Acids, Bases, and SALTS

According to Arrhenius theory,

Acid + base salt + water

Examples:

HCl + NaOH NaCl + H2O

H2SO4 + Sr(OH)2 SrSO4 + 2H2O

SALTS ~ Defined

The relationship between salts and pH

There are six strong acids~

HCl, HBr, HI, H2SO4, HNO3 HClO4

There are six strong bases~

LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2,Ba(OH)2

All other acids and bases are considered weak and do NOT dissociate

completely.

Therefore when an acid and base are combined the final pH will depend on which of the reactants is strong or weak.

Strong acid + strong base neutral salt + H2OThe acid and base will neutralize each other and create a solution with pH 7.

Strong acid + weak base salt and water (pH < 7)Because the weak base doesn’t completely dissociate, the H1+ ions will prevail in determining

pH.

Weak acid and strong base salt and water (pH> 7)Because the weak acid doesn’t completely dissociate, the OH1- ions will prevail in

determining pH.

What happens when you put a salt in water?

You would think that putting a salt in water would have nothing to do with acids or bases and pH, but that is NOT the case.

NaCl + H2O H1+(aq) + Cl1-

(aq) + Na1+(aq) + OH1-

(aq)

The hydrogen from the strong acid and the hydroxide from the strong base neutralize each other. Here the pH is 7.

Look at what happens with this salt and water combination.

NH4Cl +H2O NH4OH(aq) + H1+(aq) + Cl1-

(aq)

Notice that the product is a strong acid (which completely dissociates) and a weak base (which does NOT completely dissociate). Therefore, the strong acid is the prevailing ionic species and influences the pH.

The pH will < pH7.

The opposite will be true when the salt is derived from a weak acid and a weak base. The pH will be > pH7.

If a salt derived from both a weak acid and weak base (Ex. FeC2O4) is put in water the pH will depend on which was weaker.

Acidic Salts and Basic Salts

There is also another class of salts called acidic salts and basic salts. They will also affect pH values.

These salts occur when there is NOT complete neutralization of the acid or base.

Acidic salt ~ NaHSO4 (sodium hydrogen sulfate)

(incomplete neutralization of sulfuric acid)

Basic salt ~ BaOHCl (barium hydroxide chloride)

(incomplete neutralization of barium hydroxide)

Acidic and Basic Anhydrides• There is a class of compounds that are neither

acids nor bases nor salts, but produce either an acid or base when put in water.

• (Basic) Metallic oxides (ex. CaO, Fe2O

3, K

2O,

etc.) are solids that will make a base when put in water (Ca(OH)

2, Fe(OH)

3, KOH). These

anhydrides are all solids.

• (Acidic) Non-metallic oxides (ex. CO2, SO

2,

NO2, etc.) are gasses that will make an acid

when put in water (H2CO

3, H

2SO

3, HNO

3).

These anhydrides are all gasses.