In the previous section, we completed the discussion on Hydrogen and Chlorine. In this chapter we will see Acids and Alkalies.
We know that, when metals or non-metals react with oxygen, various oxides are formed.
We have completed our present discussion on the formation of acids from non-metallic oxides. In the next section, we will discuss about metallic oxides.
We know that, when metals or non-metals react with oxygen, various oxides are formed.
• When metals react with
oxygen, we get metallic oxides.
• When non-metals react with
oxygen, we get non-metallic oxides.
We will first consider
non-metallic oxides.
We know that carbon is a
non-metal. Nitrogen and sulphur are also non-metals. When these
non-metals react (the reaction can be combustion or ordinary
reaction) with oxygen, their oxides are produced. And these oxides
are ‘non-metallic oxides’.
Here we have to learn about 3
non-metallic oxides. CO2, NO2 and SO2. We will first consider CO2.
Let us do an experiment:
1. Take some calcium carbonate
(CaCO3) in a boiling tube. Add 5 ml dilute hydrochloric acid
(dil.HCl) to it using a thistle funnel. See fig.6.1 below:
2. The calcium carbonate reacts
with HCl. We get calcium chloride (CaCl2) and carbon dioxide (CO2) gas as products. Let us write the equation:
 |
| Fig.6.1 |
Reactants:
♦ Calcium carbonate. One molecule is CaCO3.
♦ Dilute Hydrochloric acid. One molecule is HCl.
Products:
♦ Calcium chloride. One molecule is CaCl2.
♦ Carbon dioxide. One molecule is CO2.
♦ Water. One molecule is H2O
♦ Water. One molecule is H2O
• So skeletal equation is:
CaCO3 + HCl → CaCl2 + CO2 + H2O. This is not a balanced equation. The steps for writing the balanced equation are shown below:
Step 1: CaCO3 + HCl → CaCl2 + CO2 + H2O
Step 2: CaCO3 + 2HCl → CaCl2 + CO2 + H2O
Step 1: CaCO3 + HCl → CaCl2 + CO2 + H2O
Step 2: CaCO3 + 2HCl → CaCl2 + CO2 + H2O
| REACTANTS | PRODUCTS | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ca | C | O | H | Cl | Ca | C | O | H | Cl | ||||
| Step 1 | 1 | 1 | 3 | 1 | 1 | 1 | 1 | 3 | 2 | 2 | |||
| Step 2 | 1 | 1 | 3 | 2 | 2 | 1 | 1 | 3 | 2 | 2 | |||
■ So the balanced equation is: CaCO3 + 2HCl → CaCl2 + CO2 + H2O
Thus we
successfully produced a non-metallic oxide CO2. There is more to
do:
3. Pass this CO2 through water in
a test tube. When we do that, we get a solution. The solution formed
by dissolving CO2 in water.
4. Now dip a blue litmus paper in the solution. The blue litmus turns into red.
5. So we can conclude that, the solution is acidic in nature.
4. Now dip a blue litmus paper in the solution. The blue litmus turns into red.
5. So we can conclude that, the solution is acidic in nature.
■ That means, an acid is formed
when CO2 dissolves in water.
• In fact, when the CO2 dissolves in water, a reaction takes place. The product formed is carbonic acid (H2CO3). Let us write the equation:
• In fact, when the CO2 dissolves in water, a reaction takes place. The product formed is carbonic acid (H2CO3). Let us write the equation:
Reactants:
♦ Carbon dioxide. One molecule is CO2
♦ Carbon dioxide. One molecule is CO2
♦ Water. One molecule is H2O
Product:
♦ Carbonic acid. One molecule is H2CO3
♦ Carbonic acid. One molecule is H2CO3
• So the skeletal equation is: H2O + CO2 → H2CO3. This is a balanced equation.
■ From the above experiment, we
get the following information:
The non-metallic oxide CO2 dissolves in water to give an acid
Generally, CO2 will not
dissolve in water easily. If we want to dissolve large quantities of CO2 in water, we will have to apply pressure. Soda water is produced
in this way.
Now we consider another
non-metallic oxide. That of nitrogen:
Nitrogen does not take part in
reactions easily. This is because of the triple bond in the nitrogen
molecule. But under the action of thunder and lightning, nitrogen in
the atmosphere combines with oxygen. Thus oxides of nitrogen are
produced. We saw the details in the previous chapter when we saw Nitrogen cycle. We will write the steps again:
1. First, nitrogen combines with oxygen to form nitric oxide. The balanced equation is:
N2 + O2 → 2NO
1. First, nitrogen combines with oxygen to form nitric oxide. The balanced equation is:
N2 + O2 → 2NO
2. The NO thus formed will combine with more oxygen to become Nitrogen dioxide NO2. The balanced equation is:
2NO + O2 → 2NO2
3. Thus we reach the stage of a non-metallic oxide NO2. We want to know the further actions of this NO2.
2NO + O2 → 2NO2
3. Thus we reach the stage of a non-metallic oxide NO2. We want to know the further actions of this NO2.
4. The NO2 dissolves in rain
water. It may be noted that, oxygen is required for this dissolution
to take place. The balanced equation is:
4NO2 + 2H2O + O2 → 4HNO3.
4NO2 + 2H2O + O2 → 4HNO3.
5. So, as a result of the 'dissolving of nitric oxide in water', we get HNO3, which is nitric
acid.
■ Thus so far, we have seen two cases:
■ Thus so far, we have seen two cases:
• The non-metallic oxide CO2 dissolves in water to give carbonic acid
• The non-metallic oxide NO2 dissolves in water to give nitric acid.
Now we will see yet another
non-metallic oxide. The oxide of sulphur. It is called sulphur
dioxide (SO2) We have seen it's formation when we studied about the
reactions of oxygen with non-metals. Details here. The balanced
equation for the formation of SO2 is:
S + O2 → SO2.
• This oxide also dissolves in
water to form an acid. The acid formed is sulphurous acid (H2SO3). Let us write the equation:
Reactants:
♦ Sulphur dioxide. One molecule is SO2.
♦ Water. One molecule is H2O.
Products:
♦ Sulphurous acid. One molecule is H2SO3.
• So skeletal equation is:
SO2 + H2O → H2SO3. This is a balanced equation.
So we have one more case to
add to our list. The modified list is given below:
■ Three cases seen so far:
• The non-metallic oxide CO2 dissolves in water to give carbonic acid
• The non-metallic oxide NO2 dissolves in water to give nitric acid.
• The non-metallic oxide SO2 dissolves in water to give Sulphurous acid.
We can say this: Non-metallic
oxides generally give acids when they dissolve in water
Let us now see the ill-effects
of such acid formation:
■ Factories, thermal power
plants, automobiles etc., send out large quantities of non-metallic
oxides like NO2 and SO2. These acids dissolve in rain water, and form
acids. When large quantities of acids come down with rain, we call it
an acid rain. Following are the ill-effects of acid rains:
• Acid rains destroy the leaves
of plants. The plants then will no longer be able to produce food by
photosynthesis.
• Acid rains destroys the
greenery of a region. This will increase air pollution
• Acid rains can cause deaths of
fish and other marine organisms
• Acid rains cause destruction
of substances containing calcium carbonate (CaCO3). Because, CaCO3 reacts with acids
♦Corals contain CaCO3. When
acids reach them, reaction takes place, and the corals will be
destroyed. The effects can be seen here.
♦ Marble contain CaCO3. When
acids reach them, reaction takes place, and the beauty of marble is
lost.
♦ Limestone is an important naturally occuring veriety of stone. It is essential for many industries. They are obtained from limestone quarries. The quarries as a whole will be destroyed if the acid rains reach them. The destruction takes place due to the reaction of CaCO3 with the acids. The effects can be seen here.
♦ Limestone is an important naturally occuring veriety of stone. It is essential for many industries. They are obtained from limestone quarries. The quarries as a whole will be destroyed if the acid rains reach them. The destruction takes place due to the reaction of CaCO3 with the acids. The effects can be seen here.
We have to take steps for the
prevention of acid rains. Following are some of the effective steps:
• Reduce the use of fossil fuels
like coal and petroleum
• Use those fossil fuels from
which sulphur compounds have been removed. In this way, oxides of sulphur will not be formed during the combustion of the fuel.
• Regularly check that, the
gases emitted from automobiles does not cause pollution above the
accepted level.
We have completed our present discussion on the formation of acids from non-metallic oxides. In the next section, we will discuss about metallic oxides.
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