5.2 Explore How Can Chemical Reactions Be Used to Improve Air Quality? One way to reduce the pollutants in the air is by keeping them from getting into the air in the first place. Some pollutants are solid particles, and these can be removed with filters. But other pollutants are gases. You cannot catch a gas in a filter. However, you know that the pollutants that are emitted by cars and smokestacks are unstable molecules created as the products of chemical reactions. Engineers and scientists have found ways to set up chemical reactions that transform some pollutants into other substances before they reach the environment. This is how some pollutants are removed from smokestacks of power plants and factories. And this is how pollutants are removed from internal combustion engines in cars. Using Chemicals to Clean Pollutants from Smokestacks of Power Plants and Factories Many pollutants are removed from the smokestacks of power plants and factories by using chemical processes. Typically, an assembly called a scrubber is used to transform the gases coming out of a plant into other substances. A scrubber sprays water or some other liquid at the combustion products produced by a power plant or factory. Sometimes a base, usually lime, is added to the water. Some of the gases in the polluted air, such as the sulfur dioxide and sulfur trioxide, dissolve in the spray. This makes the spray more acidic. However, the lime in the spray is a base. The dissolved acid and the lime in the water neutralize each other and produce a solid precipitate. The products of this reaction are then collected and disposed of or used for manufacturing. For example, one product, calcium sulfate dihydrate, is collected and used to make wallboard. Other scrubbers work a little differently. Water is sprayed at the combustion products from the power plant or factory, and the water becomes acidic and falls to the floor of the smokestack. The acidic water is drained out and treated with a base, often lime. The products of this process are exactly the same as the products of the first process. In both processes, many of the gaseous pollutants produced by the power plant or factory are transformed into non-polluting substances. AQ 259 AIR QUALITY
Learning Set 5 How Can Air Quality Be Improved? Stop and Think 1. Sketch what happens in a smokestack when a lime-water spray is used. Show the inputs to the smokestack, the chemical reaction and where it happens, and what happens to the products of the chemical reaction. 2. Sketch what happens in a smokestack when a plain-water spray is used. Show the inputs to the smokestack, the chemical reactions, their products, where the chemical reactions happen, and what happens to the products of the chemical reactions. Cleaning Up the Products of Internal- Combustion Engines Removing pollutants from polluted air before they get into the air you breathe is not easy. Many chemical reactions are involved in the formation of pollutants. Reversing them requires many more chemical reactions. The chemical reactions that remove acid-forming pollutants from smokestacks of power plants and factories happen relatively quickly. A factory or power plant can be built to contain these pollutants until they are transformed into non-polluting substances. Removing pollutants from the products of a car or truck s internal-combustion engine, however, is a lot more difficult. Think about a car s exhaust system. Pollutants can be caught before they enter the air only if the chemical reaction that transforms them into another substance happens before they have time to escape from a car s exhaust pipe. (Exhaust is the name for the gases produced through internal combustion.) But a car s exhaust pipe is very short, a car produces a lot of exhaust, and there is little space under a car to contain the pollutants until they are transformed. Only chemical reactions that occur very quickly will remove pollutants from a car or truck s exhaust before they enter the air you breathe. Unfortunately, the reactions that turn the pollutants from an internalcombustion engine into harmless substances are very slow. It would take days, if not longer, for those chemical reactions to complete. A way to speed up those reactions is needed. How Can Chemical Reactions Be Sped Up? To investigate how the speed of a reaction can be changed, you will consider the reaction in which hydrogen peroxide (H 2 O 2 ) breaks apart into water and oxygen. As the reaction takes place, oxygen bubbles are released and rise to the top of the solution. Project-Based Inquiry Science AQ 260
5.2 Explore Hydrogen peroxide yields water plus oxygen. slow 2HO 2HO+ O 2 2 2 2 This reaction usually happens very slowly because a lot of energy is required to break hydrogen peroxide into its parts. Procedure As you perform the investigation, watch for the bubbles of oxygen gas that show a reaction is occurring. 1. Use the wax pencil to label the test tubes A and B. 2. Use the graduated cylinder to measure 5 ml of hydrogen peroxide, and pour it into test tube A. Repeat for test tube B. 3. Observe the test tubes for 1 min. Watch for bubbles of oxygen gas. Bubbles are your evidence that a chemical reaction is happening. Record your observations on your Speeding up Chemical Reactions page. 4. Using the scoop, add a small amount of the manganese dioxide to test tube B. Use only enough of the manganese dioxide to cover the tip of the scoop. 5. Observe the test tubes again for 1 min. Look for evidence that a chemical reaction is occurring. Record your observations on your Speeding up Chemical Reactions page. Analyze Your Data 1. What chemical reaction was occurring when you first added the hydrogen peroxide to the test tubes? 2. What evidence do you have that the manganese dioxide increased the speed of the reaction? Materials 2 test tubes 10 ml of 3% hydrogen peroxide (H 2 O 2 ) small metal scoop 0.01 g of manganese dioxide (MnO 2 ) test tube rack 10-mL graduated cylinder wax pencil safety glasses stopwatch Speeding Up Chemical Reactions page 3. Manganese dioxide is an example of a catalyst. Describe what you think a catalyst does. AQ 261 AIR QUALITY
Learning Set 5 How Can Air Quality Be Improved? catalyst: a substance that increases the rate of a chemical reaction by lowering the amount of energy needed for the reaction. The amount of the catalyst is the same before and after the reaction. How Do Catalysts Work? In your investigation, you saw that the manganese dioxide sped up the breakdown of hydrogen dioxide into water and oxygen. But how? The answer has to do with energy. Energy is the ability to do work or cause change. A certain amount of energy is required for any chemical reaction to occur. The amount of energy is known as a reaction s activation energy. It is shown by the peaks in the graph below. Certain substances, known as catalysts, can lower the activation energy needed for a reaction. This means that more particles of the reactants will have enough energy to take part in the chemical reaction. Energy Reactants Activation energy (no catalyst) Activation energy (catalyst) Time Products Catalysts lower the activation energy needed for a reaction. A catalyst lowers the activation energy needed for a reaction, but is not used up during the chemical reaction. The amount of catalyst is the same at the end of a reaction as it was at the beginning. slow Test tube A: 2HO 2HO+ O 2 2 2 2 fast Test tube B: 2HO + MnO 2H O+ O + MnO ( unchanged) 2 2 2 2 2 2 How Are Catalysts Used to Remove Pollutant Gases From Air? Catalysts are used in many laboratory and industrial reactions to make reactions proceed faster. As in the example you explored, a catalyst is usually a metal. Scientists have found that the metals platinum, palladium, and rhodium can speed up the reactions that transform nitrogen oxides into Project-Based Inquiry Science AQ 262
5.2 Explore nitrogen gas. They have found that platinum and palladium can also be used to transform carbon monoxide and VOC s into carbon dioxide and water. Some of the chemical chemical equations involving platinum (Pt) are shown below. Nitrogen monoxide plus platinum yields nitrogen plus oxygen plus platinum. 2NO + Pt N2 + O2 + Pt Carbon monoxide plus oxygen plus platinum yields carbon dioxide plus platinum. 2CO + O + Pt 2CO + Pt 2 2 VOC s plus oxygen plus platinum yield carbon dioxide plus water plus platinum. VOC s+ O2 + Pt CO2 + H2O+ Pt To reduce emissions from cars and trucks, these catalysts do their work in catalytic converters. Catalytic converters remove carbon monoxide, nitrogen oxides, and VOC s produced by an internal-combustion engine. By law, every car now must have a catalytic converter to reduce pollutants released into the air. In a car, the exhaust from the internal-combustion engine is piped into the catalytic converter, which transforms the pollutants into less harmful substances before the exhaust is released from the car s exhaust pipe. catalytic converter: a piece of equipment in the exhaust line of a car that reduces the amount of pollutants released into the air. muffler catalytic converter engine As gases from combustion go from the car engine to the exhaust, they pass through a catalytic converter. tailpipe exhaust pipe AQ 263 AIR QUALITY
Learning Set 5 How Can Air Quality Be Improved? ceramic: a glasslike material that will not shatter or melt at high temperatures. A modern catalytic converter uses two separate steps to reduce pollution. The first step removes nitrogen oxides by changing them back into nitrogen gas. To make this reaction proceed, ceramic pellets are coated with small amounts of platinum, palladium, and rhodium. When the exhaust from the internal combustion engine enters the catalytic converter, the catalysts cause the reactions that transform nitrogen oxides into nitrogen and oxygen to occur. catalyst catalyst 2NO N2+ O2 and 2NO2 N 2 + 2O 2 The chemical reactions above are written using a kind of shorthand. Instead of showing the catalyst on both sides of a chemical equation, scientists use the word catalyst or list the catalyst used over the chemical reaction arrow. This notation specifies that a catalyst is used to speed up the reaction. A catalytic converter contains ceramic pellets coated with small amounts of metals. These metals act as a catalysts and cause reactions that convert nitrogen oxides to nitrogen and oxygen. In the second step, carbon monoxide and VOC s are removed from the exhaust by transforming them into CO 2 and H 2 O. A platinum or palladium catalyst is used. As the exhaust passes over the catalysts, carbon monoxide and VOC s mix with oxygen in the exhaust to form carbon dioxide and water. 2CO O catalyst catalyst + 2CO and VOC s + O CO + HO 2 2 2 2 2 In order for this second step to work, there must be oxygen left in the exhaust after the first set of reactions. To make sure a vehicle s catalytic converter can do its job completely, cars and trucks include an oxygen sensor. This sensor tells the car s computer if there is enough oxygen in the exhaust to transform all of the CO and VOC s into CO 2 and H 2 O. If there is not enough oxygen, the computer lets more air into the engine. Project-Based Inquiry Science AQ 264
5.2 Explore The sensor also makes sure that there is not too much oxygen, because this would lead to the formation of more nitrogen oxides. If there is too much oxygen, the computer reduces the amount of air going into the engine. Reflect 1. Check the equations of each reaction in a catalytic converter to see if they are balanced. Count each atom on the reactant side of the equation and match that number to the atoms for the same element on the product side of the equation. What did you find? 2. Although the technology for catalytic converters was available long ago, converters began to be added to cars only in 1975. Why do you think automobile manufacturers might not have wanted to put catalytic converters in cars? Rhodium, palladium, platinum, and gold are some of the metals on the periodic table that can be used as catalysts. Other Ways Catalysts Are Used to Reduce Pollution In addition to the catalytic converters in cars, catalysts are also used to clean up the air from industrial pollution. For example, a bakery was found to be emitting too high a concentration of VOC s in the form of ethanol. The ethanol was formed during the fermentation of sugar by yeast. To remove the ethanol, a scrubber with a catalyst was used to change the ethanol to carbon dioxide and water. Ethanol plus oxygen plus catalyst yields carbon dioxide plus water plus catalyst. CH CH OH + 3O 2CO + 3HO 3 2 2 catalyst 2 2 Commercial bakeries can produce high levels of ethanol, an air pollutant. Scrubbers with a catalyst can change the ethanol to carbon dioxide and water. AQ 265 AIR QUALITY
Learning Set 5 How Can Air Quality Be Improved? What s the Point? Increasing the rate of chemical reactions is one way of treating pollutants before they get into the air you breathe. Smokestacks of some power plants and factories have scrubbers that use water and a basic compound to trap some polluting gases as solutions or solids. Sometimes the solids that form are even usable. However, chemically removing the pollutants from car or truck exhaust is more difficult because the necessary chemical reactions could take days to complete. To speed up the chemical reactions that transform exhaust gases into non-polluting substances, cars and trucks use catalytic converters. Catalytic converters in vehicles can reduce nitrogen oxides, VOC s, and carbon monoxide emissions. Catalytic converters use a solid chemical to speed up chemical reactions that transform pollutants into gases that are not harmful. A catalyst can be used over and over again because it is not used up in a reaction. Before 1975, cars did not have catalytic converters. These cars released more pollutants into the air. Project-Based Inquiry Science AQ 266