When metals like Zn and Cu react with concentrated nitric acid, nitrogen oxides are produced as by-products. The nature of the oxide depends on the metal's reactivity.
Zinc (Zn), being more reactive, reacts with concentrated HNO$_3$ to form nitrogen dioxide (NO$_2$), a brown toxic gas. Copper (Cu), though less reactive, also forms NO$_2$ with concentrated nitric acid. In both cases, NO$_2$ is the nitrogen oxide formed due to the strong oxidizing nature of concentrated HNO$_3$.
Thus, in both reactions, the nitrogen oxide evolved is NO$_2$.
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Find A and B:
The given reaction shows that both zinc (Zn) and copper (Cu) react with concentrated nitric acid (HNO₃), producing different by-products along with an oxide of nitrogen in each case. We are to identify the oxide of nitrogen (A and B) released in both reactions.
Step 1: Reaction of Zn with concentrated HNO₃:
Zinc is an active metal and reacts readily with concentrated nitric acid. The reaction is a redox process where zinc is oxidized and nitric acid acts as an oxidizing agent. The nitrogen in nitric acid is reduced to nitrogen dioxide (NO₂), a reddish-brown toxic gas.
Reaction:
\[
\text{Zn} + 4HNO_3 \rightarrow Zn(NO_3)_2 + 2H_2O + 2NO_2
\]
So, oxide of nitrogen (A) = NO₂
Step 2: Reaction of Cu with concentrated HNO₃:
Copper is less reactive than zinc but still reacts with concentrated nitric acid. This is also a redox reaction, where copper is oxidized to Cu²⁺ and nitric acid is reduced, again releasing nitrogen dioxide (NO₂).
Reaction:
\[
\text{Cu} + 4HNO_3 \rightarrow Cu(NO_3)_2 + 2H_2O + 2NO_2
\]
So, oxide of nitrogen (B) = NO₂
Conclusion:
In both reactions, the oxide of nitrogen released is nitrogen dioxide.
Final Answer:
\[
\boxed{\text{NO}_2, \ \text{NO}_2}
\]
