Acids and Bases: Salts

Determining whether Salt Solutions are Acids or Bases

Learning GoalsStudents will be able:1) to determine whether salts form acids or

bases when dissolved into solutions2) to determine the pH of salt solutions

Success CriteriaStudents will display their understanding by:1) using a problem solving method to

determine whether salts are acids or bases in solution.

2) using ICE tables and Ka or Kb values to determine [H+] or [OH-] and therefore pH or pOH

3) knowing that the strength of an acid varies inversely with the strength of its conjugate base.

Pre-requisite KnowledgeStudent must already understand:1) Kw = KaKb

2) pH = -log[H+]; [H+] = 10-pH; pOH = -log[OH-]; [OH-] = 10-pOH

3) ICE tables, solving for concentrations given Ka (using the very small K method)

We know that compounds such as HCl, H2SO4, HC2H3O2 are acids from the H in their formulas, similarly NaOH, Ca(OH)2 and KOH are bases from the hydroxide in their formulas.

How do we know the acidity or alkalinity of salts (ionic compounds) with formulas that have neither H or OH such as NaCl, CaCO3, NH4Cl?

Laying the foundationWe can use the knowledge of

acids and their conjugate bases (and bases and their conjugate acids)

The diagram shows that as the strength of an acid increases, the strength of its conjugate base decreases.

This makes sense because strong acids release large amounts of H+ (H3O+) into solution and as a result their conjugate bases must release very little OH-.

Acids & Conjugate BasesHCl(aq) + H2O(aq) ↔ Cl-(aq) + H3O+

(aq)

HC2H3O2(aq) + H2O(aq) ↔ C2H3O2 -(aq) + H3O+

(aq)

HF(aq) + H2O(aq) ↔ F-(aq) + H3O+

(aq)

strong acid

very weak conjugate

base

weak acid

? acid ?conjugate

base

weak conjugate

base

Look at page 803 to determine whether an acid is strong or weak

Bases & Conjugate AcidsNaOH(aq) ↔ Na+

(aq) + OH-(aq)

NH3(aq) + H2O(aq) ↔ NH4 +

(aq) + OH-(aq)

N2H4(aq) + H2O(aq) ↔ N2H5+

(aq) + OH-(aq)

very weak conjugate acid

? base

weakconjugate

acid

?conjugate

acid

strong base

weak base

Let’s Look at a Salt NaF NaF(s) ↔ Na+

(aq) + F-

(aq)

Na+(aq) + F-

(aq)

since a weak acid is stronger than a very weak base, NaF forms an acidic solution.

remember: NaOH(aq) ↔ Na+(aq) +

OH-(aq)

and: HF(aq) + H2O(aq) ↔ F-

(aq) + H3O+

(aq)

conjugate acid of a strong base

conjugate base of a weak acid

very weak base

very weak acid

Try this one! KNO3 KNO3(s) ↔ K+

(aq) + NO3-(aq)

K+

(aq) + NO3-(aq)

CONCLUSION:

think: KOH(aq) ↔ K+(aq) + OH-

(aq)

and: HNO3 (aq)+ H2O(aq) ↔ NO3

-(aq)+

H3O+(aq)

conjugate acid of a _____ base

conjugate base of a _____ acid

________ acid _________ base

Try this one! NH4Br NH4Br(s) ↔

CONCLUSION:

think: ↔ and: ↔conjugate

acid of a _____ base

conjugate base of a _____ acid

________ acid _________ base

What if both the conjugate acid and conjugate base form weak solutons

NH4NO2 NH4NO2(s) ↔ NH4+

(aq) + NO2-(aq)

NH4

+(aq) + NO2

-(aq)

PROBLEM: Is this neutral? NO! We have to determine which is stronger - the weak acid or the weak base? To do this we must look at the Ka and Kb values of NH4

+(aq) + NO2

-(aq).

remember:NH3(aq) + H2O(aq) ↔ NH4

+(aq) + OH-

(aq)

HNO2(aq) + H2O(aq) ↔ NO2

-(aq) +

H3O+(aq)

conjugate acid of a weak base

conjugate base of a weak acid

weak acid weak base

Comparing Ka and Kb The Ka for NH4

+(aq) cannot be found in the Ka chart on page 803, BUT we

have the Kb value for NH3(aq). Remember, Kw = KaKb, therefore Ka = Kw/Kb.

After substituting the values:Ka = 1.0 x 10-14/1.8 x 10-5 = 5.6 x 10-10

The Kb for NO2-(aq) cannot be found in the Kb chart on page 803, BUT we

have the Kb value for HNO3(aq). Remember, Kw = KaKb, therefore Kb = Kw/Ka.

After substituting the values:Kb = 1.0 x 10-14/7.2 x 10-4 = 1.4 x 10-11

Compare the Ka and Kb values for the ions.acidic if Ka > Kb basic if Ka < Kb

neutral if Ka = Kb

Since Ka > Kb , NH4NO2(s) will form a weak acid in solution

Small, highly charged metal cations

Metallic ions with small atomic radii and high ionic charges often react with water to form acidic solutions.

For example, an aluminum salt follows this 3-step process to produce hydrogen ions in solution:

AlCl3(aq) ↔ Al3+(aq) + 3 Cl-(aq)

Al3+(aq) + 6 H2O(l) ↔ Al(H2O)6

3+(aq)

Al(H2O)63+

(aq) ↔ Al(H2O)5(OH)2+(aq) + H+

(aq)

The effect of metallic ions on the acidity of waterNote that the number of waters in the

hydrate complex is double the charge.Chromium (III) nitrate follows this 3-step

process to produce hydrogen ions in solution:

Cr(NO3)3(aq) ↔ Cr3+(aq) + 3 NO3

-(aq)

Cr3+(aq) + 6 H2O(l) ↔ Cr(H2O)6

3+(aq)

Cr(H2O)63+

(aq) ↔ Cr(H2O)5(OH)2+(aq) + H+

(aq)

Metal Oxides

Metal oxides such as CuO, CaO, PbO, and Fe2O3 will react with H2O to form basic solutions.

CuO + H2O → Cu2+ + 2 OH-

Copper (II) oxide reacts to form a basic solution.This process often occurs in mining areas

where metal ores and metal-laden mine tailings react with groundwater to form toxic alkaline run-off.

Non-metal oxidesNon-metal oxides such as SO2, CO2, N2O will react with H2O

to form acids.We often see this occur in our lower atmosphere. The

burning of fossils fuels releases a lot of SO2 into the atmosphere where it combines with water in the air (clouds) to form acids. These acids rain down upon us as acid rain

Salts: Determining Acid-Base Type of Salt pH of Solution example explanation1 Cations associated with strong bases

Anions associated with strong acids

Neutral NaClKNO3

K2SO4

NaOH → Na+ + OH-

Na+ is the very weak conjugate acid of a strong baseHCl → H+ + Cl-

Cl- is the very weak conjugate base of a strong acid2 Cations associated with weak bases

Anions associated with strong acids The conjugate acid of a weak base is itself a weak acid.

Acid NH4ClNH4NO3

NH3 + H2O → NH4+ + OH-

NH4+ is the weak conjugate acid of a weak base

HCl → H+ + Cl-

Cl- is the very weak conjugate base of a strong acid

3 Cations associated with strong bases Anions associated with weak acids The conjugate base of a weak acid is itself a weak base.

Base NaC2H3O2

KNO2

NaOH → Na+ + OH-

Na+ is the very weak conjugate acid of a strong baseHC2H3O2 → H+ + C2H3O2

-

C2H3O2- is the weak conjugate base of a weak acid

4 Anions associated with weak acids and cations associated with weak bases are acidic.The acid–base nature of the salt can be determined by comparing the relative values of Ka and Kb for the ions. In this situation, normally the cation is acidic and the anion is basic.

Acid or Base NH4C2H3O2

NH4F

NH4C2H3O2 → NH4+ + C2H3O2

-

NH3 + H2O → NH4+ + OH-

NH4+ is the weak conjugate acid of a weak base

HC2H3O2 → H+ + C2H3O2-

C2H3O2- is the weak conjugate base of a weak acid

Compare the Ka and Kb values for the ions.acidic if Ka > Kb basic if Ka < Kb

neutral if Ka = Kb

In this case, Ka for NH4

+ is 5.8 x 10-10 and the Kb for C2H3O2

- is 5.6 x 10-10, so Ka > Kb the solution is acidic

Salts: Determining Acid-Base5 Cations are small, highly charged (+2 or

+3) metal cations Anions associated with strong acids highly charged (+2 or +3) metal cations are hydrated in water and act as acidic cations.

Acid AlCl3

CuSO4

AlCl3 → Al3+ + 3 Cl-

Al3+ + 6 H2O → Al(H2O)63+

Al(H2O)63+→ Al(H2O)5(OH)2+ + H+ and a weak acid forms

HCl → H+ + Cl-

Cl- is the very weak conjugate base of a strong acid

6 Metal Oxides Basic CuOFe2O3

CuO + H2O → Cu2+ + 2 OH-

Copper (II) oxide reacts to form a basic solution.

7 Non-metal Oxides Acidic SO2

CO2

H2O + SO2 → H2SO3

Sulfur dioxide gas reacts with water to form sulfurous acid

Practice Questions

1) KCl2) K2CO3

3) NH4NO3

4) NH4NO2

5) Na2SO3

6) N2H5Cl

7) AlCl38) N2O

9) NaClO4

10)AlF3

11)NaC2H3O2

12)PbO

Problems: Identifying Salt Solutions as Acid, Neutral or Basic: