IB Docs (2) Team MC test: Lewis acids & bases
Multiple choice test on 18.1 Lewis acids & bases
Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).
If you get an answer wrong, read through the explanation carefully to learn from your mistakes.
Which are correct statements concerning the Brønsted-Lowry and Lewis theories of acids and bases?
I. Brønsted-Lowry theory is concerned with proton transfer and Lewis theory is concerned with electron pair transfer.
II. When 'curly arrows' are used in organic mechanisms it indicates a Lewis acid-base reaction.
III. All Lewis acids are also Brønsted-Lowry acids.
All Brønsted-Lowry acids are also Lewis acids since the transfer of a proton involves the movement of an electron pair. However, if a Lewis acid contains no protons then it cannot be a Brønsted-Lowry acid.
Which can act as a Lewis acid but not as a Bronsted-Lowry acid?
Zn2+ can attracts the electron pairs from ligands such as water to form complex ions e.g. [Zn(H2O)6]2+. Neither PF6− nor CH4 can accept a pair of electrons and NH4+ can also act as a Brønsted-Lowry acid.
Which reaction can be classified as an acid-base reaction only by using the Lewis theory.
The formation of sodium bromide from its elements is not an acid-base reaction. The other three are all Lewis acid-base reactions but the reactions of ammonia with water and hydrochloric acid are also Brønsted-Lowry acid-base reactions.
Which are correct statements about Lewis bases?
I. Lewis bases donate a pair of electrons.
II. All ligands can function as Lewis bases.
III. All nucleophiles can function as Lewis bases.
Ligands and nucleophiles both contain at least one non-bonding pair of electrons and function as Lewis bases by donating them when they react.
Which can act as a Lewis base?
Only a fluoride ion can donate a pair of electrons.
The mechanism for the electrophilic addition of hydrogen bromide to ethene is shown
Which row gives the correct description of how the reactants and intermediates are acting?
| Row | ethene | hydrogen bromide | carbocation intermediate | bromide ion |
| 1 | Lewis base | Lewis acid | Lewis base | Lewis acid |
| 2 | Lewis base | Lewis acid | Lewis acid | Lewis base |
| 3 | Lewis acid | Lewis base | Lewis base | Lewis acid |
| 4 | Lewis acid | Lewis base | Lewis acid | Lewis base |
The π bond in ethene is donating a pair of electrons to the δ+ on the H atom of HBr and the bromide ion is donating a pair of electrons to the intermediate carbocation.
Aluminium chloride can act as a catalyst for the reaction between benzene and chlorine to form chlorobenzene.
Which best describes how chlorine is behaving in the first concerted step?
One of the chlorine atoms is accepting a pair of electrons from the delocalized benzene π electrons and at the same time the other chlorine atom is donating a pair of electrons to the aluminium chloride catalyst.
Which is not a Lewis acid-base reaction?
The chlorination of methane in ultraviolet light proceeds via a free radical mechanism where bonds are broken homolytically and does not involve the transfer of a pair of electrons.
Which are correct statements concerning the Lewis theory of acids and bases?
I. The conjugate base of a Brønsted-Lowry acid is a Lewis base.
II. A Lewis acid must contain an empty orbital capable of accepting a pair of electrons.
III. The bond formed between a Lewis acid and a Lewis base is a coordinate (dative) covalent bond.
Since all Brønsted-Lowry acids are also Lewis acids then all Brønsted-Lowry conjugate bases must be Lewis bases. A Lewis acid must have an empty orbital capable of accepting the pair of electrons donated by the Lewis base and as both electrons in the bond formed between a Lewis acid and Lewis base come from the base the bond formed will be a coordinate bond.
Which species cannot function as a Lewis acid?
Only I− cannot function as a Lewis acid as it cannot accept a pair of electrons as it has a noble gas electron configuration. In fact it functions as a Lewis base in the reaction with iodine to form the triiodide ion, I3−.
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