Double Replacement Rxn

Chemistry I

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A closer look at  Double Replacement Reactions

• Instead of FIGHTING (Single Replacement)• Double Rep. Are all about LOVE• Evidence of the DR is either:

– A precipitate forming– an acid/base (two ionic) reaction forming covalent

water molecules• neutralization

• If neither happens, No Reaction

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Aqueous Solutions• Many times, the chemicals we are reacting

together are dissolved in water.– Mixtures of a chemical dissolved in water are

called aqueous solutions.• The chemicals must first be dissolved in water

– Compounds separate into free ions.

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• When some ionic compounds dissolve in water, The anions and cations are separated from each other.

DissociationBecause they are more attracted to polar water molecules.

• NaCl(s) Na+(aq) +Cl- (aq) • Ions are FREE to move throughout the solution.

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• When compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion.

• AgNO3 (s) Ag+ (aq) + NO3-(aq)

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Dissociation, Continued• Sodium chloride dissociates in water

NaCl(s) → Na+1(aq) + Cl-1(aq)

Silver nitrate dissociates in waterAgNO3(s) → Ag+1(aq) + NO3

-1(aq)

Na+1 Cl-1Na Cl

Ag+1NO3

-1Ag NO3

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DR Reactions

• Occurs if A cation and anion

combine to form an insoluble precipitate.

If no precipitate forms, then NR (no reaction).

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Why did a precipitate form?• Ions in AgCl are more attracted to each other than

to the polar water molecules. • Fall madly deeply in love, wrap their arms around

each other, and drop out of the solution as a solid.

AgCl(s)

What about the other couple, NaNO3?

• The ions of NaNO3 are more attracted to the polar water molecules and stay separate (dissociated)

• Soluble product is shown as (aq)• Na+ (aq) + NO3

- (aq)

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So, how do you know if a new product is (s) or (aq)?

• Solubility Rules

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Gross equation: NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)

Net equation: Cl- (aq) + Ag+ (aq) AgCl (s)

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Solubility of Ionic SaltsGeneral Rules

 1.    Sodium (Na+1), potassium (K+1), and ammonium (NH4

+1)

compounds are soluble in water. 2. Nitrates (NO3

-1), acetates (C2H3O2-1), and chlorates (ClO3

-1)

are soluble. 3. Chlorides (Cl-1) are soluble except with silver (Ag+1), mercury(I) (Hg2

+2), and lead(II) (Pb+2).

[Lead(II) chloride is soluble in hot water.] 4. Sulfates (SO4

-2) are soluble except with calcium (Ca+2),

barium (Ba+2), strontium (Sr+2), and lead(II) (Pb+2). 5. Carbonates (CO3

-2), phosphates(PO4-3), and

silicates (SiO3-2) are insoluble except with sodium (Na+1),

potassium (K+1), and ammonium (NH4+1).

 6. Sulfides (S-2) are insoluble except with calcium (Ca+2), barium (Ba+2), strontium (Sr+2), magnesium (Mg+2), sodium (Na+1), potassium (K+1), and ammonium (NH4

+1).

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Solubility Patterns…

Generally Soluble Generally InsolubleCations:

NH4+, Li+, Na+, K+

Cations:

Ag+, Pb2+, Hg2+

Anions:

Cl-, Br-, I-, NO3-, ClO3

-, C2H3O2-

Anions:OH-, S2-, CO3

2-, PO43-, CrO4

2-

Determine whether each of the salts is soluble or insoluble:

• AgBr• CaCl2

• Pb(NO3)2

• PbSO4

• CuS• Fe2(SO4 )3

• PbCO3

• NH4Cl

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• AgBr insoluble• CaCl2

soluble

• Pb(NO3)2 soluble

• PbSO4 insoluble

• CuS insoluble• Fe2(SO4 )3 soluble

• PbCO3 insoluble

• NH4Cl soluble

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Table of Solubilities of Inorganic Compounds in Wateraq - solubleppt - insoluble

ace

tate

bro

mid

e

carb

on

ate

chlo

rid

e

chro

ma

te

hyd

roxi

de

iod

ide

nitr

ate

ph

osp

ha

te

sulfa

te

sulfi

de

chlo

rate

aluminum aq aq ppt aq ppt ppt aq aq ppt aq aq Al +3

ammonium aq aq aq aq aq aq aq aq aq aq aq aq NH4+1

barium aq aq ppt aq ppt aq aq aq ppt ppt aq Ba+2

calcium aq aq ppt aq aq ppt aq aq ppt ppt aq Ca+2

copper(I) aq aq ppt aq ppt ppt aq aq ppt aq ppt aq Cu+1

iron (II) aq aq ppt aq ppt aq aq ppt aq ppt Fe+2

iron (III) aq aq aq ppt ppt aq ppt ppt Fe+3

lead (II) aq ppt ppt ppt ppt ppt ppt aq ppt ppt ppt aq Pb+2

magnesium aq aq ppt aq aq ppt aq aq ppt aq aq Mg+2

mercury (I) aq ppt ppt ppt ppt ppt aq ppt ppt ppt aq Hg+1

mercury (II) aq ppt ppt aq ppt ppt ppt aq ppt ppt aq Hg+2

potassium aq aq aq aq aq aq aq aq aq aq aq aq K+1

silver aq ppt ppt ppt ppt ppt aq ppt ppt ppt aq Ag+1

sodium aq aq aq aq aq aq aq aq aq aq aq aq Na+1

zinc aq aq aq aq aq ppt aq aq ppt aq ppt aq Zn+2

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Precipitation Reactions, Continued2 KI(aq) + Pb(NO3)2(aq) 2 KNO3(aq) + PbI2(s)

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DR Precipitation Reaction2 KI(aq) + Pb(NO3)2(aq) 2 KNO3(aq) + PbI2(s)

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No Precipitate Formation = No Reaction

KI(aq) + NaCl(aq) KCl(aq) + NaI(aq)All ions still present, no reaction.

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Process for Predicting the Products ofa Precipitation Reaction

1. Write the formula for the reactants.2. Exchange ions. (Trade dance partners)

(+) ion combines with (-) ion3. Criss-cross ionic charges of new dance couples to get

formulas. (Don’t worry about balancing the equation yet.)4. Use the solubility rules to determine solubility of each product

in water.– If product is insoluble or slightly soluble, it will precipitate.

Write (s) behind it. – If the product is soluble, write (aq) behind it. – If neither product will precipitate, no reaction.

• 5. Eliminate the spectator ions (those that are (aq) on both sides)

• 6. Write a balanced net ionic equation.

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An aqueous Solution of Sodium Carbonate Is Added to an Aqueous Solution of Copper(II) Chloride, a White Solid

Forms.

1. Write the formulas of the reactantsNa2CO3(aq) + CuCl2(aq)

2. Exchange the ions.Na2CO3 (aq) + CuCl2 (aq) Na+ + Cl- + Cu2+ + CO3

2-

3. Criss-Cross charges and reduce Na2CO3(aq) + CuCl2(aq) NaCl + CuCO3

• 4. Determine solubility of productsNaCl is soluble

CuCO3 is insoluble.

Na2CO3(aq) + CuCl2(aq) NaCl(aq) + CuCO3(s)

5. Na+ and Cl- are (aq) on both sides. Spectators. Eliminate.

6. Net ionic equation:CO3

2-(aq) + Cu2+(aq) CuCO3(s)

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Consider:AlCl3(aq) + AgNO3(aq)

Write the dissociation equation of each reactant:

AlCl3(s) Al+3(aq) + 3 Cl-1

(aq)

Why 3 Cl-1(aq) and NOT Cl3

-1 ?

AgNO3(s) Ag+1(aq) + NO3

-1(aq)

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One of two things can happen:

• All ions remain “free” • No reaction happens.

Two oppositely charged ions are attracted to each other strongly, bond

Form a precipitate.

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What are the products (possible new dance couples?

• Al+3 combines with NO3- : Al(NO3)3

• Ag+1 combines with Cl-1 : AgCl• Why NOT AlAg? Or NO3Cl ?

• Does one, both, or neither product form a precipitate?

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AgCl insoluble

• AgCl (s)

Al(NO3)3 soluble• Al+3(aq) + 3NO3

-1 (aq)

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Gross and Net Equations

Al+3 (aq) + 3 Cl-1(aq) + 3Ag+1(aq) + 3NO3-1(aq)

3 AgCl (s) + Al+3 (aq) +3 NO3- (aq)

Eliminate spectators.

Net Ionic equation:

Ag+(aq) + Cl_ (aq) AgCl (s)

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Pb(NO3)2 (aq) + KI (aq)

• Write the dissociation equations for each salt.• Pb+2(aq) + NO3-(aq) + K+ (aq) + I-(aq)

• Determine what the products are.PbI2 and KNO3

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Pb(NO3)2 (aq) + KI (aq)

• Which is the precipitate?PbI2 is insoluble (forms a precipitate)

• Gross Equation:• Pb+2(aq) + NO3-(aq) + K+ (aq) + I-(aq) PbI2 (s) +

KNO3 (aq)

Net Ionic Equation: Pb2+ (aq) + 2 I-(aq) PbI2 (s)

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FeBr2(aq) + AgNO3(aq)  

• Dissociation of reactants?• Fe+2

(aq) + 2 Br –1(aq) + Ag+1

(aq) + NO3 -1

(aq)

• New Dance Couples?Fe (NO3)2 and AgBr

• Check table for precipitateFe(NO3)2 is soluble or (aq)

AgBr is insoluble or (s)

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• Fe+2(aq) + 2 Br –1

(aq) + Ag+1(aq) + NO3

-1(aq)

Fe+2(aq) + NO3

-1(aq) + AgBr (s)

Net ionic Equation: Ag+1(aq) + Br-1(aq) 2AgBr (s)