Cations are positive ions and some of them undergo hydrolysis when they are
mixed with water. Here, we’ll examine these more closely.
Cations that do NOT hydrolyze
When discussing cations, it is best to start by learning which cations do NOT
hydrolyze.
Cations that do NOT hydrolyze
1
2
Cations that do not hydrolyze are the ions of group 1 (excluding hydrogen) and
group 2. These are the ions formed by the alkali metals and alkaline earth metals.
1
2
These ions do NOT
react with water and
are always NEUTRAL in
aqueous solutions
So these ions do NOT react with water and are always NEUTRAL in aqueous
solutions.
1
2
These ions do NOT
react with water and
are always NEUTRAL in
aqueous solutions
They are called
Spectator Ions
Because they have no acid or base reactions (click), they are called spectator ions. It’s actually
useful to memorize these as spectator ions, ions which do not undergo hydrolysis
Cations that Do hydrolyze
Now that we’ve seen which cations do not hydrolyze, we’ll now look at the cations
that (click) do hydrolyze, or undergo hydrolysis.
Cations
that
Hydrolyze
In chemistry 12, the cations that are recognized as hydrolyzing are these four.
Cations
that
Hydrolyze
The hexaaquoiron ion
Cations
that
Hydrolyze
With the formula Fe, H2O six, three plus
Cations
that
Hydrolyze
The hexaaquochromium ion
Cations
that
Hydrolyze
With the formula Cr, H2O six, three plus
Cations
that
Hydrolyze
The hexaaquoaluminum ion
Cations
that
Hydrolyze
With the formula Al, H2O six, three plus
Cations
that
Hydrolyze
And the ammonium ion,
Cations
that
Hydrolyze
With the formula NH4 plus
Cations
that
Hydrolyze
We’ll take a closer look at these three ions, the ones whose names start with
“hexaaquo…”
Many of the cations in the middle section of the periodic table, as shown here, will undergo acid hydrolysis
when added to water, producing hydronium ions. But in chemistry 12, we will not consider all of them…
Cations that
Hydrolyze
The three metallic ions that we will consider in chem 12 are the chromium(III) ion,
iron(III) ion and the aluminum ion. Notice these all have positive 3 charges.
You may remember from Chemistry 11, that as we go across a period from left to
right, (click) positive ions tend to get smaller.
These ions
have a large
charge and a
small size
So these three ions, which are near the center of the periodic table, have a large
charge and a small size.
These ions
have a high
charge
density
Because they have a large charge concentrated into a small size, they have what is
called a high charge density.
These ions have
a strong
attraction for
negative charges
This gives them a strong attraction for any negative charges that happen to be
nearby.
This ion has a
strong attraction
for negative
charges
We’ll have a closer look, using the chromium (III) ion as an example.
O
H
H
Just before we do that, we’ll look at a water molecule,
O
H
H
You may remember, that there is an unequal sharing of valence electrons, with shared
electrons closer to the oxygen atom than to the hydrogen atoms in a water molecule.
O
H
H
+
+
This causes the hydrogen atoms to have a partial positive charge, shown by the
greek letter delta with a plus sign.
–
O
H
H
+
+
And the oxygen atom has a partial negative charge, shown with a delta minus
–
O
H
H
+
+
In other words, water is a polar molecule with a partial negative charge on the
oxygen atom and partial positive charges on the hydrogen atoms.
The
chromium(III)
ion
Cr3+
We’ll have a look at what happens to a chromium (III) ion when it is added to water.
+
H
+
H
H O
O
H
+
–
+
–
+
H
H
O
–
Cr3+
–
+
–
O
H
H
O
H
H
+
+
+
+
–
O
+
H
H
+
The Cr 3 plus ion, attracts the negative oxygen ends of six water molecules (click)
forming this ion
+
H
+
H
H O
O
H
+
–
+
–
+
H
H
O
–
Cr3+
–
+
–
O
H
H
+
–
O
+
H
H
+
Water molecules have a net charge of zero,
+
O
H
H
+
+
Net charge
=0
+
H
+
H
H O
O
H
+
3+
–
+
–
+
H
H
O
–
Cr
Cr3+
–
+
–
O
H
H
O
H
H
+
+
+
+
–
O
+
H
H
+
so the plus 3 charge on the chromium (click) can be considered as the net charge of
the whole ion.
+
H
+
H
H O
O
H
+
3+
–
+
–
+
H
H
O
–
Cr
–
+
–
O
H
H
O
H
H
+
+
+
+
–
O
+
H
H
+
So this ion consists of a chromium atom in the center, surrounded by 6 water
molecules around the outside. The ion as a whole has a charge of 3 plus.
H
O
H
3+
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
H
ion
Cr  Hhexaaquochromium
O

2
6
3
The formula for this ion is Cr,
H
O
3+
H
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
hexaaquochromium ion
H2O in brackets, 6, for the six H2O’s
H
O
3+
H
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
hexaaquochromium ion
And the whole ion has a 3+ charge, so 3 plus is written here.
H
O
3+
H
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
The name of the ion is hexa, for the 6,
hexaaquochromium ion
H
O
3+
H
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
Aquo for the water,
hexaaquochromium ion
H
O
H
3+
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
And chromium for the Cr
hexaaquochromium ion
H
O
H
3+
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
hexaaquochromium ion
So the full name is the hexaaquochromium ion.
H
O
H
3+
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
A
Complex
Ion
H
Cr  H 2O  6
3
hexaaquochromium ion
An ion composed of a central metal atom surrounded by a number of other groups
weakly bonded to it is called a complex ion.
Many transition
metals tend to
form complex ions
Transition metals are found in the center section of the periodic table. Many of
these tend to form complex ions.
H
O
H
3+
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
A
Hydrated
Ion
H
Cr  H 2O  6
3
hexaaquochromium ion
When the groups surrounding the central atom are all water molecules, a complex
ion can also be called a hydrated ion.
H
O
H
3+
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
hexaaquochromium ion
The system for naming complex ions uses “aquo” to represent a water molecule
weakly bonded to a central atom.
Hydration
O
H
H
+
Cr3+
O
H
H
H
O
H
H O
O
H
H
H
H
H
O
H
H
+ 6H 2O
H
H
Cr
H O
H
H
O H
H
O
H
Cr
3+
O
O
3+
O
H
H
Cr  H 2O  6
3
The process of a positive ion being surrounded by water molecules and forming a hydrated ion
is called (click) hydration. This is different than hydrolysis, which we’ll look at soon.
Hydration
O
H
H
+
Cr3+
O
H
H
H
O
H
H O
O
H
H
H
H
H
O
H
H
+ 6H 2O
H
H
Cr
H O
H
H
O H
H
O
H
Cr
3+
O
O
3+
O
H
H
Cr  H 2O  6
3
The equation we can use for the hydration of the chromium(III) ion is (click) Cr 3
plus
Hydration
O
H
H
+
Cr3+
O
H
H
H
O
H
H O
O
H
H
H
H
H
O
H
H
Plus 6 H2O
+ 6H 2O
H
H
Cr
H O
H
H
O H
H
O
H
Cr
3+
O
O
3+
O
H
H
Cr  H 2O  6
3
Hydration
O
H
H
+
Cr3+
O
H
H
H
O
H
H O
O
H
H
H
H
H
O
H
H
gives
+ 6H 2O
H
H
Cr
H O
H
H
O H
H
O
H
Cr
3+
O
O
3+
O
H
H
Cr  H 2O  6
3
Hydration
O
H
H
+
Cr3+
O
H
H
H
O
H
H O
O
H
H
H
H
H
O
H
H
+ 6H 2O
Cr H2O six 3 plus
H
H
Cr
H O
H
H
O H
H
O
H
Cr
3+
O
O
3+
O
H
H
Cr  H 2O  6
3
H
O
H
3+
H
H O
O
H
H
Cr
H O
H
O H
H
O
H
Has 12 H
atoms
around
the
outside
H
Cr  H 2O  6
3
hexaaquochromium ion
Notice the hexaaquochromium ion has 12 hydrogen atoms around the outside. This
enables it to undergo acid hydrolysis
H
O
H
3+
H
H O
O
H
H
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
Remember, this ion is dissolved in water. We’ll draw one of the water molecules
here.
H
O
H
3+
H
H O
O
H
H
O
H
H
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
We’ll consider a collision between this water molecule and the hexaaquochromium
ion
H
O
H
3+
H
H O
O
H
H
H+
Cr
H O
H
H
O H
H
O
H
O
H
H
Cr  H 2O  6
3
This hydrogen atom will donate one of its shared electrons to the rest of the
hexaaquochromium ion, and (click) change into an H+ ion, or proton.
H
O
H
3+
2+
H
H O
O
H
H+
Cr
H O
H
H
O H
H
O
H
O
H
H
Cr  H 2O  6
3
Because the hexaaquochromium ion has gained an electron, it’s charge will now be
one less positive and (click) it will change from 3+ to 2+
H
O
H
2+
H
H O
O
H
H+
Cr
H O
H
H
O H
H
O
H
O
H
H
Cr  H 2O  6
3
This proton will now be transferred (click) to the water molecule on the right
H
O
H
2+
H
H O
O
H
H+
Cr
H O
H
O H
H
O
H
H
Cr  H 2O  6
3
Forming a hydronium ion.
O
H
H
H
O
H
2+
H
H O
O
H
Cr
H O
H
O
+
H
O H
H
O
H
H
H+
H
H
Cr  H 2O  6
3
The positive charge on this proton can now be considered as (click) a net positive
charge on the whole hydronium ion
H
O
H
2+
H
H O
O
H
Cr
H O
H
+
H
O H
H
O
H
H O
H
H
Cr  H 2O  6
3
The hydronium ion will now move (click) away from the complex ion.
H
O
H
2+
H
H O
O
H
H O
Cr
H O
H
H
+
H
H 3O
O H
H
O
H

H
Cr  H 2O  5  OH 
2
Now we’ll focus on what’s left of this ion, and have a look at its formula. (click) We
have one Cr atom
1
2
H
O
H
2+
H
H O
O
H
H O
Cr
3
H O
H
O H
H
O
4
H
H
+
H
H 3O

5
H
Cr  H 2O  5  OH 
2
Notice, we only have (click) 5 complete water molecules left, so (click) we write
H2O in brackets, 5.
H
O
H
2+
H
H O
O
H
1
Cr
H O
H
H
+
H
H 3O
O H
H
O
H O
H

H
Cr  H 2O  5  OH 
2
The water molecule that was here lost a hydrogen, so now it is just an OH group
H
O
H
2+
H
H O
O
H
1
Cr
H O
H
H
+
H
H 3O
O H
H
O
H O
H

H
Cr  H 2O  5  OH 
2
We have one OH group, so we write (click) one OH in the formula. Even though there is only one
OH, the system for writing formulas for complex ions still requires brackets around the OH.
H
O
H
2+
H
H O
O
H
H O
Cr
H O
H
H
+
H
H 3O
O H
H
O
H

H
Cr  H 2O  5  OH 
2
Finally the whole ion has a net charge of 2 plus, so we write (click) 2 plus here.
H
O
H
2+
H
H O
O
H
H O
Cr
H O
H
H
+
H
H 3O
O H
H
O
H

H
Cr  H 2O  5  OH 
2
pentaaquohydroxochromium ion
So this is the final formula for the ion left when the hexaaquochromium ion
donates a proton to a water molecule.
H
O
H
2+
H
H O
O
H
H O
Cr
H O
H
H
+
H
H 3O
O H
H
O
H

H
Cr  H 2O  5  OH 
2
Conjugate base of hexaaquochromium
This is the conjugate base of the hexaaquochromium ion. It is what remains when a
hexaaquochromium ion loses a proton.
H
O
H
2+
H
H O
O
H
H O
Cr
H O
H
H
+
H
H 3O
O H
H
O
H

H
Cr  H2O  5  OH 
2
pentaaquohydroxochromium ion
Because it has 5 waters and one OH group, it’s called pentaaquohydroxochromium
ion. But don’t worry, you won’t be required to name this ion in Chem 12.
+
+
Hydrolysis
Cr  H 2O  6
3

H 2O
H 3O  Cr  H 2O  5  OH 

2
So we can now write an equation which summarizes this process of hydrolysis. We
write (click) Cr H2O six 3 plus
+
+
Hydrolysis
Cr  H 2O  6
3
Plus water

H 2O
H 3O  Cr  H 2O  5  OH 

2
+
+
Hydrolysis
Cr  H 2O  6
3
Gives H3O plus

H 2O
H 3O  Cr  H 2O  5  OH 

2
+
+
Hydrolysis
Cr  H 2O  6
3

Plus Cr , H2O five, OH, 2 plus
H 2O
H 3O  Cr  H 2O  5  OH 

2
+
+
Hydrolysis
H+
Cr  H 2O  6
3

H 2O
H 3O  Cr  H 2O  5  OH 

Remember, the main thing that happens here is one proton from the
hexaaquochromium ion is transferred to a water molecule.
2
+
+
Hydrolysis
Cr  H 2O  6
3

Forming a hydronium ion
H 2O
H 3O  Cr  H 2O  5  OH 

2
+
+
Hydrolysis
Cr  H 2O  6
3

H 2O
H 3O  Cr  H 2O  5  OH 

And the conjugate base of the hexaaquochromium ion.
2
A Cation
Cr  H 2 O  6  H 2 O
3
H 3O  Cr  H 2O  5  OH 

Cation Hydrolysis
Because the hexaaquochromium ion is a cation
2
A Cation
Cr  H 2 O  6  H 2 O
3
H 3O  Cr  H 2O  5  OH 

Cation Hydrolysis
This is an example of (click) cation hydrolysis
2
Cr  H 2 O  6  H 2 O
3
H 3O  Cr  H 2O  5  OH 

2
Cation Hydrolysis
The species in this equation can be found on the acid table. (click) here is Cr H20
six 3 plus
Cr  H 2O  6  H 2O
3
H 3O  Cr  H 2O  5  OH 

Cation Hydrolysis
H plus is short for H3O plus
2
Cr  H 2O  6  H 2O
3
H 3O  Cr  H 2O  5  OH 

Cation Hydrolysis
And the conjugate base of hexaaquochromium is here, Cr, H2O five, OH, 2+
2
Fe  H 2O  6  H 2O
3
H 3O  Fe  H 2O  5  OH 

2
Cation Hydrolysis
You can see, we can also use the table to write the hydrolysis equation for the
hexaaquoiron ion.
Al  H 2O  6  H 2O
3
H 3O  Al  H 2O  5  OH 

Cation Hydrolysis
And the hexaaquoaluminum ion
2
Fe  H 2O  6  H 2O
3
Cr  H 2O  6  H 2O
3
Al  H 2O  6  H 2O
3
H 3O   Fe  H 2O  5  OH 
H 3O   Cr  H 2O  5  OH 
H 3O   Al  H 2O  5  OH 
2
2
2
The hydrolysis equations for these three hydrated ions are all similar to each
other.

NH4
 H 2O

H 3O  NH 3
Cation Hydrolysis
The fourth cation hydrolysis equation we can write using the table is (click) the
hydrolysis of the ammonium ion, NH4 plus

NH4
 H 2O

H 3O  NH 3
Cation Hydrolysis
NH4 plus, plus water gives H3O plus, plus NH3.
Cation Hydrolysis
In Chemistry 12, you’re likely to see only these four cations undergoing hydrolysis.
Fe  H 2O  6  H 2O
H 3O   Fe  H 2O  5  OH 
Cr  H 2O  6  H 2O
H 3O   Cr  H 2O  5  OH 
Al  H 2O  6  H 2O
H 3O   Al  H 2O  5  OH 
3
3
3
NH4  H 2O
Cation Hydrolysis
H 3O   NH 3
All of these hydrolysis
reactions produce
hydronium ions
All of these hydrolysis reactions produce hydronium ions,
2
2
2
Fe  H2O  6  H2O
H3O  Fe  H2O  5  OH 
Cr  H2O  6  H2O
H3O  Cr  H2O  5  OH 
Al  H2O  6  H2O
H3O  Al  H2O  5  OH 
3
3
3
NH4  H2O
Acid Hydrolysis
So they are all examples of acid hydrolysis.
H3O  NH3
All of these hydrolysis
reactions produce
hydronium ions
2
2
2
Cations that do NOT hydrolyze
1
2
And remember that group 1 and 2 cations do not undergo hydrolysis. They are
spectator ions in acid-base chemistry.