Topic 8.6 Notes – Molecular Structure of Acids and Bases

1. Acid and Base Strength Comes from Conjugate Stability

You already know the Brønsted-Lowry definitions:

• Acid = proton ($\ce{H+}$) donor
• Base = proton ($\ce{H+}$) acceptor
• Every acid forms a conjugate base
• Every base forms a conjugate acid

The strength relationship is always inverse:

• Strong acid → very weak, very stable conjugate base
• Weak acid → relatively stronger conjugate base
• Strong base → very weak conjugate acid
• Weak base → relatively stronger conjugate acid

So when you see a structure, mentally do this:

1. Remove $\ce{H+}$ (if analyzing an acid).
2. Add $\ce{H+}$ (if analyzing a base).
3. Ask: How stable is the resulting species?

Stability of charge is the whole game.

2. What Makes an Acid Strong or Weak

Acid strength depends on two big things:

• How easy it is to break the H-X bond
• How stable the conjugate base is

a. H-X Bond Strength and Periodic Trends

If the H-X bond is weak, it’s easier to lose $\ce{H+}$.

Down a group (like the halogens):

$$\ce{HF < HCl < HBr < HI}$$

Acidity increases going down because:

• Atoms get larger
• H-X bond gets longer and weaker
• Easier to remove $\ce{H+}$

Across a period:

• More electronegative atoms stabilize negative charge better.
• So acids generally get stronger left → right.

Students often mix this up. Down a group, size dominates. Across a period, electronegativity dominates.

b. Electronegativity and Inductive Effects

If nearby atoms are very electronegative, they pull electron density toward themselves.

That:

• Increases polarity of the O-H or H-X bond
• Stabilizes the negative charge in the conjugate base
• Increases acid strength

This is called the inductive effect.
The closer the electronegative atom is to the acidic hydrogen, the stronger the effect.

AP questions love showing you two similar structures with one extra electronegative atom and asking which is more acidic. Look for what stabilizes the negative charge.

c. Resonance Stabilization of the Conjugate Base

If the conjugate base can delocalize its negative charge, it becomes much more stable.

Here’s the classic comparison between a carboxylic acid and an alcohol. Focus on how the negative charge is distributed in each conjugate base.

Localized vs delocalized negative charge in ethoxide and acetate

The alcohol’s conjugate base has the charge confined to one oxygen. The carboxylate spreads that charge over two oxygens through resonance, which makes it much more stable.

Resonance stabilization is one of the most powerful acid-strength factors you’ll see.

d. Oxyacids and Oxidation State

Oxyacids look like $\ce{H-O-Z}$.

Acid strength increases when:

• Z is very electronegative
• Z has a high oxidation state

That pulls electron density away from O-H and stabilizes the conjugate base.

This explains why these are strong acids:

• $\ce{HClO4}$
• $\ce{HNO3}$
• $\ce{H2SO4}$ (first proton)

Their conjugate bases are extremely stabilized by resonance and inductive effects.

e. Strong Acids to Recognize

You are expected to know these without hesitation:

• $\ce{HCl}$, $\ce{HBr}$, $\ce{HI}$
• $\ce{HNO3}$
• $\ce{HClO4}$
• $\ce{H2SO4}$ (first proton only)

If you see one of these, assume complete dissociation.

3. What Makes a Base Strong or Weak

For bases, flip the thinking. Add $\ce{H+}$ and look at the conjugate acid.

a. Strong Bases: Group I and II Hydroxides

Strong bases include:

• Group I: $\ce{LiOH}$, $\ce{NaOH}$, $\ce{KOH}$
• Group II: $\ce{Ca(OH)2}$, $\ce{Sr(OH)2}$, $\ce{Ba(OH)2}$

They:

• Dissociate completely in water
• Produce $\ce{OH^{-}}$
• Have extremely weak conjugate acids ($\ce{H2O}$)

So strong base → very weak conjugate acid.

b. Nitrogenous Bases

Common weak bases:

• $\ce{NH3}$
• Amines like $\ce{CH3NH2}$

Nitrogen has a lone pair that accepts $\ce{H+}$:

$$\ce{NH3 + H2O <=> NH4+ + OH-}$$

More electron density on nitrogen → stronger base.
Alkyl groups push electron density toward N, slightly increasing basicity.

c. Carboxylate Ions as Weak Bases

Carboxylate ions ($\ce{RCOO^{-}}$) are weak bases because:

• Their negative charge is already resonance-stabilized
• They are comfortable as they are
• They don’t strongly attract $\ce{H+}$

Stable anion → weak base.

4. Predicting Strength from Structure

When comparing molecules:

1. Identify the acidic hydrogen.
2. Draw the conjugate base.
3. Look for:
   • Resonance?
   • Electronegative atoms?
   • Inductive effects?
   • Atom size?
4. More stable conjugate species → stronger parent acid.

On FRQs, they expect you to justify with structure language like “the conjugate base is stabilized by resonance” or “the negative charge is localized on a less electronegative atom.” Be specific.