posted by candy .
Metals above hydrogen in the reactivity series react with dilute hydrochloric acid or dilute sulphuric acid to give metal chloride + hydrogen.
1)Is it correct to say that we use dilute acid,not concentrated acid because the reaction can take place with dilute acid & will be too vigorous if we use concentrated acid?
2)Why don't metals react with dilute nitric acid in the same way?
First, let me correct the question. "..... to give metal chlorides + hydrogen or metal sulphates + hydrogen."
1. I think your answer is ok but you need to understand that concentrated H2SO4, especially if hot, gives a different kind of reaction to that of dilute H2SO4. In fact, most metals will react faster with dilute H2SO4 than with concd H2SO4.
2. HNO3 and HCl are different because HNO3 is an oxidizing agent as well as an acid while HCl is just an acid. So reaction with HCl has just one reaction path but reaction with HNO34 has two, one as an oxidant and the other as an acid. With this in mind, HNO3, therefore, will react with Cu (Cu is below H in the activity series) because it can go into the oxidizer mode. The products are NOT H2 and the copper nitrate but Copper nitrate + NO or NO2 (depending upon the strength of the HNO3) + H2O.