Acids and Bases

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Properties of Acids

Sour taste

Change the color of indicators

litmus - blue to red

bromcresol - green to yellow

phenolphthalein - rose to colorless

React with metals to liberate hydrogen gas

2HCl + Mg ® MgCl₂ + H₂

Properties of Acids

React with basic metal oxides & hydroxides to form a salt & H₂O

2HBr + CuO(s) ® CuBr₂ + 2H₂O

React with salts of weaker or volatile acids to give a new salt and a new acid

2HClO₄ + FeS ® H₂S + Fe(ClO₄)₂ + 2H₂O

Properties of Bases

Bitter taste

Change the color of indicators

litmus - red to blue

bromcresol - yellow to green

phenolphthalein - colorless to rose

Neutralize acids

Acid-Base Concepts: The Brønsted-Lowry Theory

Arrhenius Acid: A substance that dissociates in water to produce hydrogen ions, H⁺.

Arrhenius Base: A substance that dissociates in water to produce hydroxide ions, OH^-.

Bronsted-Lowrey Acid: A substance that can transfer hydrogen ions, H⁺. A proton donor.

Bronsted-Lowrey Base: A substance that can accept hydrogen ions, H⁺. A proton acceptor.

Conjugate Acid-Base Pairs: Chemical species whose formulas differ only by one hydrogen ion, H⁺.

Acid Strength

Strong acid: near 100% ionization

Weak acid: 10% or less ionization; Only partially dissociated in water thus being a weak electrolyte.

Base Strength

Strong acid: near 100% ionization

Weak acid: 10% or less ionization; Only partially dissociated in water thus being a weak electrolyte.

Hydrated Protons and Hydronium Ions

HA(aq) ↔ H⁺ (aq) + A^- (aq) Due to high reactivity of the hydrogen ion, it is

actually hydrated by one or more water molecules.

(For our purposes, H⁺ is equivalent to H₃O⁺.)

Dissociation of Water

Ion-Product Constant for Water: K_w = [H₃O⁺][OH^-]

At 25^oC: [H₃O⁺] = [OH^-] = 1.0 x 10^-7 M

Therefore: K_w = (1.0 x 10^-7)(1.0 x 10^-7) = 1.0 x 10^-14

The pH Scale

Acidic solution: pH < 7

Basic solution: pH >7

Neutral solution: pH = 7

Measuring pH

pH = -log[H₃O⁺] & [H₃O⁺] = 10^-pH

Example:

The hydronium ion concentration for lemon juice is approximately 0.0025. What is the pH when [H₃O⁺] = 0.0025 M?

Answer: pH = -log(0.0025) = 2.6

Example:

Calculate the pH of an aqueous ammonia solution that has an OH^- concentration of 0.0019 M.

Acid-Base Indicator: A substance that changes color in a specific pH range. Indicators exhibit pH-dependent color changes because they are weak acids and have different colors in their acid (HIn) and conjugate base (In^-) forms.

HIn(aq) + H₂O(l) ↔ H₃O⁺(aq) + In^-(aq)

Color A Color B

The pH in Solutions of Strong Acids and Strong Bases:

What is the pH of a 0.025 M solution of HNO₃?

Since HNO₃ is a strong acid, then [H₃O⁺] = [HNO₃].

So… pH = -log([H₃O⁺]) = -log(0.025) = 1.60

What is the pH of a 0.025 M solution of NaOH?

Since NaOH is a strong base, then [OH^-] = [NaOH].

So…[H₃O⁺] = 1 x 10^-14/[OH^-] = 1 x 10-14/ 0.025 = 4.0 X 10^-13 M

pH = -log([H₃O⁺]) = -log(4.0 x 10^-13 ) = 12.40

Equilibria in Solutions of Weak Acids

Acid-Dissociation Constant:

Example: The pH of 0.250 M HF is 2.036. What are the values of Ka and pKa

for hydrofluoric acid?

x = [H₃O⁺] = 10^-2.036 = 0.00920 M

Example: Calculate the pH of a 0.10 M HCN solution. At 25 °C,

Ka = 1.4 x 10-9.

Percent Dissociation in Solutions of Weak Acids

Classification of Acids

Monoprotic

Diprotic

Triprotic

Polyprotic Acids

Example: Calculate the pH of a 0.020 M H₂CO₃ solution. At 25 °C,

K_a1 = 4.3 x 10^-7.

Equilibria in Solutions of Weak Bases

Example: Calculate the pH of a 0.40 M NH3 solution. At 25 °C, Kb = 1.8 x 10-5.

Relationship between K_a andv K_b

Acid-Base Properties of Salts

Salts That Yield Neutral Solutions

The following ions do not react appreciably with water to produce either

H₃O⁺ or OH^- ions:

• Cations from strong bases:

• Alkali metal cations of group 1a (Li⁺, Na⁺, K⁺)

• Alkaline earth metal cations of group 2a

(Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺), except for Be²⁺

• Anions from strong monoprotic acids:

• Cl^-, Br^-, I^-, NO₃^-, and ClO₄^-

Salts That Yield Acidic Solutions

Salts such as NH₄Cl that are derived from a weak base (NH₃) and a strong acid (HCl) yield acidic solutions.

NH₄⁺(aq) + H₂O(l) ↔ HCN(aq) + OH^-(aq)

Ammonium ion (NH₄⁺) is the conjugate acid of the weak base ammonia (NH₃) while chloride ion (Cl^-) is neither acidic nor basic.

The acidity of hydrated main-group cations increases from left to right in the periodic table as the metal ion charge increases and the metal ion size decreases:

Li⁺ < Be²⁺

Na⁺ < Mg²⁺ < Al³⁺

Salts That Yield Basic Solutions

Salts such as NaCN that are derived from a strong base (NaOH) and a weak acid (HCN) yield basic solutions.

CN^-(aq) + H₂O(l) ↔ HCN(aq) + OH^-(aq)

Cyanide ion (CN^-) is the conjugate base of the weak acid hydrocyanic acid (HCN) while sodium ion (Na⁺) is neither acidic nor basic.

Salts That Contain Acidic Cations and Basic Anions

The pH of an ammonium carbonate solution, (NH₄)₂CO₃, depends on the relative acid strength of the cation and the relative base strength of the anion.

Three possibilities:

• K_a > K_b: The solution will contain an excess of H₃O⁺ ions (pH < 7).

• K_a < K_b: The solution will contain an excess of OH^- ions (pH > 7).

K_a ≈ K_b: The solution will contain approximately equal concentrations of H₃O⁺ and OH^- ions (pH ≈ 7).

Factors That Affect Acid Strength

The extent of dissociation of an acid is often determined by the strength and

polarity of the H-A bond.

Oxoacids containing the same number of OH groups and the same number

of O atoms have the acid strength bases on increases in the electronegativity

of the nonmetal associated with the oxygen

.

Oxoacids containing the same nonmetallic atom have their acid strength

based on different number of oxygen aton. Acid strength increases as

numbers of oxygen increases.

Lewis Acids and Bases