CHAPTER 8: CLASSIFYING CHEMiCAL REACTIONS

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1 INTRODUCTION CHAPTER 8: CLASSIFYING CHEMiCAL REACTIONS There are many different kinds of chemical reactioii~. When you first Iook at the equation for a reaction, it often looks completely new and unfamiliar. Mter you learn th' ;fhaterial in this chapter, you will be able to classify many ofthe new reactions you come across into one or more basic categories. Categorizing a reaction is important. By fitting a reaction into a specific familiar category, you automatically know some things about that reaction, even if you have never seen that specific reaction before. GOALS FOR THIS CHAPTER 1. Be able to recognize chemical reactions where the reactants transfer electrons to produce products. (Section 8.1) 2. Be able to show which reactant is losing electrons and which is gaining electrons, and how many electrons are transferred. (Section 8.1) 3. Be able to recognize electron transfer reactions between nonmetals by the presence of oxygen gas as a reactant or as a product. (Section 8.1j 4. Be able to classifyreactions into one ormore categories. (Sections 8.2 and 8.3) QUICK DEFINIT10NS Oxidation-reduction reaction Precipitation reaction Double displacement reaction Acid-base reaction Single replacement reaction Combustion reaction A reaction that involves the transferofelectrons from one substance to another. (Section 8.1) A reaction where a solid product is produced when two anions in solution are exchanged. (Section 8.2) The same thing as a precipitation reaction. (Section 8.2) The reaction between listtohg acid and a strong base. (Section 8.2) An anion is exchanged between one of the reactants and another reactant. (Section 8.2) A reaction that includes oxygen gas as a reactant and produces heat energy. A combustion reaction is a type of oxidation-reduction reaction. (Section S.3) 131

2 Synthesis reaction Decomposition reaction A reaction in which the reactants are siinpler than the product. This type of reaction is also called a combination reaction. Synthesis reactions are a type ofoxidation-reductionreaction. (Section 8.3) A reactant compound is broken down into elements or smaller compounds, A decomposition reaction isa type of oxidationreduction reaction. (Section 8.3) PRETEST 1. How many electrons do the elements below gain or lose when they form ions? a. 1 2 b. Ca 2. Write an equation showing how the ions below gain or lose electrons to form atoms or diatomic molecules.. 3. Identify the atom or ion gaining electrons and the atom or ion losing electrons; 2 Hg(l) + 02(g) HgO(s) 4. Identify the reaction below as.a precipitation reaction, an acid-base reaction, or an oxidation-reduction reaction. 2 KOH(aq) + ZnCI 2(aq) --. Zn(OH)z{s) + 2 KCI(aq) 5. Identifythe reaction below as a combustion, synthesis or decomposition reaction. 6. Which ofthe reaction classes is/are not electron transferreactions? a. combustion b. acid-base c. decomposition d. synthesis e. precipitation Classifying Chemical Reactions

3 7. Classify the reaction below as precipitation, acid-base, or oxididation-reduction, Aluminum metal reacts with oxygen gas to produce solid aluminum oxide 8. Classify the reaction below as precipitation, acid-base, or oxidation-reduction. Aqueous solutions of potassium hydroxide and nitric acid are mixed to produce aqueous potassium nitrate and liquid water. 9. In which kind of a reaction is water always a product? a. precipitation b. oxidation-reduction c. synthesis d. acid-base e. decomposition 10. In which kind of a reaction is an ionic solid always a product? a. precipitation b. oxidation-reduction c. synthesis d. acid-base e. decomposition PRETEST ANSWERS 1. a. Each iodine atom gains one electron e -. 2 r b. A calcium atom loses two electrons. Ca -. Ca e- 2. a. b. Mg e' -. Mg e- 3. Elemental Hg forms an Hg 2 + ion. Each Hg atom loses two electrons. Hg-. Hg e- Elemental 02 forms an 0 2- ion e' Each atom gains two electrons. Pretest Answers 133

4 4. In this reaction, aqueous solutions of ionic compounds form an ionic solid. This is a precipitation reaction. 2 KOH(aq) + ZnCI 2(aq) -.. Zn(OH)2(s) + 2 KCI(aq) 5. In this reaction, two smaller molecules combine to form a larger molecule. This is a synthesis reaction. 6. Combustion, decomposition and synthesis reactions are electron-transfer reactions. Precipitationand acid-base reactions are not electron-transferreactions. 7. When aluminum metal reacts with oxygen to produce solid aluminum oxide, elemental oxygen is present as a reactant. This is an oxidation-reduction reaction. 8. When aqueous solutions of potassium hydroxide and nitric acid produce potassium nitrate and water, an acid and a base are the reactant molecules. This is an acid-base reaction. 9. The correct answer is "d". In acid-base reactions, water is always a product. Water may be a product for some other reactions as well. 10. The correct answer is "a". In a precipitation reaction, an ionic solid is always formed. CHAPTER REVIEW 8.1 REACTIONS OF METALS WITH NONMETALS (OXIDATION-REDUCTION) What Kind ofa Reaction Takes Place Between Metals andnonmetals? In Chapter 7 we saw what happens when solutions containing two ionic compounds react. Now we want to see what happens when a metal and a nonmetal react. When a metal and a nonmetal react, the product is often an ionic compound. For example, when solid potassium metal reacts with liquid bromine, the product is the ionic compound, potassium bromide. 2K(s) + Br 2 (l) -.. 2KBr(s) Classifying Chemical Reactions

5 By ionic, we mean that the product potassium bromide is actually composed of potassium ions (K+) and bromide ions (BO, although we write the formula as KBr. The reactant potassium atoms and the bromine molecule are both electrically neutral, The number of protons equals the number of electrons. How are the neutral species potassium and bromine converted to ions? An electron is transferred from two potassium atoms to each atom in the bromine molecule. Let's look closer at this transfer. The potassium atom has nineteen protons and nineteen electrons. The number of positive charges is counterbalanced by an equal number of negative charges. Likewise, each bromine atom has thirty-five protons and thirty-five electrons. Bromine atoms are also electrically neutral. When the reaction between two potassium atoms and a bromine molecule occurs, one electron from each potassium atom is transferred to each bromine atom. Potassium atoms have become potassium ions, because the atoms are no longer electrically neutral. Each atom is missing one electron. This means that there is an excess positive charge of 1+ for each potassium ion. We write the formula of the ion K+. Each bromine atom in the bromine molecule becomes a bromide ion, because the atoms are no longer electrically neutral. Each bromine atom has gained one electron. The bromine ion has a 1- charge, and is written Br, The reaction between potassium and bromine is an example of an electron transfer reaction. When an electron transfer reaction occurs, one substance always loses one or more electrons, and another substance always gains them. Losing and gaining electrons always happen together in the same reaction. Reactions where electrons are transferred are also called oxidation-reduction reactions. How Can We Tell How Electrons Are Transferred in a More Complex Reaction? The reaction above is an electron transfer reaction. Let's see how electrons are transferred in this example. The product CU(N0 3 ) 2 contains two N0 3 - anions, each of which has a 1- charge, for a total negative charge of 2-. For CU(N0 3 ) 2 to be neutral the copper cation must be copper(ii) or Cu 2 +. This means that each atom of Cu loses two electrons to form the Cu 2 + cation. Chapter Review 135

6 If copper is losing electrons, some other substance in this reaction must gain electrons. Before the electron transfer occurs, silver is present as the cation Ag+. After the transfer, silver is present as neutral silver atoms. The silver cation has gained electrons to become elemental silver. We can show the overall reaction more clearly by showing only reactants and products which are gaining or losing electrons. Cu(s) + 2Ag+ + 2e Cu Z+ + 2Ag(s) + 2e- 8.2 WAYS TO CLASSIFY REACTIONS Classifying reactions can help you to understand reactions you have never seen before. When you look at the equation for a new reaction, if you are able to determine what class ofreaction it is, all the information you know about that class of reaction is true about the new reaction as well. One way to classify reactions is to examine the driving forces, the things that happen that tend to make a reaction occur. They are the formation ofa solid, the formation of water and the transfer of electrons. WhatKinds ofreactions Cause Formation ofa Solid? When two aqueous solutions are mixed and a solid forms, the reaction is called a precipitation reaction. For example, produces the solid Ag ZS04 Ionic solids produced in chemical reactions are called precipitates. When precipitation reactions occur, the two anions exchange places. This can be symbolized as AB +CD -... AD+CB Because of this double exchange, these reactions are also called double displacement reactions. To identify this kind of reaction, look for an ionic solid present as a product, but not present as a reactant. What Kinds ofreactions Cause Production ofwater? So far, you have studied one type of reaction which produces water, the reaction between strong acids and strong bases. For example, NaOH(aq) + HN0 3(aq)-'" NaNOiaq) + Hz0(l) Classifying Chemical Reactions

7 When any strong acid reacts with any strong base, the product is always water. These are called acid-base reactions. To identify this kind of reaction, look for water as a product, and a strong acid and a strong base as reactants. What Kinds ofreactions Are Electron Transfer Reactions? A third driving force we have encountered is electron transfer. An example reaction is 2K(s) + Bril)... 2KBr(s) where each K loses one electron to become K+ and each bromine atom gains an electron to become Br. Electron transfer reactions are called oxidation-reduction reactions. Some electron transfer reactions can be identified by comparing reactants and products. If a reactant is present as a neutral element or as a diatomic molecule, but the product contains ions of that element, then an electron transfer has occurred. Also, if ions are present in the reactants, but the product of that element is present either as the neutral element or as a diatomic molecule, then electron transfer has taken place. 8.3 OTHER WAYS TO CLASSIFY REACTIONS There are many categories of reaction besides precipitation, acid-base, and oxidation-reduction. Fortunately, most ofthe other categories are special cases of one of the three main types. Combustion Reactions Some reactions have oxygen gas as a reactant, and produce heat as a product. combustion reactions. An example is These are the the combustion of ethyl alcohol, a fuel additive present in gasohol. This reaction appears to be a new type, one we have not classified before. Many reactions which involve oxygen gas as a reactant are examples of oxidation-reduction reactions, although at this point in the course, it is hard to show why this is true. Combustion reactions are a kind ofoxidation-reduction reaction. Synthesis Reactions This name is a little misleading because every reaction synthesizes (makes) something. Synthesis reactions usually mean making a more complex molecule from simpler ones. It often means making a compound from individual elements. For example, in Chapter Review 137

8 molecular hydrogen gas and molecular chlorine gas react to produce the compound hydrogen chloride. Ifwe look closely, hydrogen atoms lose an electron and chlorine atoms gain an electron. Synthesis reactions are a type ofoxidation-reductionreaction. Decomposition Reactions Decomposition reactions usually refer to the breaking down of a compound into simpler (often elemental) components. Hydrogen peroxide can be broken down into molecular hydrogen and molecularoxygen. How Can You Determine the Classification ofa Reaction? The summary below can help you classify the reactions we have covered so far. Let's try an example. In p'tassiumchlorate reacts to produce potassium chloride and oxygen gas. 1. When two water soluble ionic compounds react to produce a solid ionic compound, the reaction is probably a precipitation reaction. Solid potassium chloride is formed as a product, but the reactant is a solid too, not in an aqueous solution. There is only one reactant, and two water soluble reactants are needed in a precipitation reaction. This is not a precipitation reaction. 2. If water is a product, and one of the reactants is an acid and the other reactant is a base, then the reaction is an acid-base reaction. In this reaction water is not formed, so the reaction is not an acid-base reaction Classifying Chemical Reactions

9 3. If the reaction has oxygen as a reactant, then the reaction is most likely a combustion reaction. Combustion reactions are a sub-class of oxidation-reduction reactions. In this reaction, there is 110 oxygen gas on the reactant side, so this is not a combustion reaction. 4. If the reaction has elements or small molecules as reactants and produces larger compounds, then the reaction is a synthesis reaction. Synthesis reactions are a sub-class of oxidation-reduction reaction. In this reaction larger compounds are not products, so this is not a synthesis reaction. 5. If the reaction has a large compound as reactant and the products are smaller compounds or elements, then the reaction is a decomposition reaction. In this reaction, the reactant is larger than the products. One of the products is molecular oxygen gas; the other one is the small ionic compound, potassium chloride. This is an example ofa decomposition reaction. LEARNING REVIEW 1. How many electrons do the elements below either gain or lose when they form ions? For example, potassium atoms lose one electron to form potassium ions. K... K++ea. Br 2 d. Al b. Mg e. 2 c. n, f. S 2. Write an equation showing how the ions below gain or lose electrons to form atoms or diatomic molecules. For example, a sodium ion gains one electron to form an atom of sodium. Na++e-... Na a. 2Cr d. Ca 2+ b. K+ e. 2r c. p3- f. Ae+ Learning Review 139

10 3. For each reaction below, write equations showing how each element or ion gains or loses electrons. a. Cu(s) + 2AgN0 3(aq) 2Ag(s) + Cu(N03)iaq) b. 2HCI(aq) + Zn(s) Hz(g) + ZnCliaq) c. 2NaBr(aq) + Clz(g) 2NaCI(aq) + Brz(g) 4. Classify the reactions below as a precipitation reaction, an acid-base reaction, or an oxidation-reduction reaction. a. 2NaCI(s) + Brz(l)... 2NaBr(s) + Clig) b. Nazsoiaq) + Pb(N03)laq) PbSOis) + 2NaN0 3(aq) c. 2NaOH(aq) + HzSOiaq) 2Hz0(l) + NazSOiaq) d. 2AgN0 3(aq) + Fe(s)... Fe(N0 3)z(aq) + 2Ag(s) 5. Classify the reactions below as combustion, synthesis or decomposition reactions. a. Nz(g) + 3H z (g)... 2NH 3 (g) b. C;H 16 (g) + 110z(g) 7CO z(g) + 8H zo(g) c. 16Cu(s) + Sg(s) 8Cu zs(s) d. 2NaN0 3(s)... 2NaN0z{s) + 0z(g) 6. Write balanced equations for each of the word descriptions. Classify each reaction as precipitation, oxidation-reduction, or acid-base. a. Ethyl alcohol, a gasoline additive, bums in the presence of oxygen gas to produce carbon dioxide and water vapor. b. Aqueous solutions of ammonium sulfide and lead nitrate are mixed to produce solid lead sulfide and aqueous ammonium nitrate. c. Sodium metal reacts with liquid water to produce aqueous sodium hydroxide and hydrogen gas. d. Aqueous solutions of sodium phosphate and silver nitrate are mixed to produce solid silver phosphate and aqueous sodium nitrate Classifying Chemical Reactions