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Anions are negative ions and some of them undergo hydrolysis when they are mixed with water. Here, we’ll examine these more closely.
Hydrolysis of
Anions
We will start by getting familiar with the anions that do NOT hydrolyze.
Anions that do NOT hydrolyze
Anions that do NOT hydrolyze, or undergo hydrolysis, are the top 5 ions on the right side of the acid table.
Anions that do NOT hydrolyze
The top 5 ions on the right side of the acid
table
So these ions are the conjugate bases of strong acids.
Anions that do NOT hydrolyze
They are the conjugate bases of strong acids
Even though these are on the base side of the table, (click) they are too weak to remove protons from any species, including water.
Anions that do NOT hydrolyze
Too weak to remove protons
The single arrows, pointing only to the right remind us that these reactions can only go in the forward direction.
Anions that do NOT hydrolyze
These reactions can ONLY go forward
For example, HClO4 will readily lose a proton to form ClO4 minus
Anions that do NOT hydrolyze
HClO4 will readily lose a proton to form ClO4
–
But ClO4 minus CANNOT gain a proton to form HClO4. A reaction with a single arrow on this table will not go in the opposite direction of the arrow.
Anions that do NOT hydrolyze
ClO4– cannot gain a
proton to form HClO4
They do NOT react with water and are always NEUTRAL in aqueous solutions
They do NOT react with water and are always
NEUTRAL in aqueous solutions
Therefore, they are called spectator ions.
They do NOT react with water and are always
NEUTRAL in aqueous solutionsThey are called
Spectator Ions
After all, it’s conjugate acid, H2SO4, is a strong acid
Why isn’t HSO4– a spectator ion?
Strong Acids
It’s because HSO4 minus also appears in this location on the left side of the table. That means it can act as a weak acid. So it is NOT a spectator ion.
So the spectator ions are ClO4 minus, I minus, Br minus, Cl minus, and NO3 minus. It would be useful to memorize these, or at least remember they are the top 5 ions on the right side of the acid table.
spectator anions
Now that we’ve seen which anions do not hydrolyze, we’ll now look at the anions that (click) do hydrolyze, or undergo hydrolysis.
Anions that Do hydrolyze
In this video, we’ll deal only with anions that are not amphiprotic. In other words, their formulas do not begin with an “H”. They are shown here. We’ll deal with hydrolysis of amphiprotic anions in another video
Anions that Hydrolyze
Which means they all accept a proton from a water molecule.
Act as weak
bases in water
Accept a proton
from water
Anions that Hydrolyze
Here we’ll show one of these hydrolyzing anions, carbonate, CO3 2 minus (click) and a water molecule
–O
CO
O
–
23CO
H
HO
2H O
H
This hydrogen atom will donate one of its shared electrons to the oxygen atom in the water molecule (click) and it will change to an H+ ion, or proton.
–O
CO
O
–
23CO
HO
2H O
H+
H+
Because this oxygen atom has gained an electron, it will (click) acquire a negative charge
–O
CO
O
–
23CO
HO
2H O
–
This proton (click) is now transferred to the carbonate ion as the water changes to an OH minus, or hydroxide ion.
–O
CO–
23CO
HO
2H O
–H+O
OH
An electron from this oxygen will move toward the proton, so this (click) negative charge disappears as the proton changes into a hydrogen atom
OCO–
23CO
HO–H+
–HO
3HCOOH
Now, we can write the equation for the hydrolysis of the carbonate ion, It’s (click) CO3 2 minus
23 2 3CO H O HCO OH
+ +
Hydrolysis of CO32–
Basically, this is just (click) a proton transfer from a water molecule to the carbonate ion,
23 2 3CO H O HCO OH
+ +
Hydrolysis of CO32–
H+
Looking on the right side of the acid table, we can find a few other non-amphiprotic anions that we can write hydrolysis equations for.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
A few anions that are not amphiprotic
None of these are able to donate a proton to water. They act only as bases, not acids.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
None of these can donate a
proton
So all of these can undergo base hydrolysis
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
All of these undergo base
hydrolysis
We’ll now write hydrolysis equations for these (click) starting with the nitrite ion, NO2 minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
We write down NO2 minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Because its hydrolysis, we add water.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
NO2 minus is a weak base, so it takes a proton from water,
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
weak base
Forming its conjugate acid, HNO2,
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
And a hydroxide ion.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Next, we’ll do the hydrolysis equation for F minus,
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Being a weak base, F minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Reacts with water by taking a proton from it,
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Producing its conjugate acid, HF,
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
And a hydroxide ion.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Now, we’ll write an equation for the hydrolysis of the benzoate ion, C6H5COO minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
We’ll start by writing C6H5COO minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Being a weak base, this also reacts with water and takes a proton from it.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Forming its conjugate acid, C6H5COOH
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
And a hydroxide ion.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Now we’ll do the oxalate ion, C2O4 2 minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
C2O4 2 minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
reacts with water, removing just one proton from it. In equations for hydrolysis reactions, we always remove only one proton at a time.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
the oxalate ion gains a proton to form the binoxalate or hydrogen oxalate ion, HC2O4 minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
and water loses a proton to form a hydroxide ion.
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
the last equation we’ll do is the hydrolysis equation for the acetate or ethanoate ion, CH3COO minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
CH3COO minus
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
Hydrolyzes by removing a proton from water,
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
To form its conjugate acid, CH3COOH
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
And a hydroxide ion
2 2 2NO H O HNO OH
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O H O HC O OH
3 2 3CH COO H O CH COOH OH
The anions here all undergo base hydrolysis, producing hydroxide ions. The formation of hydroxide ions in a solution, tends to make the solution more basic.
2 2 2NO H O HN OHO
Hydrolysis of Some Anions
2F H O HF OH
6 5 2 6 5C H COO H O C H COOH OH 2
2 4 2 2 4C O HO OH HC O
3 2 3CH COO H O CH COOH OH
The anions here all undergo base hydrolysis, producing
hydroxide ions.