Acid and base strength from molecular structure
ChemistryAcids, Bases & TitrationAges 16–17
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Why is one acid stronger than another? Change bond polarity and bond strength (period 2 hydrides, hydrogen halides), electronegativity and the number of oxygen atoms (HOX, chlorine oxoacids), electron-withdrawing or donating groups and their distance (chloroethanoic, butanoic and substituted benzoic acids) and charge delocalisation in phenoxide ions; show the conjugate base and read pKa on a scale. The Amines screen compares the base strength of NH₃, aliphatic amines and phenylamine in water and in the gas phase; the Nucleophiles screen runs ammonia with bromoethane to give primary to quaternary products; the Reaction cards show the Kolbe, Reimer–Tiemann, Hell–Volhard–Zelinsky and decarboxylation reactions.
Lesson: Acid and base strength: inductive effect, delocalisation and bond strength
What it shows
Acid strength depends on how easily the H–A bond breaks and how stable the conjugate base A⁻ is. Across a period, bond polarity decides (CH₄ to HF); down a group, bond strength decides (HF to HI). Electronegative atoms and extra oxygen atoms pull electron density away and spread the negative charge (HOCl, HClO₄, chloroethanoic acids), alkyl groups push it back, and in phenoxide ions the charge is delocalised into the ring. For amines the lone pair on nitrogen accepts H⁺: alkyl groups make it more available, delocalisation into a benzene ring makes it less available, and in water hydrogen bonding to the ion matters too.
How to use
On Acids, choose a set under Compare and a Compound, read pKa, Ka and the scale, and tick Show conjugate base (remove H⁺). On Amines (bases), pick an Amine, switch Phase between In water and Gas phase, and tick Show protonated form (conjugate acid). On Amines as nucleophiles, set the Mole ratio NH₃ : bromoethane and press Run; each run adds a row to the table. Reaction cards show four named reactions. Answer the Check yourself questions and press Check.
Parameters you can change
- Starting screen Acids, Amines (bases), Amines as nucleophiles, Reaction cards
- Acids: comparison set Period 2 hydrides, Hydrogen halides (group 17), Hypohalous acids HOX, Chlorine oxoacids, Number of Cl atoms, Which halogen, Position of Cl, Alkyl groups, Substituted benzoic acids, Alcohol and phenols
- Acids: compound number in the set (1 = weakest acid) 1–7
- Acids: show conjugate base
- Amines: compound NH₃, CH₃NH₂, (CH₃)₂NH, (CH₃)₃N, C₂H₅NH₂, (C₂H₅)₂NH, (C₂H₅)₃N, C₆H₅NH₂, 4-CH₃C₆H₄NH₂, 4-NO₂C₆H₄NH₂
- Amines: phase In water, Gas phase
- Amines: show protonated form
- Nucleophiles: mole ratio NH₃ : bromoethane 0.25–20
- Reaction cards: reaction Kolbe, Reimer–Tiemann, Hell–Volhard–Zelinsky, Decarboxylation
- Reaction cards: R group (HVZ, decarboxylation) R = CH₃, R = C₂H₅, R = C₆H₅
- Reaction cards: halogen (HVZ) Cl₂, Br₂
Questions to explore
- Why is HF a much weaker acid than HI even though fluorine is more electronegative?
- How does each chlorine atom make chloroethanoic acids stronger, and why does the effect fade along the chain?
- Why is phenylamine a weaker base than ammonia while ethylamine is stronger?
