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The structure of acids and bases

Why is one acid stronger than another? It comes down to how easily the molecule lets go of its proton, and that is set by structure — bond strength for binary acids, oxygen count and electronegativity for oxyacids.

§1

Structure sets strength.

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Acid strength depends on how easily the molecule releases H⁺, which is set by molecular structure. For binary acids (H–X), a weaker, longer H–X bond releases H⁺ more easily, so acidity increases down a group (bond strength dominates over electronegativity there).

For oxyacids, more oxygen atoms and a more electronegative central atom pull electron density away from the O–H bond, making the acid stronger.

And conjugate strength runs opposite: a stronger acid has a weaker conjugate base. Choosing the acidic proton means finding the H whose loss is most stabilized, not just any hydrogen.

UNIT 8 TOPIC 8.6 • MOLECULAR STRUCTURE OF ACIDS AND BASES ACID STRENGTH Structure sets acid strength through bond strength, electronegativity, and conjugate-base stability. BINARY ACIDS (H–X) Acid strength is set by the H–X bond. Weaker / longer H–X bond → releases H⁺ more easily DOWN A GROUP → stronger acid HF HCl HBr HI bond weakens · acid strengthens HI > HBr > HCl > HF Across a period, a more polar / more electronegative X also makes the acid stronger. e.g. HF < H₂O (F more en. than O) OXYACIDS (H–O–X) O O O O X H More O atoms — or a more electronegative X — pull electron density away and spread out the negative charge of the anion. stabler conjugate base → stronger acid MORE OXYGENS → stronger acid HClO < HClO₂ < HClO₃ < HClO₄ weakest ────────────→ strongest CONJUGATE-BASE STABILITY more stable A⁻ → stronger HA The charge on A⁻ is stabilized by: resonance delocalization electronegative neighboring atoms inductive electron withdrawal A stable A⁻ holds H⁺ weakly, so the acid dissociates more fully: HA ⇌ H⁺ + A⁻ stable A⁻ → easier dissociation → larger Ka, smaller pKa. KEY PRINCIPLE A more stable conjugate base (A⁻) always means a stronger acid (HA) — the unifying idea behind every case above. AP Chemistry · Unit 8 · Acids and Bases
Fig. 8.6.1 Molecular structure sets acid strength. For binary acids (H–X), a weaker, longer H–X bond releases H⁺ more easily. For oxyacids, more oxygens and a more electronegative central atom strengthen the acid. A stronger acid has a weaker conjugate base.
§2

Reasoning about strength.

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Pick the right structural factor.

  1. Classify the acid. Binary (H–X) or oxyacid? Different factors apply.
  2. For binary acids, weigh the bond. A weaker, longer H–X bond (down a group) gives a stronger acid.
  3. For oxyacids, count O and electronegativity. More oxygens and a more electronegative center strengthen the acid.
  4. Invert for the conjugate base. A stronger acid has a weaker conjugate base.
§3

The pieces you'll meet.

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Different acids, different structural rules.

binary
Binary acid (H–X)
Strength set by H–X bond strength.
bond
Bond strength
Weaker/longer H–X bond = stronger acid.
oxyacid
Oxyacid
Strength set by oxygens and central electronegativity.
conjugate
Conjugate rule
Stronger acid → weaker conjugate base.
central atom
Central atom
More electronegative center = stronger oxyacid.
acidic H
Acidic proton
The H whose loss is most stabilized.
§4

Worked example: compare acid strengths.

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Binary acids. HI is a stronger acid than HF because the H–I bond is weaker and longer, releasing H⁺ more easily — even though F is more electronegative. Bond strength wins down the group.

Oxyacids. HClO₄ is stronger than HClO because it has more oxygens pulling electron density from the O–H bond. More O (or a more electronegative central atom) strengthens the acid.

Conjugate. The stronger acid HClO₄ has the weaker conjugate base (ClO₄⁻), while the weaker acid HClO has a stronger conjugate base.

Takeaway. Match the factor to the acid type: bond strength for binary, oxygens and electronegativity for oxyacids.

§5

Mistakes that cost real points.

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Pitfall · 01

"A stronger acid has a stronger conjugate base."

It is the opposite: a stronger acid has a weaker conjugate base (the conjugate holds the proton loosely). A weak acid has a relatively strong conjugate base. Inverting this relationship reverses every conjugate comparison.

Fix. Remember the inverse: stronger acid → weaker conjugate base, and vice versa.

Pitfall · 02

"For oxyacids, fewer oxygens make a stronger acid."

More oxygens make a stronger oxyacid — additional electronegative oxygens pull electron density away from the O–H bond, easing proton loss. HClO₄ (four O) is far stronger than HClO (one O). The factors are reversed in this misconception.

Fix. For oxyacids, add strength with more oxygens and a more electronegative central atom.

Pitfall · 03

"Binary acid strength is set mainly by electronegativity."

Down a group, binary acid strength is dominated by bond strength, not electronegativity: HI is a stronger acid than HF because the weaker, longer H–I bond releases H⁺ more easily, despite fluorine being more electronegative.

Fix. For binary acids down a group, judge by H–X bond strength (weaker/longer = stronger acid), not electronegativity.

§6

Skill Check.

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