9.3
Double Replacement Reactions occur
when two compounds swap partners.
Metals replace metals ( or + charges
replace + charges)
Nonmetals replace nonmetals (or –
charges replace – charges)
These are also called precipitation or double
displacement reactions.
Double replacement reactions usually
occur in an aqueous mixture of two
ionic compounds.
Both compounds dissolve and
dissociate in the water, freeing the
ions to recombine with the other
compound’s ions.
K
F
KF and NaCl in solution
(dissolved in water)
Any positive ion could combine with any
negative ion.
Na
Cl
KF(aq)+NaCl(aq) → NaF(aq) + KCl(aq)
Basically, the Na and the K have switched
places.
Sometimes a precipitate is formed- an
insoluble (won’t dissolve) ionic compound.
AgNO3(aq) + NaCl(aq)
AgCl(s) + NaNO3(aq)
AgNO3(aq) + NaCl(aq)
NaNO3(aq) + AgCl(s)
Add the reagents to water…
AgNO3(aq) + NaCl(aq)
NaNO3(aq) + AgCl(s)
Both are soluble, so the ions
are all separated by water
molecules.
AgNO3(aq) + NaCl(aq)
NaNO3(aq) + AgCl(s)
When the Ag and Cl ions bump into each other, they
“stick” because AgCl is insoluble; this is a
precipitate. The Na and NO3 ions remain in solution.
Frequently, double replacement
reactions yield precipitates
(insoluble* compounds). But how do
you know when the products are
insoluble?
*less than 3 grams will dissolve in 100 mL water
Soluble Compounds Important Exceptions
Alkali metals
NH4+ & H+
NO3- & NO2-
C2H3O2ClO3- & ClO4-
(insoluble)
AgC2H3O2 and HgC2H3O2
are partially soluble;
record as a PPT.
Soluble Compounds Important Exceptions
(insoluble)
Binary
compounds of
Cl-, Br-, I*PbCl2 is soluble
Ag, Hg, Pb*
Soluble Compounds Important Exceptions
(insoluble)
SO42-
Ba, Ca*, Fe*,Ag*,
Pb*, Hg*, Sr*
*Partially soluble,
record as a PPT
Insoluble
Compounds
Important Exceptions
(Soluble)
sulfides
Sr and *
oxides
Ba, Sr and *
hydroxides
Ca, Sr, Ba and *
*Alkali metals, NH4+
Insoluble
Compounds
Phosphates
Carbonates
Sulfites
Chromates
Important Exceptions
(Soluble)
Alkali metals, NH4+
FYI: Table R-8 p 974
in your text has a
complete chart of
solubility rules. It
does not need to go
on your “best friend”.
Ionic Equations
For reactions taking place in
solution, an ionic equation can be
written. In an ionic equation, the
soluble reactants and products are
shown as ions.
Molecular equation
Cd(NO3)2(aq)+Na2S(aq)→CdS(s)+2NaNO3(aq)
Written as ions:
Cd2+(aq) +2 NO3-(aq)+ 2 Na+ + S2-(aq)→
CdS(s)+2 Na+ + 2 NO3-(aq)
Insoluble – does not ionize
Some of the ions remain unchanged in
the reaction; these are called spectator
ions (just watching, not playing).
Cd2+(aq) +2 NO3-(aq)+ 2 Na+ + S2-(aq)→
CdS(s)+2 Na+ + 2 NO3-(aq)
These spectator ions can be omitted in
the net ionic equation; it shows only the
ions forming compounds that do not
ionize (dissociate into ions when in
solution).
Cd2+(aq) + S2-(aq)→ CdS(s)
Ionic equation – spectator ions
= net ionic equation
Remember that water is not an
ionic compound; it is a covalently
bonded compound in its liquid state.
2H+ + O2- → H2O(l)
Water does not ionize.