Acid-Base Properties of Salt A salt is simply another name for
Solutions and Acid-Base an ionic compound.
Strength Remember, that most “salts” are
strong electrolytes that completely
dissociate in solution.
By dissociation, salts can
sometimes affect pH by increasing H+
or OH- concentrations, donating or
accepting a proton, in side reactions.
The reactions of ions with water Salts that form Neutral Solutions
are frequently called hydrolysis We know from the Bronsted-
reactions. Lowry theory that conjugate bases of
strong acids have no affinity for
For Example: protons in water.
NaF(s) → Na+(aq) + F-(aq) Example:
No apparent change in pH; However, HNO3 + H2O → H3O+ + NO3-
If NO3- had any affinity for H+,
F- + H2O ⇄ HF + OH- then this reaction would be an
By this logic, we should Salts That Form Basic Solutions
understand that the addition of salts Notice what happens when we put
containing cations of strong bases sodium acetate in solution
and anions of strong acids have no
NaCH3CO2 → Na+ + CH3CO2-
effect on the pH of a solution.
CH3CO2- + H2O ⇄ CH3CO2H + OH-
AgNO3 → Ag+ + NO3-
What happed to the pH when the
sodium acetate was added?
In this situation, the acetate anion is 1. What is the pH of a solution made
acting as a weak base. Because we by adding 3.0 g of sodium acetate to
know the Ka of the conjugate acid, make 5.0 L solution?
acetic acid, we can easily calculate
the Kb of the acetate anion from:
Notice, the solution is very basic.
Kb = Kw / Ka
= 1.0x10-14 / 1.8x10-5 = 5.6x10-10 Salts derived from a strong base and
By knowing the dissociation a weak acid create basic solutions.
constant, we can now make Examples: NaClO, LiF, etc…
calculations of the solution knowing
the initial concentration.
Salts That Form Acidic Solutions A second type of salt that produces
an acid solution is one that contains a
Observe: highly charged metal ion.
NH4Cl + H2O → NH4+ + Cl-
And then, The presence of
aluminum ions in solution
NH4 + + H O ⇄ NH + H O+ results in the formation of
2 3 3
So, we see that salts derived from a between the ion and
unshared pairs of electrons
weak base and strong acid create on the water molecules
acidic solutions These compounds are known as metal hydrates.
Because metal ions are positively Now, Look at Fe3+
charged, they attract the unshared Fe3+ + 6H2O → Fe(H2O)63+
electron pairs of water molecules.
What definition best describes
Notice, Aluminum is of the third the formation of a metal hydrate?
period and can have an expanded octet.
It has accepted 6 coordination bonds.
Al(NO3)3 → Al(H2O)63+ + NO3-
Draw the orbital notation for Al3+
When a water molecule interacts
with the positively charged metal ion,
electron density is drawn from the
oxygen causing the O-H bond to
become more polarized; as a result, Al(H2O)63+ ⇄ Al(OH)(H2O)52+ + H+
water molecules bound to the metal
ion act as a source of protons:
Al(NO3)3 + 6H2O → Al(H2O)63+ + NO3-
Al(H2O)63+ ⇄ Al(OH)(H2O)52+ + H+
From a known Ka value, the pH Metal Hydrates increase in their acid
for solutions containing hydrated ions strength as their charge increases and
can be easily calculated. atomic radius decreases
Fe3+ is a fairly strong acid compared
2. Calculate the to Al3+
pH and pOH of
Fe3+ + 6H2O → Fe(H2O)63+ Ka = 2.0x10-3
a 1.0x10-4 M
Wow, metal ions act as Acids!
Except, in most cases, Heavy Group I and
Group II metals.
Salts of binary hydrides (HS-) and
polyprotic oxyacids often have anions
that can form salts that are amphoteric.
anions of PO4 - PO43- (B-L Base)
(B-L Acid or Base)
Therefore, NaH2PO4 could be
acidic or basic depending on relative Ka
and Kb values
3. Predict whether Na2HPO4 will be 4. Determine if the following salts
acidic or basic in aqueous solution. will result in a neutral, acidic, or
basic solution when dissolved
a. Ammonium acetate
c. Iron (II) sulfate
e. Lithium phosphate
Acid and Base
Acid and Base Strength
• Strong acids are In any acid-base reaction, the
completely equilibrium will favor the reaction that
dissociated in water. moves the proton to the stronger base.
bases are quite
weak. HCl(aq) + H2O(l) → H3O+(aq) + Cl−(aq)
• Weak acids only
in water. H2O is a much stronger base than Cl−,
Their conjugate so the equilibrium lies so far to the right K
bases are is not measured (K>>1).
Acid and Base Strength It is important to remember that a
chemical species does not simply want to give
up a proton. The intermolecular forces of all
substances in the mixture must favor the
HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2−(aq)
result of a H+ being moved from one species
Acetate is a stronger base than H2O, so Factors affecting acid strength:
the equilibrium favors the left side (K<1). 1. Electronegativity
2. Bond Strength
3. Stability of the conjugate base
For any molecule X-H, the H+ can Factors Affecting Acid Strength
only be removed if X has a high
enough electronegativity to retain both • The more polar the
H–X bond and/or the
electrons across the bond. weaker the H–X bond,
the more acidic the
For hydrohalic acids, notice the compound.
difference in electronegativities: • So acidity increases
from left to right
2.1 H - F 4.0
across a row and from
2.1 H - Cl 3.0 top to bottom down a
2.1 H - Br 2.8 group.
2.1 H - I 2.5 © 2012 Pearson Education, Inc.
So: Factors Affecting Acid Strength
HCl → H+ + Cl- Hydride acids tend to all be weak
acids due to their low difference in
electronegativies in relationship to the
HF ⇄ H+ + F- hydrogen atom.
C N O F
• Very polar bond! 2.5 3.0 3.5 4.0
Si P S Cl
• Stable conjugate base The more 1.8 2.1 2.5 3.0
• What could cause HF to be a weak the stronger. Te I
Oxyacids increase in strength with the
Factors Affecting Acid Strength increasing electronegativity of the central atom
Oxyacids Strength: H3PO4 < H2SO4 < HNO3
Given the general formula: X – O – H For oxyacids with the same central atom,
acid strength increases with an increase in the
If X is a metal, it is easier to dissociate number of oxygen's
the X – O bond; therefore the compound
would be a base
If X is a non-metal, it is easier to
dissociate the O – H bond; therefore the
compound would be an acid
Factors Affecting Acid Strength Factors Affecting Acid Strength
For ployprotic acids, each Resonance in the conjugate bases of carboxylic
acids stabilizes the base and makes the
successive proton requires more conjugate acid more acidic.
energy to remove; therefore, the lower
the acid strength becomes
Strength: H1+ > H2+ > H3+ > etc…
5. Identify the stronger of the
following acid or base pairs:
a. HIO3, HBrO3
c. H2S, H3P
e. Li2O, CH3CH2CH2NH2
f. H2PO3-, HPO32-