General characteristics of oxygen and the reaction of its combustion
Why does oxygen burn?
50% of the earth’s crust consists of oxygen. The element is also present in minerals in the form of salts and oxides. Oxygen in bonded form makes up around 89% of the mass of water, and is also present in the cells of all living organisms and plants. Oxygen is also present in the air in a free state in the form O₂ and its allotropic modification in the form of ozone O₃, and makes up one fifth of the volume of air.
Physical and chemical properties of oxygen
Oxygen O₂ is a gas without color, taste or smell. It dissolves poorly in water, and boils at a temperature of -183 degrees Celsius. Oxygen in liquid form is light blue in color, and in solid form the element forms dark blue crystals. Oxygen melts at a temperature of -218.7 degrees Celsius.
On heating, oxygen enters into a reaction with various simple substances (metals and non-metals), forming oxides as a result of interaction – compounds of elements with oxygen. The interaction of chemical elements with oxygen is called the oxidation reaction. Examples of equations of reactions are:
4Na + О₂= 2Na₂O,
S + О₂ = SO₂.
Several complex substances also enter into a reaction with oxygen, forming oxides – the equation of the reaction is:
СН₄ + 2О₂= СО₂ + 2Н₂О
2СО + О₂ = 2СО₂.
Oxygen as a chemical element is obtained at laboratories and at industrial plants. Oxygen can be obtained in the laboratory by several methods:
- by the reaction of the breakdown of Berthollet’s salt (potassium chlorate);
- in the process of the breakdown of hydrogen peroxide, heating the substance in the presence of manganese oxide as a catalyst;
- by the breakdown of potassium permanganate.
Chemical reaction of the combustion of oxygen
Pure oxygen has special properties that oxygen in the air does not have. Air contains five times less oxygen than pure oxygen in the same volume. In air, oxygen is mixed with a large amount of nitrogen – a gas that does not burn itself and does not support combustion. For this reason, if the oxygen in the air around a flame is already expended, the next portion of oxygen must get through nitrogen and products of combustion. Accordingly, the more energetic combustion of oxygen in the atmosphere is explained by the swifter supply of oxygen to the place of combustion. In the course of the reaction, the process of the combination of oxygen with the burning substance takes place more energetically, and more heat is released. The more oxygen that is supplied to the burning substance in a unit of time, the more brightly the flame burns, the higher the temperature is and the more intensely the combustion process takes place.
How does the combustion process of oxygen take place? This can be tested in an experiment. Take a cylinder and turn it upside down. Then put a pipe with hydrogen under the cylinder. The hydrogen, which is lighter than air, will completely fill the cylinder. Ignite the hydrogen around the open part of the cylinder, and insert a glass pipe into the cylinder, through which oxygen gas flows. Around the end of the pipe, the flame will blaze up, while a flame will calmly burn inside the cylinder filled with hydrogen. In the course of the reaction, it is not the oxygen that burns, but the hydrogen in the presence of the small amount of oxygen coming out of the pipe.
What arises as a result of the combustion of hydrogen, and what oxide is formed? Hydrogen oxidizes to water. On the walls of the cylinder, drops of condensed water vapor gradually settle. For the oxidation of 2 molecules of hydrogen, 1 molecule of oxygen is used, and 2 water molecules form; the equation of the reaction is:
2Н₂ + O₂ → 2Н₂O
If the oxygen comes out of the pipe slowly, it burns fully in the atmosphere of hydrogen, and the experiment takes place smoothly.
As soon as the supply of oxygen increases so much that it does not manage to burn completely, part of it goes outside the flame, where mixtures of hydrogen and oxygen form, and small individual sparks appear, resembling explosions. A mixture of oxygen and hydrogen is known as detonating gas.
When detonating gas is ignited a large explosion takes place: with the combination of oxygen and hydrogen, water forms, and a high temperature develops. Water vapor expands greatly with the surrounding gases, and pressure becomes high, in which not only a fragile cylinder can explode, but also a more durable vessel. For this reason, you should be extremely careful when working with detonating mixture.
Expenditure of oxygen in the combustion process.
For this experiment, fill a glass crystallizer with a volume of 3 liters two thirds full of water and add a tablespoon of sodium hydroxide or potassium hydroxide. Color the water with phenolphthalein or another suitable dye. Pour sand into a small flask and vertically place a wire in it with cotton wool on the end. The flask is placed in the crystallizer with water. The cotton wool remains 10 cm above the surface of the solution.
Slightly wet the cotton wool with alcohol, oil, hexane or other combustible fluid and light. Carefully cover the burning cotton wool with the 3-liter flask and lower it below the surface of the alkali solution. In the combustion process, the oxygen turns to water and carbon dioxide, and as a result of the reaction the alkali solution in the bottle rises. The cotton wool soon goes out. Carefully place the bottle on the bottom of the crystallizer. In theory, the bottle should fill by one fifth, as air contains 20.9% oxygen. In combustion, the oxygen turns to water and carbon dioxide CO₂, which is absorbed by the alkali; the equation of the solution is:
2NaOH + CO₂ = Na₂CO₃ + H₂O
In practice, burning ends before all the oxygen is expended, some of the oxygen turns to carbon monoxide, which is not absorbed by the alkali, and some of the air leaves the bottle as a result of thermal expansion.
Warning! Don’t try to repeat these experiments without a professional supervision!