# Weak Acid Base Equilibrium - Lesson 5 - Salts _ Acid-Base Strength

Document Sample

```					   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
equilibrium reaction.

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.
And then,
i.e.
CH3CO2- + H2O ⇄ CH3CO2H + OH-
AgNO3 → Ag+ + NO3-
What happed to the pH when the

1
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.
i.e. Al3+
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
coordination bonds
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+

2
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
AlCl3 solution.
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.
Example:
anions of PO4 - PO43- (B-L Base)
- H2PO42-
(B-L Acid or Base)

Therefore, NaH2PO4 could be
acidic or basic depending on relative Ka
and Kb values

3
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
b.Lithium chloride
c. Iron (II) sulfate
d.Ammonium fluoride
e. Lithium phosphate

Acid and Base
Acid and Base Strength
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.
Their conjugate
bases are quite
weak.                    HCl(aq) + H2O(l) → H3O+(aq) + Cl−(aq)
• Weak acids only
dissociate partially
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).
stronger bases.

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
to another.

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

4
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
However:
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

electronegative,                                   As
2.0
Se
2.4
Br
2.8
• What could cause HF to be a weak                 the stronger.                                            Te     I
2.1   2.5
acid?

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

5
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
b.HNO2, HNO3
c. H2S, H3P
d.HF, HI
e. Li2O, CH3CH2CH2NH2
f. H2PO3-, HPO32-

6

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 12 posted: 3/4/2012 language: pages: 6
How are you planning on using Docstoc?