Gas
A state of matter in which substances exist in the form of nonaggregated molecules, and which, within acceptable limits of accuracy, satisfies the ideal gas laws; usually a highly superheated vapor
In this lesson, we will learn how to test for the presence of hydrogen, oxygen, carbon dioxide, ammonia, and chlorine.
We test for gases in the laboratory because it is nearly impossible to determine the identity of a gas just by its appearance.
As an example, hydrogen, oxygen, and carbon dioxide are all colourless and odourless.
How would we be able to determine which is which?
To test for hydrogen, place a lit splint at the mouth of the reaction vessel.
You should hear a distinctive “squeaky pop”, which confirms its presence.
This is due to the combustion reaction of hydrogen in the presence of oxygen, creating water as the only product.
To test for oxygen, place a glowing splint at the mouth of the reaction vessel.
Keep in mind that the splint should be “glowing”, not lit.
A glowing splint relights in the presence of oxygen.
Why does hydrogen burn with a “squeaky” pop? Why does oxygen relight the glowing splint? Hint: recall some properties of hydrogen and oxygen.
Please pause the lesson to think about this and resume once you are done.
Hydrogen is highly flammable and the pop sound that you hear is actually a mini-explosion.
The glowing splint relights in the presence of oxygen as there is a higher concentration of oxygen in the reaction vessel than compared with air, which is only 21% oxygen.
Carbon dioxide will extinguish a lit splint, but the same occurs in the presence of ammonia.
A more accurate test is to bubble carbon dioxide though limewater, which is an aqueous solution of calcium hydroxide, also known as slaked lime.
Limewater turns milky in the presence of carbon dioxide due to the formation of calcium carbonate.
Ammonia has a distinctive, pungent smell, though it is not good practice to identify a gas based on this property.
It also extinguishes a lit split, and turns damp red litmus paper blue.
In the presence of concentrated hydrochloric acid, a white smoke will form.
This is ammonium chloride, and confirms the presence of ammonia.
Chlorine turns damp blue litmus paper red and eventually bleaches it to white.
In conclusion, hydrogen burns with a “squeaky” pop, oxygen relights a glowing splint, and carbon dioxide turns limewater milky.
Ammonia turns damp red litmus paper blue and forms a white smoke of ammonium chloride in the presence of concentrated hydrochloric acid.
Chlorine turns damp blue litmus paper red and continues to bleach it white.
In this lesson, we will learn how to collect gases based on their density.
Have you ever wondered why some balloons float and some don't?
This is due to the density of the gas inside the balloon.
Balloons that float contain helium, which is less dense than air.
Balloons that we blow up ourselves contain mostly carbon dioxide, which is denser than air.
Therefore, these balloons fall to the ground.
From the introduction, we already know that a gas less dense than air rises.
These gases are collected using a method called upward delivery.
The collection vessel is inverted so that the gas is able to displace the air inside it as it rises upwards.
Air is pushed downwards, so this method is also called downward displacement.
Hydrogen gas and ammonia are collected using this method.
Recall the example of the balloon falling to the ground in the introduction.
Gases denser than air are collected by downward delivery.
The collection vessel is placed upright so the denser gas collected is able to displace the air inside it as it sinks.
Air is pushed upwards, so this method is also called upward displacement
Carbon dioxide and chlorine gas are collected using this method.
If we are unsure of the density of a gas, we can collect it over water. → Extraction par déplacement d'eau
Since all gases are less dense than water, displacement of water is possible.
The gas produced is bubbled through water and collected in a gas jar.
Note that this method does not work for gases that are soluble in water, such as ammonia.
What do you think is the best method to collect hydrogen chloride?
Let's pause the lesson momentarily to think about this.
Hydrogen chloride dissolves in water to produce hydrochloric acid, so collecting over water is not possible.
Since it is denser than air, it can be collected by downward delivery.
This brings us to the next topic and addresses a very important question.
What if you are unsure of both the density and solubility of the gas produced?
A method to collect a gas irrespective of these properties is by using a gas syringe.
It is a closed container so the density of the gas does not need to be taken into consideration.
A gas syringe is also used when the volume of a gas produced needs to be measured.
In summary, gases less dense than air are collected by upward delivery.
Downward delivery is used to collect gases denser than air.
Gases can also be collected over water, but this method only works for gases that are insoluble in water.
A gas syringe can be used to collect all gases and is useful for measuring the volume of gas produced.
In this video, we're going to learn how to calculate gas volumes for any reaction. In my previous video (molar volumes of gases), you found out that one mole of any gas occupies 24 litres per mol. This is the same for all gases at room temperature and atmospheric pressure.
So, if we know that one mole of any gas occupies 24 litres (think 24, 1L soft drinks bottles), then how many soft drinks bottles do you think are needed to contain two moles of any gas? Pause and choose from one of the following:
A)24 soft drinks bottles
B) 48 soft drinks bottles
C) 12 soft drinks bottles
The correct answer was B, 48 soft drinks bottles. This is because we doubled the number of moles f gas from one to two, and so this doubled the volume of gas produced. However, when chemical reactions take place, they don't always produce whole numbers of moles of gas, and so you need a different method to calculate the gas volume. This is where you can use a formula and the ratio given from a chemical reaction:
Gas volume produced (litres) = number of moles of gas x 24 litres per mole
So, let's pose a question. In my previous video, we talked about environmental chemists wanting to know how much carbon dioxide is produced from burning a fuel. A major part of petrol is a hydrocarbon called hexane. When it is combusted in a car engine, it produces carbon dioxide (a gas) and water. We're going to try and calculate the volume of carbon dioxide that is produced from this reaction?
C6H12 + 9O2 6CO2 + 6H2O
The equation tells us that for 1 molecule of hexane (1 mole), that 6 molecules (or 6 moles) of carbon dioxide are are produced. This is in a ratio of 1 to 6.
So, what volume of carbon dioxide do you think is produced if one mole of hexane reacts with oxygen? Pause, use the formula and choose from on of the following volumes. Continue, when you are ready.
A. 24 litres
B. 12 litres
C. 144 litres.
The answer is C, 144 litres. This is because 1 mol of hexane produces 6 moles of carbon dioxide. So if one mole of a gas occupies 24 litres then 6 moles of a gas occupy 24 litres x 6 mols, which is equal to 144 litres. However, what if you combust any amount of hexane? The amount of hexane used, in mass and moles, will vary according to how long the car is used. Let's say that you use the car for 20 minutes and it combusts 20.5 moles of hexane how many moles of carbon dioxide do you think will be produced? Pause, select and continue when you're ready:
A. 20.5 moles x 6 = 123 moles of CO2
B. 20.5 moles 6 = 3.42 moles of CO2
C. Neither, only one mole of CO2 is produced.
The correct answer is A) because 20.5 moles x 6 = 123 moles. Knowing this, you can use the gas volume formula to work out the volume of CO2 gas produced by combusting 20.5 moles of hexane. Pause, use the formula and then continue.
The answer is that 123 moles of CO2 x 24 litres per mol = 2952 litres of CO2.Did you get it right?
In summary, the molar volume of any gas is always 24 litres per mol at room temperature and pressure. Using the example provided, you can use the ratio from any chemical equation to tell you the number of moles of gas produced from the reactants. This means you can always calculate the gas volume using the formula: number of moles of gas x 24 litres per mol (at room temp and pressure).
In this lesson, we are going to learn about diffusion of gases. This will help you to understand how gases behave and also explain why smells spread.
Before we start, you need to recall what solids, liquids and gases are like in terms of their particles. In particular, I'd like you to ask yourself, how are the particles arranged in a gas? Pause the video and restart it when you think you have the answer.
The answer is: The particles in a gas are spaced out and move about very quickly in random directions. Did you get it right?
This, of course, explains why a gas takes the shape of it's container. It's because the particles will each continue to travel in a straight line until they hit something, in this case the wall of the container. So, the smelly gases given off by rotting vegetables in a sealed box will fill the box with rotting vegetable smell, but you won't be able to smell them outside the box.
However, take the lid off the box, and those particles will no longer bounce off the lid and be contained. They will start to escape into the air around the box. You'll soon notice the effect and will be able to smell the rotting vegetables; that's those gas particles reaching your nose by diffusion!
Diffusion is the random movement of one gas through another, from a region of high concentration (in this case, inside the box) to a region of low concentration (in this case, outside the box).
It is a process that doesn't rely on wind or air currents; it will happen even if you are inside a building and the air is completely still.
There are many examples of diffusion, but smell is probably the most obvious one. These could be food smells, body smells from other people, the scent that a plant gives off to attract pollinating insects. The same even happens with the smoke from a candle after it has been extinguished - it spreads out. It's all due to diffusion.
Here's challenge for you. If someone walks into a room with some fresh flowers, why does it take a while for the smell of the flowers to fill the room? Why can't you smell them straight away? Pause the video, have a think and restart it when you think you have the answer.
Just as the gas particles can hit the walls of a container and rebound, the particles can also hit each other. It doesn't matter if there is only a single gas or a mixture of gases, these collisions between particles are always taking place. It's very similar to trying to walk in a straight line through a busy crowd of people or a market place - it's very difficult to do without bumping shoulders with someone and changing direction.
This explains why it takes a while for a smell to get from one part of a room to another. The high concentration of gas (or smell) particles takes time to spread out due to the collisions with the other gas particles in the air.
And that... is diffusion.