chapter 9 energy for today

Nivaldo J. Tro

http://www.cengage.com/chemistry/tro

Mark Erickson • Hartwick College

Chapter 9Energy for Today

Molecules in Motion• A burn is the transfer of energy (too much of it for comfort)

from a hot object to the human body.

• The atoms and molecules are in constant motion.

• The hotter an object is, the faster its molecules move.

Fundamental Concepts• Heat is the flow of energy due

to a temperature difference.

• Energy, and our use of it, is ultimately tied to molecular motion.

• We use energy to move atoms and molecules in a nonrandom, or orderly, motion.

• This use of energy is called WORK.

Reliance on Energy

Without it, our most common tasks become impossible.

Reliance on Energy

The average U.S. citizen enjoys the energy output of 115 people at the flip of a switch or the push of a pedal. The energy output of a number of nations is shown in the graph.

Energy Vocabulary• Thermodynamics: The study of energy and its transformation

from one form to another

• Energy: The capacity to do work

• Work: A force acting over a distance

• Total energy of an object: The sum of its kinetic energy (motion) and its potential energy (position)

• Thermal energy: The energy associated with the temperature of an object

• System: The subject we are thermodynamically studying

• Surroundings: The environment with which the system is exchanging energy

Potential and Kinetic Energies• Chemical and physical

changes are accompanied by changes in energy.

• Total Energy is the sum of the kinetic and potential energies.

• Kinetic Energy: Energy of motion

• Potential Energy: Energy of position

The First Law of Thermodynamics• Energy can be neither created nor destroyed, only

transferred between the system and the surroundings.

– An exception occurs in nuclear processes where mass and energy are interchangeable as E = mc2.

Implications of the First Law• We cannot create energy that was not there to begin with; a

device that continuously produces energy, without the need for energy input, cannot exist.

You can’t get something for nothing.

Concept Check 9.1Explain how a perpetual motion machine violates the First Law of Thermodynamics.

Concept Check 9.1 Solution• A perpetual motion machine violates the First Law of

Thermodynamics by creating more energy than it possessed at the beginning.

• The energy of a system is constant, it can never be created or destroyed.

• Energy is transformed from one form to another between the system and its surroundings.

The Second Law of Thermodynamics

• Energy is dispersed (becomes arranged in a more disorderly way) in any spontaneous process.

• For any spontaneous process, the entropy of the universe (the entirety of any system and its surroundings) must increase.

Concept Check 9.2• Explain how the Second Law of Thermodynamics is

followed in the freezing of water at 0 oC, even though the entropy of the water is decreasing.

Concept Check 9.2 SolutionAlthough the entropy of the water is decreases as it freezes, it releases heat in the process. The heat released as ice forms is transferred to the molecules around it, thus increasing the entropy of the universe.

Implication of the Second Law: No spontaneous process can be 100% efficient with respect to energy.

The Second Law of Thermodynamics

Nature’s Heat Tax• Nature takes a heat tax, an unavoidable cut of every energy

transaction; additional energy is lost to the surroundings as well due to inefficiencies.

• No perpetual motion machines

• Lesson: Minimize the number of energy conversions required to achieve a particular goal

You can’t break even.

Units of Energy• Joule (J)

– James Joule demonstrated that energy could be converted form one form to another as long as total energy was conserved.

• Calorie (cal)– The amount of energy required to heat 1 g of water

by 1°C

Concept Check 9.3

• A popsicle contains 85 Cal of nutritional energy. How many joules does it contain?

Concept Check 9.3 Solution

A popsicle contains 85 Cal of nutritional energy. How many joules does it contain?

85 Cal

1000 cal

1 Cal

4.18 J

1 cal = 3.6 105 J

Power

• Power is energy per unit time, the rate of energy input or output.

• Basic unit for the expression of power is the watt (W), equivalent to 1 J/s.

Concept Check 9.4

A refrigerator consumes 815 W. (a) Calculate how many kilowatt-hours (kWh) the refrigerator consumes per month if it operates 4.50 hours per day, collectively. (b) If electricity costs $0.150 per kilowatt-hour, what is the monthly cost of operating the refrigerator?

Concept Check 9.4 Solution

First convert the watts to kilowatts. Then, calculate how many hours the appliance runs in a month. Finally, the kilowatt-hours are calculated by multiplying the watts consumed by how many hours the appliance runs.

815 W 1 kW

1000 W = 0.815 kW

4.50 h

1 day

30 day

1 month = 135 h/month

0.815 kW 135 h = 110. kWh

Concept Check 9.4 Solution

Finally, to calculate the monthly cost of running a this refrigerator, multiply the kilowatt-hours consumed in a month by the price:

110. kWh

$0.150

kWh = $16.50

Temperature• A measure of the kinetic

energy associated with the motion of the material’s composite atoms and molecules.

• Measured on different scales– Fahrenheit– Celsius– Kelvin

Concept Check 9.5

• Convert -10°F to Kelvin.

Concept Check 9.5 Solution

• Converting -10°F to Kelvin, we must first convert °F to °C:

• Next, convert from °C to Kelvin:

o o

o o o

C = 5/9[ F - 32]

C = 5/9[-10 F - 32] = -23 C

o

o

K = C + 273

K = -23 C + 273 = 250. K

Heat Capacity• The quantity of heat energy

required to change the temperature of a given amount of a substance by 1ºC– Substances with higher

heat capacities resist changes in temperature more than other substances.

• Water’s heat capacity is integral to global temperature regulation.

Chemistry and Energy• Exothermic reactions

– Chemical reactions that give off energy to the surroundings

• Endothermic reactions– Chemical reactions that absorb energy from the

surroundings

• Enthalpy of reaction (ΔHrxn)– The amount of heat absorbed or released by a chemical

reaction– Refers to the enthalpy change from the point of view of

the system– By convention, negative enthalpy values describe

exothermic reactions, and positive values describe endothermic ones.

Concept Check 9.6When water freezes at 0ºC, heat is released.

1)Is this water freezing exothermic or endothermic?

2)Is the sign of H associated with water freezing positive or negative?

Concept Check 9.6 SolutionWhen water freezes at 0°C, heat is released.

1)Is this water freezing exothermic or endothermic?• Energy is released; therefore the reaction is

exothermic.

2)Is the sign of H associated with water freezing positive or negative?

• Exothermic reactions have a negative H.

Enthalpy of Combustion• Fuels have significant exothermic enthalpies of combustion

(ΔHcom).

Concept Check 9.7

How much energy in kilocalories is emitted by the complete combustion of 100. g of isooctane (C8H18)? (Note: ΔHrxn = -36 kJ/g C8H18.)

Concept Check 9.7 Solution

• The conversion factor between mass and energy is the heat of combustion.

• The negative sign indicates that the reaction is exothermic (energy is emitted).

100. g isooctane

-36 kcal

1 g isooctane = -3.6 103 kcal

Energy for Our Society• Pre-1970s: Energy taken for granted

• 1970s: North American energy crisis

• 1981: Gas prices at historical high

• 1990s: Benign increases in energy costs

• Post-1990s: Prudence gained from the last energy crisis is largely forgotten.

Energy Consumption

Energy Consumption

Fossil Fuels• Natural gas: Mixture of methane and ethane

• Petroleum: Hydrocarbon range from 5 to about 18 carbons or even more

• Coal: Chains and rings containing upwards of 200 carbons

• The molecules that compose fossil fuels contain a large amount of energy because they were formed by endothermic reactions.

Fuels and Sunlight• Ancient plants used the sun’s energy to synthesize

energetic molecules.

• Photosynthesis yields glucose, which was ultimately converted to fossil fuels by processes taking millions of years.

• Combustion, the opposite of photosynthesis, releases the energy.

Concept Check 9.8

Write the balanced chemical equation for the combustion of heptane (C7H16), a common component in hydrocarbon fuels.

Concept Check 9.8 Solution• The unbalanced equation:

C7H16 + O2 → CO2 + H2O

• First, balance the elements present in only one compound on each side of the equation.

C7H16 + O2 → 7CO2 + H2O 7 C’s on each side

C7H16 + O2 → 7CO2 + 8H2O 16 H’s on each side

• Balance the element present as a free element last.

C7H16 + 11O2 → 7CO2 + 8H2O 10 O’s on each side

• Final balanced equation:

C7H16 + 11O2 → 7CO2 + 8H2O

Note: The absence of coefficient in front of a reactant or product implies a “1”.

Electricity from Fossil Fuels• About 70% of U.S. electricity is generated by burning fossil

fuels.

Smog• Hydrocarbon combustion should only produce carbon

dioxide and water, but impurities in fuels and combustion inefficiencies produce other products.– Carbon monoxide: Binds with hemoglobin in blood,

limiting oxygen transport.

– Nitrogen oxides: NO emitted in exhaust undergoes chemistry to form NO2, the brown gas that gives smog its characteristic color.

– Ozone and PAN: Partially burned hydrocarbons combine with NO2 and sunlight to form ozone (O3) and PAN (CH3CO2NO2), which sting eyes, damage rubber and crops, and make breathing difficult.

Catalytic Converters• Employs catalysts to promote the decomposition of exhaust

into less environmentally harmful substances.

• They address partially burned hydrocarbons, carbon monoxide, and nitrogen monoxide.

Acid Rain

• Sulfur dioxide emission– Fossil fuels contain sulfur

impurities.

– S + O2 → SO2

• Nitrogen monoxide and nitrogen dioxide emission– Air used in fossil fuel

combustion is mostly nitrogen.

– N2 + O2 → 2NO

– 2NO + O2 → 2NO2

These nonmetallic oxides form acids upon combination with rain water that fall as acid rain, damaging lakes and streams, building materials, and forests, and limiting visibility.

Global Warming• The most important greenhouse gas is carbon dioxide.• Water vapor also plays a part.

Global Temperatures and CO2 Levels

Kyoto Protocol

• Despite complexity of climate models, it is generally agreed that global warming has begun.

• National responses vary around the world.

• The 1997 Climate Change Convention formalized a plan to begin the reduction of greenhouse gas emissions.

• The only industrialized nations that did not ratify the resulting treaty were the U.S. and Australia.

• Politics as well as science play a part in the international decision-making process.

Chapter Summary

Molecular Concept

• Energy

• The transfer of energy

• Fossil Fuels and how they are used

• The laws of thermodynamics

Societal Impact

• We use energy in our daily lives.

• Fossil fuels are not ideal because they can create smog and other byproducts that are harmful to our health.