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.
1) It is correct to say that we use dilute acid instead of concentrated acid because the reaction can be too vigorous if we use concentrated acid. When metals above hydrogen in the reactivity series react with acids, they undergo a displacement reaction, where the metal displaces hydrogen from the acid to form metal chloride and hydrogen gas. This reaction generates heat and gas, causing the mixture to bubble and potentially release gas rapidly. If concentrated acid is used, the reaction can be highly exothermic and produce a large amount of gas, leading to a violent reaction. Dilute acid, on the other hand, contains a lower concentration of acid and thus reduces the risk of an uncontrollable and dangerous reaction.
2) Metals do not react with dilute nitric acid in the same way as they react with dilute hydrochloric acid or dilute sulphuric acid. Dilute nitric acid is a strong oxidizing agent, meaning it has the ability to accept electrons from other substances during a chemical reaction. In the presence of metals, dilute nitric acid undergoes a different type of reaction called oxidation, where it oxidizes the metal rather than being displaced by it. This oxidation reaction produces metal nitrate and nitric oxide gas, instead of metal chloride and hydrogen gas. Therefore, the reactivity of metals with dilute nitric acid is different from their reactivity with dilute hydrochloric acid or dilute sulphuric acid.