Preparation of Oxygen and Its Properties
Senior 2 · Chemistry
Preparation of Oxygen and Its Properties
Lesson 2 of 4
20 min
A gas is usually invisible, and it escapes the moment you give it a chance. Once you have prepared a gas, you must trap it in a container: pure enough to test, and sometimes in a measured amount. Every gas you meet in chemistry, from oxygen to ammonia, is collected by one of four methods.
You do not need to memorise a separate method for every gas. The right choice follows from two properties: how soluble the gas is in water, and whether it is denser or less dense than air.
Air is a mixture, mostly nitrogen (Mr 28) and oxygen (Mr 32), and its average relative molecular mass is about 29. At the same temperature and pressure, equal volumes of gases contain equal numbers of molecules. A gas with heavier molecules is therefore denser.
A gas jar is filled completely with water, covered, and turned upside down in a trough of water. It stands on a beehive shelf, a small support with a hole in it, and the delivery tube ends under the shelf. The gas bubbles up into the jar and pushes the water down and out.
The first jarful is usually thrown away. It is mostly the air that was already inside the flask and delivery tube.
The gas jar is held mouth downwards, and the delivery tube reaches up to the top of the jar. A gas that is less dense than air collects at the top and pushes the air down and out. Use it for light gases, especially soluble ones such as ammonia.
The gas jar stands upright, and the delivery tube reaches to the bottom. A dense gas settles at the bottom and pushes the air up and out; a lid is then placed on top. Use it for gases denser than air: carbon dioxide, chlorine, sulfur dioxide and hydrogen chloride. Poisonous gases such as chlorine and sulfur dioxide are collected in a fume cupboard.
With both air-displacement methods you cannot see when the jar is full (unless the gas is coloured, like yellow-green chlorine), and a little air always stays mixed with the gas.
A gas syringe is a large glass or plastic syringe marked in cm³, usually up to 100 cm³. The gas pushes the plunger out. Use it for any gas when you must measure the volume accurately, for example when following the rate of a reaction. It holds only a small volume.
Gas collected over water is moist. To obtain dry oxygen, pass it through a drying agent, such as concentrated sulfuric acid in a wash bottle or a U-tube of anhydrous calcium chloride, and then collect it in a gas syringe or by downward delivery, never over water. Ammonia is an exception: it reacts with both of those drying agents, so it is dried with calcium oxide instead.
| Gas | Mr | Solubility in water | Compared with air (29) | Best method |
|---|---|---|---|---|
| Oxygen, O2 | 32 | slightly soluble | slightly denser | over water |
| Hydrogen, H2 | 2 | almost insoluble | much less dense | over water (or upward delivery) |
| Carbon dioxide, CO2 | 44 | fairly soluble | denser | downward delivery (over water only if some loss does not matter) |
| Ammonia, NH3 | 17 | very soluble | less dense | upward delivery |
| Chlorine, Cl2 | 71 | fairly soluble | much denser | downward delivery, in a fume cupboard |
Reasoning for ammonia. Step 1: is it soluble? Yes, very, so collection over water is ruled out. Step 2: compare Mr with 29. Since 17 is less than 29, ammonia is less dense than air and rises. Step 3: an inverted jar traps a rising gas, so use upward delivery.
Reasoning for oxygen. Oxygen is only slightly soluble, so very little is lost in water. Its density is so close to that of air that it would mix rather than displace it. Collection over water is therefore the best method, and it lets you watch the jar fill.
Tip
Two questions choose the method
1. Does it dissolve? If it is insoluble or only slightly soluble, collect it over water. 2. If it dissolves, is it heavy or light? Compare its Mr with 29. A heavy gas goes into an upright jar (downward delivery); a light gas goes into an upside-down jar (upward delivery). If the volume must be measured, use a gas syringe. In exams, always give the reason (solubility and density), not just the name of the method.
Why is the first jar of gas collected over water usually thrown away?
Solution: It contains mainly the air that was in the flask and delivery tube before the reaction started, so it is not pure.
Explain why ammonia cannot be collected over water.
Solution: Ammonia is very soluble in water. It would dissolve in the trough instead of collecting in the jar.
Sulfur dioxide, SO2, is very soluble in water and poisonous. Which method should be used, and where? (S = 32, O = 16)
Solution: Mr = 32 + 2(16) = 64, which is greater than 29, so the gas is denser than air. Because it is soluble, water cannot be used. Collect it by downward delivery into an upright jar, working in a fume cupboard.
A student needs exactly 50 cm³ of dry oxygen. Describe how she should obtain it.
Solution: Pass the oxygen through a drying agent (concentrated sulfuric acid or anhydrous calcium chloride) and collect it in a gas syringe, reading the volume on the scale.
Nitrogen (Mr 28) is almost insoluble in water. Why is collection by displacement of air a poor choice?
Solution: Its Mr, 28, is almost the same as that of air (29), so nitrogen would mix with the air instead of pushing it out. Collect it over water.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
