Chapter 15: Energy – Introduction to Chemistry Study Notes

Chapter 15: Energy

Energy and the Life-Support System

The biosphere is the thin film of air, water, and soil where life exists, and only a small amount of the energy entering the biosphere is used to support life. Most energy on Earth originates from the sun.

• Solar radiation: 30% is reflected back to space.

• Photosynthesis: Less than 0.02% is used by green plants to power photosynthesis, producing oxygen and storing energy for all animals on the planet.

• Photosynthesis equation: $6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{chlorophyll, sunlight}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Glucose: A simple sugar produced by photosynthesis.

Energy In and Out

Almost all energy on Earth comes from the sun. The table below summarizes the main sources and sinks of energy.

Item	Energy (TW)*	Approximate Percentage
Solar radiation	174,000	99.97
Internal heat	46	0.03
Tides	3	0.002
Waste heat**	13	0.007
Energy out
Direct reflection	52,000	30
Water cycle*	40,000	23
Photosynthesis*	30	0.02

Additional info: *TW = terawatts; **Waste heat is eventually returned to space as infrared radiation.

Energy: Definition and Units

Energy is defined as the ability to do work or transfer heat. The SI unit of energy is the joule (J). Another common unit is the calorie (cal), where $1\text{cal} = 4.184\text{J}$.

• Power: The SI unit of power is the watt (W), which is energy per unit time ($1\text{W} = 1\text{J}/\text{s}$).

Power and Energy Conversion

Energy and power can be converted between units. For example, calculating the energy consumed by a light bulb:

• Example: A 75 W bulb burns for 1 hour. The energy consumed is:

$1\text{h} \times \frac{60\text{min}}{1\text{h}} \times \frac{60\text{s}}{1\text{min}} \times 75\text{J} \times \frac{1\text{s}}{1\text{W}} = 3600\text{s} \times 75\text{J} = 270,000\text{J}$

Energy Release in Chemical Reactions

Chemical reactions often release or absorb energy. The amount of heat released can be calculated using stoichiometry.

• Example: Burning methane releases heat:

$2.00\text{mol} \times \frac{803\text{kJ}}{1\text{mol}} = 1606\text{kJ}$

Energy Changes in Chemical Reactions

The energy change in a reaction can be calculated based on the mass and stoichiometry of reactants.

• Example: Combustion of propane:

$\text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(g) + 2201\text{kJ}$

For 225 g propane:

$225\text{g} \times \frac{1\text{mol}}{44.0\text{g}} \times \frac{2201\text{kJ}}{1\text{mol}} = 11,300\text{kJ}$

Potential and Kinetic Energy

Energy exists in two main forms: kinetic and potential.

• Kinetic energy: Energy an object possesses by virtue of its motion. $E_k = \frac{1}{2}mv^2$

• Potential energy: Energy an object possesses by virtue of its position or chemical composition.

Conversion of Energy

Energy can be converted from one type to another. For example, a cyclist at the top of a hill has potential energy, which is converted to kinetic energy as she descends.

• At the top: Potential energy

• At the bottom: Kinetic energy

Kinetic Energy and Particle Motion

All atoms, ions, and molecules have kinetic energy due to their motion. Temperature is a measure of the average kinetic energy of particles.

• $\text{Temperature} = \text{Average} \ E_k = \frac{1}{2}mv^2$

Energy and Chemical Reactions

Several factors affect the rate of a chemical reaction:

• Temperature: Increasing temperature increases reaction rates.

• Concentration of reactants: Reaction rates depend on reactant concentration.

• Presence of catalysts: Catalysts increase reaction rates by lowering activation energy.

Endothermic and Exothermic Changes

Energy changes in chemical reactions can be classified as endothermic or exothermic.

• Endothermic: System absorbs heat from surroundings; temperature of surroundings decreases.

• Exothermic: System releases heat to surroundings; temperature of surroundings increases.

Changes in Internal Energy of a Chemical System

The internal energy change ($\Delta E$) of a system determines whether a process is endothermic or exothermic.

• If $\Delta E > 0$, energy is absorbed (endothermic).

• If $\Delta E < 0$, energy is released (exothermic).

Examples of Endothermic and Exothermic Processes

The table below classifies common processes as endothermic or exothermic.

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

First Law of Thermodynamics (Law of Conservation of Energy)

The first law states that energy is neither created nor destroyed. The total energy of the universe is constant; energy lost by a system must be gained by the surroundings and vice versa.

Spontaneous Processes

Spontaneous processes occur naturally without external input. Examples include waterfalls running downhill, sugar dissolving in coffee, and heat flowing from hot to cold objects.

• Nonspontaneous processes require input of energy.

The Second Law of Thermodynamics

Energy flows spontaneously from hot objects to cooler objects. The entropy of the universe is always increasing.

• Entropy: A measure of dispersal of energy; increases in all spontaneous processes.

People Power: Early Uses of Energy

Early humans obtained energy by hunting, gathering, and using plant materials. Waterwheels and windmills convert kinetic energy of moving water and wind into mechanical energy.

Fossil Fuels

Fossil fuels, including coal, oil, and natural gas, provide more than 90% of the energy consumed in modern society. Fuels are substances that burn readily and release energy. Combustion is an exothermic oxidation process.

• Fuels: Reduced substances; nonfuels: Oxidized substances.

Reserves and Consumption Rates of Fossil Fuels

The table below shows estimated reserves and annual consumption of fossil fuels.

Reserves	World	United States
Coal	258	31.6
Petroleum	1031	6.5
Natural gas	6544	120

Coal: The Carbon Rock of Ages

Coal is a complex mixture of organic material, mostly carbon. It is abundant and the most plentiful fossil fuel. Coal combusts to produce carbon dioxide and is ranked from low-grade peat to high-grade anthracite.

Fuel Type	Carbon	Water	Ash (Minerals)
Lignite	25% to 35%	35% to 55%	13% to 30%
Sub-bituminous	35% to 45%	25% to 45%	10% to 15%
Bituminous	45% to 85%	10% to 20%	5% to 10%
Anthracite	85% to 98%	2% to 8%	< 5%

Natural Gas: Mostly Methane

Natural gas is one of the cleanest burning fossil fuels. Its primary component is methane (CH4), but it also contains ethane, propane, butane, and other gases.

Petroleum: Liquid Hydrocarbons

Petroleum is a complex mixture of hydrocarbons and has displaced coal as a principal fuel. Its combustion with oxygen produces carbon dioxide and water.

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

Additional info: These notes cover the main concepts of energy in chemistry, including energy forms, conversion, chemical reactions, thermodynamics, and fossil fuels, suitable for exam preparation in an introductory chemistry course.