L1-2: Acid-Base Chemistry

What is the Arrhenius definition of an acid?

A substance that dissociates in water to produce H⁺ ions.

What is the Arrhenius definition of a base?

A substance that dissociates in water to produce OH⁻ ions.

What is the general Arrhenius equation for an acid?

HA ⇌ H⁺ + A⁻ (in water)

What is the general Arrhenius equation for a base?

BOH ⇌ B⁺ + OH⁻ (in water)

When an acid releases H⁺ in water, what happens?

The H⁺ combines with water to form H₃O⁺ (hydronium).

What are the limitations of the Arrhenius theory?

- Only applies to aqueous solutions.

- Cannot explain bases that do not contain OH⁻ (e.g., NH₃).

What is the Brønsted-Lowry definition of an acid?

A proton (H⁺) donor.

What is the Brønsted-Lowry definition of a base?

A proton (H⁺) acceptor.

How is acid or base strength determined in Brønsted-Lowry theory?

By how readily a substance donates or accepts a proton (H⁺).

What is a strong acid or base?

One that completely ionizes (dissociates) in water.

What is a weak acid or base?

One that partially ionizes (dissociates) in water.

What is a monoprotic acid/base?

An acid/base that can donate/accept one H⁺.

What is a polyprotic acid/base?

An acid/base that can donate/accept more than one H⁺.

What does the strength of an acid or base depend on?

The solvent in which it is dissolved. Example: HCl is strong in water but weak in acetic acid.

What are protophilic solvents?

Solvents that accept protons (H⁺).

Examples: H₂O, NH₃

What are protogenic (protic) solvents?

Solvents that donate protons (H⁺).

Examples: H₂SO₄, CH₃COOH

What are amphiprotic solvents?

Solvents that can both donate and accept protons.

Examples: H₂O, CH₃COOH

What are aprotic solvents?

Solvents that neither donate nor accept protons.

Examples: Benzene (C₆H₆), carbon tetrachloride (CCl₄), carbon disulfide (CS₂)

What is the general equilibrium expression for an acid and base?

HA + B ⇌ HB⁺ + A⁻

In the equilibrium expression, what are the acid and its conjugate base?

HA = acid

A⁻ = conjugate base

In the equilibrium expression, what are the base and its conjugate acid?

B = base

HB⁺ = conjugate acid

What is a conjugate acid-base pair?

Two species that differ by one proton (H⁺).

What is the Lewis definition of an acid?

A substance that accepts an electron pair.

What is the Lewis definition of a base?

A substance that donates an electron pair.

How does the Lewis theory expand the definition of acids and bases?

It includes substances that do not donate H⁺ or contain OH⁻.

- Lewis acids accept electron pairs (e.g., BF₃).

- Lewis bases donate electron pairs (e.g., ethers).

Acids react with bases to ____ a pair of electrons

Share

How are oxidation numbers affected in a Lewis acid-base reaction?

They do not change.

What are coordination complexes?

Lewis acid-base complexes in which both bonding electrons are donated by the Lewis base to the Lewis acid (a coordinate covalent bond).

In a coordination complex, what is the Lewis acid?

The central metal ion (or atom).

In a coordination complex, what is the Lewis base?

The ligands (atoms, molecules, or ions) that donate one or more electron pairs.

What is a coordination number (CN)?

The number of ligand attachment points (bonds) to the central metal ion.

What is the most common coordination number?

6 (although it can range from 2 to 9).

5

What is the coordination number of the following compound?

What is a chelating agent (polydentate ligand)?

A ligand that binds to a central metal ion through two or more donor atoms.

What is a chelate?

A coordination complex consisting of one or more chelating agents bound to a central metal ion.

What coordination complexes are the most thermodynamically stable?

Those containing chelating agents (chelates).

How are Lewis acids and bases classified?

By their hardness or softness.

What is the HSAB (Hard and Soft Acids and Bases) principle?

Hard acids bind best to hard bases, and soft acids bind best to soft bases.

What do hard and soft bases stabilize?

Hard bases: high oxidation states

Soft bases: low oxidation states

What are the characteristics of hard acids and bases?

- Small atomic/ionic radius

- High oxidation state

- Low polarizability

- High electronegativity (bases)

What are the characteristics of soft acids and bases?

- Large atomic/ionic radius

- Low oxidation state

- High polarizability

- Low electronegativity (bases)

What does amphoteric mean?

Can act as either an acid or a base.

What does amphiprotic mean?

Can donate or accept a proton (H⁺).

What is the relationship between amphoteric and amphiprotic substances?

All amphiprotic substances are amphoteric, but not all amphoteric substances are amphiprotic.

What does it look like when water acts as an acid?

H₂O + NH₃ ⇌ NH₄⁺ + OH⁻

(Water donates H⁺.)

What does it look like when water acts as a base?

H₂O + HCl ⇌ H₃O⁺ + Cl⁻

(Water accepts H⁺.)

How is the strength of an acid or base determined?

By the extent to which it ionizes (dissociates) in water.

Reminder: What is a strong acid or base?

One that completely ionizes (dissociates) in water.

What is a weak acid or base?

One that partially ionizes (dissociates) in water.

Are strong acids and bases reversible or irreversible?

Irreversible

Are weak acids and bases reversible or irreversible?

Reversible

What are the two most common strong acids used in pharmacy?

- Hydrochloric acid (HCl)

- Phosphoric acid (H₃PO₄)

What are the three most common strong bases used in pharmacy?

- Sodium hydroxide (NaOH)

- Potassium hydroxide (KOH)

- Calcium hydroxide [Ca(OH)₂]

What are the three most common weak acids used in pharmacy?

- Formic acid (HCOOH)

- Acetic acid (CH₃COOH)

- Citric acid

What is the most common weak base used in pharmacy?

Ammonium hydroxide (NH4OH)

What does the equilibrium constant (K) measure?

The extent of dissociation (ionization) of an acid or base.

What do Ka and Kb represent?

Ka: acid dissociation constant

Kb: base dissociation constant

The higher the Ka or Kb, the _______ the acid or base.

Stronger

What is the equilibrium equation for Ka?

Ka = [H3O+][A−] / [HA]

What is the equilibrium equation for Kb?

Kb = [BH+][OH−] / [B]

What is Kw?

The ionization (dissociation) constant of water.

What is the equation for Kw?

Kw = [Ka][Kb] = [([H3O+][A−] / [HA]) x ([BH+][OH−] / [B])] = [H3O+][OH−]

What is the value of Kw at 25°C?

Kw = 1.0×10−14

What are the concentrations of H₃O⁺ and OH⁻ in pure water at 25°C?

[H3O+] = [OH−] = 1.0×10−7

How do you calculate pH?

pH = −log⁡[H+]

How do you calculate pOH?

pOH = −log⁡[OH−]

How do you calculate pH using activity instead of concentration?

pH = −log⁡(γ±×c)

where γ± = activity coefficient and c = concentration.

How are pH and pOH related?

pH + pOH = 14

(at 25°C)

How do you calculate pKw?

pKw = −log(Kw)

Since Kw = [Ka][Kb] = [H3O+][OH−]

pKw = -log(Ka) + -log(Kb) = -log(H3O+) + -log(OH-) = pH + pOH

At 25°C:

pKw = pH + pOH = 14

What type of scale is pH?

A logarithmic scale.

What does a change of one pH unit represent?

A 10-fold change in hydrogen ion concentration.

What pH range is acidic?

Less than 7

What pH range is basic?

Greater than 7

What is a neutral pH at 25°C?

7

What is the hydrogen ion concentration at pH 0?

1 M

What happens to [H⁺] as pH increases by one unit?

It decreases by a factor of 10.

What happens to [H⁺] as pH decreases by one unit?

It increases by a factor of 10.

Is pH temperature-dependent?

Yep, pH decreases (becomes less than 7) as temperature increases

If Kw = [Ka][Kb] = 14, relate pKw, pKa, and pKb

pKw = [pKa] + [pKb] = 14

For polyprotic acids, what Ka is the greatest?

The primary Ka is the greatest but as you continue, it gets harder to give up protons

What is pKa?

The negative logarithm of Ka that indicates how easily an acid donates H⁺.

pKa = −log⁡(Ka)

What is pKb?

The negative logarithm of Kb that indicates how easily a base accepts H⁺.

pKb = −log⁡(Kb)

The smaller the pKa, the ______ the acid

stronger

The smaller the pKa, the ______ the base

weaker

How are Ka and pKa related?

Inversely

Larger Ka → Smaller pKa → Stronger acid

The smaller the pKb, the ______ the base

stronger

How are Kb and pKb related?

Inversely.

Larger Kb → Smaller pKb → Stronger base

When does pH = pKa?

When 50% of the molecules are ionized (dissociated) and 50% are unionized (undissociated).

At the equivalence point of a weak acid-strong base titration, what is present?

The acid has been completely neutralized (0% acid, 100% conjugate base/salt).

At the half-equivalence point, what is true?

[Acid] = [Conjugate base (salt)]

pH = pKa

What is an equivalent (Eq)?

The amount of a substance that reacts with or supplies 1 mole of H⁺ (acid-base reactions) or 1 mole of electrons (redox reactions).

How do you calculate equivalents (Eq)?

Eq = moles × valence

moles = amount of substance (number of particles)

valence = magnitude of the ion's charge

How do you calculate milliequivalents (mEq)?

mEq = mmol × valence

mmol = amount of substance (number of particles)

valence = magnitude of the ion's charge

How do you determine an ion's valence?

Use the magnitude of its ionic charge.

Examples:

- Na⁺ → 1

- K⁺ → 1

- Ca²⁺ → 2

- Mg²⁺ → 2

- Al³⁺ → 3

- Cl⁻ → 1

- SO₄²⁻ → 2

If 1 mmol of NaCl and 1 mmol of CaCl₂ are dissolved in solution, what are the milliequivalents of each ion?

Na⁺: 1 mmol × 1 = 1 mEq

Ca²⁺: 1 mmol × 2 = 2 mEq

Cl⁻: 3 mmol × 1 = 3 mEq

What is normality (N)?

The concentration of equivalents per liter (Eq/L).

What is the equation to calculate the pH of a weak acid using pKa and concentration?

pH = 1/2(pKa−log⁡C)

Conditions: Weak acid, known concentration, low % ionization.

How do you find concentration (C)?

C = mol/L; measured in M

How is the ionized/unionized ratio of a weak acid calculated?

ionized / unionized = 10^(pH−pKa)

How do you find the percent unionized of a weak acid?

% unionized = (1 / 1+10^(pH−pKa)) × 100