Chapter 6: acids and bases

Explain Brønsted-Lowry acid-base theory

An acid is a proton (H+) donor, and a base is a proton (H+) acceptor. When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.

Explain Lewis definition of acidity and basicity

A Lewis acid is an electron-pair acceptor (electrophile). A Lewis base is an electron-pair donor (nucleophile). This definition includes reactions without proton transfer.

Explain approximate pKa values for functional groups and relative pKa of carbons

Carboxylic acids (~4-5), Phenols (~10), Alcohols (~16-18), Amines (~35-38). For carbons: sp (pKa ~25) is more acidic than sp2 (pKa ~44) which is more acidic than sp3 (pKa ~50) due to s-character stabilizing the negative charge; adjacent EWGs significantly lower pKa.

Explain how a buffer system works

A buffer consists of a weak acid and its conjugate base (or weak base and conjugate acid). It resists pH changes upon addition of small amounts of strong acid or base by neutralizing added H+ (via conjugate base) or OH- (via weak acid).

Explain how inductive effect affects acidity

Electronegative atoms pull electron density through sigma bonds, stabilizing the conjugate base's negative charge and increasing acidity. The effect increases with electronegativity and closeness to the acidic site.

Explain how resonance delocalization affects acidity

When the negative charge of a conjugate base can be delocalized across multiple atoms via resonance, the conjugate base is significantly stabilized, making the parent acid much stronger.

Explain why phenols are more acidic than alcohols, and how EWG/EDGs affect phenol acidity

Phenols (pKa ~10) are more acidic than alcohols (pKa ~16-18) because the phenoxide negative charge is delocalized into the aromatic ring by resonance. Electron-withdrawing groups (EWGs, like -NO2) further increase acidity; electron-donating groups (EDGs, like -OCH3) decrease acidity.

Explain relationship between conjugate acid/base strengths

The stronger the acid, the weaker its conjugate base (and vice versa). For a conjugate pair, Ka * Kb = Kw (or pKa + pKb = 14 in water).

Explain conjugate base stability as the core method to compare acid strength

Acid strength is directly determined by the stability of its conjugate base (A-). The more stable A- is (via Atom electronegativity/size (size wins down group) , Resonance, Dipole/ Inductive effect, or Orbital hybridization (s-charcter: s-orbital is closer to nucleus, electrons held more tightly) - CARDIO), the stronger the starting acid HA.

Compare acidity/basicity based on periodic table trends

Across a row (left to right): Acidity increases as electronegativity increases (OH is more acidic than NH). Down a column: Acidity increases as atomic size increases, because negative charge is not squeezed into small volume = less repulsion = lower energy = more stable conjugate base.

Compare acidity/basicity based on protonation conditions

Positively charged species are far more acidic than their neutral counterparts because donating a proton neutralizes formal charge (H3O+ pKa -1.7 vs H2O pKa 15.7; NH4+ pKa 9.2 vs NH3 pKa 38).

Identify relative basicity of nitrogen groups

Amine (most basic, lone pair sp3 local) > Imine (sp2 nitrogen) > Aniline (lone pair delocalized into ring) > Amide (lone pair delocalized into carbonyl oxygen) > Pyrrole-like (least basic, lone pair part of aromatic 6 pi-electron system).

Define polyprotic acids, multiple pKa values, and why pKa values become progressively higher

Polyprotic acids can donate more than one proton. They have multiple pKa values (pKa1 < pKa2 < pKa3) because removing a positively charged proton from an already negatively charged anion becomes exponentially more difficult.

Formula cheat sheet

K_a × K_b = K_w = 1.0 × 10⁻¹⁴

[H⁺] = √(Ka × [HA]

What happens at equivalence point?

The moles of added titrant equal the moles of the original analyte.

Neither the acid nor the base is left over; only the products.

What happens at half-equivalence?

concentration of the remaining weak acid equals the concentration of its conjugate base

pH equals the pKa

What happens when water gets added to a buffer solution?

The addition of water dilutes the [HCOOH] and the [HCOO– ] by the same factor. pH stays the same.

How to find pH from titration graph?

Find equivalence point on X axis and divide in half to find half-equivalence and trace up the y-axis.