EXPERIMENT #6
Double Replacement Reactions
OBJECTIVES:
✹ To determine if a chemical reaction occurs when pairs of reactants are mixed
✹ To recognize electrolytes, non-electrolytes, strong and weak acids, and strong and weak bases
✹ To write balanced molecular, total ionic, and net ionic equations
BACKGROUND:
There is an extraordinarily large number of known and potential chemical reactions.  Chemists can study each and every one of these in their attempt to understand Nature.  However, the time frame for performing a study like this is almost infinite.
On the other hand, chemists can try to understand chemical reactions by searching for patterns of similarity among them.  The patterns can be used to form a classification scheme to help chemists of the
next generation to recognize these patterns.  One popular classification scheme consists of about a half dozen categories.  One of these categories is double replacement reactions.
Double replacement reactions, also called metathesis reactions, involve an exchange of parts of the reactants.  For aqueous solutions of ionic compounds the parts exchanging are anions and cations.  The precipitation of silver chloride by reacting aqueous sodium chloride with aqueous silver nitrate is a double replacement reaction.
NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)
That a reaction is occurring can be observed by the formation of a white precipitate of silver chloride.  The driving force behind the double replacement reaction is the removal of ions from solution by the formation of a solid, as in the above example, a molecular compound, or a gas.  Double replacement reactions find applications in the field of analytical chemistry. This will be illustrated in EXPERIMENTS #8 and #9.
Since the nature of a metathesis reaction is the removal of ions from solution, it is important to describe such a change with chemical equations. Three types of equations can be used: molecular equations, which describe the molecular formulas of the reactants and products; ionic equations, whic
h describe whether the reactants and products are strong electrolytes (completely dissociated) or weak or non-electrolytes (essentially undissociated molecules in solution); and net ionic equations, which illustrate only the species reacting and ignore the spectator ions.  A very common type of metathesis reaction is a precipitation reaction.  This type of reaction is easy to observe, since two solutions (clear, homogenous mixtures) form a cloudy, heterogeneous mixture.  The cloudy material is insoluble and is called the precipitate.  How do we know which of the predicted products is the precipitate?  Since there is no simple way of predicting this, solubility rules have been created; they are listed in your textbook and on page 55.  These may be used to predict the formation of precipitates.  For example, solutions of Mg(NO3)2 and K2CO3 form a precipitate when mixed.  The molecular equation describing the process is:
Mg(NO3)2(aq) + K2CO3(aq) → MgCO3(s) + 2KNO3(aq)
Solubility rules tell us that magnesium carbonate is the solid (precipitate), while potassium nitrate remains in solution.
The same reaction in total ionic form is:
Notice that potassium and nitrate ions appear on both sides of the equation.  They are spectator ions. 
By subtracting the spectator ions from both sides of the equations (as one would do in algebra class), we obtain the net ionic equation:
Mg2+(aq) + CO32-(aq) → MgCO3(s)
The net ionic equation describes why the reaction occurred: magnesium ions and carbonate ions have been removed from solution.
Can we predict whether a solution of nickel(II) sulfate will react with a solution of barium chloride?
NiSO4(aq) + BaCl2(aq) →
To find out, exchange the ions.  With suitable attention to balance of charge, you will obtain BaSO4 and NiCl2.  From the solubility rules, NiCl2 is soluble in water, but BaSO4 is not.  The reaction will, therefore, proceed according to the net ionic equation:
Ba2+(aq) + SO42-(aq) → BaSO4(s)
Now consider the possibilities when CuCl2and Al2(SO4)3are mixed in water. The only possible combination of new products is CuSO4 and AlCl3, again with attention to the proper balance of charge
s in each compound.  From the solubility rules, you should be able to conclude that both of these compounds are soluble in water.  Accordingly,
CuCl2(aq) + Al2(SO4)3(aq) →no reaction (N.R.)
Recognizing the Occurrence of a Chemical Reaction
For the current experiment, it will be important to recognize when a reaction is occurring.  Sometimes recognition is obvious; other times greater powers of observation will be required.    A reaction is occurring when a precipitate forms (solution becomes cloudy), a gas evolves (steady stream of bubbles), a color changes, an odor changes, or a molecular substance forms.  A molecular substance is water, a weak acid, or a weak base. To determine if a molecular substance forms during the course of a double replacement reaction, predict the products of the double replacement.  If one of these products is water, a weak acid, or a weak base, then a reaction has occurred.
Common gases which may form during a metathesis reaction are CO2,NH3, and SO2.  These gases form indirectly via the decomposition of their corresponding weak acids or weak bases.  (See equations below.)  Carbon dioxide can be detected easily by observing the evolution of gas bubbles.  Sulfur dioxide can be detected by holding a piece of moist blue litmus paper over the top of the reactio
n vessel.  If the paper turns red, the presence of this acidic gas is indicated.  Ammonia can be detected by holding a piece of moist red litmus paper over the top of the reaction vessel.  If the paper turns blue, the presence of this basic gas is indicated.
Formation of carbon dioxide:  H2CO3(aq) → H2O(l) + CO2(g)
Formation of sulfur dioxide: H2SO3(aq) → H2O(l) + SO2(g)
Formation of ammonia: NH4OH(aq) → NH3(g) + H2O(l)
react to doSolubility Rules
Water-soluble Salts
Na+, K+, NH4+All sodium, potassium, and ammonium salts are soluble.
NO3-, ClO3-, C2H3O2-All nitrates, chlorates, and acetates are soluble.
Cl-All chlorides are soluble except AgCl, Hg2Cl2, PbCl2 and HgCl2.
Br-All bromides are soluble except AgBr, Hg2Br2, PbBr2, and HgBr2.
I-All iodides are soluble except AgI, Hg2I2, PbI2, and HgI2.
SO42-All sulfates are soluble except CaSO4, SrSO4, BaSO4, Hg2SO4, PbSO4,
and Ag2SO4
Water-insoluble Salts
CO32-, SO32-, PO43-, CrO42-All carbonates, sulfites, phosphates, and chromates are insoluble except those of alkali metals and NH4+.
OH-All hydroxides are insoluble except those of alkali metals and Ca(OH)2,
Sr(OH)2, Ba(OH)2, and NH4OH.
S2-All sulfides are insoluble except those of the alkali metals, alkaline
earths, and NH4+.
Strong Electrolytes
Salts All common soluble salts.
Acids HClO4, HCl, HBr, HI, HNO3, and H2SO4are strong electrolytes and
strong acids.  All others are weak electrolytes and weak acids.
Bases Alkali metal hydroxides, Mg(OH)2, Ca(OH)2, Sr(OH)2, and Ba(OH)2are
strong electrolytes and strong bases.  All others are weak electrolytes and
weak bases.
PROCEDURE:
The DATA AND OBSERVATIONS sheet lists ten pairs of chemicals that are to be mixed.  Use about 1 mL of 0.1 M solutions of each of the reagents.  For hydrochloric acid or sulfuric acid the concentrations used are 1.0 M or 6.0 M.Mix the solutions in small test tubes and record your observations. If there is no reaction, write N.R. Thoroughly rinse test tubes with deionized water before proceeding to the next pair of reagents.  Do not put dropping pipets on the bench top; return them directly to the reagent bottle and screw the cap shut.For each pair of reactants include the following: correctly written formulas, observations, correctly written formulas of products, and the molecular, total ionic, and net ionic equations.  No credit for equations can be given if the formula of any reactant or product is writte
n incorrectly.
NAME________________________________ Section_______ Date__________ DATA AND OBSERVATIONS:Double  Replacement Reactions
Complete the following tables by writing formulas of reactants and observations during the lab period and balancing the equations at a later time. Try to complete one table and show to your instructor before leaving the laboratory.

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