Unit 8 · Topic 8.6 Beta

Molecular Structure of Acids and Bases

An acid is strong when the ion it leaves behind is stable.

Practice 4: Model AnalysisPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Vinegar is an acid, but ethanol, which has the same O–H group, is not acidic enough to change pH at all. Swap one hydrogen in vinegar's acid for chlorine and the acid becomes about eighty times stronger. The reason sits in the structure of the ion left behind after the proton leaves.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which atom is most electronegative?

  1. F
  2. Cl
  3. O
  4. N
Show the answer

Fluorine has the highest electronegativity of all elements (4.0).

  • Correct: F:
  • Cl:
  • O:
  • N:

2. What do the two resonance structures of the acetate ion, CH₃COO⁻, show?

  1. The negative charge is shared between the two O atoms
  2. The ion switches back and forth between two shapes
  3. One O atom always carries the charge
  4. The C atom carries the negative charge
Show the answer

Resonance structures describe one real ion in which the charge is delocalized over both O atoms.

  • Correct: The negative charge is shared between the two O atoms:
  • The ion switches back and forth between two shapes:
  • One O atom always carries the charge:
  • The C atom carries the negative charge:

3. HA has Ka = 1 × 10⁻⁴ and HB has Ka = 1 × 10⁻⁸. Which is the stronger acid?

  1. HA
  2. HB
  3. They are equal
  4. It depends on concentration
Show the answer

A larger Ka means the acid ionizes more.

  • Correct: HA:
  • HB:
  • They are equal:
  • It depends on concentration:

Part 4 · See it

See it first

Chlorine oxyacids HOCl, HOClO, HOClO2 and HOClO3 with Ka rising from 3.0 × 10⁻⁸ to well above 1: each extra oxygen pulls electron density from the O–H bond and spreads the conjugate base charge over more oxygens, so the acid is stronger. Below, HOI, HOBr and HOCl: a more electronegative central atom gives a larger Ka.
Each extra oxygen, or a more electronegative central atom, stabilizes the conjugate base and strengthens the acid. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. An acid HA gives up H⁺ and leaves A⁻ behindthe more stable A⁻ is, the further HA + H₂O ⇌ H₃O⁺ + A⁻ lies to the right
  2. Electronegative atoms near the acidic group pull electron density toward themselvesthe negative charge of A⁻ is less concentrated, so A⁻ is more stable and Ka is larger
  3. Resonance spreads the charge of A⁻ over several atomscarboxylic acids are far stronger than alcohols, and oxyacids with more O are stronger
  4. Down a group, the bond to H gets longer and weakerHF < HCl < HBr < HI and H₂O < H₂S < H₂Se in acid strength
  5. An amine N has a lone pairamines are weak bases that accept a proton on nitrogen

Part 6 · Key ideas

Key ideas

  • Acid strength follows conjugate base stability. Stabilize A⁻ and Ka goes up.
  • Oxyacids: more O atoms, or a more electronegative central atom, means a stronger acid (HOI < HOBr < HOCl < HOClO).
  • Carboxylic acids: the carboxylate is stabilized by resonance; electron-withdrawing groups (Cl, F) nearby make the acid stronger.
  • Binary acids: across a period electronegativity rules (CH₄ < NH₃ < H₂O < HF); down a group bond strength rules (HF < HCl < HBr < HI).
  • Amines (–NH₂) are weak bases: the proton goes on the N lone pair.

Part 7 · Misconception

A common mistake

The wrong idea: More hydrogen atoms in a formula make a stronger acid.

What actually happens: Only hydrogens that can leave as H⁺ count, and strength depends on how stable the conjugate base is. CH₄ has four H atoms and is not acidic; HCl has one and is strong.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Data table

Acetic acid and three substituted versions

Each acid loses the H on its –COOH group. In the substituted acids, one or two H atoms on the CH₃ carbon are replaced by halogen atoms.

Ka at 25 °C
AcidCondensed formulaKa
Acetic acidCH₃COOH1.8 × 10⁻⁵
Chloroacetic acidClCH₂COOH1.4 × 10⁻³
Dichloroacetic acidCl₂CHCOOH5.5 × 10⁻²
Fluoroacetic acidFCH₂COOH2.6 × 10⁻³

1. Why is chloroacetic acid a stronger acid than acetic acid?

  1. Electronegative Cl draws electron density away and stabilizes the conjugate base
  2. The Cl atom gives the molecule a second acidic hydrogen atom that can ionize too
  3. Chloroacetic acid has a larger molar mass, so its O–H bond is longer
  4. The Cl atom makes the C=O bond into a single bond, which releases the proton
Show the answer

An electron-withdrawing Cl two carbons away draws electron density from the –COO⁻ group, so the negative charge of ClCH₂COO⁻ is less concentrated and the ion is more stable. A more stable conjugate base means a larger Ka.

  • Correct: Electronegative Cl draws electron density away and stabilizes the conjugate base: Right: electron withdrawal stabilizes the conjugate base.
  • The Cl atom gives the molecule a second acidic hydrogen atom that can ionize too: The acidic H is still the one on –COOH; Cl replaces a C–H hydrogen, which is not acidic.
  • Chloroacetic acid has a larger molar mass, so its O–H bond is longer: Molar mass does not set acid strength; the effect comes from electronegativity.
  • The Cl atom makes the C=O bond into a single bond, which releases the proton: The C=O bond is still there; Cl changes electron density, not the bonding pattern.

2. Fluoroacetic acid has a larger Ka than chloroacetic acid. What does this show?

  1. A more electronegative substituent gives a stronger acid
  2. A larger halogen atom in the same position gives a stronger acid
  3. The H–F bond in fluoroacetic acid ionizes
  4. Fluorine adds extra resonance structures to the conjugate base
Show the answer

F (electronegativity 4.0) pulls harder than Cl (3.2), so FCH₂COO⁻ is more stabilized and FCH₂COOH ionizes more.

  • Correct: A more electronegative substituent gives a stronger acid: Right: the stronger the pull, the more stable the anion.
  • A larger halogen atom in the same position gives a stronger acid: F is smaller than Cl, yet its acid is stronger, so size is not the cause here.
  • The H–F bond in fluoroacetic acid ionizes: There is no H–F bond: F is bonded to carbon, and the acidic H is on –COOH.
  • Fluorine adds extra resonance structures to the conjugate base: F does not create new resonance forms of the carboxylate; its effect is a pull through the bonds.

3. What is the pH of 0.10 M chloroacetic acid? (Check the small-x assumption.)

Type a number.

Show the answer

Small x: √(1.4 × 10⁻³ × 0.10) = 1.2 × 10⁻² M, 12% of 0.10, too large. Quadratic: x² + (1.4 × 10⁻³)x − 1.4 × 10⁻⁴ = 0 gives x = 1.12 × 10⁻² M; pH = 1.95.

  • Answer: 1.95

4. Trichloroacetic acid, Cl₃CCOOH, is not in the table. Which Ka is most consistent with the trend?

  1. 2.2 × 10⁻¹
  2. 5.5 × 10⁻²
  3. 1.4 × 10⁻³
  4. 1.8 × 10⁻⁷
Show the answer

Each Cl added pulls more electron density from the carboxylate, so Ka rises with every substitution: 1.8 × 10⁻⁵ → 1.4 × 10⁻³ → 5.5 × 10⁻² → larger still for three.

  • Correct: 2.2 × 10⁻¹: Right: three Cl atoms, the largest Ka of the series.
  • 5.5 × 10⁻²: That is the value for two Cl atoms; a third Cl should increase it.
  • 1.4 × 10⁻³: That is the value for one Cl atom.
  • 1.8 × 10⁻⁷: This would make it weaker than acetic acid, against the trend.

Data table

Oxyacids of chlorine and the halogens

In every acid below the acidic H is bonded to an O atom, and that O is bonded to the central atom (H–O–X). Electronegativity values are on the Pauling scale.

Structure, electronegativity of the central atom, and Ka at 25 °C
AcidStructureCentral atom electronegativityKa
HOClH–O–Cl3.23.0 × 10⁻⁸
HOClOH–O–Cl–O3.21.1 × 10⁻²
HOBrH–O–Br3.02.0 × 10⁻⁹
HOIH–O–I2.72.3 × 10⁻¹¹

5. Why is HOClO a much stronger acid than HOCl?

  1. In ClO₂⁻ the negative charge is spread over two O atoms
  2. HOClO has two acidic hydrogen atoms that can both ionize
  3. The extra O atom makes the O–H bond shorter and stronger
  4. Cl has a higher electronegativity in HOClO than in HOCl
Show the answer

In ClO₂⁻ the negative charge is shared between two O atoms (two resonance forms), and the extra O pulls electron density from the O–H bond. Both make the conjugate base more stable and the acid stronger.

  • Correct: In ClO₂⁻ the negative charge is spread over two O atoms: Right: more O, a more stable conjugate base.
  • HOClO has two acidic hydrogen atoms that can both ionize: HOClO has one H, bonded to O; it is monoprotic.
  • The extra O atom makes the O–H bond shorter and stronger: The extra O weakens the O–H bond by pulling electron density away from it.
  • Cl has a higher electronegativity in HOClO than in HOCl: Electronegativity is a property of the element; Cl is the same atom in both.

6. Based on the table, how does acid strength change from HOCl to HOBr to HOI, and why?

  1. It decreases, because a less electronegative central atom pulls less electron density from the O–H bond
  2. It increases, because larger atoms hold the H more weakly
  3. It decreases, because the O–H bond gets shorter
  4. It stays the same, because each acid has one O atom
Show the answer

Ka falls from 3.0 × 10⁻⁸ to 2.0 × 10⁻⁹ to 2.3 × 10⁻¹¹ as the central atom's electronegativity falls from 3.2 to 2.7. A weaker pull leaves more electron density in the O–H bond and on the O of the anion, so the anion is less stable.

  • Correct: It decreases, because a less electronegative central atom pulls less electron density from the O–H bond: Right: the Ka data match the electronegativity order.
  • It increases, because larger atoms hold the H more weakly: In oxyacids the H is on O, not on the halogen, so the halogen's size is not the deciding factor; the data show the acids getting weaker.
  • It decreases, because the O–H bond gets shorter: The O–H bond length barely changes; the trend follows electronegativity.
  • It stays the same, because each acid has one O atom: The Ka values span three powers of ten, so strength is not the same.

Model

Three organic molecules

Ethanol, CH₃CH₂–O–H: Ka ≈ 1 × 10⁻¹⁶.

Acetic acid, CH₃–C(=O)–O–H: Ka = 1.8 × 10⁻⁵.

Methylamine, CH₃–NH₂ (N has one lone pair): Kb = 4.4 × 10⁻⁴.

7. When methylamine acts as a base in water, which atom accepts the proton?

  1. The N atom, using its lone pair
  2. The C atom, which has four bonds
  3. An H atom on the N
  4. The C atom, using a lone pair
Show the answer

A Brønsted-Lowry base needs a lone pair to bond to H⁺. The N lone pair forms the new N–H bond: CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻.

  • Correct: The N atom, using its lone pair: Right: the nitrogen lone pair.
  • The C atom, which has four bonds: Carbon has four bonds and no lone pair, so it cannot accept a proton here.
  • An H atom on the N: An H atom cannot accept another proton; the new bond forms to N.
  • The C atom, using a lone pair: The carbon in methylamine has no lone pair.

Part 9 · Summary

Summary

An acid is strong when the ion it leaves behind is stable. Electronegative atoms pull electron density from the acidic group, resonance spreads the charge of the conjugate base, and a large atom holds charge over a larger volume; each makes Ka larger. Amines are weak bases that accept a proton on the nitrogen lone pair.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections