Question

Which metal can react with an acid? Cu, Ag, Al, Au (Answer is Al) Can someone explain why?

Which of the following is the most likely to act as a Lewis Acid? PH3, AlCl3, HCN, BeO2 (Answer is AlCl3) Can someone explain why? Im assuming its because AlCl3 does not have a longe pair to donate while the rest do?

Answer 1

An acid is represented generally as HA, where H is proton and A is the anion corresponding to that acid. Reacting with an acid means that the metal M should replace H to form MA. Thus, only those metals which are capable of replacing H can react with the acid. This capability is determined experimentally for all elements and a series known as an electrochemical series has been formulated. All the elements with -ve reduction potential ( i.e. all the elements above H in the series ) can replace H from an acid.

A Lewis acid is one which can accept a lone pair of electrons, which is possible only if the central metal atom in that compound has less than * electrons around it ( i.e. its octet is incomplete). In AlCl3, there are 6e- around the Al ion. Thus it is deficient of electrons and can accept a pair of e-. Rest of the compounds either have their octet complete or have lone pairs of e- on the cental metal, for instance PH3.