1 Classifying Chemical Reactions Analyzing and Predicting Products Introduction The power of chemical reactions to transform our lives is visible all around us-in our cars, even in our bodies. Chemists try to make sense of the great variety of chemical reactions the same way that biologists organize their knowledge of life, by sorting reactions into groups and classifying them. Classifying chemical reactions allows us to predict what chemical reactions will occur when different substances are mixed. Concepts Chemical reactions Combination vs. decomposition Single vs. double replacement combustion reactions Background A chemical reaction is defined as any process in which one or more substances are converted into new substances with different properties. Chemical reactions change the identity of the reacting substance(s) and produce new substances. Observing the properties of the reactants and products is therefore a key step in identifying chemical reactions. Some of the observations that may be associated with a chemical reaction include: (1) release of a gas; (2) formation of a precipitate; (3) color changes; (4) Energy gained or lost (temperature changes, emission or absorption of light). As these observations suggest, chemical reactions can be dynamic and exciting events. The essence of any chemical reaction-reactants being transformed into products-is summarized in the form of a chemical equation. Consider the reaction represented by Equation 1, the burning of natural gas (methane, CH 4 ) in a laboratory burner. CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (g) Equation 1 The reactants or, more specifically, their formulas are written on the left side of the equation, the products on the right side of the equation. An arrow represents the direction of the reaction and is read as yields or produces. Other symbols are used to describe the physical state of the reactants and products and to describe the reaction conditions (see Table 1). Chemical reactions may be classified by considering the number of reactants and products in the reaction, the physical or chemical nature of the reactants and products, and the rearrangement of atoms in the conversion of the reactants into products (see Table 2).
2 Symbol Table 1. Symbols in Chemical Equations Translation Yields or produces (separates reactants from products) + Reacts with or forms alongside (separates two or more reactants or products) Reaction mixture is heated (written over the arrow) NR No reaction takes place when reactants are mixed (s) Pure substance (reactant or product) is a solid (l) Pure substance (reactant or product) is a liquid (g) Pure substance (reactant or product) is a gas Aqueous solution (reactant or product is dissolved in water) Cat Catalyst a substance needed to initiate a reaction (formula is written over the arrow) (aq) Type of Reaction Combination Decomposition Single Replacement Double Replacement Combustion General Description and Example(s) Two reactants combine to form a single product. The reactants may be elements or compounds. Also called a synthesis reaction Zn(s) + I 2 (s) ZnI 2 (s) CaO(s) + H 2 O(l) Ca(OH) 2 (s) One reactant, a compound, breaks down to give two or more products. BaCO 3 (s) CO 2 (g) + BaO(s) An element reacts with a compound and replaces one of the elements in the compound. Metals replace hydrogen or other metals; nonmetals replace nonmetals. Zn(s) + 2HCl(aq) H 2 (g) + ZnCl 2 (aq) Cu(s) + 2AgNO 3 (aq) 2Ag(s) + Cu(NO 3 ) 2 (aq) Cl 2 (aq) + 2NaI(aq) I 2 (aq) + 2NaCl(aq) Two ionic compounds (or compounds that break apart to form ions in solution) exchange ions to form new compounds. Examples include precipitation reactions (precipitate forms), acid-base reactions (water and salt form), and gas-forming reactions (evolution of a gas). NaCl + AgNO 3 AgCl + NaNO 3 H 2 SO 4 (aq) + 2NaOH(aq) Na 2 SO 4 (aq) + 2H 2 O(l) Na 2 SO 3 (aq) + 2HCl(aq) 2NaCl (aq) + H 2 O(l) + SO 2 (g) A compound burns in the presence of oxygen, producing energy in the form of heat and light. The combustion of organic compounds (hydrocarbons) produces carbon dioxide and water. C 4 H 8 (l) + 6O 2 (g) 4CO 2 (g) + 4H 2 O(g)
3 Purpose 1. Perform chemical reactions and identify products. 2. Classify chemical reactions. 3. Write balanced equations for each reaction. 4. Perform flame tests to determine the presence of a gas. Equipment Burner Striker Crucible tongs Microspatula Test tubes, 15 x 180 mm (7) Test tube holder Test tube rack Wood splints Pipettes Evaporating dish Safety goggles Lab apron or coat Materials Zinc, mossy (Zn) 3% Hydrogen Peroxide (H 2 O 2 ) Magnesium ribbon, 5 cm (Mg) 1 M Silver Nitrate Solutions (AgNO 3 ) Copper(II) carbonate (CuCO 3 ) 0.1 M Calcium Chloride Solution (CaCl 2 ) 6 M hydrochloric acid (HCl) 1 M Copper (II) Sulfate Safety In this investigation you will be working with open flames, heating chemicals, handling acids, and producing gaseous products. Burning magnesium produces a very bright, hot flame. Make sure you hold the burning metal at arm s length and do not look directly at it. Wear safety goggles and protective clothing at all times when working in the lab.
4 Procedure: Station 1 1. Light the Bunsen burner. (CH 4, methane, is the flammable gas in the Bunsen Burner) 2. Using a test tube holder, invert a clean, dry test tube over the flame. Record your observations. Keep the test tube inverted over the flame. 3. Test for formation of a gas. Light a wooden splint. Turn the test tube right side up and place a burning splint inside the test tube. 4. Record your observations 5. Clean your work station. Leave it as you found it. Station 2 1. Place an evaporating dish near the base of the burner. Examine a piece of magnesium ribbon. 2. Hold the piece of magnesium with crucible tongs and heat the metal in the Bunsen burner flame. Caution: DO NOT look directly at the burning magnesium ultraviolet light that is produced may damage your eyes. 3. When the magnesium ignites, remove it from the flame and hold it over an evaporating dish until the metal has burned completely. Let the produce fall into the evaporating dish. 4. Turn off the burner. Examine the product carefully and record in your table. 5. Clean your work station. Leave it as you found it. Station 3 1. Place a small amount (about the size of a jelly bean) of copper carbonate (CuCO 3 ) in a clean, dry test tube. Note the appearance of the sample. 2. Using a test tube holder, heat the CuCO 3 for about 30 seconds. Point the mouth of the test tube AWAY from you and your partner. 3. Test for the formation of a gas: Insert a burning wood splint halfway down into the test tube. Record results. 4. Clean your work station. Leave it as you found it. Station 4 1. Light the Bunsen burner. 2. Add 2 pipettes of hydrogen peroxide (H 2 O 2 ) to a test tube. Add a small scoop (about the size of half of a jelly bean) of manganese (IV) oxide to the test tube. 3. Test for the formation of a gas. As the reaction occurs, light a wood splint and allow it to burn freely for about 5 seconds. Blow out the flame and place the glowing splint halfway down into the test tube. Record your results.
5 4. Pour the liquid out of the test tube and leave the solid in the test tube. Station 5 1. Stand a clean, dry test tube in the test tube rack. Add about 1 pipette of hydrochloric acid (HCl) to the tube. CAUTION. Handle acids with care. They can cause painful burns. Do not inhale any HCl fumes. 2. Light the Bunsen Burner. 3. Now carefully drop a small piece of zinc metal (Zn) into the test tube. 4. Feel the sides of the test tube as you observe the chemical reaction 5. Test for the presence of a gas: Using a test tube holder, cover the mouth of the test tube with another test tube inverted over the top. Hold it there for about 30 seconds to collect the gas that is being released. Quickly insert a burning wood splint into the mouth of the tube. zinc 6. Clean your work station. Leave it Hydrochloric acid as you found it. Station 6 1. Add 2 pipettes of copper (II) sulfate solution to a clean, dry test tube. 2. Add one piece of zinc metal in the solution. Observe reaction for 2 3 minutes and record your observations. 3. Clean your work station. Leave it as you found it. Station 7 1. Add 5 8 drops of silver nitrate (AgNO 3 ) to a clean, dry test tube. Next, add 5-8 drops of calcium chloride (CaCl 2 ) solution to the test tube. Observe what happens and note any changes in the mixture. 2. Clean your work station. Leave it as you found it.
6 Observations and Data Data Table 1: Reaction Observations Copy table into your lab notebook. Leave room in your table to record your observations! Station Reactants Before Reaction Evidence of Reaction 1 CH 4 + O 2 Test for Presence of Gas After Reaction 2 3 4 5 Mg + O 2 X CuCO 3 H 2 O 2 Zn + HCl 6 7 Zn + CuSO 4 AgNO 3 + CaCl 2 X X Analysis and Conclusions Questions 1. Write an equation for each reaction observed. Indicate the state (s, l, g, aq) for each reactant and product, then balance the equation. Write the reaction type for each. 2. List the five types of chemical reactions named in the background section. Explain how to distinguish each of these types of reactions. 3. List the tests for gases used to identify the three gases produced in this experiment. Explain how each test is done and how a positive result is determined.
7 Station 1 1. Light the Bunsen burner. (CH 4, methane, is the flammable gas in the Bunsen Burner) 2. Using a test tube holder, invert a clean, dry test tube over the flame. Record your observations. Keep the test tube inverted over the flame. 3. Test for formation of a gas. Light a wooden splint. Turn the test tube right side up and place a burning splint inside the test tube. 4. Record your observations 5. Clean your work station. Leave it as you found it. Station 2 1. Place an evaporating dish near the base of the burner. Examine a piece of magnesium ribbon. 2. Hold the piece of magnesium with crucible tongs and heat the metal in the Bunsen burner flame. Caution: DO NOT look directly at the burning magnesium ultraviolet light that is produced may damage your eyes. 3. When the magnesium ignites, remove it from the flame and hold it over an evaporating dish until the metal has burned completely. Let the produce fall into the evaporating dish. 4. Turn off the burner. Examine the product carefully and record in your table.
8 5. Clean your work station. Leave it as you found it. Station 3 1. Place a small amount (about the size of a jelly bean) of copper carbonate (CuCO 3 ) in a clean, dry test tube. Note the appearance of the sample. 2. Using a test tube holder, heat the CuCO 3 for about 30 seconds. Point the mouth of the test tube AWAY from you and your partner. 3. Test for the formation of a gas: Insert a burning wood splint halfway down into the test tube. Record results. 4. Clean your work station. Leave it as you found it. 1. Light the Bunsen burner. Station 4 2. Add 2 pipettes of hydrogen peroxide (H 2 O 2 ) to a test tube. Add a small scoop (about the size of half of a jelly bean) of manganese (IV) oxide to the test tube. 3. Test for the formation of a gas. As the reaction occurs, light a wood splint and allow it to burn freely for about 5 seconds. Blow out the flame and place the glowing splint halfway down into the test tube. Record your results.
9 4. Pour the liquid out of the test tube and leave the solid in the test tube. Station 5 1. Stand a clean, dry test tube in the test tube rack. Add about 1 pipette of hydrochloric acid (HCl) to the tube. CAUTION. Handle acids with care. They can cause painful burns. Do not inhale any HCl fumes. 2. Light the Bunsen Burner. 3. Now carefully drop a small piece of zinc metal (Zn) into the test tube. 4. Feel the sides of the test tube as you observe the chemical reaction 5. Test for the presence of a gas: Using a test tube holder, cover the mouth of the test tube with another test tube inverted over the top. Hold it there for about 30 seconds to collect the gas that is being released. Quickly insert a burning wood splint into the mouth of the tube. 6. Clean your work station. Leave it as you found it. zinc Hydrochloric acid
10 Station 6 1. Add 2 pipettes of copper (II) sulfate solution to a clean, dry test tube. 2. Add one piece of zinc metal in the solution. Observe reaction for 2 3 minutes and record your observations. 3. Clean your work station. Leave it as you found it. Station 7 1. Add 5 8 drops of silver nitrate (AgNO 3 ) to a clean, dry test tube. Next, add 5-8 drops of calcium chloride (CaCl 2 ) solution to the test tube. Observe what happens and note any changes in the mixture. 2. Clean your work station. Leave it as you found it.
11 Classifying Chemical Reactions Worksheet Write a balanced chemical equation for each reaction and classify the reaction. 1. Copper metal heated with oxygen gas gives solid copper(ii) oxide. 2. Mixing ammonium nitrate and sodium hydroxide solutions gives aqueous sodium nitrate, ammonia gas, and water. 3. Mercury(II) nitrate solution reacts with potassium iodide solution to give a mercury(ii) iodide precipitate and potassium nitrate solution. 4. Aluminum metal and sulfuric acid yield aqueous aluminum sulfate and hydrogen gas. 5. Acetic acid and lithium hydroxide solution produce water and aqueous lithium acetate. 6. Sulfur dioxide gas reacts with oxygen gas on a platinum catalyst surface to produce sulfur trioxide gas. 7. Sodium metal reacts with water to give sodium hydroxide solution and hydrogen gas.
12 8. Heating solid nickel(ii) chloride dihydrate (NiCl 2 2H 2 O) yields solid nickel(ii) chloride and water vapor. 9. Heating solid potassium chlorate in the presence of manganese dioxide catalyst produces potassium chloride and oxygen gas.