Introduction toAcids and Bases

Chapter 19

What is and Acid?• Arrhenius Acid • Defined as any chemical that increases

the concentration of hydrogen ions (H+)in solution. (usually by a dissociation reaction

Examples • Hydrochloric acid HCl H+ + Cl-

• Sulfuric acid H2SO4 2H+ + SO42-

• Phosphoric acid H3PO4 3H+ + PO43-

Not all Hydrogens are acidic

• The hydrogen must be part of a polar bond in order to dissociate.

• For example:• HF is acidic, but CH4 is not.• In CH4, the hydrogen is part of a non-polar

covalent bond and does not dissociate in solution!

• HF is a polar bond and HF H+ + F-

Bronsted – Lowry Acid• Defined as a molecule or ion that is a hydrogen

ion donor.

• Also known as a proton donor because H+ is a proton.

• The acid will donate its H+ ion to a base in an acid base reaction.

H+ + OH- H2O

Acid + Base

What is a Base?• Arrhenius Base• Defined as any chemical that increases the

hydroxide ions (OH-) concentration in solution.

• Examples-• NaOH Na+ + OH- • KOH K+ + OH-

• Ca(OH)2 Ca2+ + 2OH-

Bronsted-Lowry Base• Defined as a hydrogen ion acceptor.• In an acid-base reaction the base

“accepts” the hydrogen ion from the acid.

NH3 + H+ NH4+

NH3 accepts the H+ from the acid.

Physical Properties of Acids• Taste Sour• Feel Sticky• React with metals to produce hydrogen gas• Conduct electricity – because they produce

ions in solution• Are corrosive (they can burn skin and other

materials)• Are found in many naturally colored solutions• Turn Litmus Indicator Red

Physical Properties of Bases• Taste Bitter• Feel slippery or soapy• Also conduct electricity• Rarely found in colored solutions• Most do not have a smell (except

ammonia)• Turn Litmus indicator Blue• Turn Phenolphthalein Indicator Pink

Physical Properties of Neutral Compounds

• Do not taste bitter or sour, but rather have a sweet or oily taste, or no taste at all.

• Many feel like water, or feel greasy/ oily• May or may not conduct electricity• They have varied reactivities.• Many have a strong chemical smell or no

smell at all.

Using pH to identify Acids and Bases

The pH Scale

What is pH?• pH = -log [H+] pH= -log[H3O+]

• pH stands for the “power of the hydrogen ion.”

• It is based on a logarithmic scale which has a base power of 10.– A pH of 1 differs from a pH of 2 by a factor of

10.

pH Scale - Continued• Created to express acidity as a more

simple number than molarity.• pH = - log [H+]• If . . .• [H+] = 1 x 10-7 then pH = 7 (neutral)• [H+] = 1 x 10-3 then pH = 3 (acid)• [H+] = 1 x 10-10 then pH = 10 (base)

Why is the pH scale 0 – 14?

The Self-Ionization of WaterH2O (l) ↔ H+ (aq) + OH-(aq)

• The equilibrium expression for this reaction is

• Kw = [H+][OH-] = 1 x 10-14

• This low equilibrium constant means very few water molecules ionize.

• Water is considered neutral because in water• [H+] = [OH-] = 1 x 10-7 M• pH = -log (1 x 10-7 ) = 7• *All aqueous solutions have H+ and OH- ions

• Acids HCl H+ + Cl-• Acidic solutions increases the H+

concentration and [H+] > [OH-]• Bases NaOH Na+ + OH-• Basic solutions increases the OH-

concentration and [H+] < [OH-]

• pH scale ranges from 0-14• pH<7 indicates an acidic solution• pH=7 indicates a neutral solution• pH>7 indicates a basic solution• Strong Acids have pH of 0 or 1• Strong Bases have a pH of 13 or 14

Strength of Acids and Bases“Strength” refers to how much an acid or

base ionizes in a solution.

STRONG WEAK

Ionize completely (~100%)

Example:HCl H+ + Cl-

NaOH Na+ + OH-

Ionize partially (usually <10%)An equilibrium reaction!

Example:HF ↔ H+ + F-

NH3 + H2O ↔ NH4+ + OH-

Strengths of Acids and Bases