Unit 8 Notes Topic 6 AP CHEMISTRY

🧪 Relative Strength of Acids & Bases

Unit 8 | Topic 6 – What Makes an Acid or Base Stronger?

🔍 What Determines Acid Strength?

Acid strength is defined by how easily a substance can donate a proton (H⁺) in water. The easier it donates, the stronger the acid.

Key Principle: The strength of an acid is linked to the stability of its conjugate base. The more stable the conjugate base, the more willing the acid is to give up its proton.

HA ⇌ H⁺ + A⁻

• Strong acids → weak conjugate bases (A⁻ is stable and doesn’t grab the proton back)
• Weak acids → stronger conjugate bases (A⁻ wants the H⁺ back → less ionization)

Common Strong Acids and Their Conjugate Bases

Acid	Conjugate Base
HNO₃	NO₃⁻
H₂SO₄	HSO₄⁻
HCl	Cl⁻
HBr	Br⁻
HI	I⁻

⚡ Factors That Affect Acid Strength

1. Bond Polarity

The more polarized the H-A bond, the easier the H⁺ breaks away. This means acid strength increases with greater bond polarity.

A–OH + H₂O ⇌ A⁻ + H₃O⁺

Acid	Polarity of O–H	pKa
HOCl	Most Polar	7.5
HOBr	Moderate	8.7
HOI	Least Polar	10.7

Why? More electronegative elements (like Cl) pull electrons toward themselves, weakening the O–H bond. This “positive inductive effect” makes proton loss easier.

2. Bond Strength

Weaker H-A bonds mean stronger acids — the proton can escape more easily.

Acid	Bond Enthalpy (kJ/mol)	pKa
HF	567	3.2
HCl	431	–7.0
HBr	366	–8.0
HI	299	–10.0

As you move down Group 17, the bond length increases → bond weakens → acid strength increases.

3. Stability of Conjugate Base

The more stabilized A⁻ is, the more willing HA is to give up H⁺. This makes HA a stronger acid.

Example: Chlorine-substituted acids

Acid	pKa
Ethanoic acid (CH₃COOH)	4.8
Chloroethanoic acid	2.9
Dichloroethanoic acid	1.3
Trichloroethanoic acid	0.6

As more Cl atoms are added, the conjugate base becomes more stable due to increased charge delocalization. Hence, acidity increases dramatically.

💥 What About Base Strength?

The same concepts apply in reverse!

• Strong bases have weak conjugate acids (e.g., NaOH → Na⁺, nearly inert)
• Weak bases have stronger conjugate acids (e.g., NH₃ ⇌ NH₄⁺ + OH⁻)

Examples:

Base	pKb	Conj. Acid	pKa
NaOH	0.2	Na⁺	14.00
NH₃	4.75	NH₄⁺	9.26

The weaker the base (larger pKb), the stronger the conjugate acid (lower pKa).

💡 AP Chemistry Tip: Always look at the stability of the conjugate base/acid. More stable = stronger acid/base. Use pKa/pKb trends to predict strength fast!