2 Gases in the atmosphere Flashcards
What are the four most abundant gases present in unpolluted, dry air?
- Nitrogen.
- Oxygen.
- Argon.
- Carbon dioxide.
What is the percentage of nitrogen in air?
78.1%
What is the fraction of nitrogen in air?
About 4/5
What is the percentage of oxygen in air?
21.0%
What is the fraction of oxygen in air?
About 1/5
What is the percentage of argon in air?
0.9%
What is the percentage of carbon dioxide in air?
0.04%
Are these the only gases in the air?
There are also very small amounts of other noble gases in the air.
What is the basic principle followed by the practicals which show that air contains about one-fifth oxygen?
We react something with the oxygen in air and look at how much the volume decreases as the oxygen is removed.
What are the practicals that we use to show that air contains about one-fifth oxygen?
- Using copper.
- Using the rusting of iron.
- Using phosphorus.
What is the apparatus used to find the percentage of oxygen in the air?
How do we know that the apparatus contains only 100cm3 of air?
The plunger on one of the gas syringes is pushed all the way in and the other moved out to 100cm3. We now know that the apparatus contains 100cm3 air.
What is step 1 of this experiment?
The silica tube is heated strongly (roaring bunsen flame).
What is step 2 of this experiment?
The plunger in the left-hand gas syringe is pushed in. This causes the air to pass over the heated copper. This pushes out the plunger on the right-hand gas syringe.
What is step 3 of this experiment?
The plungers are pushed in sequence so that the air in the system keeps passing over the heated copper. The pink-brown copper turns black as copper(II) oxide is formed.
What happens as the copper reacts?
The Bunsen Burner is moved along the tube so that it is always heating fresh copper.
What happens to the volume of the gas as oxygen is consumed
The volume of gas in the syringes falls as the oxygen is consumed.
What is step 4 of this experiment?
We keep pushing the plungers in and out until there is no change in volume.
What is step 5 of this experiment?
The apparatus is then allowed to cool to room temperature again before taking the final volume of gas (because gases expand as they are heated).
What do we need to make sure about the copper in this experiment?
We need to make sure that we use enough copper to react with all the oxygen in the air. So the copper must be in excess.
How do we know that the copper is in excess?
There will be some unreacted copper metal at the end.
What happens if we don’t use excess copper?
If we do not use enough copper then the value we get for the percentage of oxygen will be too low because not all of the oxygen will have been removed.
What is an example of what the results table from this experiment will look like?
Why have we put inverted commas around the word air?
Because we are now talking about air from which the oxygen has been removed, so it is mostly nitrogen.
What are the properties of nitrogen?
Nitrogen is very unreactive.
What happens if you put a lighted split into this ‘air’?
It would be extinguished.
Why would a lighted splint be extinguished if we put it into this ‘air’?
This is because oxygen is needed for things to burn.
Why has the volume of air decreased?
Because the oxygen has been removed as it reacts with the copper.
What is the symbol equation for this reaction?
2Cu + O2 —> CuO
What is the word equation for this reaction?
Copper + oxygen —> Copper (II) oxide.
How can we work out what the volume of oxygen is from this data?
We can work out from these data that the volume of oxygen that reacted was 100 - 79 = 21cm^3
How can we work out the percentage of oxygen in the air (method)?
The original volume of air was 100cm^3 so we can work out the percentage of oxygen in the air?
What is the formula that we use to find out the percentage oxygen in the air?
Percentage oxygen = 21/100 x 100 = 21%
Why can we use the rusting of iron to show the percentage of oxygen in air?
Iron rusts in damp air, using up oxygen as it does so. We can use this reaction to determine how much oxygen there is in the air.
What is an image of the apparatus that we use during the practical which uses the rusting of iron?
What do we need to do before we start the practical?
We need to know the volume of air present in the apparatus.
How can we find out the volume of air present in the apparatus?
We can find this by filling up the conical flask and connecting tube with water and then transferring the water to a measuring cylinder.
How do we know how far up to fill the conical flask up with water?
We mark the position of the bung and only fill with water to that point.
What do we do about the volume of the iron fillings?
We will assume that the small volume occupied by the iron fillings is negligible (very small compared to the overall volume).
What is the first step of this practical?
Set up the apparatus.
What is the second step of this practical?
Put wet iron filings into the conical flask.
What is the third step of this practical?
Record the initial reading on the gas syringe.
What is the fourth step of this practical?
Leave the apparatus in place for about a week, until the reading on the gas syringe stops changing.
What is the fifth step of this practical?
Record the final reading on the gas syringe.
What is an example of the results table from this practical?
How would we calculate the total volume of air inside the apparatus at the beginning of the experiment?
Volume of air in conical flask + volume of air in connecting tube + Initial reading on gas syringe.
130 + 12 + 92 =234cm^2