11.2. Lesson Types of Chemical Reactions. Overview

Transcription

1 11.2 Lesson Types of Chemical Reactions Objectives Identify the five general types of reactions. Lesson Links Ch. 11 Lab 14: Types of Chemical Reactions Ch. 11 Lab 15: Reactivity of Metals Ch. 11 Small Scale Lab 15: Balancing Chemical Equations Ch. 11 Core TR: Section 2 Review 11.2 Lesson Overview (PowerPoint file) 11.2 Chemistry Tutorial: Writing Equations for Double- Replacement Reactions Chapter 11 Online Student Edition 11.2 Kinetic Art: Types of Reactions Ch. 11 Concepts in Action: A Balanced Equation for Fire Study WB Chapter 11 Lesson 2 Overview/Materials Overview In this lesson you will cover the five general types of chemical reactions: combination reactions, decomposition reactions, single-replacement reactions, double-replacement reactions, and combustion reactions. Students will practice writing equations for each of these reaction types. Classroom Materials Standard 2 Block 1 Single-Replacement Reactions: magnesium ribbon, 1-mL graduated cylinder, 1M HCl, sodium, 2 beakers, tongs, water Combustion Reactions: superfine steel wool, plastic sandwich bag, balance, ring stand, utility clamp, matches Combination Reactions: magnesium ribbon, large crucible, lab burner, cobalt blue glass or exposed photographic film, crucible tongs, matches or igniter page 1 of 9

2 Standard There are no items. Chemistry & You Engage Have students study the photograph and read the Chemistry & You text on p Ask students to predict what happens to the wax as the candle burns. (When the wick is lit, it melts the nearby wax. The wax is then absorbed by the wick and pulled upward. The heat of the flame vaporizes the wax, and the wax vapor becomes the fuel for the combustion reaction.) Chapter 11 Online Student Edition Activate Prior Knowledge Engage Review with students the key steps in writing balanced equations. Have them use a flowchart to describe the best way to subdivide the task. Explain Figure 11.4 Have students study the photographs in Figure Point out that many combination reactions release large amounts of energy. Explain that the reaction of Mg with O2 was once used in flash photography because it releases energy in the form of a bright white light. Then, write a number of equations for combination reactions on the board and have students practice balancing them. Rewrite some of the equations with the product on the reactant side and the reactants on the product side. page 2 of 9

3 Ask Is it possible to reverse the process? ( yes) Ask What is the name of the reverse process? (a decomposition reaction) Ask Why is energy usually needed for a decomposition reaction to occur? ( Decomposition involves the breaking of bonds, which requires energy.) Help students create a list of criteria for identifying composition and decomposition reactions. For example, decomposition reactions are characterized by one molecule on the reactant side and smaller molecules or elements on the product side. Emphasize to students that decomposition reactions consist of one substance forming two or more different substances. Students tend to limit decomposition reactions to the decomposition of a compound into its component elements. Explain that a compound can also break down into an element and a compound or two or more compounds. Provide students with the following chemical equations as examples of decomposition reactions that result in products other than elements: 2H2O2 2H2O + O2 and H2CO3 H2O + CO2. Extend Hydrogen peroxide is capable of gently cleansing wounds, but it also has the power to launch small rockets. The secret to its versatility lies in its concentration. Hydrogen peroxide solutions sold in the first-aid section of grocery stores are very dilute (3% v/v). When a dilute solution comes into contact with the broken cells of a wound, an enzyme called catalase catalyzes the decomposition of the hydrogen peroxide: H2O2(aq) H2(g) + O2(g). The resulting bubbles of hydrogen and oxygen gas help loosen dirt and debris and bring them to the surface of the wound. The decomposition reaction generates more than gas bubbles; it is exothermic as well. While the heat produced by a 3% solution is barely perceptible, the heat generated by highly concentrated solutions can turn water into steam. While this ability to generate heat is useful under certain circumstances, it has sometimes led to disastrous results. Ask students to research one of the following hydrogen peroxide accidents: the 1 page 3 of 9

4 934 rocket explosion in Germany, the 1955 sinking of the HMS Sidon or the 2000 Russian submarine explosion. Explore Use a teacher demonstration to allow students to observe single-replacement reactions. Place a 2-cm piece of magnesium ribbon in 10 ml of 1M HCl. Using tongs, place a small piece of sodium in 250 ml of cold water. In both reactions, the metal replaces hydrogen in a compound. Hydrogen gas is released. The reaction using sodium is much more dramatic and rapid than the reaction involving magnesium. Safety HCl(aq) is corrosive and can cause severe burns. The piece of sodium should be no larger than a match head. Wear plastic gloves and use tongs to avoid contact between sodium and your skin. Materials: magnesium ribbon, 1-mL graduated cylinder, 1M HCl, sodium, 2 beakers, tongs, water Explain Direct students to Table 11.2 and review the definition of a single-replacement reaction. Explain that single-replacement reactions can be compared to cutting in at a dance. A person who is alone approaches a couple and cuts in. The person replaces one partner, who is now left alone. In a chemical reaction, however, only certain substances can replace other substances in a given compound. The activity series of metals shows which metals can replace another metal in a given compound. Review the reactions on page 361. Show students how to use Table 11.2 to predict whether a reaction will occur. Direct students' attention to hydrogen's treatment in the table. Remind students that they Ch. 11 Lab 15: Reactivity of Metals Table 11.2 page 4 of 9

5 learned in Chapter 6 that hydrogen is a nonmetal, even though it is located in period 1A of the periodic table. However, it can replace some metals and be replaced by others, so it is included in the table for reference. For this reason, hydrogen is shown enclosed by parentheses. Have students complete Ch. 11 Lab Manual: Lab 15 Reactivity of Metals. Explain Figure 11.7 Have students examine the reaction in Figure Ask What characteristics of KI and Pb(NO3)2 share? (They are both in aqueous solution, and they are both ionic compounds.) 11.2 Chemistry Tutorial: Writing Equations for Double-Replacement Reactions Explain to students that a doublereplacement reaction always involves two ionic compounds in aqueous solution. In addition, one of the products must be a precipitate, a gas, or a molecular compound. Ask Does one of the products in this reaction meet the second qualification? Why? (Yes; PbI2 is a solid, so it will precipitate from solution when it forms.) Have students complete the 11.2 Chemistry Tutorial: Writing Equations for Double-Replacement Reactions found on PersonChem.com. Explain Ask students to consider the three things necessary for a fire to burn. (fuel, oxygen, and energy to initiate combustion) Emphasize that one of the reactants must be oxygen. Relate this fact to everyday experience by recalling how removing oxygen from a combustion reaction for example, snuffing out a candle causes the reaction to stop. Ask students to infer why it is important that combustion reactions, Ch. 11 Concepts in Action: A Balanced Equation for Fire page 5 of 9

6 such as those used to heat a home or run an automobile, take place in properly ventilated areas. (Without proper ventilation and enough available oxygen, the combustion may be incomplete, and poisonous carbon monoxide may be produced.) Ask Why is it not safe to use a single kind of fire extinguisher on all fires? (A fire extinguisher that controls one type of fire may actually enhance other types of combustion reactions. For example, water is not sprayed on burning magnesium because the intense heat can decompose water producing flammable hydrogen and oxygen gas.) Ask Is a piece of paper combustible? Is it flammable? (Paper is combustible but not flammable) Explain that these two terms are often used synonymously, but combustible means that the material will burn, and flammable means a material may easily burst into flame. Point out to students that an important product of most combustion reactions is energy, which is usually in the form of heat or light. Explain that some combustion reactions are also combination reactions in which an element or compound combines with oxygen to form a single product plus energy. For example: 2Mg(s) + O2(g) 2MgO(s), and 4Fe(s) + 3O2(g) 2Fe2O3(s). Have students complete the Ch. 11 Concepts in Action: A Balanced Equation for Fire on PearsonChem.com. Explore Allow students to observe the combustion of iron with a teacher demonstration. Place a superfine steel wool pad in a plastic sandwich bag, measure the mass, and write the value on the board. Take the steel wool out of the bag and unfold it to full length. Clamp it to a ring stand with a utility clamp. Set the steel wool on fire. Ask students to predict what effect the burning will have on the mass of the sample. As soon as the steel wool stops burning and is cool, place the remains in the same plastic sandwich bag and measure the mass again. Write the new value on the board, next to the original mass. Discuss why the sample page 6 of 9

7 gained mass. Two elements, Fe(s) and O2(g), will combine in a combustion reaction to form a binary compound, Fe2O3. The sample will gain mass because initially only the mass of the iron is determined. During the reaction, oxygen from the air will combine with the iron. Materials: superfine steel wool, plastic sandwich bag, balance, ring stand, utility clamp, matches Explore Allow students to observe a combination reaction with a teacher demonstration. Explain to students that in a combination reaction, the reactants combine to make one new product. Measure a 5- to 7-cm strip of magnesium ribbon. Light the burner. Hold one end of the magnesium ribbon with a pair of crucible tongs. Ignite the magnesium and hold it above the crucible. Ask students to note any evidence of chemical change. Have students note the condition of the residue compared to the original magnesium. Metallic magnesium and oxygen gas from the air form a white powder, magnesium oxide. Other evidence of chemical change includes release of energy as heat and light. Tell students that small amounts of magnesium nitride also form. Ask What are the reactants in this combination reaction? (Mg and O2) What is the product of this reaction? (MgO) Does this reaction obey the law of conservation of mass? Explain. (Yes. The sum of the masses of magnesium and oxygen is equal to the mass of the magnesium oxide formed.) Have students write the balanced equation for the reaction. (2Mg(s) + O2(g) 2MgO(s)) Challenge students to write the balanced equation for the formation of the small amount of magnesium nitride that forms during this reaction. (3Mg(s) + N2(g) Mg3N2(s)) Safety and Disposal Students should not look directly at burning magnesium. Have them observe through page 7 of 9

8 pieces of cobalt blue glass or exposed photographic film. Another option is to conduct the reaction inside a large, metal can. The product mat be disposed of in the trash can. Materials: magnesium ribbon, large crucible, lab burner, cobalt blue glass or exposed photographic film, crucible tongs, matches or igniter Assess and Remediate Evaluate Create a matching assessment involving types of reactions in one column and balanced chemical equations in the other column. As a challenge, write on the board only the products of combination, decomposition, and replacement reactions, and challenge students to fill in the reactants. Chapter 11 Online Student Edition Then have students complete the 11.2 Lesson Check. Remediate Help student develop a branched flowchart of chemical reactions that is similar to flowcharts used in qualitative analysis. Start by asking if there is a single reactant. If so, the reaction is a decomposition. If not, proceed to the next step. Ask if oxygen is one of the reactants. If so, the reaction is either combination or combustion. If not, proceed to the next step. Continue through the five general types of reactions. Differentiated Instruction Less Proficient Readers Make sure that students understand clearly the five terms that describe general types of chemical reactions: combination, decomposition, single-replacement, doublereplacement, and combustion. Help them to spell and pronounce each term as well as to define it. Encourage them to ask questions about any term they do not fully understand. Study WB Chapter 11 Lesson 2 Ch. 11 Core TR: Section 2 Review Struggling Students page 8 of 9

9 Provide students with general statements that represent each of the first four types of chemical reactions discussed in this lesson, such as X + Y XY for a combination reaction, XY X + Y for a decomposition reaction, X + CD XD + C for a singlereplacement reaction, and AB + CD AD + CB for a double-replacement reaction. Tell students that they can compare actual reactions to these general statements to help them identify the reaction type. Advanced Students When hydrochloric acid (HCl) is dropped on calcium carbonate (CaCO3), a doublereplacement reaction occurs. Ask What are the products of this reaction? (CaCl2 and H2CO3) Neither of these products is a gas, but bubbles of gas are released during this reaction. Ask students to infer the identity of the gas. (CO2) Ask What type of reaction could produce this gas? (the decomposition reaction: H2CO3 H2O + CO2) Explain that geologists often make use of this pair of reactions to test rock samples for the presence of limestone, which is formed from the calcium carbonate skeletons of prehistoric corals. Focus on ELL Comprehensible Input Assign students to five groups. Assign each group a different type of reaction and have the students create a poster describing their reaction type. Be sure they include pictures, important facts, and some examples. Display the posters around the room. Ch. 11 Core TR: Section 2 Review Study WB Chapter 11 Lesson 2 My Notes Homework page 9 of 9