Topic 6C discusses the relationship between molecular structure and the strengths of acids. The same ideas can be applied to bases.

(a) Explain the relative strengths of the Brønsted bases OH^–, NH₂ ^–, and CH₃ ^– (see Table 6C.3).

(b) Explain why NH₃ is a weak base in water, but PH₃ forms essentially neutral solutions.

(c) If you were ranking the species in (a) or (b) as Lewis bases, would your rankings be the same or different? Explain your reasoning.

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TABLE 6C.3 Conjugate Acid-Base Pairs Arranged by Strength pK₂ Acid name Acid formula Strong acid hydroiodic acid perchloric acid hydrobromic acid hydrochloric acid sulfuric acid chloric acid nitric acid hydronium ion hydrogen sulfate ion nitrous acid 1.92 3.37 3.45 4.75 6.37 6.89 hydrosulfuric acid 9.25 ammonium ion 9.31 hydrocyanic acid 10.25 hydrogen carbonate ion 10.56 methylammonium ion hydrofluoric acid acetic acid carbonic acid water ammonia hydrogen methane hydroxide ion Very weak acid HI HCIO. HBr HCI H₂SO4 HClO3 HNO3 H₂O+ HSO HNO₂ HF CH₂COOH H,CO, H₂S NH₂+ HCN HCO, CH3NH3* H₂O NH3 H₂ CH₁ OH™ Base formula I™ clo₁™ Br CI- HSO clo, NO₂™ H₂O SO₂² NO₂™ F- CH,CO, HCO, HS™ NH, CN CO3²- CH,NH, OH NH₂™ H™ CH3 0²- Base name Very weak base iodide ion perchlorate ion bromide ion chloride ion hydrogen sulfate ion chlorate ion nitrate ion water sulfate ion nitrite ion fluoride ion acetate ion hydrogen carbonate ion hydrogen sulfide ion ammonia cyanide ion carbonate ion methylamine hydroxide ion amide ion hydride ion methide ion oxide ion Strong base pKb 12.08 10.63 10.55 9.25 7.63 7.11 4.75 4.69 3.75 3.44