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Net Ionic Equations Tutorial
In a previous tutorial, you learned about double
replacement reactions and how to predict what
products will form.
In this tutorial, you will learn that a reaction does
not always occur when two solutions are mixed.
You will also learn how to write what are called “net
ionic equations” for the reactions that do occur.
There are three possible reasons that a double
replacement reaction might happen:
1) The mixture of ions forms something insoluble.
2) The mixture of ions forms a weak acid.
3) The mixture of ion forms a compound that
decomposes spontaneously.
Net Ionic Equations Tutorial
First let’s learn more about solubility.
These are the solubility rules:
SOLUBLE
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
INSOLUBLE
except
except
except
except
(no exceptions)
Pb2+, Ag1+, Hg22+
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
O2-, S2-, OH1-, CO32-, CrO42-, PO43-
These rules would enable you to predict, for example, that Mg(NO3)2
is soluble – since all NO31- compounds are soluble – no exceptions.
On the other hand, CaS would be insoluble, since S2- compounds are
insoluble. (There are exceptions, but Ca is not among them.)
Most Br1- compounds are soluble, but Pb2+ is an exception , so that
indicates that PbBr2 is insoluble.
Net Ionic Equations Tutorial
Before we proceed, make sure you can apply the solubility rules.
Determine whether each of the compounds below is soluble or
SOLUBLE
INSOLUBLE
insoluble:
1)
2)
3)
4)
5)
6)
7)
8)
9)
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
except
except
except
except
(no exceptions)
Pb2+, Ag1+, Hg22+
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
O2-, S2-, OH1-, CO32-, CrO42-, PO43-
Insoluble (rule #3: SO4’s are soluble, but Ba is an exception.)
BaSO4
(NH4)3PO4 Soluble (rule #1: NH4’s are soluble – no exceptions.)
Soluble (rule #2: Cl’s are soluble, Sr is not an exception.)
SrCl2
Soluble (rule #1: Group 1’s are soluble, no exceptions.)
K2CO3
Insoluble (rule #4: OH’s are insoluble, Fe is not an exception.)
Fe(OH)3
NaMnO4 Soluble (rule #1: Group 1’s are soluble, no exceptions.)
Pb(C2H3O2)2 Soluble (rule #1: C2H3O2’s are soluble, no exceptions.)
Insoluble (rule #2: I’s are soluble, but Ag is an exception.)
AgI
Ca3(BO3)2 ???? (Rule #3 mentions Ca as an exception to the SO4 solubility, but
that doesn’t necessarily apply to Ca3(BO3)2
Net Ionic Equations Tutorial
But what does it mean that KF is soluble, but AgBr is insoluble?
It means that when KF crystals are placed in water, they break apart
and the ions distribute themselves throughout the sample of water:
But an AgBr crystal does not: (at least not very much…)
K+ F- K+ F-
Ag+ Br- Ag+ Br-
F- K+ F- K+
Br- Ag+ Br- Ag+
K+ F- K+ F-
Ag+ Br- Ag+ Br-
Net Ionic Equations Tutorial
A better way to think of this: if a bunch of dissolved K+ and F- ions are
mixed together, nothing happens: they just stay dissolved – because
KF is soluble. But, when a bunch of Ag+ & Br- ions are mixed together,
most of them bond together into crystals, because AgBr is insoluble.
Here is what
that would
look like: no
reaction at all.
Here is what
that would look
like: a definite
chemical
mixture to turn reaction.
These tiny crystals reflect light and cause the
instantly cloudy. This is known as a “precipitate.”
Net Ionic Equations Tutorial
Now let’s look at the second reason a double replacement reaction
might occur: the formation of a weak acid.
An acid is a compound that has an H+ ion bonded to some negative
ion: HNO3 for example is nitric acid. HF is hydrofluoric acid.
All acids fall into one of two categories: strong acids and weak acids.
A strong acid is one that dissociates 100% in water. That is, 100% of
the molecules in solution are broken up into its ions. For example,
HNO3 breaks apart 100% into its ions: H+ and NO3-. Of all the acids
that exist, only six are strong: HCl, HBr, HI, HNO3, HClO4, H2SO4.
A weak acid is one that only dissociates this way to a small extent –
usually less than 5%.
All acids other than those six listed above are weak.
Net Ionic Equations Tutorial
But what does it mean that HNO3 is strong, but HF is weak?
It means that when HNO3 molecules are dissolved in water, they
break apart 100% like this: But when HF molecules are dissolved in
water, only a relatively small percentage of them break apart into
ions:
H
O
O NO H
-
+
O
HO N
O
+
+
-
O
O NO
-
+
F
H
HF
-
+
O
N
HO O
O
N
O OH
+
F
H
HF
F
H
-
HF
Net Ionic Equations Tutorial
Again, starting with the ions: if a bunch of dissolved H+ and NO3- ions
are mixed together, nothing happens: because HNO3 is a strong acid,
these ions stay dissociated. But when H+ & F- ions are mixed together,
most of them bond together into HF molecules, because HF is a weak
acid.
As before,
this is what
that would
look like: no
reaction at
all.
Even though it
looks like nothing
happened, this
is a chemical
reaction. Something new was formed: HF molecules. These molecules are just too
small to reflect light the way the insoluble precipitate did before.
Net Ionic Equations Tutorial
Now: the third and final cause of a double replacement reaction:
the formation of a compound that decomposes spontaneously.
This one is fairly simple: there are three specific compounds that
decompose spontaneously : H2CO3, H2SO3 and NH4OH.
Each one decomposes into water (H2O) and a specific gas.
Here are those three decomposition reactions:
H2CO3  H2O(l) + CO2(g)
H2SO3  H2O(l) + SO2(g)
NH4OH  H2O(l) + NH3(g)
When one of these specific compounds forms, you just have to
recognize it as one that undergoes this type of decomposition.
By the way, only the first one would show visible bubbling and the
CO2 gas forms. The other two don’t show bubbling: the gases are
given off at the surface of the liquid, but they do have very distinctive
odors they produce: SO2 smells like burnt matches and NH3 is
ammonia and has the pungent smell associated with Windex.
Yay Ag+!
You da best!
Go Br-!
So, how do we represent these reactions? NO Solubility
3
RULES!
These reactions are best represented with net ionic equations.
Net Ionic Equations Tutorial
Net ionic equations show just those ions that are involved
in the
K+
reaction. For example, when AgNO3 solution and KBr solution are
mixed together, the Ag+ ions react with the Br- ions to form the
insoluble compound: AgBr
The net ionic equation is simply: Ag+(aq) + Br-(aq)  AgBr(s)
Notice how the NO3- and K+ ions are left out of the net ionic equation.
Because they remained dissolved in the solution, they really don’t do
anything. Such do-nothing ions are known as “spectator ions,” and
they are left out of the net ionic equation.
The “net” weight for a bag of chips refers to just the weight of the
chips – not the bag, since the bag “doesn’t count.”
In a similar way, the “net” ionic equation refers to just the ions that
are doing something – not the spectator ions, since they don’t count.
Net Ionic Equations Tutorial
So, let’s work through some problems to see how this works:
1) Pb(NO3)2 + NaI  When we mix the positive ion from one compound with the
negative ion from the other, and vise-versa, we get what are
sometimes referred to as “cross-products.”
Here the possible cross-products are Pb with I and Na with NO3.
Let’s check whether either of these meets one of the three criteria:
something insoluble, a weak acid, or something that decomposes.
NaNO3 is not insoluble (by rule #1); it is not a weak acid (since it
doesn’t start with H, it’s not an acid at all); and it is not a decomposer.
If a reaction does happen here it will not include the Na+ or the NO3-.
So let’s check the other cross-product: the Pb with I.
We can see by rule #2 that Pb with I is insoluble, so we have found the
Strong acids
two ions that react.
(weak acids are the ones that
SOLUBLE
1)
Group I1+,
NH4
1+,
NO3
1-,
INSOLUBLE
C2H3O2
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
1-
except
except
except
except
(no exceptions)
Pb2+, Ag1+, Hg22+
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
O2-, S2-, OH1-, CO32-, CrO42-, PO43-
are not on this list)
HCl
HBr
HI
HNO3
HClO4
H2SO4
Compounds
that
decompose
H2CO3
H2SO3
NH4OH
Net Ionic Equations Tutorial
So, let’s work through some problems to see how this works:
1) Pb
Pb(NO3)2 + NaI
I  When we mix the positive ion from one compound with the
negative ion from the other, and vise-versa, we get what are
sometimes referred to as “cross-products.”
So let’s pull these two ions out of the problem with their charges.
First the Pb. Pb sometimes has a charge of 2+ and sometimes 4+, but
here we know its charge is 2+ since it took two NO3’s to balance it.
Now the I. I always has a charge of 1-. And they both start off
dissolved in water. They react to form the insoluble compound: PbI
PbI must be charge balanced: And finally, let’s balance the entire
(s) for “solid” to show that the
equation:
12+
reaction has formed crystals that
This is the
completed net
(aq) + 2 (aq)  PbI2 (s) are no longer dissolved.
ionic equation:
SOLUBLE
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
except
except
except
except
Strong acids
(weak acids are the ones that
INSOLUBLE
are not on this list)
Compounds
(no exceptions)
HCl
that
HBr
Pb2+, Ag1+, Hg22+
decompose
HI
H2CO3
HNO3
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
H2SO3
HClO4
221223O , S , OH , CO3 , CrO4 , PO4
NH4OH
H2SO4
Net Ionic Equations Tutorial
Here’s another one:
2) K3PO4 + Ag2SO4 
By the way, the order
of the reactants
doesn’t matter.
Is this what you came up with:
1+(aq) +
3-(aq)  Ag PO (s)
33PO
Ag
+ 3PO
Ag41+
3
4
4 (aq)
This answer
is just as
correct.
This time the possible cross-products are K with SO4 and Ag with PO4.
Let’s check whether either of these meets one of the three criteria:
K-SO4 is not insoluble (by rule #1); it is not a weak acid (no H); and it is
not a decomposer. So we know K and SO4 are out of the running.
We can see by rule #4 that Ag with PO4 is insoluble, so we have found
the two ions that react.
Try right now to write the balanced net ionic equation.
Did you include charges on both ions? Did you balance charges?
Did you include (s), (aq)…? And did you balance the overall equation?
SOLUBLE
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
except
except
except
except
Strong acids
(weak acids are the ones that
INSOLUBLE
are not on this list)
Compounds
(no exceptions)
HCl
that
HBr
Pb2+, Ag1+, Hg22+
decompose
HI
H2CO3
HNO3
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
H2SO3
HClO4
221223O , S , OH , CO3 , CrO4 , PO4
NH4OH
H2SO4
Net Ionic Equations Tutorial
Here’s another one:
3) HNO3 + Li3BO3 
Is this what you came up with:
3 H1+(aq) + BO33-(aq)  H3BO3(aq)
Since H3BO3
is soluble,
we keep
this as “aq”.
This time the possible cross-products are H with BO3 and Li with NO3.
Let’s check whether either of these meets one of the three criteria:
H-BO3 is not insoluble (by rule #1); but it is a weak acid (notice how it
starts with H and it’s not on the list below), so we can stop right there.
We have found the two ions that react. H and BO3
Try writing the balanced net ionic equation for this reaction.
(Hint: if you do not know the charge on BO3, look at the compound
you are given above. The number of Li’s should give away the charge.)
SOLUBLE
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
except
except
except
except
Strong acids
(weak acids are the ones that
INSOLUBLE
are not on this list)
Compounds
(no exceptions)
HCl
that
HBr
Pb2+, Ag1+, Hg22+
decompose
HI
H2CO3
HNO3
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
H2SO3
HClO4
221223O , S , OH , CO3 , CrO4 , PO4
NH4OH
H2SO4
Net Ionic Equations Tutorial
Here’s another one:
4) (NH4)2S + KBr 
NR
NR stands for NO REACTION. When neither of the cross
products meet any of the the three criteria, it is an “NR”
When everyone is sitting around spectating, nothing gets done!
This time the possible cross-products are NH4 with Br and K with S.
NH4-Br is not insoluble (by rule #1); it is not a weak acid; and it is not a
decomposer. So we know NH4 and Br are not involved in this reaction.
How about K-S? K-S is not insoluble (again, by rule #1); ); it is not a
weak acid; and it is not a decomposer.
So it looks as though none of the ions is doing anything in this problem.
So, what do we write for the balanced net ionic equation?????
SOLUBLE
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
except
except
except
except
Strong acids
(weak acids are the ones that
INSOLUBLE
are not on this list)
Compounds
(no exceptions)
HCl
that
HBr
Pb2+, Ag1+, Hg22+
decompose
HI
H2CO3
HNO3
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
H2SO3
HClO4
221223O , S , OH , CO3 , CrO4 , PO4
NH4OH
H2SO4
Net Ionic Equations Tutorial
Here’s another one:
5) MgCl2 + NaOH 
Did you remember
to include the ( )
around the OH ion?
Is this what you came up with:
Mg2+(aq) + 2 OH1-(aq)  Mg(OH)2(s)
This time the possible cross-products are Mg with OH and Na with Cl.
Let’s check whether either of these meets one of the three criteria:
Mg-OH is insoluble (by rule #4); so we can stop there.
Go ahead and write the balanced net ionic equation.
SOLUBLE
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
except
except
except
except
Strong acids
(weak acids are the ones that
INSOLUBLE
are not on this list)
Compounds
(no exceptions)
HCl
that
HBr
Pb2+, Ag1+, Hg22+
decompose
HI
H2CO3
HNO3
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
H2SO3
HClO4
221223O , S , OH , CO3 , CrO4 , PO4
NH4OH
H2SO4
Net Ionic Equations Tutorial
This is how we show a
compound forming but then
right away decomposing into
something else
Here’s another one: Is this what you came up with:
6) KOH + NH4NO3  NH 1+ (aq) + OH1-(aq)  NH OH  H O(l) + NH (g)
4
4
2
3
This time the possible cross-products are K with NO3 and NH4 with OH.
K-NO3 is not insoluble (by rule #1); it is not a weak acid (no H); and it is
not a decomposer. So we know K and NO3 are sitting this one out.
NH4-OH is not insoluble (by rule #1); it is not a weak acid (it doesn’t
start with an H); and it is not … wait a second… IT IS A DECOMPOSER.
Try right now to write the balanced net ionic equation.
SOLUBLE
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
except
except
except
except
Strong acids
(weak acids are the ones that
INSOLUBLE
are not on this list)
Compounds
(no exceptions)
HCl
that
HBr
Pb2+, Ag1+, Hg22+
decompose
HI
H2CO3
HNO3
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
H2SO3
HClO4
221223O , S , OH , CO3 , CrO4 , PO4
NH4OH
H2SO4
Net Ionic Equations Tutorial
Here’s another one:
8) K2CO3 + AlBr3 
This one was
somewhat trickier
to balance.
Is this what you came up with:
2 Al3+(aq) + 3 CO32-(aq)  Al2(CO3)3(s)
This time the possible cross-products are K with Br and Al with CO3.
K-Br is not insoluble (by rule #1); it is not a weak acid (no H); and it is
not a decomposer. So K and Br are sitting this dance out.
We can see by rule #4 that Al with CO3 is insoluble, so we have found
the two ions that react.
Try right now to write the balanced net ionic equation.
SOLUBLE
1)
Group I1+, NH41+, NO31-, C2H3O21-
2)
Cl1-, Br1-, I1-
3)
SO42-
4)
Group I1+, NH41+
except
except
except
except
Strong acids
(weak acids are the ones that
INSOLUBLE
are not on this list)
Compounds
(no exceptions)
HCl
that
HBr
Pb2+, Ag1+, Hg22+
decompose
HI
H2CO3
HNO3
Pb2+, Hg22+ Ca2+, Sr2+, Ba2+
H2SO3
HClO4
221223O , S , OH , CO3 , CrO4 , PO4
NH4OH
H2SO4