Video: Detection of the Gas Produced from the Decomposition of Hydrogen Peroxide

A student uses a test tube to collect the gas produced by a chemical reaction. After lighting a splint, they blow out the flame and then insert the splint into the test tube. It reignites and glows brightly. What is the probable identity of the gas? Why does the splint glow more brightly than it did when burning in air? Which of the following reactions could have produced the gas? [A] Zinc + sulfuric acid [B] Silver oxide + hydrochloric acid [C] Barium carbonate + nitric acid [D] Hydrogen peroxide with manganese oxide catalyst [E] Auminum chloride + sodium hydroxide.

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Video Transcript

A student uses a test tube to collect the gas produced by a chemical reaction. After lighting a splint, they blow out the flame and then insert the splint into the test tube. It reignites and glows brightly. What is the probable identity of the gas?

Before answering the question, let’s have a quick review of the procedure. The student is performing a chemical reaction of some sort. Some gas is being produced. And the gas is being collected inside a test tube. They’re then taking a splint, setting it alight, and then blowing out the flame, leaving embers behind on the end of the splint. They then inserted that split into the test tube. And the splint reignited. From this, we can deduce that the gas inside the tube was reactive. A splint is made of wood which is largely composed of lignin and cellulose. This means it’s mostly composed of carbon, hydrogen, and oxygen.

For this question, we’re assuming that the gas produced is a pure substance. So there’s one more piece of information we need in order to make a good guess about the identity of the gas. The process occurring at the end of the splint when the embers are burning is combustion. The fact the flame returns indicates that the gas accelerates or is involved in combustion. Combustion of the carbon in the splint will produce carbon dioxide. Combustion of the hydrogen will produce water. This occurs normally by reaction with oxygen in the air. Oxygen typically makes up 21 percent of the air we breathe. However, if the gas in the test tube were pure oxygen, it would vastly accelerate the rate of combustion of the splint. This would be consistent with our findings.

So the probable identity of the gas is oxygen. Just to be safe, I’m going to have a look at the other common gases which you may have come across and see whether any of them fit. Hydrogen is an explosive gas when mixed with oxygen. However, a flame is needed in order to trigger the detonation. Pure hydrogen in this circumstance would quench the embers on the splint and would not cause reignition. It would block the access of oxygen in the air to the splint and stop the embers reacting. Hydrogen is usually detected by treatment with a lit splint because it generates a loud pop.

What about carbon dioxide? Carbon dioxide is a nonreactive gas. So it too would put out the embers. Carbon dioxide is usually detected with limewater in which it forms a precipitate of calcium carbonate. Chlorine is another gas that might be produced in the lab. However, it would react too slowly and too incompletely with a splint in order to cause reignition and would quench the embers like hydrogen and carbon dioxide. Chlorine is usually detected with blue litmus paper. Damp blue litmus paper will turn red upon contact with chlorine gas and then slowly turn white as it’s bleached.

And finally, of the common gases that can be tested in a laboratory setting, we come to ammonia. Ammonia too would not support the combustion of the splint and would quench the embers. Since ammonia is basic, it can be detected with damp red litmus paper which turns blue upon exposure to ammonia gas. Otherwise, it can be detected by its distinctive smell. While there are many more gases we could look at, these are the ones most likely to be produced by laboratory setup in these quantities. So the probable identity of the gas is most certainly oxygen.

Why does the splint glow more brightly than it did when burning in air?

Let’s consider the scenario where the splint is burning in air. Oxygen from the air is supporting combustion of the splint. But the majority of the air is made up of nitrogen, while, in the test tube, there’s 100 percent oxygen. This means that the combustion of the splint is much much faster. This produces a lot more heat and much more light. So the reason the splint glows much more brightly when in 100 percent oxygen than it does in air is because it is burning faster. There is a misconception that oxygen itself can burn. But that’s not true. Oxygen is necessary to support combustion. Oxygen is also consumed in the process. So it’s not acting as a catalyst.

Which of the following reactions could’ve produced the gas? A) Zinc and sulfuric acid, B) silver oxide and hydrochloric acid, C) barium carbonate plus nitric acid, D) hydrogen peroxide with manganese oxide catalyst, or E) aluminum chloride plus sodium hydroxide.

Just to recap, the gas the student produced was oxygen, O₂. So we’re looking for a reaction that would generate oxygen. Let’s start with option A, zinc plus sulfuric acid. This is an example of a metal plus acid reaction. The expected outcome is a metal salt plus hydrogen gas. In this case, the salt will be zinc sulphate. As this reaction does not generate oxygen, it is not a correct answer.

What about option B? Silver oxide is a metal oxide. And most metal oxides are bases. So what we have is an acid base reaction. So the products are metal salt and water. In this case, the salt is silver chloride. Again, this answer does not produce oxygen as a product. So it cannot be a correct answer. Barium carbonate is an example of a metal carbonate. When treated with acid, we expect the products to be a metal salt, carbon dioxide, and water. In this case, the salt is barium nitrate, but unfortunately, no oxygen.

So onto option D, hydrogen peroxide is well-known to slowly decompose into water and oxygen. Left on its own, this reaction is too slow. So a catalyst is introduced when large quantities of oxygen are needed. In this case, manganese four oxide is being used with symbol MnO₂ to break down the hydrogen peroxide. Crucially, this reaction generates the oxygen we need. So this is the correct answer. But just to be safe, let’s have a look at option E.

This reaction you might not be so familiar with. But it’s a reaction of a metal chloride with sodium hydroxide. In many cases, the outcome is a metal hydroxide plus sodium chloride. Here’s the chemical equation, aluminum chloride plus three equivalents of sodium hydroxide react to form aluminum hydroxide plus three equivalents of sodium chloride. If excess hydroxide is present, aluminum hydroxide can react to form sodium aluminate. You need not know all the details of this chemistry, only that the reaction does not produce oxygen. So this option is not a correct answer. Of the five options given, only one produces hydrogen gas. And that is hydrogen peroxide with manganese oxide catalyst.

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