Theories of acids and bases
4 marks · NormalThe hydroxonium ion, H_3O^+, plays a key role in acidbase reactions.
Draw a dot-and-cross diagram to show the bonding in a hydroxonium ion, H_3O^+.
Explain, in terms of proton transfer, how H_3O^+ is formed from HCl and H_2O.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show 3 O-H bonds correctly shown, Lone pairs on O, H^+ added via dative covalent bond
(b) The HCl donates a proton (H^+) to a water molecule, which acts as a base and accepts the proton, forming H_3O^+. · must show Identifies proton comes from HCl, Explains H_2O accepts the proton to become H_3O^+
Must show: Three O-H covalent bonds in H_3O^+, Lone pairs on O, Correct positive charge, Dative bond identified or clear third bond, Description of H^+ transfer from HCl to H_2O
Also accept: H^+ joins H_2O to make H_3O^+
Common mistakes: Incorrect number of electrons/bonds; Not explaining the bond transfer
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Marking points
D1 (a) · 2 marks · 1 mark: correct diagram structure, 1 mark: correct showing of all valence electrons/lone pairs and positive charge.
E1 (b) · 1 mark · States HCl donates a proton to H_2O.
E2 (b) · 1 mark · H_2O acts as a base, accepts proton and forms H_3O^+.
Worked solution
a) Dot-and-cross diagram: O at center, three H atoms bonded, lone pairs on O, with one bond (from H^+) formed by oxygen donating a lone pair (dative covalent bond). Total charge +1.
b) HCl donates a proton (H^+) to a water molecule. Water accepts the H^+ on its lone pair to form the hydroxonium ion, H_3O^+.