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A-Level Competing definitions of acids and bases
What the A-Level syllabus expects for Competing definitions of acids and bases, and how to practise it.
What the syllabus expects
- Grasp the Arrhenius account of acids and bases and put it into practice.
- Grasp and apply the Bronsted-Lowry model, including how a conjugate acid pairs with a conjugate base.
- Grasp and apply the Lewis theory, which reaches even to non-aqueous cases such as BF3 with NH3.
How it's examined
Questions on this topic most often ask you to explain, state. About 2% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Place phenol (C6H5OH), butanoic acid (CH3CH2CH2CO2H) and 2-bromobutanoic acid (CH3CH2CHBrCO2H) in order of increasing acid strength, and explain your ordering.
Show the worked answer
Increasing acid strength = weakest first. Phenol (C6H5OH): the phenoxide ion is stabilised by delocalisation into the ring, but O-H of phenol is far weaker as an acid than a carboxylic acid (pKa ~10 vs ~4-5) because carboxylate delocalises negative charge over two electronegative O atoms. Butanoic acid (CH3CH2CH2CO2H, pKa ~4.8): a carboxylic acid, much stronger than phenol. 2-bromobutanoic acid (CH3CH2CHBrCO2H, pKa ~2.9): the electron-withdrawing Br on the alpha carbon has an inductive (-I) effect that pulls electron density away from the carboxylate, stabilising the anion and further stabilising the conjugate base, so it is the strongest acid. Order of increasing acid strength: phenol < butanoic acid < 2-bromobutanoic acid.
Example 2 (2 marks)
Trichloroethanoic acid has pKa 0.52 compared with 4.76 for ethanoic acid. Account for why trichloroethanoic acid has the lower pKa.
Show the worked answer
In trichloroethanoic acid the three Cl atoms are electronegative and electron-withdrawing (inductive effect). They pull electron density away from the carboxylate group, dispersing/stabilising the negative charge on the CCl3COO- anion. The more stable anion means the acid dissociates more readily, so it is a stronger acid with a lower pKa than ethanoic acid, whose CH3 group is not electron-withdrawing.
Example 3 (2 marks)
Imidazole can behave as both an acid and a base. Write balanced equations that demonstrate each of these two aspects of its amphoteric behaviour.
Show the worked answer
Imidazole (C3H4N2) has a basic pyridine-type N (lone pair) and an acidic N-H. As a base it accepts a proton at the pyridine-type nitrogen: C3H4N2 + H2O -> C3H5N2+ + OH- (or C3H4N2 + H+ -> C3H5N2+, the imidazolium ion). As an acid it donates the N-H proton: C3H4N2 + H2O -> C3H3N2- + H3O+ (or C3H4N2 -> C3H3N2- + H+, the imidazolate ion).
More worked questions on this topic
- In step 2 of the iodoform mechanism the hydrogens are replaced one after another because OH- re (2 marks)
- Isopropylamine, (CH3)2CHNH2, is widely used as a feedstock for making insecticides and pharmace (2 marks)
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · Enthalpy, entropy and the feasibility of reactions · all of A-Level H2 Chemistry