Chemical Reactions and Equations 10

Transcription

1 Chemical Reactions and Equations 10 LABORATORY GOALS Observe physical and chemical properties associated with chemical changes. Give evidence for the occurrence of a chemical reaction. Write a balanced equation for a chemical reaction. Identify a reaction as a combination, decomposition, replacement, or combustion reaction. LAB INFORMATION Time: 2-2% h Comments: Read all the directions and safety instructions carefully. Match the labels on bottles and containers with the names of the substances. Label your containers with the formulas of the chemicals you place in them. A Bunsen burner is a potential hazard. Keep your work area clear of books, papers, backpacks, and other flammable items. Be sure that long hair is tied back. Tear out the report sheets and place them beside the matching procedures. Related Topics: Chemical change, chemical equation, balancing chemical equations CHEMICAL CONCEPTS When a substance undergoes a physical change, it changes its appearance but not its composition. For example, when silver (Ag) melts and forms liquid silver (Ag), it undergoes a physical change from solid to liquid. In a chemical change, a substance is converted to one or more new substances with different properties. For example, when silver (Ag), which is a shiny substance, becomes tarnished it forms a dull-gray silver sulfide (Ag 2S), a new substance with different properties. Evidence of this and other chemical reactions is observed by the formation of bubbles, a solid, a change in color or a change in heat energy. Silver experienced the formation of a solid, the silver sulfide (Ag 2S). (see Table 10.1). Ag A chemical change produces new substances. Ag2S Copyright 2014 Pearson Education, Inc. 109

2 110 Laboratory Manual for General, Organic, and Biological Chemistry TABLE 10.1 Evidence of Chemical Change Formation of a gas (bubbles) Formation of a solid (precipitates) Change in color Heat produced or absorbed Chemical Equations In a chemical reaction, atoms in the reactants are rearranged to produce new combinations of atoms in the products. In an equation for a chemical reaction, the reactants are written on the left and the products on the right. An arrow between them indicates that a chemical reaction takes place. Reactants > Products For example, we write the equation for the reaction of carbon and oxygen, which forms carbon dioxide, as C(s)+0 2 (g) --- CO 2 (g) The formula of each substance is followed by its physical state (s, 1, g) in parentheses. If heat is required for the reaction, a triangle, which is a symbol for heat, is written over the arrow (see Table 10.2). TABLE 10.2 Some Symbols Used in Writing Equations Symbol Meaning (s) Solid (1) Liquid (g) Gas or vapor (aq) Aqueous Separates two or more formulas Reacts to form products Reactants are heated Balancing Chemical Equations In a balanced equation, the total number of atoms of each element in the reactants is equal to the total number of atoms in the products. This balance is achieved by writing a coefficient in front of a formula containing that particular element. For example, we balance the equation for the reaction of hydrogen and oxygen as follows: H2 (g)+02 (g) > H 20(g) Unbalanced equation H2 (g)+02(g) ---> 2H 20(g) A coefficient of 2 in front of H 2 O balances the 0 atoms. 2H 2 (g)+02 (g) ---> 2H20(g) A coefficient of 2 in front of H2 balances the H atoms. Copyright 2014 Pearson Education, Inc.

3 Chemical Reactions and Equations 1 l 2H2(g) 02 (g) 2H20(g) H 0 va. kid *20 Reactant atoms = Product atoms If we count the number of atoms of H and 0 in the reactants, we find they are equal to the number of H and 0 atoms in the products. We say the equation is balanced. Types of Reactions There are many different chemical reactions, but most can be classified into the types of reactions shown in Table TABLE 10.3 Common Types of Chemical Reactions Type of Reaction Description Example Equation Combination Elements or simple compounds form Cu(s) + S(s) > CuS(s) a more complex product. Decomposition A reacting substance is split into simpler substances. CaCO 3 (s) > Ca0(s)+ CO 2 (g) Single replacement One element takes the place of another element in a compound. Mg(s) + 2HC1(aq) > MgC1 2 (aq) +H2 (g) Double replacement Elements in two compounds switch AgNO 3 (aq) + NaCl(aq) places. AgCl(s)+ NaNO3 (aq) Combustion Hydrocarbon fuel and oxygen form CH 4 (g)+ 202 (g) CO2(g)+2H20(g) carbon dioxide and water. Copyright 2014 Pearson Education, Inc.

4 112 Laboratory Manual for General, Organic, and Biological Chemistry EXPERIMENTAL PROCEDURES GOGGLES REQUIRED! A. Magnesium and Oxygen Materials: Magnesium ribbon (2-3 cm long), tongs, Bunsen burner 1. Obtain a small strip (2-3 cm) of magnesium ribbon. Record its initial appearance. 2. Using a pair of tongs to hold the end of the magnesium ribbon, ignite it by carefully placing it into the tip of the inner blue cone of the flame from the Bunsen burner. As soon as the magnesium ribbon ignites, remove it from the flame. Shield your eyes as the ribbon burns and do not look directly at the flame. Record your observations of chemical changes for the reaction. 3. Balance the equation given for the reaction. Use 1 as a coefficient when one unit of that substance is required. 4. Identify the type of reaction. For experiments B F, use small quantities. For solids, use the amount of compound that fits on the tip of a spatula or a small scoop. Place 3 ml of water in a test tube. Use this same volume as a reference volume when you need 3 ml of a solution. Do not dip droppers or stirring rods into reagent bottles. They may contaminate a reagent for the entire class. Discard unused chemicals as indicated by your instructor. B. Zinc and Copper(II) Sulfate Materials: Two test tubes, test tube rack, 1 M CuSO 4 (aq), Zn(s), small graduated cylinder 1. Pour 3 ml of 1 M CuSO 4 (aq) into a test tube by matching the reference volume. Obtain a small piece of zinc(s) metal. Describe the initial appearance of the CuSO 4 (aq) solution and the zinc(s) metal. 2. Add the Zn metal to the test tube containing the CuSO 4 (aq) solution. Place the test tube in your test tube rack. After 30 min has elapsed, observe any color change of the CuSO 4 solution and the piece of Zn metal. 3. Balance the equation given for the reaction. 4. Identify the type of chemical reaction that has occurred. Pour the CuSO4 (aq) solutions into a waste container or dispose of as directed by your instructor. Rinse the piece of zinc with water and place it in a recycling container or as directed by your instructor. C. Reactions of Metals and HC1 Materials: Three test tubes, test tube rack, small pieces of Zn(s), Cu(s), and Mg(s) metal, 1 M HC1 Caution: HCl is a corrosive acid. Handle carefully! 1. Obtain a small piece each of zinc, copper, and magnesium. Describe the appearance of each. 2. Place 3 ml of 1 M HC1 (match your reference volume) in each of three test tubes. Carefully add a metal piece to the HC1 solution in each of the three test tubes. Observe the reaction and record any evidence of chemical change. Carefully feel the test tubes to detect any change in the heat energy. 3. Balance the equation for each metal that gave a chemical reaction. If there was no reaction, cross out the products and write NR. 4. Identify the type of reaction for each chemical reaction that occurred. Carefully pour off the acid and follow with large quantities of water to dilute. Dispose of as directed by your instructor. Rinse the metal pieces with water and place them in a recycling container or as directed by your instructor. Copyright 2014 Pearson Education, Inc.

5 Chemical Reactions and Equations 113 D. Reactions of Ionic Compounds Materials: Four test tubes, test tube rack, 0.1 M solutions: CaC1 2 (aq), Na 3PO4 (aq), FeC13 (aq), KSCN(aq) Dl Reaction of CaC1 2 and Na3PO4 1. Place 3 ml each of 0.1 M CaC1 2 (aq) and 0.1 M Na 3PO4(aq) into separate test tubes. Record the appearance of each solution. 2. Pour the contents of one test tube into the other and describe any evidence of a chemical reaction. 3. Balance the equation. 4. Identify the type of chemical reaction. Dispose of the solutions and solids as directed by your instructor. D2 Reaction of FeC1 3 and KSCN 1. Place 3 ml each of 0.1 M FeC1 3 (aq) and 0.1 M KSCN(aq) into separate test tubes. Record the appearance of each solution. 2. Pour the contents of one test tube into the other and describe any evidence of a chemical reaction. 3. Balance the chemical equation. 4. Identify the type of chemical reaction. Dispose of the solution as directed by your instructor. E. Sodium Carbonate and HC1 Materials: Test tubes, test tube rack, 1 M HC1(aq), Na 2CO3 (s), matches or wood splints 1. Place 3 ml of 1 M HC1(aq) in a test tube. Record its appearance. Obtain a small amount of Na 2CO3 (s) (about the size of a pea). Record its appearance. 2. Carefully add the Na 2CO3 (s) to the test tube containing 1 M HC1(aq). Record any evidence of a chemical reaction. 3. Light a match or wood splint and insert it inside the neck of the test tube. Record whether the flame goes out or stays lighted. 4. Balance the chemical equation. 5. Identify the type of chemical reaction. Dispose of the solutions and solids as directed by your instructor. F. Hydrogen Peroxide Materials: Test tubes, test tube rack, 3% H 202, 0.1 M KI(aq) 1. Place 3 ml of 3% H 202 (aq) in a test tube. Record its appearance. 2. Place 3 ml of 0.1 M KI in a second test tube. Pour the KI solution (catalyst) into the first test tube. Record any evidence of a chemical reaction. 3. Balance the chemical equation. Note that KI is included in the equation above the reaction arrow because a catalyst only speeds up a reaction, it does not change the quantities of reactants. 4. Identify the type of chemical reaction. Dispose of the solutions as directed by your instructor. Copyright 2014 Pearson Education, Inc.

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7 Date Section Instructor Name Team Pre-Lab Study Questions Why are burning candles and rusting nails examples of chemical change? 2. What is included in a chemical equation? 3. How does a combination reaction differ from a decomposition reaction? 4. Balance each of the following reactions, and identify the type of reaction: Unbalanced Equation Type of Reaction a. Al(s) + Fe 2O3 (s) --L--> A Fe(/) b. KCIO3 (s) ) KC1(s) (g) c. Li(s) + C12 (g) '' - ' LiC1(s) d. C2H4(g)+ 02 (g) ---`2'.--> co2 (g) +1120(g) e. CrC13 (aq) + H2S(g) --> Cr2S3 (s) + HC1(aq) 5. Complete and balance each of the following reactions: Reactants Type of Reaction a. K(s) + N2 (g) --> Combination b. C51112 (g) + 02 (g) '1> Combustion c. Al(s) + CuSO4(aq)--> Single Replacement d. CoC13 (aq) + AgNO3 (aq) > Double Replacement e. MgCO3 (s) Decomposition Copyright 2014 Pearson Education, Inc. 115

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9 Date Section Instructor Name Team REPORT SHEET LAB Chemical Reactions and Equations 10 A. Magnesium and Oxygen 1. Initial appearance of Mg 2. Evidence of chemical reaction 3. Balance: Mg(s) (g) --> MgO(s) 4. Type of chemical reaction: B. Zinc and Copper(II) Sulfate Time CuSO 4 (aq) Zn(s) Appearance Evidence of a Appearance Chemical Reaction 1. initial 2. after 30 min 3. Zn(s) + CuSO4 (aq) Cu(s) + ZnSO4 (aq) 4. Type of chemical reaction: C. Reactions of Metals and HC1 Metal 1. Appearance of Metals 2. Evidence of a Chemical Reaction Zn(s) Cu(s) Mg(s) Copyright 2014 Pearson Education, Inc. 1 17

10 118 Laboratory Manual for General, Organic, and Biological Chemistry 3. Zn(s) + HC1(aq) ZnC1 2 (aq) + H2 (g) Cu(s) + HC1(aq) CuC1 2 (aq)+ H2 (g) Mg(s) + HC1(aq) MgC1 2 (aq) + H2 (g) 4. Type of chemical reaction: Zn Cu Mg D. Reactions of Ionic Compounds D1 Reaction of CaC1 2 and Na3 PO4 Reactants 1. Appearance of 2. Evidence of a Chemical CaC1 2 (aq) Na 3 PO4 (aq) Solutions Reaction 3. CaCl 2 (aq) + Na 3 PO 4 (aq) ---> Ca3 (PO4 )2 (s) + NaCl(aq) III 4. Type of reaction. D2 Reaction of FeC1 3 and KSCN Reactants 1. Appearance of 2. Evidence of a Chemical FeC1 3 (aq) KSCN(aq) Solutions Reaction 3. FeC13 (aq) + KSCN(aq) ---> Fe(SCN) 3 (aq) + KC1(aq) 4. Type of reaction: E. Sodium Carbonate and HCl Reactants 1. Appearance of 2. Evidence of a Chemical Reactants Reaction HC1(aq) Na 2CO3 (s) Copyright 2014 Pearson Education, Inc.

11 3. Observation of burning match or splint What caused the change in the burning match or splint? Chemical Reactions and Equations Na2CO3 (s) + HC1(aq) --> CO2 (g) + H20(1) + NaCI(aq) 5. Type of reaction. F. Hydrogen Peroxide Reactants 1. Appearance of 2. Evidence of a Chemical H202 (aq) Reactants Reaction 3. H202 (aq) H 20(/) (g) 4. Type of chemical reaction: Questions and Problems Q1 What evidence of a chemical reaction might you see in the following cases? Refer to Table a. dropping an Alka-Seltzer tablet into a glass of water b. bleaching a stain c. burning a match d. rusting of an iron nail Q2 Balance the following equations: a Mg(s) + HC1(aq) 1-12(g) + MgC1 2 (ag) b. Al(s) + 02 (g) Al (s) Copyright C 2014 Pearson Education, Inc.

12 120 Laboratory Manual for General, Organic, and Biological Chemistry c. Fe203 (s) + H 20(l) Fe(OH)3 (s) d. Ca(OH)2 (aq) + HNO 3 (aq) --4 Ca(NO3 )2 (aq) + H 2 0(/ ) Q3 Write a balanced equation for each of the following reactions. Write the correct formulas of the reactants and products and the states of each. a. Liquid pentane (C 5H12 ) and oxygen (0 2 ) gas react to form carbon dioxide and water. b. Sodium and water react to form sodium hydroxide and hydrogen gas (H 2 ). c. Iron and oxygen (0 2 ) gas react to form iron(iii) oxide. Q4 Classify each reaction as combination, decomposition, single replacement, double replacement, or combustion. a. Ni(s) + F2 (g)---> NiF2 (s) b. Fe 203 (s)+ 3C(s) > 2Fe(s) + 3C0(g) c. CaCO 3 (s) ---> CaO(s) + CO 2 (g) d. H2SO 4 (aq) + 2KOH(aq) K 2SO 4 (aq)+ 2H 20(/) e. C2H (g) 2CO2 (g)+ 2H20(g) Q5 Complete and balance each of the following chemical reactions by writing the correct formulas of the product(s) that form: Type of Reaction a. Zn(s) + CuBr2 (aq) single displacement b. H 2 (g) + C12 (g) > combination c. MgCO 3 (s) > + decomposition d. KC1(aq) + AgNO 3 (aq) ---> + double displacement Copyright 2014 Pearson Education, Inc.