Molecular structure of acids and bases — AP Chemistry
AP Chemistry · AP Chemistry CED Unit 8: Acids and Bases · 14 min read
1. Binary Acid Strength Trends★★☆☆☆⏱ 3 min
Binary acid strength follows two consistent periodic trends driven by different structural properties:
**Same group (vertical) trend**: Acid strength increases down the group. This is driven by bond dissociation energy: as atomic radius of $\ce{X}$ increases down a group, the $\ce{H-X}$ bond becomes longer and weaker, so it is easier to break to release $\ce{H^+}$. Electronegativity does not drive this trend.
**Same period (horizontal) trend**: Acid strength increases left to right across the period. This is driven by electronegativity: as electronegativity of $\ce{X}$ increases left to right, the $\ce{H-X}$ bond becomes more polar, pulling electron density away from hydrogen and making it easier to lose $\ce{H^+}$.
Exam tip: When comparing binary acids, always first confirm if they are in the same group or same period, and apply the corresponding trend. Do not mix the two and incorrectly use electronegativity for same-group comparisons.
2. Inductive Effects and Oxyacid Strength★★★☆☆⏱ 4 min
Oxyacid strength is governed almost entirely by the **inductive effect**: the pull of electron density through covalent bonds caused by electronegativity differences. Electron-withdrawing groups (highly electronegative atoms like O, Cl, F) pull electron density away from the acidic $\ce{O-H}$ bond, weakening it and stabilizing the negatively charged conjugate base after deprotonation, which increases acid strength.
**Same central atom $\ce{X}$**: Acid strength increases with the number of non-hydroxyl oxygen atoms ($m$). Non-hydroxyl oxygens are not bound to acidic H, so each acts as an additional electron-withdrawing group. For example: $\ce{HClO < HClO2 < HClO3 < HClO4}$.
**Same number of non-hydroxyl oxygens**: Acid strength increases with the electronegativity of the central atom $\ce{X}$. A more electronegative central atom pulls more electron density away from $\ce{O-H}$ bonds, increasing strength. For example: $\ce{HIO < HBrO < HClO}$.
Exam tip: Always separate oxygen atoms into hydroxyl (bound to acidic H) and non-hydroxyl (only bound to the central atom) when comparing oxyacids. Only non-hydroxyl oxygens contribute to inductive electron withdrawal, so never count all oxygen atoms for ranking.
3. Resonance Effects on Acid/Base Strength★★★☆☆⏱ 3 min
Resonance effects describe the delocalization of electrons across multiple bonds, and they have a large impact on acid strength because they can stabilize the negative charge of a conjugate base. If the negative charge on a conjugate base can be spread out over multiple atoms via resonance, it is more stable than a conjugate base with all charge localized on a single atom. Greater conjugate base stability leads to a stronger parent acid.
For bases, resonance delocalization of the base's lone pair (used to accept a proton) makes the base weaker, because the lone pair is less available to accept $\ce{H^+}$. For example, aniline (aromatic amine) has a nitrogen lone pair delocalized into the benzene ring, making it a much weaker base than aliphatic amines like methylamine.
Exam tip: When asked to justify acid strength with resonance on the AP exam, you must explicitly connect resonance to stabilization of the conjugate base, not the neutral acid. AP readers will not give credit for vague statements without this connection.
4. AP Style Practice Problems★★★★☆⏱ 4 min
Common Pitfalls
Why: Students confuse the same-period trend with the same-group trend, incorrectly applying electronegativity instead of bond strength for same-group comparisons.
Why: Students forget only non-hydroxyl (non-acidic) oxygens contribute to inductive withdrawal.
Why: Students mix up which species gains the stabilization effect from deprotonation.
Why: Students confuse the direction of inductive effects for electron-donating vs electron-withdrawing groups.
Why: Students forget that resonance delocalization of the N lone pair into the benzene ring dominates basicity for aromatic amines, overriding weak inductive effects.