570 Lesson 4

• Describe how hydrogen fuel can be produced.• Explain the way fuel cells work and how they are

used.

Reading Strategy Construct a Venn diagram comparing electrolysis and the breakdown of methane as ways to obtain hydrogen.

Vocabulary electrolysis, fuel cell

Guiding Question: How can we use hydrogen as a source of energy?

Energy From HydrogenLE

SSO

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Hydrogen is the simplest and the most abundant element in the universe. In its elemental form, hydrogen is a gas with the chemical formula H2. Hydrogen gas can be burned to release energy that, in turn, can gener-ate electricity. When it is burned to produce energy that people can use, hydrogen gas is called hydrogen fuel. Its combustion is clean compared to fossil fuels. The U.S. space flight programs have used hydrogen fuel since the 1960s.

Hydrogen gas is rare on Earth. Hydrogen atoms bind to the atoms of other elements, forming compounds such as water (H2O) and methane (CH4). To release hydrogen, the compounds that contain it must be bro-ken down chemically.

Why are environmental scientists interested in hydrogen as a source of energy? One reason is that hydrogen fuel can be stored and transported from one place to another, unlike most other sources of renewable energy. We can’t transport the wind from North Dakota to North Carolina, and we can’t move the sun from Arizona to the state of Washington. Some day, however, energy from the wind in North Dakota may be used to create hydrogen fuel. The hydrogen fuel can then be transported to places like North Carolina. The portability of hydrogen fuel is the key to its potential usefulness.

FIGURE 21 Power From Hydrogen Hydrogen fuel powered the rockets that blasted vehicles such as the space shuttle into space.

18.4 LESSON PLAN PREVIEWInquiry Students build ball-and-stick models to help understand the reactions that produce hydrogen fuel.Differentiated Instruction Less proficient readers use a diagram to help them under-stand fuel cells.

18.4 RESOURCESLesson 18.4 Worksheets • Lesson 18.4 Assessment • Chapter 18 Overview Presentation

FOCUS Show students several pho-tos of vehicles that run on hydrogen fuel cells. Explain that these vehicles are uncommon and ask students why they think this might be the case. Tell them that, in this lesson, they will learn the costs and benefits of using hydrogen as an energy source.

GUIDING QUESTION

Renewable Energy Alternatives 571

Producing Hydrogen Fuel Hydrogen fuel can be produced from the breakdown of water

or other hydrogen-containing compounds.

To produce hydrogen fuel, hydrogen-containing compounds must be forced to release their hydrogen atoms. The chemical breakdown of hydrogen-containing compounds requires an input of energy.

Electrolysis One process that releases hydrogen is electrolysis. In electrolysis, water molecules are broken down into oxygen gas (O2) and hydrogen gas (H2) by an electric current that runs through the water. This reaction can be expressed by the following chemical equation:

2 H2O ➝ 2 H2 + O2

Electrolysis produces pure hydrogen gas. The process does not emit greenhouse gases or pollutants. However, the electrolysis of water is a costly way to obtain hydrogen.

ReadingCheckpoint

What is electrolysis?

Breakdown of Methane Hydrogen can also be extracted from the methane in natural gas. This process is less expensive than electrolysis and is now the most common way of obtaining hydrogen. The following equation summarizes the reaction:

CH4 + 2 H2O ➝ 4 H2 + CO2

This reaction produces carbon dioxide, a greenhouse gas. Therefore, energy experts do not see methane as a desirable long-term source of hydrogen.

Other Methods There are disadvantages to both methods described above. Therefore, scientists are investigating other ways to obtain hydro-gen. For example, the heat given off by a nuclear reactor may one day be used to split water into hydrogen and oxygen. Researchers are also look-ing into using algae to produce hydrogen (Figure 22).

FIGURE 22 Hydrogen From Algae Microorganisms called algae give the solutions in the bottles their green color. The algae in the solutions are producing hydrogen from water.

ANSWERS

Reading Checkpoint A process in which water molecules are broken down by an electric current

Hydrogen fuel, H2

The reactions produce electricity.

Solution

Positiveelectrode

Oxygen, O2

Water, H

2O

Negativeelectrode

H2

H

H

572 Lesson 4

Benefits and Costs of Energy From Hydrogen As a fuel, hydrogen has a number of benefits. Because it is the most abundant el ement in the universe, it will never run out. Depending on how it is made, it produces few greenhouse gases or pollutants. Water and heat may be the only waste products generated by using hydrogen as fuel. Hydrogen can be stored and transported from one place to another. Hydrogen gas can catch fire, but if it is stored properly, it is probably no more dangerous than gasoline.

However, processes that break compounds down to release hydro-gen, such as electrolysis, require energy inputs. With the technology now available, hydrogen fuel is expensive to produce. In addition, to be useful in a motor vehicle, hydrogen needs to be compressed, or squeezed into a smaller volume. Scientists are working on ways to do this safely.

ReadingCheckpoint

Why are hydrogen-powered motor vehicles uncommon?

Fuel Cells Fuel cells are used to generate electricity.

Hydrogen gas can be used to produce electricity within fuel cells. The chemical reaction that takes place in a fuel cell is the reverse of the reaction for electrolysis. Two hydrogen molecules and an oxygen mol-ecule combine to form two water molecules:

2 H2 + O2 ➝ 2 H2O

Inside a Fuel Cell A fuel cell has a positive (+) electrode and a negative (–) electrode, as you can see in Figure 24. Reactions in a fuel cell result in an electric current flowing through a wire from one termi-nal to the other.

FIGURE 24 A Fuel Cell A fuel cell uses hydrogen and oxygen to produce electricity. Inside the fuel cell, hydrogen molecules are split into ions, and an electric current flows from one terminal of the cell to the other.

FIGURE 23 Hydrogen-Powered Scooter This electric scooter runs on a fuel-cell battery.

ANSWERS

Reading Checkpoint The processes that generate electricity require ener-gy; the fuel is expensive to produce; it needs to be compressed to be used in a motor vehicle.

Renewable Energy Alternatives 573

1. Apply Concepts Write the equation for the reac-tion in which hydrogen is obtained from methane. Use this equation to explain why this method of obtaining oxygen could contribute to global climate change. (Hint: Look at the products of the reaction.)

2. Explain Explain how electricity is produced inside a fuel cell.

3. When hydrogen fuel is ignited, the result can be a huge explosion that causes enormous damage. Suppose you are on a commission evaluating the safety of hydrogen fuel. Your specific focus is transportation of hydrogen. What safety measures would you recommend? Think about issues such as the design of transpor-tation vehicles, the routes they might take, and the time of day during which the transportation would take place.

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Hydrogen gas enters the side of the cell with the negative electrode. Each hydrogen molecule then splits into two positively charged hydro-gen ions (H+). Meanwhile, oxygen gas enters the cell on the side with the positive electrode. A series of reactions occurs that makes two things happen. The reactions cause an electric current to flow from the negative terminal to the positive terminal. As this is taking place, the oxygen and hydrogen ions combine to form molecules of water.

How Fuel Cells Are Used Fuel cells can be used in many ways. They provide power for moving vehicles. The city of Hamburg in Germany has buses powered by fuel cells as shown in Figure 25. In fact, vehicles ranging from cars to space vehicles use hydrogen fuel cells as an energy source. However, such vehicles are uncommon.

Fuel cells can supply power in places that are far from conventional utility companies that produce electricity. Other technologies can be combined with fuel cells to accomplish this. One example is the system built on Stuart Island in the state of Washington. There, solar power pro-duces hydrogen, which is then used in fuel cells to produce electricity.

FIGURE 25 Fuel Cell Bus The city of Hamburg in Germany has buses that run on energy obtained from hydrogen.

ANSWERS

Lesson 4 Assessment1. CH4 + 2 H2O → 4 H2 + CO2; the

breakdown of methane produces carbon dioxide, which is a green-house gas.

2. Hydrogen gas is ionized; the hydrogen ions combine with the oxygen to produce water, and an electric current is produced.

3. Answers will vary, but should reflect knowledge of safety issues of using hydrogen as a fuel.