Video Transcript
A small amount of concentrated sulfuric acid is carefully mixed with a solution suspected to contain sodium nitrate in a preliminary test for nitrate ions. Later in this test, copper filings are added to the test tube. Why is this the case? (A) To oxidize any acid present in the mixture. (B) To form a brown ring on the surface of the solution. (C) To increase the amount of nitrogen dioxide being produced. (D) To form a freshly prepared solution of copper sulfate. (E) To react with the oxygen gas being generated to form copper(II) oxide.
Concentrated sulfuric acid has the chemical formula H2SO4. It can be involved in the chemical test for halide ions and nitrate ions. In this question, we see that the sulfuric acid is part of the test for nitrate ions that originate from some sodium nitrate. Since sodium nitrate is very soluble in water, it will release nitrate ions into a solution. Nitrate ions have the chemical formula NO3−. To answer this question, first, we’ll see how the preliminary test from nitrate ions works and then we’ll test the validity of each statement.
Since most nitrate salts are highly soluble in water, it’s very difficult to detect nitrate ions in solution using precipitation methods. If various solutions containing metal ions were added separately to solutions of nitrate ions, no precipitates would be observed at all. Instead, we rely on the formation of a visible gas in the test for nitrate ions by using concentrated sulfuric acid. In this question, concentrated sulfuric acid is being added to some sodium nitrate solution. The purpose of this part of the test is to produce nitric acid from the nitrate ions in the solution. The concentrated sulfuric acid is a very strong acid. It reacts with the sodium nitrate solution to produce sodium sulfate solution and nitric acid. So after this part of the test, the test tube will contain nitric acid and sodium sulfate solution.
Nitric acid has the chemical formula HNO3. If we heat the mixture up, the nitric acid will decompose. The products of the thermal decomposition of nitric acid are water, nitrogen dioxide, and oxygen. At the temperature of this reaction, all of these products are in a gaseous state. But in particular, nitrogen dioxide will be visible as it’s a brown gas. Nitrogen dioxide is also a very toxic gas, so this part of the test should be performed in a fume hood or a fume cupboard. So far, we haven’t seen how copper filings were involved in this test, but they’re mentioned in the question.
Copper filings are in fact added to increase the rate of nitrogen dioxide formation, that is, the formation of the brown gas that we’re going to observe. By adding copper filings to the nitric acid that was present in the mixture, we get a chemical reaction that produces copper nitrate, water, and more nitrogen dioxide. In this reaction, the copper gets oxidized. It changes from copper metal to aqueous copper two plus ions by losing two electrons. The nitric acid is oxidizing the copper in this reaction. So, the purpose of the copper filings in this test is to speed up the production of the brown nitrogen dioxide gas so that we can detect it easily. It is the observation of a brown gas that gives us a positive result for nitrate ions in this part of the test.
Now, let’s test the validity of each statement to find the correct answer. Statement (A) suggests that the copper is added to oxidize any nitric acid present in the test. We have seen that actually the copper filings get oxidized and the nitric acid gets reduced in this test. This statement is therefore not the correct answer. Statement (B) suggests that the copper is added to form a brown ring on the surface of the solution. In fact, a brown gas is observed during the test. A brown ring is not formed in this test, and it’s not the correct answer. Statement (C) suggests that the copper is involved in increasing the amount of nitrogen dioxide gas being produced. We have seen that this is true, and it could be the correct answer.
Statement (D) suggests that the copper is added to prepare copper sulfate solution. The copper reacts with nitric acid in the test, and in fact, copper(II) nitrate is formed. Since we don’t form copper sulfate in this test, this is not the correct answer. Statement (E) suggests that the copper reacts with oxygen gas that is in fact produced when the nitric acid decomposes. However, if there were any oxygen gas present, the copper would not react with it at the temperature at which this test is performed at. Statement (E) is not the correct answer. The purpose of the copper filings in this test is, in fact, to increase the amount of nitrogen dioxide being produced. To increase the amount of nitrogen dioxide being produced is the correct answer.
