Unit 4: Acids and Bases (Part 1)

Arrhenius Theory

• An acid is any compound that produces hydrogen ions, H⁺_(aq), in water.
  → Ex: HCl_(g) → H⁺_(aq) + Cl^-_(aq)

• A base is any compound that produces hydroxide ions, OH^-_(aq), in water.
  → Ex: NaOH_(s) → Na⁺_(aq) + OH^-_(aq)

• Note: Organic acids have formulas that end in “COOH”; do not confuse them with bases.

Neutralization reaction

A neutralization is simply a special case of a double replacement reaction in which water is always one of the products.

• Acid + Base → Salt + Water

Properties of ACIDS

• Taste SOUR

• Conduct an electric current

• Turn litmus paper RED

• Produce hydrogen gas (H₂) when reacted with certain metals, such as magnesium

• Neutralized by bases

When an acid such as HCl is dissolved in water, it produces a hydrogen ion or proton, H⁺_(aq).

• Protons do not exist on their own in water, but rather readily attach themselves to water molecules (H₂O) to produce hydronium ions or hydrated protons, H₃O⁺_(aq).

• HCl_(g) → H⁺_(aq) + Cl^-_(aq)

• HCl_(g) + H₂O_(l) → H₃O⁺_(aq) + Cl^-_(aq)

Properties of BASES

• Taste like “SOAP”

• Conduct an electric current

• Turn litmus paper BLUE

• Feel slippery

• Neutralized by acids

Bronsted-Lowry Theory

• ACID is any substance that can DONATE a proton (H⁺) to another substance.
  → Proton donor

• BASE is any substance that can ACCEPT a proton (H⁺) from another substance.
  → Proton acceptor

HCN_(g) + H₂O_(l) ⇌ H₃O⁺_(aq) + CN^-_(aq)

• Forward reaction:

  • HCN is an acid: it loses “H” and a “+1” charge to become CN^-.

  • H₂O is a base: it gains “H” and a “+1” charge to become H₃O⁺.

• Reverse reaction: H₃O⁺ is the acid and CN^- is the base.

Conjugate Acid-Base Pairs

A pair of chemicals that differ by only ONE PROTON.

• CONJUGATE ACID has an extra proton.

• CONJUGATE BASE lacks a proton.

HCN_(g) + H2O_(l) ⇌ H3O⁺_(aq) + CN^-_(aq)

• HCN and CN^- are the conjugate acid-base pair

  • HCN is the acid, and CN^- is the conjugate base.

  • H₂O is the base, and H₃O⁺ is the conjugate acid.

Amphiprotic

• Substances that can act as either an acid or a base, depending on the kind of substances they react with.

• HCN_(g) + H₂O_(l) ⇌ H₃O⁺_(aq) + CN^-_(aq)

  • H₂O gains a proton to become H₃O⁺, so it is acting as a BASE.

• NH_3(aq) + H₂O_(l) ⇌ NH₄⁺_(aq) + OH^-_(aq)

  • H₂O loses a proton to become OH^-, so it is acting as an ACID.

→ Water can be considered AMPHIOROTIC.

Recognizing amphiprotic substances

A substance is amphiprotic if it:

• possesses a NEGATIVE CHARGE

• still has an easily removable HYDROGEN*

Strengths of acids and bases

• STRONG acids and bases ionize completely, 100% IONIZED.

• WEAK acids and bases do not ionize completely, LESS THAN 100% IONIZED.
  → Weak acids are better represented by an equilibrium system where the conjugate acid and base pairs both exist.

• The terms strong or weak refer to the degree of ionization of the acid or base, not the molar concentration.
  → Ex: 0.0010 M HCl is a STRONG acid, while 6.0 M HF is a WEAK acid.

Strong ACIDS

• HBr, HCl, HI, HNO₃, HClO₄, H₂SO₄

• All six reactions have ONE-WAY reaction arrows, pointing to the right.

• Aqueous molecules of the acid do not exist, and the conjugate bases cannot accept a proton from water.

  • Ex: HCl_(aq) + H₂O_(l) → H₃O⁺_(aq) + Cl^-_(aq)

Strong BASES

• O₂ and NH₂

• These two reactions have ONE-WAY reaction arrows, pointing to the left.

• Aqueous molecules of the base do not exist, and the conjugate acids cannot donate a proton to water.

  • NH_3(aq) + OH^-_(aq) ← NH₂^-_(aq) + H₂O_(l)

• Hydroxides are also strong bases: NaOH, LiOH, KOH, Ca(OH)₂, Ba(OH)₂, Sr(OH)₂

Weak ACIDS

• Weak acids are the species on the left side of the table from HIO₃ down to H₂O.

• Aqueous molecules of the conjugate acid and base coexist in solution.
  → Ex: HIO_3(aq) ⇌ H⁺_(aq) + IO₃^-_(aq)

• The last two species on the left, OH^- and NH₃, cannot act as acids in aqueous solutions.

Weak BASES

• Weak bases are the species on the right side of the table from H₂O down to PO₄^3-.

• Aqueous molecules of the conjugate base and acid coexist in solution.

• The top six species on the right, Br^-, Cl^-, I^-, NO₃^-, ClO₄^-, and HSO₄^-, cannot act as bases in aqueous solutions.

Levelling effect

• All strong acids and bases have IDENTICAL STRENGTHS because they are 100% ionized in aqueous solutions.

• Strong acids and bases have a higher electrical conductivity because they ionize completely.

• Six strong acids, HBr, HCl, HI, HNO₃, HClO₄, and H₂SO₄, cannot exist as molecules in aqueous solutions because they ionize completely to produce H₃O⁺ and an anion.
  → H₃O⁺ is the strongest acid that can exist in aqueous solution.

Equilibrium constant for the ionization of water

• ACIDIC: [H₃O⁺] > [OH^-]

• NEUTRAL: [H₃O⁺] = [OH^-]

• BASIC: [H₃O⁺] < [OH^-]

SELF-IONIZATION of water can be represented as: energy + H₂O + H₂O ⇌ H₃O⁺ + OH^-

Since H₂O is pure liquid, [H₂O] is constant and can be ignored in the expression of the EQUILIBRIUM CONSTANT OF WATER, K_w:
K_w = [H₃O⁺][OH^-] = 1.00 × 10^-14 (at 25°C)

K_w only varies with temperature change.

pH and pOH

• pH of a solution is the negative logarithm of the molar concentration of hydronium ion (H₃O⁺)

  • pH = - log [H₃O⁺]

  • [H₃O⁺] = antilog (-pH) = 10^-pH

• pOH is the negative logarithm of the molar concentration of hydroxide ion (OH^-)

  • pH = - log [OH^-]

  • [OH^-] = antilog (-pOH) = 10^-pOH

NOTE: For pH and pOH, only the numbers after the decimal point are significant.

pH + pOH = pK_w = 14.00

• pH < 7.00 or pOH > 7.00 → acidic solution

• pH > 7.00 or pOH < 7.00 → basic solution

• pH = pOH = 7.00 → neutral solution

1. Low pH and pOH values mean relatively high values of [H₃O⁺] and [OH^-], respectively.

2. High pH and pOH values mean relatively low values of [H₃O⁺] and [OH^-], respectively.

ACID IONIZATION CONSTANT

• Written as K_a

• The greater the value of K_a, the stronger the acid.

• Large K_a value indicates that the equilibrium favours products, hence these acids are stronger because they have greater ionization.

• The K_a values for STRONG acids are not listed in the table of Strengths of Acids since these acids are 100% ionized, and the concentration of the unionized acid in the denominator of the K_a expression is zero.

BASE IONIZATION CONSTANT

• Written as K_b

• The greater the K_b value, the stronger the base.

• K_b values must be calculated using K_a values of the conjugate acids.

Relationship between K_a and K_b

K_a x K_b = K_w (at 25°C, K_w = 1.0 x 10^-14)

K_a ∝ 1/K_b (Reciprocal nature between K_a and K_b)

• The conjugate base of a weak acid is a “strong” base.

• The conjugate base of a “strong” acid is a weak base.

→ The stronger the acid, the weaker the conjugate base.

The relative strengths of Acids and Bases

HCO₃^- + H₂PO₄^- ⇌ H₂CO₃ + HPO₄^2-

• If solutions containing amphiprotic ions such as HCO₃^- and H₂PO₄^- are mixed, the stronger of the two acids will donate a proton.

• HCO₃^- (K_a = 5.6 × 10^-11) and H₂PO₄^- (K_a = 6.2 × 10^-8). Since H₂PO₄^- has a larger K_a, it will donate a proton to HCO₃^-.

• There is a “proton competition” between the acids in the solution.

• Since H₂PO₄^- has a greater tendency to donate protons, it will be the stronger acid and the equilibrium will favour the products.

Hydrolysis

• Reaction between water and the cation or anion (or both) contained in the salt to produce an acidic or basic solution.

• SPECTATOR IONS do not hydrolyze and remain the same in the reaction.

  • SPECTATOR CATIONS (+): alkali metals (group 1) and alkaline earth metals (group 2)

  • SPECTATOR ANIONS (-): first five ions at the top right of the table of Relative Strengths of Acids (conjugate bases of strong acids): ClO₄^-, I^-, Br^-, Cl^-, and NO₃^-

    • HSO₄^- is not a spectator ion since it is a weak acid.

Anionic (-) hydrolysis

• Anions (-) hydrolyze to produce OH^-_(aq) and give a basic solution.

• Act as BASES when reacting with water.

B^-_(aq) + H₂O_(l) ⇌ HB_(aq) + OH^-_(aq)

Cationic (+) hydrolysis

• Cations (+) hydrolyze to produce H₃O⁺_(aq) and give an acidic solution.

• Act as ACIDS when reacting with water.

HA⁺_(aq) + H₂O_(l) → A^-_(aq) + H₃O⁺_(aq)

Summary of Hydrolysis

• STRONG ACID - STRONG BASE salts → NEUTRAL

• STRONG ACID - weak base salts → ACIDIC

• weak acid - STRONG BASE salts → BASIC

Metal vs. Nonmetal Oxides

• METAL OXIDES (group 1 & 2 metals) produce BASIC solutions when dissolved in water (basic anhydrides).

  • Na₂O_(s) → 2Na⁺_(aq) + O^2-_(aq)

  • O^2-_(aq) + H₂O_(l) → 2OH^-_(aq)

  • Na₂O_(s) + H₂O_(l) → 2NaOH_(aq)

• NONMETAL OXIDES produce ACIDIC solutions when dissolved in water (acidic anhydrides).

  • CO_2(g) + H₂O_(l) → H₂CO_3(aq)

  • H₂CO_3(aq) + H₂O_(l) ⇌ HCO₃^-_(aq) + H₃O^+_(aq)