In the previous section, we saw the relation between non-metallic oxides and acids. In this section we will see metallic oxides.
Let us do an experiment:
For this experiment, we need 'quick lime'. Quick lime is the common name for calcium oxide (CaO). It is obtained by heating naturally occuring substances like limestone or sea shells. Limestone and sea shells are calcium carbonates (CaCO3). When it is heated, carbon dioxide escapes, and CaO is left behind. Let us write the equation:
The above classification is based on the following two facts:
■ Non-metallic oxides are acidic in nature
■ Metallic oxides are alkaline in nature
Following are some of the
properties of acids and alkalies:
We have seen some of the important details about acids. In the next section, we will discuss about alkalies.
Let us do an experiment:
For this experiment, we need 'quick lime'. Quick lime is the common name for calcium oxide (CaO). It is obtained by heating naturally occuring substances like limestone or sea shells. Limestone and sea shells are calcium carbonates (CaCO3). When it is heated, carbon dioxide escapes, and CaO is left behind. Let us write the equation:
Reactants:
♦ Calcium carbonate. One molecule is CaCO3
♦ Calcium carbonate. One molecule is CaCO3
Products:
♦ Calcium oxide. One molecule is CaO
♦ Calcium oxide. One molecule is CaO
♦ Carbon dioxide. One molecule is CO2
• So the skeletal equation is: CaCO3 → CaO + CO2 . This is a balanced equation.
1. So we have an oxide CaO. An oxide
of a metal calcium. So it is a metallic oxide. We are going to do the
experiment with it.
2. Take water in a beaker. Add some quick lime to it
and stir well. A reaction takes place. It is between CaO and water. If we touch the sides of the beaker, we can feel heat. This is because the reaction releases heat. In other words, it is an exothermic reaction. Let us write the equation:
Reactants:
♦ Calcium oxide. One molecule is CaO
♦ Calcium oxide. One molecule is CaO
♦ Water. One molecule is H2O
Product:
♦ Calcium hydroxide. One molecule is Ca(OH)2
♦ Calcium hydroxide. One molecule is Ca(OH)2
• So the skeletal equation is: CaO + H2O → Ca(OH)2. This is a balanced equation.
3. How are we going to use this
calcium hydroxide?
• We added quick lime to water.
The quick lime was converted into Ca(OH)2 (calcium hydroxide).
• Some water will be used up for the reaction. But there will still be some excess water. Because all the water will not be used up.
• So the product, which is Ca(OH)2, will dissolve in the excess water. And we get a solution of Ca(OH)2 in water.
• But all the Ca(OH)2 will not dissolve in water. Some of it will precipitate to the bottom of the beaker. The clear portion above the precipitate is called 'supernate'.
■ We see a separation as a precipitate and a supernate because, there is not enough water for all the Ca(OH)2 to dissolve. That portion of Ca(OH)2, which is unable to dissolve, will form the precipitate. That means, the supernate is a solution of Ca(OH)2 in water.
• Some water will be used up for the reaction. But there will still be some excess water. Because all the water will not be used up.
• So the product, which is Ca(OH)2, will dissolve in the excess water. And we get a solution of Ca(OH)2 in water.
• But all the Ca(OH)2 will not dissolve in water. Some of it will precipitate to the bottom of the beaker. The clear portion above the precipitate is called 'supernate'.
■ We see a separation as a precipitate and a supernate because, there is not enough water for all the Ca(OH)2 to dissolve. That portion of Ca(OH)2, which is unable to dissolve, will form the precipitate. That means, the supernate is a solution of Ca(OH)2 in water.
Take some of this supernate in a test tube. Add a drop of red litmus to
it. We can see that, a blue colour appears in the solution. So we can
infer that Ca(OH)2 is alkaline in nature. Let us analyse what we have done:
• We took quick lime. It is an
oxide of calcium, which is a metal. So quick lime is a metallic
oxide.
• This metallic oxide, when added to water, gave an alkali. We can say this: Metallic oxides are generally alkaline in nature.
• This metallic oxide, when added to water, gave an alkali. We can say this: Metallic oxides are generally alkaline in nature.
■ So, if we are given a list of
oxides, we can classify them as acidic or alkaline. Given below is a
list:
SO3, NO2, CaO, K2O, P2O5, Na2O, CO2, MgO
Let us classify them into acidic and alkaline:
| Acidic oxides | Alkaline oxides |
|---|---|
| SO3 | CaO |
| P2O5 | K2O |
| CO2 | Na2O |
| NO2 | MgO |
■ Non-metallic oxides are acidic in nature
■ Metallic oxides are alkaline in nature
■ Acids:
• Have sour taste
• Changes blue litmus to red
• Produces carbon dioxide gas on
reacting with carbonates
• Produces hydrogen gas on
reacting with metals like Fe, Mg etc.,
■ Alkalies:
• Have bitter taste
• Changes red litmus to blue
• Slippery to touch
Let us explore some more about acids:
Acids can be classified as
Mineral acids and Organic acids
■ Mineral acids are also known
as inorganic acids. They are prepared from one or more inorganic
compounds, and are stronger than organic acids. Most common mineral
acids are Sulphuric acid, Nitric acid and Hydrochloric acid. Mineral
acids should not be tasted.
■ Organic acids are those acids found in naturally occuring organic compounds. They are generally weak acids. The sour taste of some of the food items is due to the presence of organic acids. Some examples are given below:
■ Organic acids are those acids found in naturally occuring organic compounds. They are generally weak acids. The sour taste of some of the food items is due to the presence of organic acids. Some examples are given below:
• Lime – Citric acid
• Tamarind – Tartaric acid
• Tomato – Oxalic acid
• Sour milk – Lactic acid
• Vinegar – Acetic acid
• From the above list, we find
that hydrogen is present in all the acids. So hydrogen is a common
factor in acids.
The common factor in acids
Let us write the chemical
formula of some commonly known acids:
| Name of acid | Chemical formula |
|---|---|
| Hydrochloric acid | HCl |
| Nitric acid |
HNO3
|
| Acetic acid | CH3COOH |
| Carbonic acid | H2CO3 |
| Sulphuric acid | H2SO4 |
Hydrogen is the
component present in all acids and is responsible for the common
properties of acids.
Let us see a reaction in which
an acid is a reactant:
Hydrochloric acid reacts with
iron to give ferric chloride and hydrogen. Let us write the equation:
Reactants:
♦ Iron. One molecule is Fe
♦ Dilute Hydrochloric acid. One molecule is HCl.
Products:
♦ Ferric chloride. One molecule is FeCl2.
♦ Carbon dioxide. One molecule is CO2.
• So skeletal equation is:
Fe + HCl → FeCl2 + H2. This is not a balanced equation. The steps for writing the balanced equation are shown below:
Step 1: Fe + HCl → FeCl2 + H2
Step 2: Fe + 2HCl → FeCl2 + H2
■ So the balanced equation is: Fe + 2HCl → FeCl2 + H2
Step 1: Fe + HCl → FeCl2 + H2
Step 2: Fe + 2HCl → FeCl2 + H2
| Reactants | Products | |||||
|---|---|---|---|---|---|---|
| Fe | H | Cl | Fe | H | Cl | |
| Step 1 | 1 | 1 | 1 | 1 | 2 | 2 |
| Step 2 | 1 | 2 | 2 | 1 | 2 | 2 |
We can say this:
The acid HCl, reacts with the
metal iron to liberate hydrogen. In fact, it is a common property of
all acids:
■ All acids react with metals to
liberate hydrogen
We saw another example in the
preparation of hydrogen in the lab. Details here.
We will now discuss about
acids in detail:
We know that in HCl:
We know that in HCl:
• There is a covalent bonding between H and Cl.
• Cl is more electronegative, and so attracts the shared pair of electrons towards itself
• So the hydrogen has a partial positive charge, and chlorine has a partial negative charge.
• We have seen the above details here.
• When HCl is mixed with water, the bond between H and Cl breaks. The hydrogen will have to leave with out taking it's electron.
• This is because, Cl is more electronegative, and so, will keep both the shared electrons
• The H atom thus becomes H+ ion and Cl becomes Cl-
• The H+ ions will go towards the water molecules. They will attach with the water molecules.
• The water molecules will then become ions because of the positive charge brought by the H+ ions.
• The new ion is written as H3O+. It is called the hydronium ion.
• These hydronium ions are responsible for the characteristics of acids.
Consider the following scenario:
• Cl is more electronegative, and so attracts the shared pair of electrons towards itself
• So the hydrogen has a partial positive charge, and chlorine has a partial negative charge.
• We have seen the above details here.
• When HCl is mixed with water, the bond between H and Cl breaks. The hydrogen will have to leave with out taking it's electron.
• This is because, Cl is more electronegative, and so, will keep both the shared electrons
• The H atom thus becomes H+ ion and Cl becomes Cl-
• The H+ ions will go towards the water molecules. They will attach with the water molecules.
• The water molecules will then become ions because of the positive charge brought by the H+ ions.
• The new ion is written as H3O+. It is called the hydronium ion.
• These hydronium ions are responsible for the characteristics of acids.
Consider the following scenario:
We have a situation in which
we want the effects of an acid. We have a bottle of HCl. If we keep
the HCl in the bottle itself, or pour it into another suitable
container, there is no effect. To see the effects of acid, the acid
must come in contact with other materials.
Note that it is very important
to handle acids carefully. All safety measures should be taken. Some
of the compulsory safety measures are:
• Wearing lab coat or lab apron
• Wearing safety goggles for
protection of eyes
• Using acid resistant gloves
Experiments should be
performed only under the guidance of authorised professionals
• We have seen the effect when
HCl comes into contact with water: The hydronium ions (H3O+)are formed,
and that gives the acidic effect.
• If more H+ ions are liberated,
more hydronium ions will be formed, and there will be greater acidic effect
Eg: HCl → H+ + Cl-.
HNO3 → H+ + NO3-
■ If one molecule of an acid gives two H+ ions, It is a dibasic acid
Eg: H2SO4 → 2H+ + SO42-
It may be noted that, the two H+ ions are obtained as a result of two steps as shown below:
Step 1: H2SO4 dissociates into H+ and HSO4-
Step 2: HSO4- dissociates into H+ and SO42-
Step 1 gives one H+ and one HSO4-. But
after step 2, the HSO4- is no longer present. It has dissociated into H+ and SO42-
So we get a total of two H+ ions
Another example:
H2CO3 → 2H+ + CO32-.
Here also, the two H+ ions are obtained as a result of two steps as shown below:
Step 1: H2CO3 dissociates into H+ and HCO3-
Step 2: HCO3- dissociates into H+ and CO32-.
So we get a total of two H+ ions
■ If one molecule of an acid gives three H+ ions, It is a tribasic acid
Eg: H3PO4 → 3H+ + PO43-
Here, the three H+ ions are obtained as a result of three steps as shown below:
Step 1: H3PO4 dissociates into H+ and H2PO4-
Step 2: H2PO4- dissociates into H+ and HPO42-.
Step 3: HPO42- dissociates into H+ and PO43-.
So we get a total of three H+ ions
We have seen some of the important details about acids. In the next section, we will discuss about alkalies.
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