An analyst observed evolution of bubbles with colourless gas by adding copper metal to two-molar HNO₃. In a similar experiment, large amounts of brown gas evolved after adding copper metal to 10-molar HNO₃. From these observations, it can be concluded that blank. A) The reaction product depends on the concentration of nitric acid. B) Copper can be oxidised. C) The colourless gas was oxygen. D) Copper dissolves in 0.5-molar HNO₃. E) Copper reduces nitric acid to ammonium.
This question describes two experiments. In the first, copper metal is added to two-molar HNO₃, or nitric acid, which produced a colourless gas. In the second experiment, copper metal is added to 10-molar HNO₃, which produced large amounts of a brown gas. So what changed between these two experiments?
Well, the concentration of nitric acid increased quite dramatically between the first experiment to the second experiment. And in the first experiment, the product is colourless. But in the second experiment, the product is brown, which indicates that they’re probably different chemical species.
From what we discussed so far, this matches answer choice A. The reaction product depends on the concentration of nitric acid. When we changed the concentration of nitric acid from two molar to 10 molar, the product changed from a colourless gas to a brown gas.
If we look at the chemical equations for this reaction, we’ll see that this is in fact the case. When copper metal reacts with dilute nitric acid, the products are copper nitrate, nitrogen oxide, and water. But when copper metal reacts with concentrated nitric acid, the products are copper nitrate, nitrogen dioxide, and water.
So the identity of the colourless gas that was produced as a result of the first experiment is nitrogen oxide. And the identity of the brown gas that was produced as a result of the second experiment was nitrogen dioxide.
So of our answer choices, it can only be concluded that the reaction product depends on the concentration of nitric acid. In dilute nitric acid, we produced the colourless nitrogen oxide gas. And in concentrated nitric acid, we produce the brown toxic nitrogen dioxide gas. The remaining answer choices are either simply untrue or there’s no way that we could reach a conclusion based on the information provided in the problem alone.