In the previous section, we saw some reactions of oxygen with non-metals. In this section we will see some reactions of oxygen with metals.
A freshly cut surface of metals like aluminium and iron will have a metallic lustre. But this lustre will fade away gradually. This is due to the formation of a new substance on the surface. This new substance is formed because of the reaction of the metal with oxygen.
Reaction with Aluminium:
Aluminium reacts with the oxygen in the atmosphere, and forms aluminium oxide. Let us write the equation:
Reaction with Iron:
Reaction with magnesium:
We have completed our present discussion on oxygen. In the next section, we will discuss about Nitrogen.
A freshly cut surface of metals like aluminium and iron will have a metallic lustre. But this lustre will fade away gradually. This is due to the formation of a new substance on the surface. This new substance is formed because of the reaction of the metal with oxygen.
Reaction with Aluminium:
Aluminium reacts with the oxygen in the atmosphere, and forms aluminium oxide. Let us write the equation:
• One molecule of aluminium is
Al. One molecule of oxygen is O2.
We will write each of them on the left side
• One molecule of aluminium
oxide is Al2O3.
We will write it on the right side
• So the skeletal equation is Al +
O2
→ Al2O3
. This is not a balanced equation
• The steps for balancing the
equation are given below:
So the balanced equation is:
4Al + 3O2
→ 2Al2O3
Reaction with Iron:
Iron
reacts with the oxygen in the atmosphere, and forms iron oxide. Let
us write the equation:
• One molecule of iron is Fe.
One molecule of oxygen is O2.
We will write each of them on the left side
• One molecule of iron oxide is
Fe2O3.
We will write it on the right side
• So the skeletal equation is Fe +
O2
→ Fe2O3
. This is not a balanced equation
• The steps for balancing the
equation are given below:
• So the balanced equation is:
4Fe + 3O2
→ 2Fe2O3
• The iron oxide is commonly known as 'rust'Reaction with magnesium:
When magnesium burns, it
reacts with oxygen. We can do a simple experiment to demonstrate
this. [Note that all experiments should be done only under the
supervision of teachers. All safety precautions should be taken. In
the following experiment, avoid looking directly at the flash]
1. Take a magnesium ribbon about
2 cm long. Clean it's surface using sand paper. This is to remove any
impurities. Because we want the reaction to take place directly
between magnesium and oxygen. We do not want any impurities in
between.
2. Hold the ribbon with a pair of
tongs. Burn it using a spirit lamp or burner. Note that it should be
burnt while keeping it as far away as possible from the eyes.
3. The ribbon will burn with
bright white light. Do not look directly at the light. When the
burning is complete, we will see that the ribbon has changed into a
white powder. This powder is magnesium
oxide.
• One molecule of magnesium is Mg. One molecule of oxygen is O2. We will write each of them on the left side
• One molecule of aluminium oxide is MgO. We will write it on the right side
• So the skeletal equation is Mg + O2 → MgO . This is not a balanced equation.
• The steps for balancing the equation are given below:
So the balanced equation is: 2Mg + O2 → 2MgO
We have seen some of the reactions of oxygen with metals as well as non-metals. Next we will discuss about Ozone layer.
■ Some elements are found in two or more physical forms. This is called allotropy. The different physical forms are called allotropes of the element. Example:
Ozone is present mostly in the stratosphere of the atmosphere. Let us see the steps in the formation of Ozone:
We have seen some of the reactions of oxygen with metals as well as non-metals. Next we will discuss about Ozone layer.
■ Oxygen (O2) is found as a
diatomic molecule. That is., each molecule of oxygen will have two
atoms of oxygen.
■ But some times, three atoms of oxygen will combine
together to form a single molecule. That molecule is entirely
different from oxygen in physical and chemical properties. It is
called Ozone.
■ Some elements are found in two or more physical forms. This is called allotropy. The different physical forms are called allotropes of the element. Example:
• Diamond and graphite are two
allotropes of carbon.
• Oxygen (O2) and Ozone (O3) are
two allotropes of oxygen
Ozone is present mostly in the stratosphere of the atmosphere. Let us see the steps in the formation of Ozone:
1. The ordinary oxygen (O2)
molecules absorbs high energy ultraviolet radiation, and dissociates
into individual oxygen atoms.
2. These individual oxygen atoms combine
together to form molecules of ozone (O3).
3. Ozone molecules so formed,
absorb low energy ultraviolet radiations and decompose into oxygen
molecules (O2).
4. The O2 molecules so formed
will again undergo the process in step (1)
■ So we see that it is a
cyclic process. The cycle is shown in the fig.5.2 below:
Let us see the
important points to be noted in the cycle:
 |
| Fig.5.2 |
• In (1), the O2 absorbs
high energy ultraviolet radiation. But in (3), the O3 absorbs low energy ultraviolet radiation.
• In (1), the O2 dissociates
into individual atoms. But in (3), the O3 decomposes into O2.
Importance of the ozone cycle:
We see that ultraviolet
radiations are absorbed during the cyclic process. As a result, those
harmful radiations do not reach the surface of the earth. Such
radiations can cause diseases like cancer. So the ozone layer is very
important for sustaining life on earth.
Ozone layer depletion
The proper functioning of Ozone layer is under threat. Let us see why:
• Compounds known as CFC or
chlorofluorocarbons, are special compounds that contain chlorine,
fluorine and carbon.
• They have a special property: They are normally
in the gaseous state. But they can be easily brought into liquid
state by applying pressure.
• Once they are in the liquid state, they
absorb heat from the surroundings. Using this heat, they evaporate
and gets converted back into gaseous state.
• When they absorb heat,
the surroundings are cooled. So they are used in refrigerators and
air conditioners.
• After long periods of usage,
the appliances like refrigerators, air conditioners etc., become worn
out. They are then abandoned.
• The CFC in them will escape into the
atmosphere. They reach the stratosphere.
• The molecules of CFC will
break down under the action of ultraviolet radiations. When they
break down, individual chlorine atoms become available.
• These
chlorine atoms react with ozone molecules. This reaction will cause
the decomposition of ozone molecules. The ozone molecules thus
decomposed will be out of the ozone cycle.
• So if more CFC escapes
into the atmosphere, more chlorine will become available, and so more
molecules of ozone will be decomposed. Gaps will begin to appear in the ozone layer. This is known as ozone layer
depletion.
•When more molecules of ozone
is decomposed, only a lesser quantity of ozone will remain to take
part in the cycle. This remaining ozone will not be able to absorb
all the harmful radiations coming from the sun. As a result, much of
those radiations will reach the surface of the earth.
■ In order to create awareness
about the need to preserve the ozone layer, September 16th
is celebrated as International Ozone Day.
■ Now a days many countries
have introduced strict rules to avoid using harmful CFC. Safer
substances should be used instead. We must all abide by the rules so
that, the ozone layer can be restored back to it's original state.
We have completed our present discussion on oxygen. In the next section, we will discuss about Nitrogen.
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