8.6 Molecular Structure of Acids and Bases

Acid or base strength comes from how easy it is to lose or gain a proton—which you can read from the molecule’s structure. Strong acids have acidic protons whose conjugate bases are highly stabilized (so the proton “wants” to leave). Stabilizing features include high electronegativity (pulls negative charge toward an atom), inductive effects (electron-withdrawing groups nearby), and resonance delocalization (spreads the negative charge over the molecule). That’s why HCl, HBr, HI, HClO4, H2SO4, and HNO3 are strong: their conjugate bases are very stabilized. Weak acids (like carboxylic acids) have less-stabilized conjugate bases; their acidic proton is less easily lost. For bases, strong bases (group I and II hydroxides) have very weak, unstable conjugate acids, so they grab protons readily; common weak bases include ammonia and carboxylate ions. For AP study, focus on linking conjugate-base stability to acid strength (CED 8.6.A). For a quick review and practice problems on Topic 8.6, see the Fiveable study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and Unit 8 resources (https://library.fiveable.me/ap-chemistry/unit-8).

Acid strength comes down to how easily H+ leaves and how stable the conjugate base is afterward. Strong acids like HCl completely dissociate because their conjugate base (Cl–) is very stable—the negative charge is spread over a large, highly polarizable atom and the H–Cl bond is relatively weak. Weak acids like acetic acid (a carboxylic acid) only partially dissociate because the conjugate base (acetate) is less stabilized: resonance does help (so acetate is stabilized compared with many other bases), but not enough to make CH3COOH fully dissociate. Key factors from the CED: electronegativity and inductive effects (more EN atoms near the negative charge stabilize it), resonance stabilization (increases acid strength), and bond strength to H. Remember: stronger acid → weaker, more stabilized conjugate base (8.6.A.1). For a clear review, see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and extra practice (https://library.fiveable.me/practice/ap-chemistry).

Think of acid strength as how easily a molecule gives up H+. A stronger acid has a conjugate base that’s more stable (less willing to grab H+ back). Structure tells you that stability: - Electronegativity/ bond polarity: More electronegative atom bonded to H (H–Cl vs H–C) pulls electron density away, making H more acidic; the conjugate base is stabilized by that electronegativity. - Size (bond strength): Larger atoms (I) make weaker H–X bonds so H+ leaves more easily → stronger acid (HI > HBr > HCl). - Resonance: If the negative charge on the conjugate base is delocalized (carboxylate ions), the base is stabilized → acid is stronger. - Inductive effect: Electron-withdrawing groups (–NO2, –Cl) near the acidic proton stabilize the conjugate base, raising acidity. - Hybridization: More s-character (sp) stabilizes negative charge, so an sp C–H is more acidic than sp2 or sp3. For bases: strong bases have very weak (unstable) conjugate acids (e.g., OH–’s conjugate acid is H2O). When asked on the AP, always link acidity/basicity to conjugate-base stability and cite resonance/inductive/electronegativity/size (CED 8.6.A). For extra examples and practice, check the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and AP practice problems (https://library.fiveable.me/practice/ap-chemistry).

Strong acids are strong because their conjugate bases are unusually stable—so once the proton leaves, the resulting anion “wants” to stay deprotonated. Stability comes from things the CED calls out: high electronegativity, inductive effects, resonance, or combinations of these (8.6.A.1). - HClO4: when H+ leaves you get ClO4–, which has full resonance delocalization of the negative charge over four oxygens and strong inductive withdrawal by the central Cl. That makes ClO4– very weakly basic, so HClO4 is very strong. - H2SO4 (first proton): removal gives HSO4–, whose negative charge is delocalized over oxygens and stabilized by S=O bonds (resonance/inductive), so the first proton is very acidic. The second proton is less acidic because the anion is already negative. - HCl, HBr, HI: large, weakly basic conjugate bases (Cl–, Br–, I–) and weaker H–X bonds (especially down the group) make them strong acids. This is exactly what AP asks you to connect: molecular structure → conjugate-base stability → acid strength (Topic 8.6). For a focused review, see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and more practice at Fiveable (https://library.fiveable.me/practice/ap-chemistry).

Look for the H that, when removed, gives the most stable conjugate base. Quick checks: - Atom type: H bonded to very electronegative atoms (O, N, halogens) is more acidic than H on C. Strong acids are H–X (HCl, HBr, HI, HClO4, H2SO4, HNO3)—their conjugate bases are highly stabilized (CED 8.6.A.1.i). - Resonance: If the negative charge can be delocalized (carboxylates, phenoxide), that H is acidic (e.g., carboxylic acids, CED 8.6.A.1.ii). - Inductive effects: Nearby electron-withdrawing groups (-NO2, halogens) stabilize the conjugate base and increase acidity. - Bond strength: Weaker H–X bonds (e.g., H–I) give stronger acids. - Exceptions: C–H is usually nonacidic except when the resulting carbanion is stabilized (α-to-C=O, allylic, benzylic). So: find H on O/N/X, ask “Is the conjugate base resonance- or inductively-stabilized?” If yes → that proton reacts. For more practice and AP-aligned examples, see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and Unit 8 review (https://library.fiveable.me/ap-chemistry/unit-8). Want problems to try? Check the AP practice sets (https://library.fiveable.me/practice/ap-chemistry).

Short answer: the difference is how well the conjugate base is stabilized by the molecule’s structure. - Strong acids (HCl, HBr, HI, HClO4, H2SO4, HNO3) have very weak conjugate bases that are highly stabilized—often by large electronegativity, strong inductive effects, or extensive resonance/delocalization—and/or by being inherently nonbasic (very polar H–X bond). Their pKa values are << 0, so they fully dissociate in water. This matches CED 8.6.A.1.i: very weak, stabilized conjugate bases → strong acid. - Carboxylic acids (R–COOH) are weak acids (typical pKa ~4–5). Their conjugate base (R–COO–) is resonance-stabilized across two oxygens, which makes them acidic, but that stabilization is weaker than for the strong acids above. Acid strength for carboxylic acids also varies with R by inductive/electron-withdrawing effects (CED 8.6.A.1.ii, v). For AP prep, focus on relating acid strength to conjugate-base stability and structural effects (resonance, electronegativity, inductive). For a quick topic review, see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and practice problems (https://library.fiveable.me/practice/ap-chemistry).

Electronegative atoms make acids stronger because they stabilize the conjugate base that forms after H+ leaves. At the molecular level two things matter: (1) bond polarity and bond strength—attaching H to a less electronegative atom gives a less polar, weaker H–A bond that’s easier to break; attaching H to a more electronegative atom makes the bond more polarized so H+ is more readily released; and (2) conjugate-base stabilization—an electronegative atom (or atoms nearby) withdraws electron density by the inductive effect and lowers the negative charge density on the conjugate base, making that base less reactive (weaker). A weaker conjugate base means the forward acid-dissociation reaction is favored, so the acid is stronger. This is exactly what the CED says: electronegativity and inductive effects stabilize conjugate bases and increase acid strength (Topic 8.6, EK 8.6.A.1 and 8.6.A.1.v). For more examples and practice, see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and extra problems (https://library.fiveable.me/practice/ap-chemistry).

Resonance stabilizes a conjugate base by spreading out (delocalizing) the negative charge over more than one atom instead of keeping it all on one atom. That lower, spread-out charge makes the conjugate base lower in energy and more stable. Because acid strength is linked to how stable its conjugate base is (CED 8.6.A.1), greater resonance stabilization → stronger acid. Classic example: removing H+ from acetic acid gives the acetate ion; the negative charge is shared between two oxygens by resonance, so acetate is more stable than, say, ethoxide (where the negative charge sits mainly on one O). If the conjugate base can’t delocalize charge, it’s less stable and the acid is weaker. For more AP-aligned practice and examples on Topic 8.6, see the Topic study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and Unit 8 overview (https://library.fiveable.me/ap-chemistry/unit-8). For extra practice, check the problems page (https://library.fiveable.me/practice/ap-chemistry).

Pick the proton that can be lost (the “acidic proton”), then ask: how stable is the conjugate base? More stable conjugate base = stronger acid. Use these quick structure rules from the CED (8.6.A): - Electronegativity/size: H–X acids get stronger as X is more electronegative or larger (HCl < HBr < HI because the X–H bond weakens and the conjugate base is stabilized). Strong acids in the CED: HCl, HBr, HI, HClO4, H2SO4, HNO3. - Inductive effect: Electron-withdrawing groups (-NO2, halogens) near the acidic H pull electron density away and stabilize the conjugate base → stronger acid. - Resonance: If the negative charge can delocalize (carboxylates), acidity increases. Carboxylic acids are common weak acids because their conjugate bases are resonance stabilized. - For bases: strong bases are group I/II hydroxides (very weak conjugate acids); common weak bases are nitrogenous bases like NH3 and carboxylates. On the exam you’ll often be asked to “explain” acid strength by naming the acidic proton and citing conjugate-base stabilization (CED LO 8.6.A). For a concise review and practice, see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and extra practice problems (https://library.fiveable.me/practice/ap-chemistry).

Inductive effects are the pull (or push) of electron density through sigma bonds caused by electronegative atoms or groups. When an electronegative substituent (like F, Cl, or NO2) is near an acidic H, it withdraws electron density through the bond network, stabilizing the negative charge of the conjugate base. More stabilized conjugate base → stronger acid (CED 8.6.A.1, “conjugate base stability” and “electronegativity, inductive effects, resonance”). The effect gets weaker with distance (e.g., a substituent on the α-carbon has a bigger impact than one two carbons away). For bases, electron-withdrawing inductive groups lower basicity by making the lone pair less available; electron-donating groups increase basicity. On the exam you may be asked to compare acid strengths by citing conjugate-base stability from inductive effects (Topic 8.6). For a clear review and examples (like haloacetic acids vs. acetic acid), see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i). For extra practice, check Fiveable’s AP Chem practice problems (https://library.fiveable.me/practice/ap-chemistry).

Group I and II hydroxides (like NaOH, Ca(OH)2) are ionic solids that, when dissolved, release OH− essentially completely. OH− is a very strong base because its conjugate acid (H2O) is very weak—so the equilibrium lies far to the right and you get a high [OH−]. In CED terms: these bases have very weak conjugate acids (8.6.A.1.iii), and the metal–oxygen bonding is ionic, so there’s no stable, neutral species left to hold onto the proton. Ammonia (NH3) is a neutral, covalent molecule that acts as a base by accepting a proton to form NH4+. That protonation is an equilibrium (NH3 + H+ ⇌ NH4+); NH4+ is a moderately weak conjugate acid (pKa ≈ 9.25), so the equilibrium doesn’t go to completion—NH3 is a weak base (8.6.A.1.iv). Put simply: strong bases give OH− directly and fully; weak bases like NH3 must compete in an equilibrium where their conjugate acid isn’t extremely unstable. For more AP-aligned review on molecular reasons for strength, see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and practice problems (https://library.fiveable.me/practice/ap-chemistry).

Electronegativity affects acid strength because a more electronegative atom stabilizes negative charge on the conjugate base, making deprotonation easier. When an acid HA loses H+, you form A–. If A– can spread or hold that negative charge (by electronegativity, inductive pull from nearby EN atoms, resonance, or size/polarizability), the conjugate base is more stable and the acid is stronger (CED 8.6.A.1, especially 8.6.A.1.v). Quick rules you can use: - H–X where X is more electronegative (e.g., H–O vs H–C): O–H acids are acidic; C–H bonds are not. - Oxyacids: increasing electronegativity of the central atom or adding more O atoms (more electron-withdrawing groups) increases acid strength. - Halogen acids: across HCl, HBr, HI—size/polarizability (not just EN) stabilizes A–, so HI is strongest even though I is less EN. This is exactly what AP expects you to explain for Topic 8.6 (connect acid strength to conjugate-base stability). For a focused review, see the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and try related practice problems (https://library.fiveable.me/practice/ap-chemistry).

Carboxylate ions (RCO2–) are weaker bases than hydroxide (OH–) because their negative charge is stabilized and less available to grab a proton. In a carboxylate the negative charge is delocalized by resonance over two oxygen atoms, and nearby electronegative atoms/groups can further stabilize that charge by the inductive effect. That resonance + inductive stabilization makes the conjugate base of a carboxylic acid unusually low in energy, so the acid (RCO2H) is relatively acidic (pKa ≈ 2–5 for many carboxylic acids). By contrast, OH– holds its negative charge more locally on one oxygen (no equivalent resonance), so it’s much more willing to accept H+ (water’s pKa ≈ 15.7), i.e. OH– is a much stronger base. This explanation is exactly what the CED calls out (resonance, inductive effects, electronegativity → conjugate-base stabilization) for Topic 8.6 (see the Topic 8.6 study guide: https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i). For extra practice, try problems on conjugate-base stability in the Unit 8 practice set (https://library.fiveable.me/practice/ap-chemistry).

Look for the acidic proton and ask: “Is its conjugate base stabilized?” Strong acids have H attached to atoms or groups that make the conjugate base very stable (so H+ leaves easily). Quick rules of thumb: - Memorize common strong acids: HCl, HBr, HI, HClO4, H2SO4, HNO3—these have very weak, stabilized conjugate bases (CED 8.6.A.1.i). - Binary H–X acids: acid strength increases down a column (H–I > H–Br > H–Cl) because bond strength falls. - O–H acids: more electronegative atoms or more O’s near the O–H (inductive effect) → stronger acid (e.g., HClO4). Resonance in the conjugate base (carboxylates) also stabilizes it → increases acidity (carboxylic acids are common weak acids; conjugate carboxylate is resonance-stabilized, CED 8.6.A.1.ii, v). - Strong bases: group 1 & 2 hydroxides (NaOH, KOH, Ca(OH)2) because their conjugate acids are very weak (CED 8.6.A.1.iii). - Weak bases include N-containing bases (NH3) and carboxylate ions (CED 8.6.A.1.iv). If you want practice applying these ideas to formulas, check the Topic 8.6 study guide (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i) and more problems at the Unit 8 page (https://library.fiveable.me/ap-chemistry/unit-8) or practice set (https://library.fiveable.me/practice/ap-chemistry). On the exam, justify strength by citing conjugate-base stabilization (electronegativity, inductive, resonance, bond strength).

Strong acids are strong because the H+ leaves easily and the leftover conjugate base is unusually stable—that stability is the molecular reason the conjugate base is very weak (it won’t pick H+ back up). Key stabilizing factors: - Size/electronegativity: in HX (HCl, HBr, HI) the X– is large or highly polarizable, so the negative charge is spread out and stabilized. - Inductive effects: electron-withdrawing atoms/groups pull electron density away from the negative charge (e.g., HClO4, HNO3), stabilizing the anion. - Resonance: delocalizing the negative charge over multiple atoms (like in HNO3 or HSO4–/SO42– family) lowers energy of the base. - Strong H–A bond weakness: if the H–A bond is weak or easily heterolytically cleaved, H+ leaves more readily. Because the conjugate base is low in energy and well stabilized, it has very little tendency to re-donate electrons to accept H+, so it’s a very weak base. This is exactly what the CED expects you to explain (8.6.A.1)—see the Topic 8.6 study guide for examples and practice (https://library.fiveable.me/ap-chemistry/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i). For more problems to practice these ideas, check the AP Chem practice set (https://library.fiveable.me/practice/ap-chemistry).