Air, Combustion and Rusting

AIR

The atmosphere which covers the earth is made of air.

Air is composed of several gases.

Noble gases

Is air a compound or a mixture?

1. Air has no chemical formula. Any compound must have a specific formula.
2. The composition of the air varies slightly in different localities. In the same locality the composition is different at different times. If it were a compound, it would always have a definite composition by mass everywhere and all the time.
3. When the constituents of the air are mixed in their right proportions, there is no energy (heat or light) change. A compound is always made by a chemical change involving energy changes.
4. The gases in air can be separated by purely physical means (i.e. Fractional distillation). Elements of a compound can never be separated by physical means.

SEPARATION OF GASES FROM ITS CONSTITUENTS

Main stages of separating air

1. Air is filtered to remove dust.
2. Carbon dioxide and water vapor are removed by using special chemicals and drying agent. This is because if carbon dioxide and water vapour would be left when the air is cooled, they would solidify and block up the pipes.
3. The air is then cooled to -200◦C. At this temperature air a liquid
4. The liquid air is passed into a fractionating tower where it is slowly warmed up.
• The first gas to boil off is nitrogen (-196◦C.)
• At -186◦C argon is boiled off.
• Oxygen is boiled off at -183◦C.
All those gases are boiled off separately and are collected stored under pressure in cylinder.

USES OF GASES IN AIR

1. Oxygen
• Animals and plants need oxygen for respiration
• It is used by divers and mountain climbers for breathing
• Burning of substance need oxygen.
2. Carbon dioxide
• It is a raw material in photosynthesis
• It used in fire extinguisher
3. Nitrogen
• Component of nitrogenous fertilizes
4. Noble gases
• For filling electric light bulbs
• Used advertising signs
• In the helium –neon gas laser
• To provide an inert atmosphere for welding

DETERMINATION OF PERCENTAGE OF OXYGEN IN AIR

• A known volume of air is passed through tube with burning copper powder and oxygen in air will react with hot copper to produce black copper oxide.
  2Cu(s) + O2 (g) →2CuO­(S)
• If oxygen is depleted the readings on both syringes will be steady and the reaction has completed. Hence to find the volume of oxygen in air collected in syringes.

Volume of O2 = Initial vol. of air – final volume of air vol. of air

For instance, the initial volume of air in one syringe is 80cm3 and the final volume is 63.20cm3. hence the percentage volume of O2 in air is :-
80cm³ - 63.20cm³=16.8cm³


 % vol. of oxygen = Vol. of O2 in one syringe    x100%
                 Initial vol. of gas in one syringe                       

                                          = 16.8cm3 x 100%
                                               80cm3

                                            =21%

TESTS OF SOME GASES IN AIR

1. Oxygen: relights / rekindles a glowing splint
2. Carbon dioxide: turns lime water milky
3. Water vapour : the white anhydrous copper (II) sulphate turns blue in the presence of water.

COMBUSTION

Combustion is the process of burning of substance in the presence of oxygen to give heat and light.
OR
Combustion is the chemical reaction of a substance with oxygen, usually producing heat and light.

Combustion can take place in the open, such as an open fire or in a closed system, such as car engine.


Combustion that results in a flame is very fast and is called burning.
Materials that catch fire and burn easily are said to be combustible.

APPLICATION OF COMBUTION

AREA	APPLICATION
Industries	In engines In large boilers Incinerators for burning wastes During welding and smelting
Domestic	Cooking Heating homes Burning wastes
Laboratory	Sterilization of equipment · During experiment

FIRE

Fire is the state or process of combustion in which ignited material combines with oxygen and gives off light, heat and flame.

COMPONENTS NEEDED TO START A FIRE

A fire will start or continue to burn if three factors are satisfied.
• there must be fuel to burn
• there must be air to supply oxygen
• There must be heat (ignition temperature) to start and continue the combustion process.
The three components can be summarized by the fire triangle
fuel
heat
Oxygen


RUSTING

Rusting is the chemical process by which the surface of iron or steel become coated with rust.

Rust is the reddish brown substance which forms on the surface of iron or steel when exposed to air and water.

Conditions necessary for rusting

1. Iron or steel
2. Water or moisture
3. Oxygen or air
The chemical name for rust is called hydrated iron (III) oxide The chemical reaction takes place in two stages:
1. Iron reacts with oxygen/air to form iron (III) oxide
   Fe + O₂ → Fe₂O₃
   Iron + oxygen → iron (III) oxide
2. Iron (III) oxide reacts with moisture/water to form hydrated iron (III)oxide
   Fe₂O3 + x H₂O → Fe₂O₃ .x H₂O (hydrated iron (III) oxide)

   Disadvantages of Rusting

   1. It increases friction in machine parts thereby reducing the efficiency of the machine.
   2. Reduces the lifespan(durability) of the materials made of iron or steel
   3. Increases the maintenance costs of structures or materials made of iron
   4. Weakens the infrastructures made of iron eg iron bridges

   Methods of preventing rusting

   Some of the methods used to prevent rusting are
   1. Painting
   This is the coating of item with special pigment paints. The coating prevents oxygen and water from coming into contact with iron. These parts are protected from rusting by oiling or greasing regularly or keeping them in oil/grease.
   2. Oiling or greasing
   This involves coating iron with oil or grease. Commonly used to protect the moving parts of the machines from rusting.
   3. Galvanization
   This is the process of coating iron or steel with zinc. Zinc protects iron by reacting with oxygen to form an impermiable coat of zinc oxide.
   4. Alloying
   This involving mixing iron with other metals the resulting mixture is called alloy does not rust. For example, stainless steel contains chromium, nickel and magnesium mixed with iron. Stainless steel does not rust.

   Some utensils such as cutlery and spoons are made of stainless steel. They are constantly placed in water and even if thousands of years they could be in water still they won’t rust.

   5. Electroplating
   This involves coating iron or steel with other metal by using electricity.
   6. Sacrificial protection
   In this method surface of iron is covered with a layer of more active metal like zinc and magnesium of more active metals get corroded in preference to iron and hence prevent the rusting of iron. For example, bars of zinc are attached to the hulls of ships and to oil rigs. These can be renewed once corroded.
   7. Use of silica gel
   Silica gel is a substance in the form of grains that absorbs moisture. It is usually used in some fragile instruments such as cameras which are made of iron or steel parts.

   Advantages of Preventing Rusting

   1. Increase the durability of materials made of iron
   2. Reduces maintenance costs of materials/structures made of iron that is repairing costs
   3. Reduces/removes friction in machine parts thereby increasing efficiency
   4. Maintain the shape of the structure made of iron.

   ---

   ---