The reaction for tarnish formation due to contact with hydrogen sulphide.

$2A{g}_{\left(s\right)}+H_2{S}_{\left(g\right)}\to A{g}_2{S}_{\left(s\right)}+H_{2\left(g\right)}$  and the above tarnish can be removed by using Al metal as

$2Al+3A{g}_{2}S\to A{l}_2{S}_3+6Ag$

The above two reactions appears to be a displacement reactions. But ‘Ag’ is less reactive than ‘H’ in reactivity series. Then what might be the reason.

i) ‘Ag’ can displace ‘H’ under certain conditions even though it is less reactive

ii) Ag cannot react directly with $H_{2}S$  to displace ‘H’ from it.

iii) ‘Ag’ first oxidised to its oxide and then it reacts with $H_{2}S$  to form $A{g}_{2}S$

iv) The formation of $A{g}_{2}S$  is possible by a decomposition reaction.