7 Changes in Matter Chapter 21 Introduction to Chapter 21 Types of There are many different types of reactions that occur around you all of the time. For example, when you breathe you take in oxygen, which reacts with the sugar in your cells. This produces carbon dioxide and water. It also releases energy for your Reactions cells to use or store. This type of reaction is called a combustion reaction. In this chapter you will learn how to classify reactions based on the reactants and products, and whether or not they produce or use energy. Investigations for Chapter 21 21.1 Classifying Reactions How can you predict the products in a reaction? A double-displacement reaction is a chemical reaction in which the ions from the two reactants change places. One of the new compounds formed is sometimes insoluble and forms a cloudy precipitate. In this Investigation, you will develop a set of rules for precipitate formation that will allow you to make predictions about the types of products in double-displacement reactions. 21.2 Energy in Reactions How can you classify reactions based on energy? You will measure the energy changes in reactions and classify reactions according to how they use or produce energy. 365 Chapter 21: Types of Reactions Learning Goals In this chapter, you will: D Distinguish between different types of reactions. D Given the reactants, identify the type of reaction that will occur and predict the products that will be produced. D Analyze energy changes that accompany chemical reactions and classify them as exothermic or endothermic. D Observe reactions of household chemicals to develop a set of rules for precipitate formation. Use these rules to make predictions. D Demonstrate safe practices during laboratory investigations. Vocabulary addition reaction dissolution reaction exothermic reaction precipitate combustion reaction double-displacement reaction insoluble single-displacement reaction decomposition reaction endothermic reaction polymerization 366 Chapter 21 21.1 Classifying Reactions Most of the products you use every day are the result of chemical reactions. How do manufacturers know which chemical reactions to use when they make their products? In this section, you will learn how to classify reactions. Being able to recognize the types of reactions will help you predict what types of substances will be produced. Addition reactions Compounds are In an addition reaction, two or more substances combine to form a new made in addition compound. A good example of an addition reaction is the formation of rust: reactions A general equation for addition reactions is: A and B are elements or compounds, and AB is a compound qAcid rain Some fossil fuels, like coal, contain sulfur. When these fuels are burned, the sulfur reacts with oxygen in the air to form the compound symbol meaning sulfur dioxide in the following addition reaction: (s) substance is a solid (l) substance is a liquid (g) substance is a gas In air polluted with sulfur dioxide, acid rain is produced in the reaction substance is below: (aq) dissolved in solution (aqueous) Figure 21.1: What do the symbols The “x” indicates that the subscript for oxygen varies. shown in parenthesis in equations mean? 21.1 Classifying Reactions 367 Chapter 21 Polymerization is In the last chapter, you learned that polymers are large molecules made up of an addition repeating segments. Polymerization, or the formation of polymers, is a series of polymer products reaction addition reactions taking place to produce a very large molecule. Polymers are made by joining smaller molecules called monomers. polystyrene foam containers polyethylene food packaging Decomposition reactions polyester clothing Compounds are A chemical reaction in which a single compound is broken down to produce two polyvinyl plumbing broken down in or more smaller compounds is called a decomposition reaction. The simplest kind chloride (PVC pipes) decomposition of decomposition is the breakdown of a binary compound into its elements, as in reactions the decomposition of water with electricity into hydrogen and oxygen: polyvinyl chewing gum acetate Figure 21.2: Polymers you’ll find Larger compounds can also decompose to produce other compounds, as in the around your house. decomposition of baking soda with heat: The general equation for decomposition is: AB is a compound, and A and B are elements or compounds. 368 Chapter 21 Single-displacement reactions In single In single-displacement reactions, one element replaces a similar element in a displacement compound. For example, if you place an iron nail into a beaker of copper (II) reactions, one chloride, you will begin to see reddish copper forming on the iron nail. In this element replaces reaction, iron replaces copper in the solution and the copper falls out of the another in a solution as a metal: compound Single-displacement reactions can be represented with the general equation: Where AX is a compound, B is an element, BX is a compound, and A is an Figure 21.3: The reaction element. between iron and copper chloride is a single-displacement reaction. Double-displacement reactions In double- In double-displacement reactions, ions from two compounds in solution exchange displacement places to produce two new compounds. One of the compounds formed is usually a reactions, ions precipitate that settles out of the solution, a gas that bubbles out of the solution, or switch places a molecular compound such as water. The other compound formed often remains dissolved in the solution. The general formula for a double-displacement reaction is: Where AB and CD are ionic compounds in a solution, and AD and CB are ionic compounds in a solution as well. What is a The formation of a precipitate occurs when one of the compounds formed in a precipitate? double-displacement reaction is insoluble, or does not dissolve in water. Figure 21.4: The formation of a Precipitates are first recognizable by the cloudy appearance they give to a cloudy precipitate is evidence that a solution. double-displacement reaction has occurred. 21.1 Classifying Reactions 369 Chapter 21 If undisturbed in a beaker, a precipitate will settle to the bottom. Depending on the compound formed, the precipitate can be many different colors from white to fluorescent yellow, as in the reaction between lead (II) nitrate and potassium iodide: lUse your head... Which product is the “precipitate” in the m dried fruit reaction to Consumer chemistry: Preserving dried fruit the left? Have you ever opened up a box of dried fruit such as golden raisins or apricots and smelled a slight “sulfur” odor, like a lit match? The odor is caused by sulfur dioxide, a gas that is used to preserve the color of dried fruits. This gas is produced in a double-displacement reaction between sodium sulfite and hydrochloric acid: The fruit is exposed to the gas, which is absorbed into the skin of the fruit. When you open the box for the first time, some of the gas that has escaped the fruit may not escape your nose! Combustion reactions In a combustion It’s hard to imagine where our society would be without combustion reactions. A reaction, a substance such as wood, natural gas, or propane combines with oxygen, releasing substance a large amount of energy in the form of light and heat, thus completing a combines with combustion reaction. For example, in the combustion of natural gas to heat a oxygen to release house, methane (natural gas) combines with oxygen to produce carbon dioxide energy and water: The general formula for a combustion reaction is: Figure 21.5: The combustion of meathane gas, in oxygen, produces carbon dioxide and water. 370 Chapter 21 qEnergy and the environment: Hydrogen-powered cars Not all combustion reactions produce carbon dioxide. For example, when l hydrogen gas is burned in oxygen, water is the only product: Try this... You get energy from a combustion reaction known as respiration. In this Perhaps in the future, some of our cars will run by this reaction. In fact, reaction, you “burn” glucose automobile manufacturers are developing hydrogen-powered cars right now (C H O ) in oxygen to using a technology called “fuel cells.” 6 12 6 produce carbon dioxide and • How would hydrogen-powered cars impact global warming? water. Can you write the • Do some Internet research to find out about fuel cells and hydrogen- balanced equation for this powered cars. reaction? Summary of the types of reactions m Use your head... Table 21.1: Summary of the types of reactions Have you ever noticed water Type General equation Example dripping from the tailpipe of a car? Could combustion be → addition A + B → AB 2H2 + O2 2H2O responsible for some of that water? → decomposition AB → A + B 2NaHCO3 H2 + 2NaCO3 → single-displacement AX + B → BX + A Fe + CuCl2 FeCl2 + Cu → double-displacement AB + CD → AD + CB Pb(NO3) + 2KI PbI2 + 2KNO3 → → combustion carbon cpd + O2 CO2 + H2OC6H12O6 + 6O2 6CO2 + 6H2O 21.1 Classifying Reactions 371 Chapter 21 Example: predicting the products of a reaction Can you predict Silver becomes tarnished as time passes. Have you ever eaten something with a the products of tarnished fork or spoon? The black tarnish is silver sulfide, the result of a reaction this reaction? between the silver metal and sulfur in foods. Some products claim to be able to remove the tarnish without destroying the silver. One product contains aluminum metal in a solution. Write the complete reaction for removing tarnish with this product. 1. Write the chemical formulas for the reactants Silver sulfide reacts with the aluminum metal in the product. Silver = Ag+ ion; sulfide = S2- ion The chemical formula for silver sulfide is Ag2S Aluminum is an element so its formula is Al Chemical formulas for reactants: Ag2S + Al 2.