Reaction Types and Chemical Equations

Transcription

1 Cool Chemistry Show Section 4 Reaction Types and Chemical Equations What Do You See? Learning Outcomes In this section you will Represent chemical changes using word equations and chemical equations. Distinguish between different classes of chemical reactions. Predict the possible products of single-replacement and doublereplacement reactions. Determine whether a reaction has occurred based on evidence observed. Use the law of conservation of matter to balance chemical reactions. What Do You Think? When people communicate with each other, they often like to do so in a compact way. Below are a few examples. Can you interpret these? From the Internet: CUl8r Wat RU ^ 2? TMI LOL License plates: 14U2C 10SNE1 EDUC8R What are some situations (other than the examples given) when language is compressed or abbreviated? Record your ideas about this question in your log. Be prepared to discuss your responses with your small group and the class. Investigate 1. Watch closely as your teacher shows you some cool chemistry. a) Record your observations in your 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: Beaker One 20 drops of the indicator phenolphthalein solution; 490

2 Section 4 Reaction Types and Chemical Equations 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. A chemical reaction occurs when substances change to form new substances. There are many chemical reactions that can occur. You have already observed some of them. One way chemists group reactions is into the following four categories. synthesis reactions; decomposition reactions; single-replacement reactions; double-replacement 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 tells you the number of atoms in one molecule. Oxygen is diatomic. 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 to the left, 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 is the problem with the equation? Write a two in front of the MgO. Mg O 2 2MgO (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 is the problem with the equation? 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? 491

3 Cool Chemistry Show 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 2Mg(s) O 2 (g) 2MgO(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, which 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 Decomposition of water into H 2 (g) and O 2 (g) by electrolysis Water (liquid) and energy produces hydrogen (gas) and oxygen (gas). 2H 2 O(l) energy 2H 2 (g) O 2 (g) a) Is the equation properly balanced? How did you check? When a substance breaks down into its component parts, the process is called a decomposition reaction. AB A B Write word and balanced chemical equations for the following decomposition reactions: (Remember that some elements are diatomic H 2, N 2, and so on.) 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 form products. Such was the case when solid zinc was dropped into hydrochloric acid in Section 1, forming hydrogen gas and aqueous zinc chloride solution. 492

4 Section 4 Reaction Types and Chemical Equations 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 by completing the table in your log. The reaction with zinc and hydrochloric acid is called a singlereplacement 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) metal is added to an aqueous solution of copper (II) sulfate (CuSO 4 ) and produces iron (II) sulfate (FeSO 4 ) and copper metal. c) Solid lead (Pb) metal is added to an aqueous solution of silver nitrate (AgNO 3 ) and produces lead (II) nitrate (Pb(NO 3 ) 2 ) and silver metal. 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 metal. Balance each of the equations, if you have not done so. Number of Atoms Before After Balanced Zn 1 1 yes H 2 Cl You can use a chart similar to the one above, as you did before. 8. Another type of reaction is a double-replacement reaction. AB CD CB AD You may have already done double-replacement reactions in a previous chapter. You will soon try some more with 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 shown below. Safety goggles and a lab apron must be worn at all times in a chemistry lab. Silver nitrate (AgNO 3 ) Copper (II) sulfate (CuSO 4 ) Magnesium sulfate (MgSO 4 ) Sodium hydroxide (NaOH) potassium carbonate (K 2 CO 3 ) sodium hydroxide (NaOH) potassium iodide (Kl) iron (III) chloride (FeCl 3 )

5 Cool Chemistry Show Safety goggles and a lab apron must be worn at all times in a chemistry lab. Silver nitrate will stain skin and clothing. Handle with care. The iron, silver, and copper solutions contain heavy metals. Dispose of them as directed by your teacher. Wash hands and arms thoroughly after the investigation. 9. Mix three drops of one solution (for example, potassium carbonate) with three drops of another solution (for example, silver nitrate), as indicated by the first box on the chart. You can mix these solutions in a well of a well-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 (potassium carbonate with copper (II) sulfate; then potassium carbonate with magnesium sulfate; and so on). b) Record all your observations in your log. 10. Clean all equipment 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 doublereplacement 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 (aq)) 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)) * These are more complex than indicated above. 494

6 Section 4 Reaction Types and Chemical Equations Chem Talk CHEMICAL REACTIONS Kinds of Chemical Reactions A chemical reaction takes place when starting materials (reactants) change to new materials (products). Synthesis, decomposition, singlereplacement, and double-replacement reactions are some common kinds of chemical reactions. Synthesis and Decomposition Reactions Synthesis means putting together. In a synthesis reaction two or more substances (elements or compounds) combine to form one or more compounds. In this investigation you explored 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. Electricity can be used to decompose water into hydrogen and oxygen. Single-replacement Reactions A single-replacement reaction is one in which an element reacts with a compound to produce a new element and an ionic compound. For example, a single-replacement reaction occurs when you put a strip of zinc into hydrochloric acid. Hydrogen gas and aqueous zinc chloride are formed, as you observed in Section 1. Activity Series Chem Words synthesis reaction: a chemical reaction in which two or more substances combine to form a compound. decomposition reaction: a chemical reaction in which a compound breaks down into two or more simple substances. single-replacement reaction: a reaction in which an element displaces or replaces an ion of another element in a compound. A single-replacement reaction occurs when you put a strip of zinc in a copper (II) sulfate solution. The zinc metal exchanges places with the copper cations. If you put copper metal in a zinc sulfate solution you would find that no reaction would take place, as expected. Zinc atoms exchange places with copper ions, but copper atoms will not exchange places with zinc ions. You have learned something about a property of copper and zinc. Zinc is more reactive 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. 495

7 Cool Chemistry Show Activity Series of Metals (most active to least active) lithium (Li Li + e ) potassium (K K + e ) calcium (Ca Ca 2+ 2e ) sodium (Na Na + 1e ) magnesium (Mg Mg 2+ 2e ) aluminum (Al Al 3+ 3e ) zinc (Zn Zn 2+ 2e ) iron (Fe Fe 2+ 2e ) lead (Pb Pb 2+ 2e ) hydrogen (H 2 (g) 2H + 2e ) copper (Cu Cu 2+ 2e ) mercury (Hg Hg 2+ 2e ) silver (Ag Ag + e ) gold (Au Au 3+ 3e ) The table looks like the one on this 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 metal. Zinc replaced the copper in the copper (II) sulfate solution 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 in the silver nitrate solution 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. 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) 2KOH(aq) (Note: Water, H 2 O is written as HOH.) Chem Words double-replacement reaction: a chemical reaction in which two ionic compounds exchange cations to produce two new compounds. Double-replacement Reactions Double-replacement reactions are different from singlereplacement reactions in that you start with two aqueous phase solutions and when they are mixed they switch partners. 496

8 Section 4 Reaction Types and Chemical Equations An example of this type of reaction is Ba(NO 3 ) 2 (aq) Na 2 SO 4 (aq) 2NaNO 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 precipitate. When examining double-replacement reactions, you know that a reaction has taken place if you see a precipitate or a gas. 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 having both cations and anions. In the example above, barium sulfate formed as 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. Solubility rule #2 tells you that silver chloride is insoluble (a precipitate will form) and solubility rule #3 tells you that sodium nitrate is soluble and stays in solution. Using these rules, you can now predict whether a mixture will produce a precipitate or not. Solubility Rules 1. All salts (defined as ionic compounds) of the alkali metals (Group 1 on the periodic table) 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. Chem Words salt: an ionic compound that is produced by the neutralization of an acid and base. 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 doublereplacement reaction. 4. What evidence would you look for to determine if a doublereplacement 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. 497

9 Cool Chemistry Show What Do You Think Now? At the beginning of the section you were asked the following: What are some situations (other than the examples given) when language is compressed or abbreviated? As a student chemist, you know that an obvious answer to this question is chemistry. Do you think the use of chemical equations simplifies the communication of chemical change? Explain. Do you think you understand the basic format for how chemical change is communicated in a compact way? What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. MACRO NANO SYMBOLIC If you add silver nitrate solution to sodium chloride solution, what will you observe that convinces you that a chemical reaction has taken place? Chem Essential Questions Why do chemical equations have to be balanced? Explain what occurs on the atomic level when potassium iodide reacts with silver nitrate. How can you represent the reaction of silver nitrate solution with potassium iodide solution using chemical symbols? How do you know? Water can be decomposed to form two products: hydrogen and oxygen. What evidence was given in this section for the chemical composition of water? Why do you believe? In mathematics, the symbol is used to represent sides of an equation. In chemistry the symbol is used in chemical equations. Why do you believe this symbol is better for chemistry than the symbol? Why should you care? Being able to represent chemical changes in a compact way simplifies chemistry. When you perform your chemistry show, how might you use chemical equations to make it easier for your audience to understand the chemical changes that you are demonstrating? 498

10 Section 4 Reaction Types and Chemical Equations Reflecting on the Section and the Challenge In this section you have learned about a number of different types of reactions: synthesis, decomposition, single-replacement and double-replacement 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 reaction 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 equations to your elementary school audience. 1. Baking soda (NaHCO 3 ) has been used in several reactions in previous sections. 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. Use the solubility rules to explain why these reactions do not form precipitates. 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) Chem to Go 499