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15 Types of Chemical Reactions

PRE-LAB DISCUSSION

There are many kinds of chemical reactions and several ways of classifying them. Onepopular and useful method is to classify reactions into four major types. These are: (1)direct combination, or synthesis; (2) decomposition, or analysis; (3) single replacement;and (4) exchange of ions, or double replacement. Not all chemical reactions can be put intoone of these categories. However, many can.

In a synthesis reaction, two or more substances (elements or compounds) combine toform a more complex substance. Equations for synthesis reactions have the general formA + B -+ AB. An example of such a reaction is the formation of water from its constituentelements: 2H2 + O2 -+ 2H20.

A decomposition reaction is exactly the opposite of a synthesis reaction. In adecomposition reaction, a compound breaks down into two or more simpler substances(elements or simpler compounds). The general form of the equation for a decompositionreaction is AB -+ A + B. The breakdown of water into its elements is an example of such areaction: 2H20 -+ 2H2 + O2.

In a single replacement reaction, one substance in a compound is replaced by another,more active, substance (an element). Equations for single replacement reactions have twogeneral forms. In reactions in which one metal replaces another metal, the general equationis X + YB -+ XB + Y. In reactions in which one nonmetal replaces another nonmetal, the

,----,' general form is X + A Y -+ AX + Y. The following equations illustrate each of these typesof single replacement reactions:

Zinc metal replaces copper (II) ion:

Zn(s) + CuS04(aq) -+ ZnS04(aq) + Cu(s)Chlorine (a nonmetal) replaces bromide ions:

Cb(g) + 2KBr(aq) -+ 2KCI(aq) + Br2(l)In a double replacement reaction, the metal ions of two different ionic compounds can

be thought of as "replacing one another." Equations for this type of reaction have thegeneral equation AB + CD -+ AD + CB. Most 'replacement' reactions (both single anddouble) take place in aqueous solutions containing free ions. In a double replacementreaction, one of the products formed is a precipitate, an insoluble gas, or water.

All of the types of reactions discussed here may be represented by balanced molecularequations. Reactions involving ion exchanges may also be represented by ionic equations.In this investigation you will be concerned only with molecular formulas and equations. Ina balanced equation, the number of atoms of any given element must be the same on bothsides of the equation. Multiplying the coefficient times the subscript of an element mustyield the same product on both sides of the balanced equation.

In this experiment you will observe examples of the four types of chemical reactionsdescribed here. You will be expected to balance the equations representing the observedequations.

PURPOSE

Observe some chemical reactions and identify reactants and products of thosereactions. Classify the reactions and write. balanced equations.

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EQUIPMENTburnercrucible tongsmicrospatulatest tubes, 15x180-mm (7)test tube holdertest tube rackwood splintssandpaper, fineevaporating dishsafety glasses

MA TERIALSzinc, mossy (Zn)copper wire, 10 cm (Cu)magnesium ribbon, 5 cm (Mg)copper (II) carbonate (CuC03)6 Mhydrochloric acid (HCI)1 M copper (II) sulfate (CUS04)0.1 M zinc acetate (Zn(CzH30z)z)

0.1 M sodium phosphate (Na3P04)1M sodium sulfite (Na2S03)

PROCEDURE ~ ~A. Synthesis1. Use fine sandpaper to clean a piece of copper wire until the wire

is shiny. Examine the wire and note its appearance.Using crucible tongs, hold the wire in the hottest part of theburner flame for 1-2 minutes. Examine the wire and note anychange in its appearance caused by the heat.

3. Place an evaporating dish handy to the base of the burner.Examine 'a piece of magnesium ribbon. Using crucible tongs,hold the sample in the burner flame until the magnesium startsto burn. DO NOT LOOK DIRECTL Y AT THE FLAME.HOLD THE BURNING MAGNESIUM AWAY FROM YOUAND DIRECTLY OVER THE EVAPORA TING DISH. Whenthe ribbon stops burning, put what is left of it in the evaporatingdish. Examine this product thoroughly.

B. Decomposition4. Place 2 heaping microspatulas of copper (II) carbonate (CuC03)

in a clean, dry test tube. Note the appearance of the sample.5. Using a test tube holder, heat the CuC03 strongly in a burner

:','" flame for about 3 minutes. Extinguish the flame and insert a

"~ burning wood splint in the test tube to test for the presence of~ carbon dioxide gas. (C02 will put the flame out.) Note anychange in the appearance of the material in the test tube.

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EXPERIMENT 15. TYPES OF CHEMICAL REACTIONS (CONTINUED)

C. Single Replacement

~6. Stand a clean, dry test tube in a test tube rack. Add about 5 mL of

6 M hydrochloric acid (HCI) to the tube. CAUTION: Handle~IT" acids with care. They can cause painful burns. Carefully drop a

small piece of zinc metal (Zn) into the acid in the test tube.Observe and record what happens.

7. Using a test tube holder, invert a second test tube over the mouthof the test tube in which the reaction is taking place. (See Figure15-1.) Remove the inverted tube after about 30 seconds andquickly insert a burning wood splint into the mouth of the tube.(A "pop" indicates the presence of hydrogen gas.) Note theappearance of the substance in the first test tube.

8. Add about 5 mL of 1 M copper (II) sulfate (CUS04) solution toa clean, dry test tube. Place a small piece of zinc metal in thesolution. Note the appearance of the solution and the zincbefore and after the reaction.

D. Double Replacement9. Add about 2 mL of 0.1 M zinc acetate (Zn(CzH30z)z) to a clean,

dry test tube. Next, add about 2 mL of 0.1 M sodium phosphatetribasic (Na3P04) solution to the same test tube. Observe whathappens and note any changes in the mixture.

,10. Add about 5 mL of 1 M sodium sulfite (Na2S03) solution to a:::~: clean, dry test tube. To this solution, add about 1 mL of 6 MI-~-··:('HC!. Observe the odor given off by the mixture by wafting some

of the gas toward your nose. DO NOT SMELL THE GASDIRECTLY.

OBSERV A TIONS AND DATA

Figure 15-1

SampleBefore reactionAfter reaction

A. Synthesis1. Cu2. Mg

B. Decomposition3. CuC03

C. Single Replacement

4. Zn + HCI5. Zn + CuSO.D. Double Replacement

6. Zn(C2H302h +Na3PO•.7. Na2S03 + HCI

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CALCULATIONS

Balance each of the equations by inserting the proper coefficientswhere needed. Write the names of the reactant(s) and product(s) inthe space provided below the molecular equation for the reaction.

A. Synthesis

1. Cu(s) + 02(g)- CuO(s)

2. Mg(s) + MgO(s)

B. Decomposition

3.

CuC03(s)- CuO(s) +CO2(g)

C. Single Replacement

4.

Zn(s) +HCI(aq) -ZnCb(aq) +H2(g) 1

5.

Zn(s) +CuS04(aq)-ZnS04(aq) +Cu(s)

D. Double Replacement

6.

7. HCl(aq)- NaCI(aq) + H20(l) + S02(g) 1

CONCLUSIONS AND QUESTIONS1. In this experiment, what method was used to test for the presence of CO2 gas? What is another

test for CO2 gas? Write a balanced equation for this second test.

2. What test was used to check for the presence of hydrogen gas? Write a balanced equation torepresent this test.

3. Balance the equations below and identify the type of reaction represented by each equation.a. AgN03(aq) + Cu(s)- Cu(N03h(aq) + Ag(s)b. BaCb(aq) + Na2S04(aq)- BaS04(s)! + NaCI(aq)c. CI2(g) + NaBr(aq)- NaCI(aq) + Br2i(1)d. KCI03(s)- KCI(s) + O;(g:)1

e. AICI3(aq) + NH40H(aq)- NH4c](aq) + Al(OHhCs)!f. H2(g) + 02(g)- H20 (g)

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