Grade 12 Lesson 15.10 Sallts

What is a solution of HCl called ?

Salt is an ionic compound formed by acid-base neutralization reaction.

Acid + Base → salt + water

Lesson 15.10:
Acid-Base Properties of Salts

Hint to identify the acid and base that form the salt

1- Write the dissociation reaction of salt.
2- Always add H⁺ to negative ion to know the acid.
3- If the positive ion is group 1A or 2A add OH^- to know the base.
- If Not remove H⁺ to know the base.

Example: NaNO3 → Na⁺ + NO3^-

-

(NaOH) ( HNO3)

Strong base Strong acid

SHOW ME

identify the acid and base that form the following salts:

a- NH4Cl

b- NaClO3

●

●What will happen when salt is dissolved in water?

●Conjugate bases of strong acid or cations of group 1A and 2A cannot make hydrolysis with water.
These are spectator ions: Cl^-, NO^3-,Na⁺.

●Conjugate bases of weak acids and conjugate acids of weak bases can behave as acids or bases by reacting with water.

This reaction is hydrolysis reaction

NaNO3 is a salt:

Dissociation reaction: NaNO3→ Na⁺ + NO3

-

A Solution containing Na⁺ and NO3^-

ions is: neutral, PH= 7

A) Salts produce neutral solutions:

pH=7

Strong acid + Strong base = neutral salt

LiOH NaOH

KOH RbOH

CsOH Ba(OH)2

Sr(OH)2

B) Salts that produce basic solution

•The solution of a salt derived from strong base and weak acid is basic.
•acetic acid + sodium hydroxide 🡪 sodium acetate + water

CH₃COOH + NaOH 🡪 CH₃COONa + H₂O

(weak acid) (strong base) (Salt)

•Dissociation reaction of sodium acetate in water:

CH₃COONa (s) 🡪 CH3COO^- (aq) + Na⁺ (aq)

Na+ : neither donates or accepts H+ ions

CH3COO^- : is conjugate base of weak acid CH3COOH stronger than H2O
in base property and therefore has affinity for H⁺ .

•Hydrolysis reaction of CH3COO^- :

CH3COO^- + H2O ⇄ CH3COOH + OH^-

Because this reaction produce OH^- ions, the
sodium acetate solution is basic

To find the pH of basic salts use the following
shortcut:
pH= 14 + log ⎷Kb x [salt]0

•The solution of a salt derived from strong acid (HCl) and weak
base (NH3) is acidic.

•For example when solid NH4Cl is dissolved in water, NH4 ⁺ and Cl^- ions are present

•Dissociation reaction of NH4Cl:

NH₄Cl 🡪 NH₄ ⁺ + Cl^-

Cl^-: conjugate base of strong acid HCl 

Cl^- have no affinity for H⁺ in water

NH4⁺ : strong conjugate acid of weak base NH3

NH4⁺ is a donor for H+ (stronger than water in acidity)

C) Salts that produce acidic solution

Hydrolysis reaction :

NH₄⁺ ⇄ NH₃ + H⁺

solution is acidic

Because this reaction
produces H+ , NH4Cl

solution is acidic

To find the pH of acidic salts use the
following shortcut:

PH= - log ⎷Ka x [salt]0

Salts in which both anion and cation hydrolyze

•Salts derived from weak acid and weak base , both cation and anion hydrolyze

•pH of the solution containing such a salt depends on the relative strength of
weak acid and weak base.

•Ka = Kb solution will be neutral

•Ka > Kb solution will be acidic because cation hydrolysis will be more
important than anion hydrolysis

•Kb > Ka Solution will be basic because the anion will hydrolyze to greater
extent than the cation. At equilibrium, there will be more OH- than H+ ions

Predict whether aqueous solution of each of the following salts will be
acidic, basic, or neutral

A- NH4C2H3O2

B- NH4CN

C- NaNO2

A- NH4C2H3O2

Ions in solution are NH4

+ and C2H3O2

-

•Ka for NH4

+ is 5.6 x 10 -10

•Kb for C2H3O2

- is 5.6 x 10 -10

Ka = Kb the solution will be neutral

-

B- NH4CN

Ions in solution are NH4

+ and CN-

Ka for NH4

+ is 5.6 x 10 -10

Kb for CN- is 1.6 x 10-5

Kb > ka the solution will be basic

C- NaNO2

Ions in solution are Na⁺ and NO2

-

Na⁺ : do not hydrolyze

NO2^- : weak conjugate base of weak acid HNO2

Hydrolysis reaction:

NO₂^- + H₂O ⇄ HNO₂ + OH^-

This hydrolysis reaction produce:
OH^- the pH of NaNO2 solution will be basic

Some anions can act either as an acid or as a base.

EXAMPLE:

HCO3

- ion that is amphoteric and acts like acid or base

Because Kb > ka we predict that more OH- are produced.

Thus a solution of sodium bicarbonate NaHCO3 will be basic

​                     HCO₃ ^- + H₂O      ⇄     CO₃ ^2-  + H₃O⁺   Ka= 4.8 x 10 ^-11                      HCO₃ - + H₂O      ⇄     H₂CO₃  + OH^-    Kb= 2.4 x 10 ^-8 

Arrange the following from the most acidic to the most
basic:

CaBr2 , KNO2 , HClO4 , NH4ClO4 , HNO2

Arrange the following from the most acidic to the most
basic:

CaBr2 , KNO2 , HClO4 , NH4ClO4 , HNO2

solution:

HClO4 ---HNO2 ---NH4ClO4 ---CaBr2 ---KNO2

Strong acid weak acid acidic salt neutral salt basic salt