Question

For NH4+, write an equation that shows how the cation acts as an acid

Answer 1

Cations and anions are fancy words for positive and negative ions respectively. I like to remember it by saying to myself 'which electrode would it be attracted to?'. A positively charged ion would move towards the negatively charged cathode, hence 'cation', and the reverse is true for 'anion'.

Okay, by the Bronsted-Lowry theory of acids and bases, an acid is a species that donates a proton and a base is a species that accepts a proton. Given that a proton is simply a H+ cation, any substance containing a hydrogen atom has the capacity to behave as an acid. The first answer is:

NH4+ --> NH3 + H+

CO3+ cannot by itself act as an acid, for the simple reason that it does not contain any hydrogen atoms. Hence this seems to be a trick question!

CH2NH3+ will most likely lose a proton from the positively charged NH3 part of the molecule, so as to neutralise the charge of the organic molecule:

CH2NH3+ --> CH2NH2 + H+

I hope this helps you :)

Answer 2

According to  Bronsted-Lowry theory acid are donor of protons and bases are acceptors of protons (the hydrogen cation or H⁺).

1) NH₄⁺(aq) + H₂O(l) ⇄ NH₃ + H₃O⁺(aq); the ammonium cation (NH₄⁺) gives proton to water.

2)  [Co(H₂O)₆]³⁺ + H₂O ⇌ [Co(H₂O)₅(OH)]²⁺ + H₃O⁺; cobalt cation (Co³⁺) forms complex cation with six molecules of water, which act like acid and gives protons.

3) CH₂NH₃⁺(aq) + H₂O(l) ⇄ CH₂NH₂ + H₃O⁺(aq); H⁺ is cationic form of atomic hydrogen, the hydrated form of the hydrogen cation is the hydronium ion H₃O⁺(aq).