Experiment: OBSERVING CHEMICAL REACTIONS/WRITING CHEMICAL EQUATIONS
Introduction
The notion of chemical change is expressed by the use of balanced chemical equations. Chemical changes, or chemical reactions, are evidenced in one of several ways: evolution of heat, a color change, formation of a gas (as evidenced by the production of bubbles in solution), or formation of an insoluble solid (a precipitate.)
Simple chemical reactions can be categorized into several groups:
A single displacement reaction occurs when a free element A (usually a metal) displaces another element, B, from a compound to produce a different compound and a different free element (eq 1):
A + BY → AY + B (1)
A common example of this type of reaction is the reaction of an elemental active metal with an aqueous solution of an acid. The metal displaces hydrogen on the acid and combines with the anion of the acid, liberating elemental hydrogen gas,
A double displacement (metathesis) reaction (eq 2) occurs when two compounds exchange ions to form new compounds. In all such reactions, one of the products is a compound that will separate from the reaction mixture in some way, frequently as a solid or gas, or.as a stable molecular compound, like water.
AX + BY → AY + BX (2)
To deduce whether the products AY and BX of a double-displacement reaction are soluble or insoluble, we use a solubility table, as shown in Table 1.
Table I. Solubility Guidelines for Ionic Compounds in Water.
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Soluble Ionic Compounds
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Important Exceptions
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Compounds Containing NO3-
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None
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CH3COO-
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None
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Cl-
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Compounds of Ag+, Hg22+,Pb2+
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Br-
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Compounds of Ag+, Hg22+, Pb2+.
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1-
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Compounds of Ag+, Hg22+,Pb2+
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SO42-
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Compounds of Sr2+, Ba2+, Hg2+, Pb2+
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Insoluble Ionic Compounds
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Important Exceptions
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Compounds Containing S2-
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Compounds of NH4-, the alkali metal cations, and Ca2+', Sr2+, and Ba2+.
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CO32-
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Compounds of NH4+ and the alkali metal cations
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PO43-
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Compounds of NH4+ and the alkali metal cations
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OH-
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Compounds of the alkali metal cations, and Ca2+, Sr2+, and Ba2+, NH4+
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A neutralization reaction is a double displacement reaction between an acid and a base, An acid and a base react to form a salt and water. These reactions occur because of the formation of the stable water molecule. Neutralization reactions are accompanied by the liberation of heat.
In this experiment, you will be observing these three types of reactions.
Procedure
Make the additions indicated below, carefully noting all observations. If you are uncertain of a particular result, repeat the addition. Keep good notes on how the reagents look before and after you mix them; feel the test tube to see if any heat is evolved after combining two reagents. Look carefully for the formation of solids and gases; record what you see. In general, use all of your senses (except taste) to get information.
Use approximately 1 mL of each solution unless otherwise indicated. One mL gives about }.4 inch of solution in the bottom of the smaller test tubes in your lab drawer.
Don't rush! Give the reactions time to occur. Double displacement reactions
DD1. To 0.1 M MgCl2 add 0.1 M NaOH dropwise, mixing after each addition. Continue until an equal volume of NaOH has been added.
DD2 Quickly, add up to 2 mL of 2.0 M NaOH to an equal volume of 1.0 M H2SO4.
DD3 To 0.1 M ZnCl2, add an equal volume of 0.1 M Na2S dropwise.
DD4 To solid CaCO3, add 1.0 M HCl dropwise.
DD5 To solid CaCO3, add 1.0 M H2SO4 dropwise.
DD6 To 0.1 M CuCl2, add 0.1 M NH4OH dropwise, shaking and observing carefully what happens with the addition of each drop.
Single displacement reactions
SD1 Drop a piece of zinc metal into 3.0 M H2SO4.
SD2 Drop a piece of zinc metal into 0.1 M CuSO4.
Report
Results Section
For each reaction obsened, write out the chemical equation in word form, and then in balanced form using chemical formulas. For example. in reaction DD 1:
Aqueous magnesium chloride reacted with aqueous sodium hydroxide to form solid magnesium hydroxide and aqueous sodium chloride:
MgCl2(aq)- 2 NaOH(ag) → Mg(OH)2(s) + 2 NaCl (aq)
(FYI: The arrow in the reaction was obtained using Insert. Symbol from the pull-down menu and chosen from the Times New Roman font. Arrows subset in Word 2000)
Discussion Section
In the discussion section. include and discuss the following points:
Reiterate the categories of reactions studied in this experiment
Discuss the physical changes that indicate the occurrence of a reaction
Discuss the use of the solubility tables to help identify a product formed in a chemical reaction