Acids, Bases, and pH

Acids and Bases

• Acids produce H+ ions which react with water to form hydronium (H3O+) ions

• Bases produce OH- ions

Macroscopic properties of acids and bases

• Taste and feel– Acids taste sour (lemon juice, vinegar)– Bases taste bitter– Bases are slippery (soap)

• Acids react with bases – the reaction of acids and bases are central to the chemistry of living systems, the environment, and many important industrial processes

• Litmus test and other color changes– Indicators change colors in the presence of an acid

or a base– Litmus: acid = red, base = blue

Submicroscopic behavior of acids

• Hydronium ion formation

HCl + H2O H3O+ + Cl-

HNO3 + H2O H3O+ + NO3-

• Acidic hydrogen atoms– Reaction of acid with water is a transfer of a

hydrogen ion from an acid to a water molecule– Any hydrogen that can be transferred to water is

called an acidic hydrogen– It is possible for acids to have more than one

acidic hydrogen

• Acids are electrolytes– Substance that dissolve in pure water to form ions

& conduct electricity – Not all acids conduct electricity equally well• Strong acids conduct electricity better than weak acids

Submicroscopic behavior of bases

• Hydroxide ion formation can happen two ways

• Simple bases: NaOH Na+ + OH-

• Bases that accept H+:NH3+ H2O NH4

+ +OH-

• Bases are electrolytes– Strong bases conduct electricity better than weak

bases

Macroscopic-Submicroscopic Acid-Base Connection

• The properties of acids and bases are determined by the submicroscopic interactions between the acid or base and water

Strengths of Acids and Bases

• Strong acids and bases ionize completely while weak acids and bases ionize only partially

Strong Bases

• A base that completely dissociates into ions when dissolved in water

• Ionic compounds that contain hydroxide ions

NaOH Na+ + OH-

Strong Acids

• An acid that completely ionizes in water

HCl + H2O H3O+ + Cl-

Weak Acids

• An acid that only partially ionizes in solution• Usually when acidic hydrogen is bonded to an

oxygen

H2CO3 + H2O H3O+ + CO32- + H2CO3

Weak Bases

• Only partially ionize in solution

NH3 + H2O NH4+ + OH- + NH3 + H2O

Weak is not insignificant

• Most acids and bases are classified as weak

• Most of the acid-base chemistry in living systems occurs between weak acids and weak bases

Strength is not concentration

• Weak & strong compare strength• Dilute and concentrated compare

concentration• Combination of strength and concentration

determine the behavior of solution

The pH Scale

• A mathematical scale in which the concentration of H+ ions in a solution is expressed as a number from 0 – 14

Interpreting the pH scale

• pH < 7 = acidic• pH = 7 = neutral• pH > 7 = basic

• Each unit of pH represents a power of 10– Something with pH of 2 is 10 times more acidic

than something with a pH of 3

pH = -log [H+]

• What is the pH of solutions having the following ion concentrations?

1. [H+] = 1.0 x 10-2 M

2. [H+] = 3.0 x 10-6 M

• Relating H+ and OH- ion concentration

pOH = -log [OH-]

• What is the pOH of a solution having the following ion concentration?

1. [OH-] = 1.0 x 10-6 M

2. [OH-] = 6.5 x 10-4 M

pH + pOH = 14

• What is the pOH of a solution whose pH is 5?

• What is the pH of a solution whose [OH-] = 4.0 x 10-3 M

• Calculate the pH and pOH of the following solutions: 1. [H+] = 0.000033 M

2. [OH-] = 0.0095 M