In this scenario, we are dealing with a chemical reaction involving concentrated sulfuric acid and an unknown salt, which produces two distinct gases. Let's break down the process step by step to identify gases A and B.
Identifying Gas A
When concentrated sulfuric acid is added to certain salts, particularly those containing nitrate ions, a brown gas is often produced. This gas is nitrogen dioxide (NO2), which is known for its characteristic brown color. The reaction typically occurs when sulfuric acid reacts with a nitrate salt, such as sodium nitrate (NaNO3). The reaction can be summarized as follows:
- 2 H2SO4 + 2 NaNO3 → 2 NaHSO4 + 2 NO2 + H2O
In this case, gas A is nitrogen dioxide (NO2).
The Role of Copper Turnings
When copper turnings are introduced into the test tube, they react with the nitrogen dioxide gas. This reaction leads to the formation of copper(II) nitrate and can also produce additional nitrogen oxides. The presence of copper enhances the reaction, resulting in a more vigorous evolution of gas A, which is still nitrogen dioxide.
Transition to Gas B
Upon cooling, nitrogen dioxide (NO2) can dimerize to form dinitrogen tetroxide (N2O4), which is a colorless gas. This process occurs because NO2 is in equilibrium with N2O4 at lower temperatures:
Thus, as the temperature decreases, more of the brown NO2 gas converts into the colorless N2O4 gas. Therefore, gas B is dinitrogen tetroxide (N2O4).
Summary of Gases Identified
To summarize:
- Gas A: Nitrogen dioxide (NO2) - the brown gas evolved initially.
- Gas B: Dinitrogen tetroxide (N2O4) - the colorless gas formed upon cooling.
This sequence of reactions illustrates the fascinating interplay between temperature, chemical reactivity, and the properties of gases. Understanding these transformations not only helps in identifying the gases involved but also deepens our appreciation for the principles of chemistry at work.
