up	>350	Paraffin, asphalt

Larger fractions can be cracked into smaller, more useful molecules.

Octane Ratings of Gasoline

The octane rating measures a fuel's resistance to knocking in engines. Isooctane is rated 100, heptane is 0. Gasoline with an octane rating of 90 behaves like a mixture of 90% isooctane and 10% heptane.

Alternative Fuels

Engines can use various fuels, including diesel (C9–C12 hydrocarbons), biodiesel (from fats/oils), and ethanol (E85 is 85% ethanol, 15% gasoline).

Electricity and Energy Conversion

Coal Gasification and Liquefaction

Coal can be converted into gas or liquid fuels for convenience. Electricity is a highly versatile energy carrier.

Electric Power Generation

Coal-burning power plants generate electricity by converting chemical energy to heat, then to mechanical, and finally to electrical energy.

In 2003, U.S. electricity generation sources included coal (45%), gas (23%), nuclear (20%), hydroelectric (7%), and others.

Nuclear Energy

Nuclear Fission

Nuclear power plants use fission reactions to produce heat, generating steam to turn turbines and produce electricity. Nuclear energy produces minimal air pollution but requires strict safety and waste management.

Country	% Electricity from Nuclear	Operating Plants	Plants Under Construction
France	73.3%	58	1
United States	19.4%	104	5
Sweden	42.7%	10	0
Ukraine	43.6%	15	2
China	2.1%	20	28
Other countries	NA	72	13

Renewable Energy Sources

Solar Energy

Solar energy is abundant but diffuse. It must be concentrated for practical use. Solar collectors heat water, and photovoltaic cells convert sunlight directly into electricity.

Biomass Energy

Biomass energy is derived from burning plant material or converting it to fuels like ethanol or methane. This process utilizes the energy stored by photosynthesis.

Hydrogen as a Fuel

Hydrogen can be burned cleanly or used in fuel cells to produce electricity. The reaction is:

$2\ \mathrm{H}_2(g) + \mathrm{O}_2(g) \rightarrow 2\ \mathrm{H}_2\mathrm{O}(g) + 572\ \mathrm{kJ}$

Other Renewable Sources

• Wind: Wind turbines convert kinetic energy to electricity.

• Water: Hydroelectric plants use falling water to turn turbines.

• Geothermal: Uses Earth's internal heat to generate steam and electricity.

• Ocean energy: Includes thermal, tidal, and wave energy conversion.

Global Energy Consumption

Trends and Challenges

Global energy demand is increasing, while many traditional sources are finite. This drives the search for sustainable and renewable energy solutions.

Country	Primary Energy Consumption per Capita (quadrillion BTU, 2011)
China	119.613
United States	92.896
Russia	29.625
India	25.27
Canada	14.361
Germany	13.203
Brazil	12.687
Saudi Arabia	10.657
France	10.261
United Kingdom	8.111
Mexico	7.62
Thailand	5.07
Nigeria	1.369
Guatemala	0.239
Zimbabwe	0.164