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Química Geral · Acids and bases

Ácidos, bases e pH

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O que você vai aprender

  • Identify Brønsted and Lewis acids and bases, conjugate pairs and amphoteric species, and draw proton transfers with curved arrows.
  • Predict the direction of an acid–base equilibrium from pKa values and estimate its equilibrium constant.
  • Rank acids and bases using the charged atom, resonance, induction and orbital hybridization.
  • Calculate pH, pOH, Ka and pKb for strong and weak acids and bases at 25 °C.
  • Design and analyse buffers with the Henderson–Hasselbalch equation, including after addition of strong acid or base.
  • Follow the stepwise ionization of polyprotic acids and identify the major species at a given pH.

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Brønsted and Lewis acids

A Brønsted acid donates a proton (H⁺) and a Brønsted base accepts one, always with a lone pair. When acetic acid meets hydroxide, a hydroxide lone pair forms a new O–H bond and the old O–H bonding pair stays on the acetate oxygen. Each side of the equation has an acid and a base: acetic acid and acetate are a conjugate pair (they differ by exactly one H⁺), and so are water and hydroxide. The Lewis definition is broader: a Lewis base donates an electron pair and a Lewis acid accepts one. Boron trifluoride has an empty p orbital on boron, so it accepts the nitrogen lone pair of ammonia to give the adduct F₃B–NH₃ even though no proton moves. Every Brønsted base is also a Lewis base; H⁺ itself is the Lewis acid in a proton transfer.

pKa and the direction of equilibrium

pKa = −log Ka; the smaller the pKa, the stronger the acid and the weaker its conjugate base. An acid–base equilibrium always favours the side with the weaker acid (higher pKa). For a proton transfer HA + B⁻ ⇌ A⁻ + HB, K = 10^(pKa of HB − pKa of HA). Acetic acid (pKa 4.76) with hydroxide gives water (pKa 15.7), so K ≈ 10¹¹ and the reaction is effectively complete. The value 15.7 for water is the one used when comparing organic acids; it treats water as a solute at 55.5 M. Taking the activity of water as 1, as Kw does, gives 14.0 instead, and every equilibrium direction in this topic is the same on either scale. A base can fully deprotonate an acid only if its conjugate acid is weaker than that acid: hydroxide cannot deprotonate a terminal alkyne (pKa ≈ 25), but amide ion, whose conjugate acid ammonia has pKa ≈ 38, can. In water the strongest acid that can exist is H₃O⁺ and the strongest base is OH⁻; stronger acids such as HCl, HBr and HClO₄ are all levelled to H₃O⁺.

What makes an acid strong

Compare conjugate bases: the more stable A⁻ is, the stronger HA is. Four factors decide, in roughly this order of importance. Atom: across a period acidity rises with electronegativity (CH₄ ≈ 50, NH₃ ≈ 38, H₂O 15.7, HF 3.2); down a group it rises with atom size because a larger ion spreads its charge (HF < HCl < HBr < HI). Resonance: acetate shares its charge equally over two oxygens (drawn), so acetic acid (4.76) is about 10¹¹ times more acidic than ethanol (≈16); phenoxide spreads its charge into the ortho and para ring carbons, giving phenol a pKa of 10.0. Induction: electronegative atoms pull electron density away from the charge, an effect that fades quickly with distance (2-chlorobutanoic acid 2.86, 3-chloro 4.05, 4-chloro 4.52, butanoic 4.82); extra oxygens on the central atom do the same in oxyacids (HOCl < HClO₂ < HClO₃ < HClO₄). Orbital: a lone pair in an orbital with more s character is held closer to the nucleus, so C–H acidity rises sp³ < sp² < sp (ethane ≈ 50, ethene ≈ 44, ethyne ≈ 25).

pH of strong and weak acids and bases

pH = −log[H₃O⁺] and pOH = −log[OH⁻]. At 25 °C water autoionizes with Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴, so pH + pOH = 14.00 and, for a conjugate pair, Ka × Kb = Kw, or pKa + pKb = 14.00. A strong acid or base ionizes completely: 0.010 M HCl has pH 2.00, and 0.0050 M Ba(OH)₂ releases 0.010 M OH⁻, pOH 2.00, pH 12.00. A weak acid ionizes only slightly; when x is small compared with the starting concentration C, Ka = x²/C gives [H₃O⁺] = √(Ka·C). For 0.10 M acetic acid (Ka = 1.8 × 10⁻⁵), [H₃O⁺] = √(1.8 × 10⁻⁶) = 1.34 × 10⁻³ M and pH = 2.87; only about 1.3% of the acid is ionized. The same equation with Kb gives [OH⁻] for a weak base such as ammonia. Very dilute strong acids are the exception: below about 10⁻⁶ M the H₃O⁺ from water itself cannot be ignored, and an acid solution never becomes basic.

Buffers

A buffer contains a weak acid and its conjugate base in comparable amounts. Rearranging Ka gives the Henderson–Hasselbalch equation, pH = pKa + log([A⁻]/[HA]). When the two are equal, pH = pKa; a tenfold excess of base raises the pH by one unit, and a tenfold excess of acid lowers it by one. Added H₃O⁺ is consumed by A⁻ (acetate + H₃O⁺ → acetic acid + water) and added OH⁻ by HA, so a strong acid or base is exchanged for a weak one and the ratio changes only a little. Example: 1.00 L holding 0.100 mol acetic acid and 0.100 mol acetate takes 0.020 mol NaOH to give 0.080 mol acid and 0.120 mol acetate, and the pH moves only from 4.76 to 4.76 + log 1.5 = 4.94. Choose a pair whose pKa lies within one unit of the target pH. In a titration of a weak acid, the pH at half the equivalence volume equals its pKa.

Polyprotic acids and amphoteric ions

A polyprotic acid loses its protons one at a time, and each step is weaker than the one before because the proton must leave an increasingly negative ion: phosphoric acid has pKa 2.15, 7.20 and 12.35. Between two pKa values one species dominates; at pH 10.0 the main phosphate species is HPO₄²⁻ (drawn), because the pH is above pKa2 but below pKa3. The intermediate ions, such as H₂PO₄⁻, HPO₄²⁻ and HCO₃⁻, are amphoteric: they can donate a proton or accept one. A solution of an amphoteric ion alone has pH ≈ ½(pKa1 + pKa2) for the two pKa values that bracket it, so sodium bicarbonate (carbonic acid pKa 6.35 and 10.33) gives pH ≈ 8.34. Bicarbonate neutralizes acid by becoming carbonic acid, which then loses CO₂, and neutralizes base by becoming carbonate.

What to carry forward

Find the acid and the base, identify each conjugate pair, and let the pKa values decide where the equilibrium lies: it favours the weaker acid. Explain acid strength by the stability of the conjugate base (atom, resonance, induction, orbital). For numbers, use pH = −log[H₃O⁺], pH + pOH = 14.00 at 25 °C, [H₃O⁺] = √(Ka·C) for a weak acid, and pH = pKa + log([A⁻]/[HA]) for buffers and for each step of a polyprotic acid.

Problemas de prática (40)

Praticar é grátis com uma conta.

Desenhe o mecanismo 10

pH, Ka and buffers

  • An acetate buffer neutralizes added acidDraw acetate in a buffer capturing a proton from hydronium, and explain why the pH changes only slightly.

Acid strength and pKa

  • Ammonia deprotonates acetic acidDraw the proton transfer from acetic acid to ammonia with curved arrows, decide from pKa values which side is favoured, and show the two equivalent resonance contributors of acetate.
  • Hydroxide deprotonates 4-methylphenol; phenoxide resonanceDraw the deprotonation of 4-methylphenol by hydroxide and push curved arrows through the resonance contributors of the phenoxide to show which atoms share the negative charge.
  • Amide ion deprotonates a terminal alkyneDraw amide ion removing the terminal sp C–H proton of propyne and justify the choice of base from pKa values.

Brønsted and Lewis acids

  • Boron trifluoride and ammonia form a Lewis adductDraw the single curved arrow by which ammonia donates its lone pair to boron trifluoride, and assign the formal charges of the adduct.
  • Hydroxide adds to carbon dioxideDraw hydroxide acting as a Lewis base toward the electrophilic carbon of carbon dioxide to form bicarbonate.
  • Autoionization of waterDraw the proton transfer between two water molecules that gives hydronium and hydroxide, and relate it to Kw and the pH of pure water.

Polyprotic acids and amphoteric ions

  • Bicarbonate acting as a baseDraw bicarbonate accepting a proton from hydronium to give carbonic acid, and predict what carbonic acid does next.
  • Bicarbonate acting as an acidDraw bicarbonate donating its proton to hydroxide to give carbonate, showing that bicarbonate is amphoteric.
  • Second ionization of phosphoric acidDraw hydroxide removing a proton from dihydrogen phosphate to give hydrogen phosphate, and connect this step to pKa2 of phosphoric acid.

Preveja o produto 1

  • Major phosphate species at pH 10Predict the dominant form of a polyprotic acid at a given pH from its successive pKa values.

Classificar 4

Polyprotic acids and amphoteric ions

  • Spot the amphoteric ionRecognize an amphoteric species as one that has both a removable proton and a lone pair that can accept one.

Brønsted and Lewis acids

  • Find the conjugate pairIdentify a conjugate acid–base pair as two species that differ by exactly one proton.
  • A Lewis acid that is not a Brønsted acidDistinguish an electron-pair acceptor without protons from Brønsted proton donors.

pH, Ka and buffers

  • Which mixture is a buffer?Decide whether a mixture contains a weak conjugate pair in comparable amounts after any neutralization.

Comparar 7

Acid strength and pKa

  • Most acidic O–HCombine resonance and inductive effects to pick the most acidic of four O–H acids.
  • Hybridization and C–H acidityIdentify the most acidic hydrocarbon from the hybridization of the carbon that keeps the lone pair.
  • Rank bases from conjugate-acid pKaRank anions by base strength using the pKa values of their conjugate acids.
  • Chlorine oxyacidsRank oxyacids of the same central atom by acidity from the number of oxygen atoms on that atom.
  • Inductive effect and distanceRank chlorinated carboxylic acids by acidity from the distance between the chlorine and the carboxyl group.
  • Acidity down the halogen groupRank the hydrogen halides by intrinsic acid strength using atom size and bond strength.
  • Acidity across the second periodRank the second-period hydrides by acidity using the electronegativity of the atom that keeps the charge.

Calcular 12

pH, Ka and buffers

  • Base-to-acid ratio for an ammonia bufferFind the base-to-acid concentration ratio needed for an ammonia–ammonium buffer at a target pH.
  • Buffer pH after adding NaOHCalculate the pH of a buffer after a strong base reacts with its acid component.
  • pH of an acetic acid–acetate bufferUse the Henderson–Hasselbalch equation to find the pH of a buffer with unequal acid and base concentrations.
  • Percent of acetic acid ionized at pH 5.76Calculate the percentage of a weak acid present as its conjugate base at a fixed pH.
  • pH of 0.10 M acetic acidCalculate the pH of a weak acid solution from Ka and concentration using the small-x approximation.
  • pH of 0.10 M ammoniaCalculate the pH of a weak base solution from Kb, converting through pOH.
  • pH of 0.0050 M Ba(OH)₂Calculate the pH of a strong base that releases two hydroxide ions per formula unit.
  • pH of 0.010 M HClCalculate the pH of a strong monoprotic acid from its concentration.

Polyprotic acids and amphoteric ions

  • pH of sodium bicarbonate solutionEstimate the pH of a solution of an amphoteric ion from the two pKa values that bracket it.
  • Phosphate ratio at pH 7.40Choose the correct pKa of a polyprotic acid and calculate the base-to-acid ratio at a given pH.

Acid strength and pKa

  • log K for phenol plus ammoniaCalculate the equilibrium constant of a proton transfer from the pKa values of the acids on each side, including an unfavourable case.
  • pKb of acetateConvert the pKa of an acid into the pKb of its conjugate base using Kw.

Encontre o erro 1

  • A basic pH for an acid?Diagnose the error of ignoring water autoionization for a very dilute strong acid.

Interprete as evidências 1

  • Reading pKa from a titration curveRead the pKa of a weak acid from the pH at half the equivalence volume of its titration with strong base.

Planeje uma síntese 1

  • Pick a buffer for pH 9.0Choose a conjugate acid–base pair whose pKa makes an effective buffer at a target pH.

Explique por quê 3

Acid strength and pKa

  • Why carboxylic acids beat alcoholsExplain the acidity difference between a carboxylic acid and an alcohol by resonance in the conjugate base.
  • Separating a carboxylic acid from a phenolUse pKa values to explain why a weak base extracts a carboxylic acid but not a phenol.

Brønsted and Lewis acids

  • Why strong acids look equally strong in waterExplain the levelling effect of water on acids stronger than hydronium.

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