1. Predicting the pH of salt solutions

2. Salt Formation
Salts are formed from the neutralization reaction between an acid and a base.
Acid + Base  Salt + Water

3. Salt Formation A salt is an ionic compound that:
comes from the anion(-) of an acid
comes from the cation(+) of a base
Ex. HCl + NaOH  NaCl + Water
H Cl- Na OH-

4. Hydrolysis means: reacts with water. ‘lysis’- break water apart.
“Salt hydrolysis” - a salt that reacts with water to produce an acidic or basic solution.
When salts are placed in water some salts affect the overall pH of the solution.

5. KOH + H2CO3  K2CO3 + H2O KOH + H2CO3  K2CO3 + H2O
potassium hydroxide + carbonic acid  Neutralization Reaction:
KOH + H2CO3  K2CO3 + H2O
To see if the resulting salt is acidic or basic, check the “parent” acid and base that formed it.
KOH H2CO  K2CO3 + H2O

7. Dissociation (Ionization) Equation
Ionization Equation of Salt:
K2CO3  2K+ + CO3-2
Cation from the salt acts as Bronsted-Lowry acid
Parent Base: ____________
Anion from the salt act as Bronsted-Lowry base
Parent Acid: _________

8. Hydrolysis Equations K+ + H2O  NO hydrolysis
Hydrolysis Equation with Cation:
K+ + H2O  NO hydrolysis
Hydrolysis Equation with Anion:
CO2-2 + H2O HCO2-1 + OH-
Since OH- ions created, final solution will be basic with pH higher than 7.

9. Hydrolysis (Dissociation) Equations:
Cations that are weak acids come from strong bases and don’t hydrolyze (do not interact with ions from water)
Cations that are strong acids come from weak bases and do hydrolyze
Cations that are transition metals such as Al3+, Sn2+ and Fe3+ form acidic solutions (pg 582 – high charge density)
Anions that are weak bases come from strong acids and don’t hydrolyze
Anions that are strong bases come from weak acids do hydrolyze (do interact with ions from water)

11. Lets try another..
E.g. (NH4)2SO4 is added to water what pH will the final solution be?

12. The Degree of hydrolysis
Strong Acids + Strong Bases create salts that are neutral Both cation and anion created are weak and do not hydrolyze
Strong Acid + Weak base create salts that are acidic since Cation formed is strong acid (hydrolysis creating H3O+)
Anion formed is weak base (no hydrolysis)
Weak Acid + Strong base create salts that basic since
Cation formed is weak acid (no hydrolysis)
Anion formed is strong base (hydrolysis creating OH-)
Weak Acid + Weak Base create salts that are neutral, acidic or basic.
Both cation and anion are strong. Depends on degree of hydrolysis. Consult Ka and Kb

13. Salt created from Weak acid and Weak base
Eg. Ammonium hydroxide plus hydrogen cyanide create the salt NH4CN and water. What will the pH of the final solution be.

15. Determining pH of salt created from a weak acid and weak base
Ka > Kb Acidic Solution
Kb > Ka Basic Solution

16. Buffers
Buffers are solutions in which the pH remains relatively constant, even when small amounts of acid or base are added
made from a pair of chemicals: a weak acid and one of it’s salts; or a weak base and one of it’s salts

17. Buffers
A buffer system is better able to resist changes in pH than pure water
Since it is a pair of chemicals:
one chemical neutralizes any acid added, while the other chemical would neutralize any additional base
AND, they produce each other in the process!!!

18. Buffers
The two buffers that are crucial to maintain the pH of human blood are:
1. carbonic acid (H2CO3) & hydrogen carbonate (HCO31-)
2. dihydrogen phosphate (H2PO41-) & monohydrogen phoshate (HPO42-)

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Buffers - summary
Solutions with buffers resist changes in pH, when small amounts of acid or base are added Buffers are important in blood, cells, resisting the effects of acid rain on lake ecosystems. A buffer is created when a weak acid is mixed with a salt that contains the identical ion. Two equilibria contribute to the consistent [H+] HA  H+ + A–
NaA + 
Na+ A–
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