Acids and BasesUnit 12

Properties of an Acid Sour taste Turns litmus paper red (and responds

uniquely to other indicators) Reacts with:

Hydroxide bases to produce water and an ionic compound.

Pure metals (i.e. Zn & Mg) to produce H2(g)

Carbonates to produce CO2(g).

Properties of a Base Bitter taste Slippery, soapy feeling Turns litmus paper blue (and responds

uniquely to other indicators)

Arrhenius Definition of an Acid/Base Acids produce hydrogen ions, or protons (H+) in aqueous

solution. HA represents a generic acid.

HA ⇄ H+ (aq) + A- (aq)Example: HCl (g) ⇄ H+ (aq) + Cl- (aq)

Bases produce hydroxide ions (OH–) in aqueous solution. MOH represents a generic base.

MOH ⇄ M+ (aq) + OH- (aq)Example: NaOH ⇄ Na+ (aq) + OH- (aq)

The Arrhenius definition is very limited – Not all bases contain OH-

Example – NH3 or ammonia is a common base.

(1859 – 1927)

Neutralization of an Arrhenius Acid and Base An Arrhenius acid and an Arrhenius base will react to form

a salt and water.

Example: HCl (aq) + NaOH (aq) ⇄ NaCl (aq) + H2O (l)

The salt is formed from the positive ion of the base (Na+) and the negative ion of the acid (Cl-).

The water is formed from hydrogen ion of the acid (H+) and the hydroxide ion of the base (OH-).

If such a reaction results in a solution with no hydrogen or hydroxide ions, the products would be neither an acid nor a base. This type of reaction is called a neutralization reaction.

Bronsted-Lowry Definition Arrhenius’ model was limited because it only

applied to substances with hydrogen and hydroxide ions.

Bronsted and Lowry proposed a new definition for acid and bases: Acids are proton donors (give off an H+) Bases are proton acceptors

What do we mean by a proton donor?

Hydrogen! What is H+?

Bronsted-Lowry Acid/Base Below is the general reaction that occurs

when an acid (HA) is dissolved in water. HA (aq) + H2O (l) ⇄ H3O+ (aq) + A- (aq)

The acid (HA) is donating a proton to the base

(H2O).

When the H2O accepts the proton it become H3O+ which is known as the hydronium ion.

Acid Base

Conjugate Acid/Base Pairs

HA (aq) + H2O (l) ⇄ H3O+ (aq) + A- (aq)

This is a reversible reaction. If we look at the reverse reaction, which substance would donate a proton?

Because the H3O+ is a proton donor in the reverse reaction, it is considered the conjugate acid.

If we look at the reverse reaction, which substance would accept a proton?

Because the A- is a proton acceptor in the reverse reaction, it is considered the conjugate base.

Acid donates proton, becomes the conjugate base.Base accepts proton, becomes the conjugate acid.

Acid Conjugate BaseBase Conjugate Acid

Is water an acid or a base? Pure water is neutral! Water can act as an acid or a base:

H2O (l) + H2O (l) H3O+ (aq) + OH- (aq) One water molecule acts as an acid by donating a

proton; the other water molecule acts as a base by accepting the proton.

The reverse reaction dominates in normal conditions, indicated by the longer arrow.