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Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.16

Lewis Theory of Acids and Bases.

Mix boron trifluoride with ammonia and they react, though no H⁺ changes hands. By the Brønsted-Lowry theory that is not an acid-base reaction at all. Lewis changed the question: not who gives away a proton, but who takes an electron pair and who gives it.

1. Whoever takes the pair is the acid

In 1923 G. N. Lewis gave the electronic theory of acids and bases. His definition takes two lines: a Lewis acid is a compound or ion that accepts an electron pair, and a Lewis base is an electron-pair donor.

This definition stretches the list of acids a long way. CO₂, SO₂, SO₃, BF₃, AlCl₃ and Cu²⁺ carry no hydrogen, yet all are Lewis acids. The link to Brønsted-Lowry fits in one line: every Brønsted-Lowry acid is a Lewis acid, but not every Lewis acid is a Brønsted acid. All cations, H⁺ included, and neutral molecules with an empty valence shell are Lewis acids.

Now the bases: a molecule with a lone pair, or a negative ion, is a Lewis base. NH₃, H₂O, OH⁻, CN⁻ and Cl⁻ are the examples; so is NF₃, since its N has a lone pair. On the other side, B in BCl₃ has an incomplete octet, so BCl₃ is a Lewis acid just like BF₃. Watch one trap: H⁺ is an acid because it takes a pair, and H₂O is a base because it gives one. Do not mix the two up.

Sort fourteen species into the two boxes.

sort them who takes the electron pair

Takes a pair or gives one

Pick up a species, then drop it in a box. One question decides it: will it take an electron pair or give one?

2. Where the pair goes

The book's examples keep returning to one picture. With a cationic Lewis acid: Cu²⁺ + 4NH₃ → [Cu(NH₃)₄]²⁺, and the solution turns deep blue. Cu²⁺ takes the pairs, NH₃ gives them.

A non-metal oxide can be a Lewis acid too: SO₂ + H₂O → H₂SO₃.

The neatest example is two neutral molecules joining: BF₃ + :NH₃ → F₃B←NH₃. NH₃ has a lone pair, so it is the Lewis base. BF₃ has an incomplete octet, so it is the Lewis acid. NH₃ gives the pair, BF₃ takes it, and a covalent (dative) bond forms between them.

Solved problem 1.69 asks you to spot the acid and base in four reactions. (a) CO₂ + OH⁻ → HCO₃⁻, CO₂ is the acid. (b) B(OH)₃ + OH⁻ → B(OH)₄⁻, B(OH)₃ is the acid. (c) Fe³⁺ + 6CN⁻ → [Fe(CN)₆]³⁻, Fe³⁺ is the acid, d²sp³. (d) HCl + NH₃ → NH₄Cl, where the acid is H⁺ and the base is NH₃. Below are six reactions: tap the acid first, then the base.

reactions the arrow points to the acid

Whose pair goes where

3. From both ends of a tube

Problem 1.70: BF₃ (M = 10.8 + 57 = 67.8) and NH₃ (M = 17) are released together from the two ends of an 18 cm tube. In the same time the distances covered are in the ratio of the rates, so r₁/r₂ = √(17/67.8) = 0.50. If BF₃ goes x cm, NH₃ goes (18 − x) cm, so x/(18 − x) = 0.5 and x = 6 cm. The heavy BF₃ travels 6 cm and the light NH₃ travels 12 cm.

The calculator holds the book's case and one practice case, NH₃ and HCl. You can type your own numbers too. Guess first, then check.

calculator the lighter gas moves faster

From both ends

The drawing is a hand-drawn model, not a measurement. Nothing here goes anywhere; the notebook is yours.

4. Check yourself

Three questions in the exam's style. The first is on the definition.

check yourself pick one answer

Question 1

An example of a Lewis acid is-

In the second, find who takes the pair.

check yourself pick one answer

Question 2

Which pair is Lewis acids?

The last one builds on the calculation.

check yourself pick one answer

Question 3

In CO₂ + OH⁻ → HCO₃⁻, which is the Lewis acid?

The whole note on seven cards.

cut from the note tap to turn over

Seven cards

QuestionWhat are a Lewis acid and a Lewis base?
AnswerAn acid accepts an electron pair; a base donates one.
QuestionGive four Lewis acids that have no hydrogen.
AnswerCO₂, SO₂, SO₃, BF₃, AlCl₃ and Cu²⁺ (any four).
QuestionHow are Brønsted and Lewis acids related?
AnswerEvery Brønsted-Lowry acid is a Lewis acid, but not every Lewis acid is a Brønsted acid; all cations including H⁺ are Lewis acids.
QuestionName five Lewis bases.
AnswerNH₃, H₂O, OH⁻, CN⁻, Cl⁻.
QuestionWhat bond forms in BF₃ + NH₃ and why is BF₃ a Lewis acid?
AnswerA covalent (dative) bond, F₃B←NH₃; BF₃ has an incomplete octet while NH₃ has a lone pair.
QuestionIn CO₂ + OH⁻ → HCO₃⁻, which is the Lewis acid and which the base?
AnswerCO₂ is the Lewis acid (accepts a pair), OH⁻ the Lewis base (gives a pair).
QuestionBF₃ and NH₃ are released from the ends of an 18 cm tube; how far does BF₃ go?
AnswerM(BF₃) = 67.8, M(NH₃) = 17; r₁/r₂ = √(17/67.8) = 0.50; x/(18−x) = 0.5, so x = 6 cm.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.16.