IB Chemistry HL R3.4 Electron-pair sharing reactions questions

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Question 1

Paper 2Hard12 marks

Boron trifluoride reacts with ammonia to form a boron-containing adduct. A sample initially contains 1.00 mmol1.00\ \mathrm{mmol} of BF3\mathrm{BF_3} Ammonia is added in measured amounts, and the amount of adduct formed is determined.

00.20.40.60.811.21.41.600.20.40.60.811.2Amount of NH3 added / mmolAmount of adduct formed / mmol

The following two stages occur when propanone reacts with cyanide ions in the presence of hydrogen cyanide:

(CH3)2C=O+CN−⟶(CH3)2C(O−)(C≡N)\mathrm{(CH_3)_2C{=}O + CN^- \longrightarrow (CH_3)_2C(O^-)(C{\equiv}N)} (CH3)2C(O−)(C≡N)+H−C≡N⟶(CH3)2C(OH)(C≡N)+CN−\mathrm{(CH_3)_2C(O^-)(C{\equiv}N) + H{-}C{\equiv}N \longrightarrow (CH_3)_2C(OH)(C{\equiv}N) + CN^-}

In the product of the first stage, the carbon atom of CN−\mathrm{CN^-} is bonded to the carbon atom that was part of the carbonyl group.

(a)

Use Figure 1 to estimate the amount of NH3\mathrm{NH_3} at which the amount of adduct formed first becomes approximately constant. Hence, deduce the stoichiometric ratio of BF3\mathrm{BF_3} to NH3\mathrm{NH_3} in the adduct.

[2]
(b)

Write an equation for the formation of the adduct, showing the coordinate bond, and identify the Lewis acid and Lewis base.

[2]
(c)

State why the formation of the adduct in (b) is not a Brønsted–Lowry acid–base reaction.

[1]
(d)

A student claims, “Since each stage in the propanone reaction involves an electron-pair donor and an electron-pair acceptor, both stages must also be Brønsted–Lowry acid–base reactions.”

Annotate the two stages with curly arrows to show electron-pair movement. Identify the Lewis acid and Lewis base in each stage, and evaluate the student’s claim.

[7]

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