Video Transcript
Concentrated sulfuric acid is a more potent oxidizing agent than some of the group 17 halides. When adding a few drops of concentrated sulfuric acid to solid halide samples, fumes of different gases may be produced and further tested with different reagents. Which of the following related statements is correct? (A) Pale green fumes of chlorine gas turn red litmus paper blue. (B) Violet fumes of iodine gas turn red litmus paper blue. (C) Orange-red fumes of bromine gas turn starch indicator yellow. (D) Violet fumes of iodine gas turn starch indicator yellow. (E) Orange-red fumes of bromine gas turn starch indicator blue.
The elements in group 17 that we most commonly encounter in chemistry are fluorine, chlorine, bromine, and iodine. Collectively, these elements are referred to as the halogens. Because these nonmetal elements are highly reactive, in nature, they’re most commonly encountered as halide ions. These negatively charged ions are known as fluoride, chloride, bromide, and iodide. The halide ions are often encountered in samples of solid halide salts. For example, potassium iodide contains iodide ions. Solid sodium chloride, which is used as table salt, contains chloride ions. If we add a few drops of concentrated sulfuric acid to samples of various solid halides, some interesting reactions can be observed.
In all cases with the solid halide salts, steamy white fumes are observed. The steamy white fumes are caused by the formation of hydrogen halides. These are hydrogen fluoride, hydrogen chloride, hydrogen bromide, and hydrogen iodide, respectively. All of these hydrogen halides are acidic gases, and they will immediately turn damp blue litmus paper red in their presence. It’s important to note here that during the formation of these hydrogen halide gases, no redox reactions have actually occurred. In fact, in the case of solid fluorides and solid chlorides, the fluoride and chloride ions cannot be oxidized by concentrated sulfuric acid at all. Concentrated sulfuric acid is not a sufficiently potent oxidizing agent to oxidize fluoride or chloride ions. With fluoride and chloride ions, no further products are formed and no further observations are made.
In the reaction of a solid bromide salt and concentrated sulfuric acid, further redox reactions take place. Orange-to-red fumes of bromine are observed, and the colorless acidic gas sulfur dioxide is also produced. In the reaction of concentrated sulfuric acid with a solid iodide salt, a purple vapor would be observed. The reaction is quite exothermic, and iodine is produced, which vaporizes to give the purple fumes. In addition to the iodine formed, the reduction products of the sulfuric acid may include sulfur dioxide, sulfur, and hydrogen sulfide. Hydrogen sulfide, or H2S, smells like rotten eggs. One of the most notable features of these reactions is the formation of orange-red fumes of bromine gas in the case of a solid bromide reacting with concentrated sulfuric acid.
The presence of bromine gas can be tested with damp starch indicator paper, which will turn yellow. The other notable feature in the reaction of a solid iodide with concentrated sulfuric acid is the production of violet or purple fumes of iodine vapor. These fumes will turn damp starch indicator paper a blue or blue-to-black color. Notice again that in the cases of solid fluorides and solid chlorides, no fluorine gas or chlorine gas is produced in these reactions.
Now let’s test each statement to see which one used the correct answer. The first statement suggests that pale green fumes of chlorine gas will be produced that will turn red litmus paper blue. Chlorine gas is a pale green gas. It will turn damp blue litmus paper to red before bleaching it. It will not turn red litmus paper to blue though. And in fact, no chlorine gas is produced from solid chlorides in the reaction with concentrated sulfuric acid. So this is not a correct answer.
The next statement suggests that violet fumes of iodine gas could be produced, which will turn red litmus paper to blue. We’ve seen that iodine can be produced in these reactions. And in the vapor phase, it does indeed have a purple or violet color. But the test for iodine vapor is that it turns damp starch indicator paper to blue. It won’t turn red litmus paper to blue, so this is not a correct answer.
The next statement suggests that orange-to-red fumes of bromine vapor will be produced, which will turn starch indicator yellow. We’ve seen that bromine vapor can be produced in the reaction of solid bromides with concentrated sulfuric acid. Bromine vapor is an orange-to-red color, and it does turn starch indicator paper yellow. This could be the correct answer.
We just need to test the last two statements to be sure of our answer. Statement (D) suggests that violet fumes of iodine gas will turn starch indicator paper yellow. We know iodine gas can be produced in these reactions, but it will turn starch indicator blue. It’s therefore not the correct answer. The last statement suggests that bromine vapor formed will turn starch indicator paper blue. Since bromine vapor turns starch indicator paper yellow, it’s not the correct answer.
The only correct statement that relates to a solid halide reacting with concentrated sulfuric acid is that orange-red fumes of bromine gas turn starch indicator yellow when they’re formed. “Orange-to-red fumes of bromine gas turn starch indicator yellow” is the correct answer.
