Acids and BasesAcids and Bases

• Acids and bases are some of the most important industrial compounds on Earth

• ACIDIC SOLUTIONS:• taste sour• Carbonic and phosphoric acid give carbonated

drinks their sharp taste• Citric acid and ascorbic acid give lemons and

grapefruit their mouth-puckering tartness• Acetic acid makes vinegar taste sour

Acids and BasesAcids and Bases

• Basic solutions taste sour and feel slippery• Think of soap slipping out of your hands in

the shower• The litmus in litmus paper is one of the dyes

commonly used to distinguish solutions of acids and bases (turns red = acid; turns blue = base)

• Acid and base solutions conduct electricity

Ions in SolutionIons in Solution

• Why are some aqueous solutions acidic, others basic, and others neutral?

• All water (aqueous) solutions contain hydrogen ions (H+) and hydroxide ions (OH-)

• The relative amounts of the two ions determines whether an aqueous solution is acidic, basic, or neutral

• An acidic solution contains more hydrogen ions than hydroxide ions

• A basic solution contains more hydroxide ions than hydrogen ions

Ions in SolutionIons in Solution

• What do you think a neutral solution contains?• The usual solvent for acids and bases is water• Water produces equal numbers of H+ ions and OH-

ions in a process known as self-ionization• Self-ionization: two water molecules react to form a

hydronium ion (H3O+) and a hydroxide ion according to its equilibrium

H2O(l) + H2O(l) H3O+(aq) + OH-(aq) Water molecules Hydronium Hydroxide ion ion

Ions in SolutionsIons in Solutions

• Simplified version of the equation for self-ionization of water is:

H2O (l) H+ (aq) + OH- (aq)

• ARRHENIUS MODEL:

states that an acid is a substance that contains hydrogen and ionizes to produce hydrogen

ions in aqueous solution

HCl(g) → H+(aq) + Cl-(aq)

Bronsted-Lowry ModelBronsted-Lowry Model

• Bronsted-Lowry Model: states that an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor

HX(aq) + H2O(l) H3O+(aq) + X-(aq)

• The water molecule accepts a H+ ion and becomes an acid

• On donating its H+ ion, the acid HX becomes a base X-

ConjugatesConjugates

• Conjugate acid: species produced when a base accepts a hydrogen ion from an acid

• Conjugate base: species that results when an acid donates a hydrogen ion to a base

HX(aq) + H2O(l) H3O+(aq) + X-(aq)Acid Base Conjugate

AcidConjugate Base

ConjugatesConjugates

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

• Amphoteric: substances that can act as both an acid or base

• Monoprotic acid: an acid that donate only one hydrogen ion

• Polyprotic acid: an acid that can donate more than one hydrogen ion

Base Acid Conjugate Acid

Conjugate Base

Equilibrium ConstantEquilibrium Constant

• Equilibrium constant (Keq): the numerical value of the ratio of product concentration to reactant concentrations, with each concentration raised to the power corresponding to its coefficient in the balanced equation

aA + bB cC + dD• A and B are reactants; C and D the products. the

coefficients in the balanced equation are a, b, c and d

[C]c[D]d

[A]a[B]b Keq =

Strengths of Acids and BasesStrengths of Acids and Bases

• Acids that ionize completely are called strong acids.

• Because strong acids produce the maximum number of ions, they are good conductors of electricity

• Weak acid: an acid that ionizes only partially in dilute aqueous solution

• Weak acids cannot conduct electricity as efficiently because they produce fewer ions

Strengths of Acids and BasesStrengths of Acids and Bases

• Ionization equation for hydrocyanic acid:

HCN(aq) + H2O(l) H3O+(aq) + CN-(aq)

[H3O+][CN-]

[HCN][H2O]

• The concentration of liquid water in the denominator is constant, so it can be combined with Keq to give a new equilibrium constant Ka

[H3O+][CN-]

[HCN]

Keq =

Keq [H2O] = Ka =

Strengths of Acids and BasesStrengths of Acids and Bases

• Ka is called the acid ionization constant

• The smaller the Ka, the weaker the acid

• Strength of Bases:• Strong bases dissociate entirely into metal ions and

hydroxide ions

Ca(OH)2(s) Ca2+(aq) + 2OH-(aq)

• Weak base: ionizes only partially in dilute aqueous solution to form the conjugate acid of the base and hydroxide ion

Strength of BasesStrength of Bases

CH3NH2(aq) + H2O(l) CH3NH3+(aq) + OH-(aq)

Base ionization constant: Kb = [CH3NH3+][OH-]

[CH3NH2]

• Kb is called the base ionization constant

• The smaller the value of Kb, the weaker the base

Base Acid Conjugate Base

Conjugate Acid

pH and pOHpH and pOH

• The pH of a solution is the negative logarithm of the hydrogen ion concentration

pH = -log[H+]• Acids have a pH below 7• Base solutions have a pH above 7• Neutral solutions have a pH = 7• Using pOH:

The pOH of a solution is the negative logarithm of the hydroxide ion concentration

pOH = -log[OH-]

pH and pOHpH and pOH

• pH + pOH = 14.00• Knowing the value of one enables you to find the

other• Suppose the pH of a solution is 3.50 and you must

determine the concentrations of H+ and OH-.

pH = -log[H+]• First you need to multiply both sides of the equation

by -1

-pH = log[H+]

pH and pOHpH and pOH• To calculate [H+] using this equation you must take

the antilog of both sides of the equation.• antilog(-pH) = [H+]• To calculate [H+], substitute 3.50 for pH in the

equation• antilog(-3.50) = [H+]• Use a log table or your calculator to determine the

antilog of -3.50. The antilog is 3.2 x 10-4. [H+] = 3.2 x 10-4 mol/L• You can calculate [OH-] using the relationship [OH-] =

antilog (-pOH)