Cool Chemistry Show Activity 4 Chemical Equations GOALS In this activity you will: Represent chemical changes using word equations and chemical equations. Distinguish between different classes of chemical reactions. Predict the possible products of single-displacement and double-displacement reactions. Determine whether a reaction has occurred based on evidence observed. Use the principle of conservation of matter to balance chemical reactions. What Do You Think? In Activity 1 you mixed zinc metal and hydrochloric acid. You noticed that a gas was produced from the fizzing that occurred. How could you communicate (tell about) the reactants and products of this reaction in a compact way? Record your ideas about this question in your Active Chemistry log. Be prepared to discuss your responses with your small group and with the class. Investigate 1. Watch closely as your teacher shows you some cool chemistry. a) Record your observations in your Active Chemistry log. 2. Here s how the cool chemistry was done. Into each of three beakers that appeared empty, your teacher added about 45 ml of 2.0 M ammonium hydroxide solution. Before beginning the demonstration, your teacher had also added the following to each beaker: Active Chemistry 486
Activity 4 Chemical Equations Beaker One 20 drops of the indicator phenolphthalein solution; Beaker Two 15 drops of 1 M magnesium sulfate solution; Beaker Three 15 drops of 1 M copper (II) sulfate solution. a) Did a chemical reaction take place in each beaker? What evidence do you have to justify your answer? 3. There are many chemical reactions that can occur. You have already observed some of them. Chemists group most of these reactions into four main categories. They are: synthesis reactions decomposition reactions single-displacement reactions double-displacement reactions a) What do the words synthesis and decomposition mean? 4. In a synthesis reaction two or more chemicals combine to form a compound. A B AB Here is an example of a synthesis reaction. When magnesium and oxygen react, a white solid, magnesium oxide is formed. This can be written as a word equation: Magnesium (solid) and oxygen (gas) produce magnesium oxide (solid). This can also be written using a chemical equation:?mg?o 2?MgO (The subscript communicates the number of atoms in one molecule. Oxygen is diatomic, that means that it exists as a molecule made up of two atoms.) a) What do you think are the advantages of writing a reaction using chemical symbols? Any equation in chemistry must follow scientific laws or principles. The number of atoms of each element must be equal before and after the reaction. b) In the equation above, how many atoms of oxygen are in the reactants (before the reaction)? c) How many oxygen atoms are in the product (after the reaction)? d) What appears to be the problem with the equation? How could you correct this problem? Write a 2 in front of the MgO.?Mg O 2 2MgO 487 Coordinated Science for the 21st Century
Cool Chemistry Show (The number in front of a chemical formula, called a coefficient, communicates the number of molecules or formula units that are involved in the reaction. In this equation there are two molecules of magnesium oxide represented. That is, there is a total of two magnesium atoms and two oxygen atoms.) e) How many magnesium atoms are now represented in the product? f) How many reactant atoms of magnesium are shown? g) What now appears to be the problem with the equation? How could you correct this problem? Write a two in front of the Mg. 2Mg O 2 2MgO h) The chemical equation above is now balanced. (The number of magnesium and oxygen atoms in the product is equal to the number in the reactant.) In your own words, explain the meaning of a balanced equation. How does the chemical equation communicate what happened in the reaction, and how does it follow the Law of Conservation of Matter? When writing a chemical equation the states of the reactants and products are also given. The following symbols are used: (s) for solid (l) for liquid (g) for gas (aq) for aqueous, meaning in a water solution The complete balanced chemical equation for the reaction of magnesium and oxygen is: 2 Mg(s) O 2 (g) 2 MgO(s) 5. Write a word equation and a balanced chemical equation for each of the following synthesis reactions. Note that there are eight elements that are diatomic, that means that they exist as a molecule comprised of two atoms. They are hydrogen (H 2 ), nitrogen (N 2 ), oxygen (O 2 ), fluorine (F 2 ), chlorine (Cl 2 ), bromine (Br 2 ), iodine (I 2 ) and astatine (At 2 ). If you need to include any of these elements in an uncombined state in a chemical equation, don t forget the 2 as a subscript. a) Solid carbon (C) burns in air (oxygen gas) to form carbon dioxide gas (CO 2 ). b) Hydrogen gas reacts with oxygen gas to form liquid water (H 2 O). c) A piece of solid iron (Fe) over time will react with oxygen to form iron (III) oxide (Fe 2 O 3 ). d) A piece of solid sodium (Na) is dropped into a container of chlorine gas to produce solid sodium chloride (NaCl). 6. Water can be separated into its elements with an input of energy. The equation for this reaction is: Water (liquid) and energy produces hydrogen (gas) and oxygen (gas). 2H 2 O(l) energy 2 H 2 (g) O 2 (g) a) Is the equation properly balanced? How did you check? Active Chemistry 488
Activity 4 Chemical Equations Water When a substance breaks down into its component parts, the process is called a decomposition reaction. form products. Such was the case when solid zinc was dropped into hydrochloric acid in Activity 1, forming hydrogen gas and aqueous zinc chloride solution. The equation for this reaction is: Zinc (solid) and hydrochloric acid (aqueous) produces hydrogen (gas) and zinc chloride (solution). Zn(s) 2HCl(aq) H 2 (g) ZnCl 2 (aq) a) Check to ensure that the chemical equation is properly balanced. Complete the following table in your log. Number of Atoms Before After Balanced Zn 1 1 yes H Cl AB A B Write word and balanced chemical equations for the following decomposition reactions. (Remember that some elements are diatomic H 2, N 2, etc.): b) sodium chloride solid (NaCl(s)) c) potassium iodide solid (KI(s)) d) magnesium bromide solid (MgBr 2 (s)) 7. The reactions mentioned above involve elements combining to form compounds or compounds breaking up to form elements. There are other reactions that involve elements reacting with compounds to The reaction with zinc and hydrochloric acid is called a single-displacement reaction because zinc replaces the hydrogen in the acid. A BC B AC Write word and chemical equations for the following: b) A piece of iron (Fe) is added to an aqueous solution of copper (II) sulfate (CuSO 4 ) and produces iron sulfate (FeSO 4 ) and copper. c) Solid lead (Pb) is added to an aqueous solution of silver nitrate (AgNO 3 ) and produces lead nitrate Pb(NO 3 ) 2 and silver. 489 Coordinated Science for the 21st Century
Cool Chemistry Show Safety goggles and a lab apron must be worn during this activity. d) Aluminum foil (Al) is placed in a beaker of aqueous copper (II) hydroxide (Cu(OH) 2 ) and produces aluminum hydroxide (Al(OH) 3 ) and copper. Balance each of the equations, if you have not done so. 8. Another type of reaction is a double-displacement reaction. AB CD CB AD Try some double-displacement reactions on your own in your group. Use the chart below to guide your work. The compounds are in water solution. a) Record your observations of the reactants before you mix them. For example, record your observations of potassium carbonate and silver nitrate before you mix them. b) Create a chart in your log to record your observations after you mix the reactants. You may wish to use a chart similar to the one below. 9. Mix three drops of one solution (i.e., potassium carbonate) with three drops of another solution (i.e., silver nitrate), as indicated by the first box on the chart. You can mix these solutions in the well of a spot plate or on a plastic surface. Do not allow the tip of the dropper of one solution to come in contact with another solution. This is important to prevent contamination of solutions. a) In the chart in your log, record your observations after mixing the reactants. Continue with the other reactants (i.e., potassium carbonate with copper (II) sulfate; then potassium carbonate with magnesium sulfate; and so on). b) Record all your observations in your Active Chemistry log. Silver Copper (II) Magnesium Sodium Nitrate Sulfate Sulfate Hydroxide (AgNO 3 ) (CuSO 4 ) (MgSO 4 ) (NaOH) Potassium 1. 2. 3. 4. Carbonate (K 2 CO 3 ) Sodium 5. 6. 7. 8. Hydroxide (NaOH) Potassium 9. 10. 11. 12. Iodide (KI) Iron (III) 13. 14. 15. 16. Chloride (FeCl 3 ) Active Chemistry 490
Activity 4 Chemical Equations 10. Clean all apparatus and the laboratory bench when you are finished. Dispose of all chemicals as directed by your teacher. Wash your hands. 11. Look at the data from the doubledisplacement reactions that you observed. a) Do you think a chemical reaction took place in each case? Explain your answer. b) Are you able to predict or identify any of the products that were formed? If so, which ones? c) Write word equations and balanced chemical equations for each reaction that you observed. In all cases, assume only two reactants are used and only two products are formed. Use the following formulas to help you write your equations: copper (II) carbonate (CuCO 3 (s)); copper (II) iodide (CuI 2 (s)); iron (III) hydroxide (Fe(OH) 3 (s)); silver carbonate (Ag 2 CO 3 (s)); silver hydroxide (AgOH(s)); potassium sulfate (K 2 SO 4 (aq)); silver iodide (AgI(s)); sodium nitrate (NaNO 3 (aq)); copper (II) hydroxide (Cu(OH) 2 (s)); iron (III) nitrate (Fe(NO 3 ) 3 (s)); magnesium carbonate (MgCO 3 (s)); silver chloride (AgCl(s)); potassium nitrate (KNO 3 (aq)); magnesium hydroxide (Mg(OH) 2 (s)); sodium chloride (NaCl(aq)); sodium sulfate (Na 2 SO 4 (aq)); 491 Coordinated Science for the 21st Century
Cool Chemistry Show CHEMICAL REACTIONS Kinds of Chemical Reactions A chemical reaction takes place when starting materials (reactants) change to new materials (products). Synthesis, decomposition, singledisplacement, and double-displacement reactions are some common kinds of chemical reactions. Synthesis and Decomposition Reactions Chem Words synthesis reaction: a chemical reaction in which two or more substances combine to form a compound. decomposition: a chemical reaction in which a single compound reacts to give two or more products. single-displacement reaction: a reaction in which an element displaces or replaces another element in a compound. double-displacement reaction: a chemical reaction in which two ionic compounds "exchange" cations to produce two new compounds. Synthesis means putting together. In a synthesis reaction two or more elements combine to form one or more compounds. In this activity you investigated the reaction of magnesium in oxygen to form magnesium oxide.the opposite kind of reaction is a decomposition reaction. In chemical decomposition a compound is separated into its elements. Single-displacement Reactions A single-displacement reaction is one in which an element reacts with a compound to produce a new element and an ionic compound. For example, a single-displacement reaction occurs when you put a strip of zinc in a copper (II) sulfate solution.the zinc metal exchanges places with the copper cations.you also observed that the free copper metal atoms now plate on the zinc strip. Activity Series If you put copper metal in a zinc sulfate solution you would find that no reaction would take place, as expected. Zinc exchanges places with copper, but copper will not exchange places with zinc.you have learned something about a property of copper and zinc. Zinc is more active than copper. If you were to experiment with different metals and metallic solutions, you should be able to create an activity series of metals.the activity series of metals can be put into a table that you can use to predict if a reaction will take place.the table looks like the one on the following page.the table permits you to determine how a metal will react in a metal solution.a metal that is more active than another will dissolve into the metal solution and plate out the less Active Chemistry 492
Activity 4 Chemical Equations Activity Series of Metals (Most Active to Least Active) 1. Lithium (Li Li e ) 2. Potassium (K K e ) 3. Calcium (Ca Ca 2 2e ) 4. Sodium (Na Na 1e ) 5. Magnesium (Mg Mg 2 2e ) 6.Aluminum (Al Al 3+ 3e ) 7. Zinc (Zn Zn 2+ 2e ) active metal. Zinc replaced the copper and the copper plated the zinc. For example, let s say that you place a strip of copper in a silver nitrate solution. According to the table, the copper will dissolve into copper ions (Cu 2+ ) and the silver ions will plate out on the copper as silver. In addition to metals, you will notice that hydrogen gas is also listed in the activity series. In your investigation you found that different metals produced hydrogen gas and metal ions when they reacted with hydrochloric acid.you read above that metals can replace less active metals in metal salt solutions. Metals that are more active than hydrogen can replace the hydrogen from water to form metal hydroxides. As an example, if you were to react potassium metal with water, you would get hydrogen gas and potassium hydroxide solution. The equation is: 2K(s) 2HOH(l) H 2 (g) 2 KOH(aq) 8. Iron (Fe Fe 2+ 2e ) 9. Lead (Pb Pb 2+ 2e ) 10. Hydrogen (H 2 (g) 2H + 2e ) 11. Copper (Cu Cu 2+ 2e ) 12. Mercury (Hg Hg 2+ 2e ) 13. Silver (Ag Ag + e ) 14. Gold (Au Au 3+ 3e ) Double-displacement Reactions Double-displacement reactions are different from singledisplacement reactions, in that you start with two aqueous phase solutions and when they react they switch partners. An example of this type of reaction is: Ba(NO 3 ) 2 (aq) Na 2 SO 4 (aq) 2 NaNO 3 (aq) BaSO 4 (s) Note that the cation of the one compound (Ba 2+ ) exchanged places with the cation (Na+) of the other compound.the solid BaSO 4 is a 493 Coordinated Science for the 21st Century
Cool Chemistry Show Chem Words salts: ionic compounds in which the anion is not a hydroxide ion (OH ) and the cation is not a hydrogen proton (H+). precipitate.when examining double-displacement reactions, you know that a reaction has taken place if you see: a precipitate a gas water If none of these are present, then no reaction had occurred. Solubility Rules A precipitate will form if the compound is not soluble in water. In the example above, barium sulfate was not soluble in water.this was noted in the equation by referring to it as a solid (s). Chemists have created a set of solubility rules for salts. Salts are classified as ionic compounds in which the anion is not a hydroxide ion (OH ) and the cation is not a hydrogen proton (H + ). Solubility Rules Checking Up 1. What is a synthesis reaction? Provide an example. 2. What is a decomposition reaction? Provide an example. 3. Distinguish between a single and a double-displacement reaction. 4. What evidence would you look for to determine if a double-displacement reaction has occurred? 5. Will hydrochloric acid react with a clean strip of copper? Explain your answer. 6. Is calcium sulfate soluble in water? Justify your answer. 1. All salts (defined as ionic compounds) of the alkali (Group 1 on the periodic table) metals and the ammonium ion are soluble in water. 2. All chlorides, bromides, and iodides are soluble with the exception of silver, lead, and mercury halides. 3. All nitrate, chlorate, perchlorate, and acetate salts are soluble. 4. All sulfates are soluble with the exception of calcium, barium, strontium, and lead. 5. All carbonates, phosphates, chromates, hydroxides, and sulfides are insoluble except when they are combined with alkali metals or the ammonium ion. In the example above, barium sulfate formed a precipitate. Since barium sulfate is insoluble, this agrees with the solubility rules. If you mixed silver nitrate with sodium chloride, would you expect to get a precipitate? The two products that would form are silver chloride and sodium nitrate. The solubility rule #2 tells you that silver chloride is insoluble and solubility rule #3 tells you that sodium nitrate is soluble. Using these rules, you can now predict whether a mixture will produce a precipitate or not. Active Chemistry 494
Activity 4 Chemical Equations Reflecting on the Activity and the Challenge In this activity you have learned about a number of different types of reactions: synthesis, decomposition, single-displacement and doubledisplacement reactions. Knowing these types of reactions can help you predict the products of some chemical reactions. You ll need to decide if you want the audience for the Cool Chemistry Show to learn about these reactions types. You also learned how to write balanced equations for some of the reactions you observed. Think about a creative way of showing how you can explain balancing chemical equation to your elementary school audience. In this activity you have learned how to write the formulas for many compounds and how to name some compounds. You have also investigated both ionic and molecular compounds. As you prepare your presentation for your Cool Chemistry Show, you will want to include your knowledge of formulas, the names of compounds, and the different kinds of compounds. Remember that you will be providing the teacher with an explanation of why you included certain demonstrations, and you will also want to include explanations that are grade-appropriate. Think about how much information you will need to provide each. 1. Baking soda (NaHCO 3 ) has been used in several reactions in previous activities. When heat is applied to baking soda, three compounds are produced. Two of the compounds are gases and the other is a solid. If the two gases are water and carbon dioxide, what is the third product? Explain how you arrived at your answer. 2. When solutions of sodium hydroxide and potassium carbonate are mixed together, no apparent reaction takes place. The same is true when you mix sodium hydroxide and potassium iodide together. Explain this observation. 3. If you mix sodium sulfate and barium nitrate solutions together, you get a white precipitate. What is the precipitate that formed? What information did you use to arrive at your answer? 4. There were five combinations that you mixed together that did not form a precipitate. Complete and balance the following equations: a) K 2 CO 3 (aq) NaOH(aq) b) KI(aq) MgSO 4 (aq) c) KI(aq) NaOH(aq) d) FeCl 3 (aq) CuSO 4 (aq) e) FeCl 3 (aq) MgSO 4 (aq) 5. Use the solubility rules to explain why the products in Question 4 do not form precipitates. 495 Coordinated Science for the 21st Century