Double Displacement Rxns

• 2 soluble ionic compounds trade cations (+ve ions)

• General EquationAB + CD CB +AD

• 3 possible products– Precipitate– Gas– Water

Will a precipitate form?

• If the product is insoluble in water , a precipitate will form (the compound is solid).

• If the product is soluble in water, no precipitate forms (the compound is aqueous)

• How do we know which compounds are soluble?– Use a solubility table

Rule ExceptionNitrates (NO3

-) are soluble None

Halides (Cl- , Br- , I-) are soluble

Ag+, Hg2+, Pb2+

Sulfates (SO42-) are soluble Ca2+, Ba2+, Pb2+, Hg2+, Ag+

Sulfides (S2-) are insoluble NH4+ and ions of groups 1

and 2Carbonates (CO3

2-)are insoluble

NH4+ and ions of group 1

Phosphates (PO43-)are

insolubleNH4

+ and ions of group 1

Hydroxides (OH-)are insoluble

Ca2+, Ba2+, Sr2+, and ions of group 1

Example• Aqueous solution of lead(II) nitrate is added an

aqueous solution of potassium iodide. Predict the products, balance the chemical equation and predict the solubility of the products.

• ProductsPb2+ NO3-

K+ I-

Example Con’t• Skeleton Equation

Pb (NO3)2 + KI PbI2 + KNO3

• Balanced Equation:

• Add states to the equation:• Reactants are both aqueous• Go to the table– Lead (II) iodide– Potassium nitrate

Insoluble due to lead(II) ion (precipitate forms)

Soluble – all nitrates are soluble

Determine the products, balance and predict state of the products

• Silver(I) nitrate (aq) + potassium sulfate (aq)

• Li2CO3 (aq) + NaCl (aq)

• Mercury(II) nitrate(aq) + Lithium bromide(aq)

Double Displacement Rxns Producing Gases

• 3 situations where gases are produced:1.Carbonates react with acids2.Sulfites react with acids3.Ammonium salts react with bases

• In all cases the product containing the carbonate, sulfite or ammonium ion breaks down into H2O and a gas.

Examples

• CaCO3(s) + HCl(aq) CaCl2(aq) + H2CO3(aq)

CO2(g) + H2O(l)

• The final equation is:

CaCO3(s) + HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

Decomposes

Examples Con’t

• NH4Cl(aq) + NaOH(aq) NaCl(aq) + NH4OH(aq)

NH3(g) + H2O(l)

• The final equation is:

NH4Cl(aq) + NaOH(aq) NaCl(aq) + NH3(g) + H2O(l)

Decomposes

Examples Con’t

• Na2SO3(s) + HCl(aq)

+ H2O(l)

• The final equation is:

• Na2SO3(s) + HCl(aq)

Decomposes

How do we know when a gas will form?

• If one of the products is:• H2CO3 (ag) CO2(g) + H2O(l)

• H2SO3(aq) SO2(g) + H2O(l)

• NH4OH(aq) NH3(g) + H2O(l)

= decomposes into

Remember, we still need to show the other product of the double displacement rxn.

Practice: Determine the products, states and balance the equation

• Na2CO3 (aq) + HBr(aq)

Neutralization Reactions

• Double displacement rxn involving an acid and a base as reactants

• Products are a salt and water• Eg. HNO3(aq) + KOH(aq) KNO3(aq) + H2O(l)

For the following neutralization rxn: Predict the products, states and

balance the equation

HCl(aq) + NaOH(aq)

H2SO4(aq) + CaOH(aq)

Wrap-up of Double Displacement Rxns

Remember!!!• Double displacement rxns only happen when:– A precipitate is formed– A gas is formed– Water is formed

• In all other cases, the ions stay in solution!!!– No rxn happens!!!

Animations for Double Displacement Rxns

• http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/clm2s3_4.swf

• http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/crm3s2_3.swf

Net Ionic Equations

• Used when a precipitate is formed in a double displacement rxn.

• Soluble ionic compounds are present in solution as aqueous ions.

disolves in water

• NaCl(s) Na+ + Cl-

disolves in water

• K2SO4(s) 2K+ + SO42-

Why use a Net Ionic Equation• It is the ions that will join to create the

precipitate, by looking at the ions we get a better idea of what is happening in the solution.

Eg. For the reaction:K2CO3 (aq) + BaCl2 (aq)

What are the potential products? Are they soluble?

Total Ionic Equations

• K2CO3(aq) + BaCl2(aq) 2KCl(aq) + BaCO3(s)

• KCl soluble From a • BaCO3 insoluble table of solubility

• We can write the equation using ions for all aqueous compounds

• K2CO3(aq) + BaCl2(aq) 2KCl(aq) + BaCO3(s)

Becomes,2K+

(aq) + CO32-

(aq) + Ba2+ (aq) + 2Cl-

(aq) 2K+ (aq) + 2Cl-

(aq) + BaCO3(s)

From Total Net Ionic Equations

• Total ionic equation – contains all ions/compounds2K+

(aq) + CO32-

(aq) + Ba2+ (aq) + 2Cl-

(aq) 2K+ (aq) + 2Cl-

(aq) + BaCO3(s)

• The potassium and chloride ions we unchanged by the rxn (in red)

• Because they do not take part in the rxn, we call them spectator ions

• If we show only the ions involved in the rxn, we get a net ionic equation

Net Ionic Equations

• Total ionic equation – contains all ions/compounds2K+

(aq) + CO32-

(aq) + Ba2+ (aq) + 2Cl-

(aq) 2K+ (aq) + 2Cl-

(aq) + BaCO3(s)

• Net ionic equation – contains only ions involved in the reaction and the solid they form.

• For the above reaction the net ionic equation is:

CO32-

(aq) + Ba2+ (aq) BaCO3(s)

Down side to the Net Ionic Equation

• Do not show what aqueous were mixedEx. CO3

2- (aq) + Ba2+

(aq) BaCO3(s)

• We don’t know which barium compound and which carbonate compound were mixed.

• We do know that any reaction involving the combination of barium ions and carbonate ions will produce a precipitate! Try one out.

Rules for Net Ionic Equations1. Solutions of soluble salts

(KCl, NaNO3 etc.) All written as ions2. Solutions of strong acids

(HCl, HNO3, H2SO4 and HClO4) 3. Solutions of strong bases (NaOH, KOH, Ba(OH)2)

4. All covalent compounds left in molecular form, states are written beside them.– Including all gases, liquids (eg. H2O) and soluble covalent

compounds (eg. ethanol)

Steps for Writing a Net Ionic Equation

1. Identify Type of Reaction and Possible Products

2. Look Up Solubility of Both Products3. Indicate States of Reactants and Products4. Write Chemical Equation for Reaction5. Balance Equation6. Write Total Ionic Equation7. Write Net Ionic Equation

Ex. Write the total ionic equation and net ionic equation for the reaction of barium sulfide with sodium sulfate.