In the previous section, we discussed about sulphuric acid. In this section we will discuss about Hydrogen chloride.
In the next section, we will see Nitric acid.
Hydrogen chloride
Hydrogen chloride is an important compound of hydrogen and chlorine. The preparation of hydrogen chloride in the laboratory is shown in fig.7.3 below:
The reactants used are sodium chloride (NaCl) and concentrated sulphuric acid. First, NaCl is taken in a flask. Then the concentrated sulphuric acid is added through the thistle funnel. The mixture is heated. Equation of the reaction is:
NaCl + H2SO4 → NaHSO4 + HCl. This is a balanced equation
• The HCl gas formed is passed through concentrated sulphuric acid. Why is this?
We know that concentrated sulphuric acid is a drying agent. We have seen the details here. So we will get dry HCl in the gas jar.
• HCl gas is denser than air. So it will be collected in the gas jar by the upward displacement of air.
• HCl gas is colourless. So how do we know whether the gas jar is full or not? The method is as follows:
We know that:
♦ Acids turn blue litmus paper red
♦ Bases turn red litmus paper blue
HCl is acidic. So show a wet blue litmus paper at the mouth of the gas jar. If the gas jar is full, the HCl gas will come in contact with the litmus paper, and it's colour will become red.
• The same experiment can be used to prove the solubility of HCl gas in water. The apparatus is shown in the fig.7.4 below:
• The only difference is that, blue litmus is added to the water in the trough. This blue colour will change to red colour when it becomes the fountain. We can explain this colour change as follows:
When the water enters the flask, the HCl gas will dissolve in it. The water will thus become acidic. But blue litmus is already added to the water. So this blue colour will change to red. Thus we get a red coloured fountain.
NH3 + HCl → NH4Cl
This is a balanced equation
■ Note that the same reaction was used earlier in the identification of ammonia. Details here.
• There we dipped the glass rod in ammonia and introduced it to HCl
• Here we dip the glass rod in HCl and introduce it to ammonia
• It is a volatile acid. That is., the acid will easily change to gaseous state. If the bottle containing concentrated HCl is kept open, white fumes will be formed. This is due to the dissolution of HCl gas in the water vapour of the air. A video can be seen here.
Some reactions of hydrochloric acid:
HCl reacts with almost all metals and metallic compounds. Let us see some examples:
■ Reaction with the metal zinc:
• We have seen the above reaction in the preparation of hydrogen. Details here.
• The balanced equation is: Zn + 2HCl → ZnCl2 + H2 .
• So, when HCl reacts with a metal, two things happen:
(i) The chloride of the metal is formed
(ii) Hydrogen is formed
■ Reaction with sodium hydroxide:
We have seen the above reaction when we learned neutralisation. Details here.
■ Reaction with calcium carbonate:
• This reaction gives calcium chloride. The balanced equation is:
2HCl + CaCO3 → CaCl2 + H2O + CO2.
1. Make an aqueous solution of the given salt.
2. Add a little silver nitrate (AgNO3) solution to this
3. If a curdy white precipitate is formed, the given salt can be a chloride salt. The balanced equation is:
NaCl + AgNO3 → AgCl ↓ + NaNO3.
4. But we need to confirm. For that, add ammonium hydroxide solution to the white curdy precipitate
5. If the white precipitate dissolves, we can confirm that, the given salt is a chloride salt.
A video showing the formation of white curdy precipitate can be seen here.
 |
| Fig.7.3 |
NaCl + H2SO4 → NaHSO4 + HCl. This is a balanced equation
• The HCl gas formed is passed through concentrated sulphuric acid. Why is this?
We know that concentrated sulphuric acid is a drying agent. We have seen the details here. So we will get dry HCl in the gas jar.
• HCl gas is denser than air. So it will be collected in the gas jar by the upward displacement of air.
• HCl gas is colourless. So how do we know whether the gas jar is full or not? The method is as follows:
We know that:
♦ Acids turn blue litmus paper red
♦ Bases turn red litmus paper blue
HCl is acidic. So show a wet blue litmus paper at the mouth of the gas jar. If the gas jar is full, the HCl gas will come in contact with the litmus paper, and it's colour will become red.
Solubility of Hydrogen chloride gas in water
• We have seen the fountain experiment for ammonia gas. Details here. It proved that the ammonia gas is soluble in water.• The same experiment can be used to prove the solubility of HCl gas in water. The apparatus is shown in the fig.7.4 below:
 |
| Fig.7.4 |
When the water enters the flask, the HCl gas will dissolve in it. The water will thus become acidic. But blue litmus is already added to the water. So this blue colour will change to red. Thus we get a red coloured fountain.
Properties of Hydrogen chloride gas
• It is a colourless gas • It has a pungent smell • It is denser than air • It is soluble in water • It is acidic in natureIdentification of hydrogen chloride gas
Introduce a glass rod dipped in ammonia solution to the HCl gas. Thick white fumes of ammonium chloride shows the presence of HCl gas. The fumes are due to the formation of ammonium chloride (NH4Cl). The equation of the reaction is:NH3 + HCl → NH4Cl
This is a balanced equation
■ Note that the same reaction was used earlier in the identification of ammonia. Details here.
• There we dipped the glass rod in ammonia and introduced it to HCl
• Here we dip the glass rod in HCl and introduce it to ammonia
Hydrochloric acid
• The aqueous solution of hydrogen chloride gas is hydrochloric acid. Molecular formula of hydrochloric acid is HCl.• It is a volatile acid. That is., the acid will easily change to gaseous state. If the bottle containing concentrated HCl is kept open, white fumes will be formed. This is due to the dissolution of HCl gas in the water vapour of the air. A video can be seen here.
Some reactions of hydrochloric acid:
HCl reacts with almost all metals and metallic compounds. Let us see some examples:
■ Reaction with the metal zinc:
• We have seen the above reaction in the preparation of hydrogen. Details here.
• The balanced equation is: Zn + 2HCl → ZnCl2 + H2 .
• So, when HCl reacts with a metal, two things happen:
(i) The chloride of the metal is formed
(ii) Hydrogen is formed
■ Reaction with sodium hydroxide:
We have seen the above reaction when we learned neutralisation. Details here.
■ Reaction with calcium carbonate:
• This reaction gives calcium chloride. The balanced equation is:
2HCl + CaCO3 → CaCl2 + H2O + CO2.
Identification of chloride salts
The following procedure can be used:1. Make an aqueous solution of the given salt.
2. Add a little silver nitrate (AgNO3) solution to this
3. If a curdy white precipitate is formed, the given salt can be a chloride salt. The balanced equation is:
NaCl + AgNO3 → AgCl ↓ + NaNO3.
4. But we need to confirm. For that, add ammonium hydroxide solution to the white curdy precipitate
5. If the white precipitate dissolves, we can confirm that, the given salt is a chloride salt.
A video showing the formation of white curdy precipitate can be seen here.
In the next section, we will see Nitric acid.
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