Nazarsir.blogspot.com: 5. Acids, Bases and Salts

5. Acids, Bases and Salts :

Acids : Some compounds are seen to have H+ as the basic radical in their molecules. These are all acids.
Bases : Some compounds are seen to have OH - as the acidic radical in their molecules. All these compounds are bases.
Salts : These ionic compounds having basic radicals other than H + and acidic radicals other than OH - are called salts

There are three types of ionic compounds and these are

acids,
bases and
salts

Ionic compounds : A recapitulation

The molecule of an ionic compound has two constituents, namely, cation (postive ion/ basic radical) and anion (negative ion /acidic radical).
There is a force of attraction between these ions as they are oppositely charged, and that is called the ionic bond.
The force of attraction between one positive charge on a cation and one negative charge on an anion makes one ionic bond.
there is a natural tendency of any body to change from an electrically charged state into an electrically neutral state.

The outermost shell of a sodium and a chlorine atoms is not a complete octet.
However outermost shells in both the Na+ and Cl- ions are complete octet.
An electronic configuration with a complete octet indicates a stable state.
Further, an ionic bond is formed between the oppositely charged Na+ and Cl- ions and therefore an ionic compound NaCl having very high stability is formed

Dissociation of ionic compounds :

When an ionic compound begins to dissolve in water, the water molecules penetrate in between the ions of the compound and separate them from each other.
That is to say, an ionic compound dissociates during formation of an aqueous solution.
Each of the dissociated ions in the aqueous solution is surrounded by water molecules.
This state is indicated by writing (aq), meaning aqueous, on the right of the symbol of the ion.

Arrhenius theory of acids and bases :[1887].

Acid :

An acid is a substance which on dissolving in water gives rise to H+ ion as the only cation. For example, HCl, H2SO4 , H2CO3 [Carbonic Acid ].

NH3 - Ammonia

Na2O - Sodium Oxide

CaO - Calcium Oxide

Base :

A base is a substance which on dissolving in water gives rise to the OH- ion as the only anion. For example, NaOH, Ca(OH)2

Classification of acids and bases :

1. Strong and weak acids, bases and alkali

Strong acid :

On dissolving in water, a strong acid dissociates almost completely and the resulting aqueous solution contains mainly H+ ions and the concerned acidic radical.
For example, HCl, HBr, HNO3 , H2SO4 .

Weak acid :

Does not dissociate completely.
Contains H+ ion and the concerned acidic radical in small proportion along with large proportion of the undissociated molecules of the acid.
For example, CH3COOH (Acetic Acid), CO2 .

Strong base :

On dissolving in water, a strong base dissociates almost completely and the resulting aqueous solution contains mainly OH- ions and the concerned basic radicals.
For example, NaOH, KOH, Ca(OH)2 , Na2O.

Weak base :

Does not dissociate completely.
Contains a small proportion of OH- ions and the concerned basic radical along with a large proportion of undissociated molecules of the base.
For example, NH3 .

Alkali :

The bases which are highly soluble in water are called alkali.
For example, NaOH, KOH, NH3 .
Here, NaOH and KOH are strong bases while NH3 is a weak base

2. Basicity and acidity :

Basicity of acids :

The number of H+ ions obtainable by the dissociation of one molecule of an acid is called its basicity.

Acidity of bases :

The number of OH- ions obtainable by the dissociation of one molecule of a base is called its acidity.

Concentration of acid and base :

The proportion of a solute in a solution is called the concentration of the solute in the solution.
When the concentration of a solute in its solution is high, it is a concentrated solution
Solution is called a dilute solution when the concentration of the solute is low.

Units :

First unit is the mass of solute in grams dissolved in one litre of the solution. (grams per litre, g/L).
The second unit is the number of moles of the solute dissolved in one litre of the solution. This is also called the molarity (M) of the solution.
The molarity of a solute is indicated by writing its molecular formula inside a square bracket.
For example ‘[NaCl] = 1’ means the molarity of this solution of common salt is 1M (1 Molar)

pH of solution :

Acids and bases dissociate to a smaller or larger extent on dissolving in water forming H+ and OH- ions respectively.
H+ and OH- ions are found in different proportions in all natural aqueous solutions, and that determines the properties of those solutions.

Pure water also undergoes dissociation to a very small extent and gives rise to H+ and OH- ions in equal proportion.

The range of H+ ion concentration is very wide from 10⁰ to 10^-14 mol/L.
The concentration of H+ ions formed by dissociation of water is 1 x 10^-7 mol/L at 25⁰C.
At the same temperature, the concentration of H+ ions in 1M solution of HCl is 1 x 10⁰mol/L,
In a 1M NaOH solution, the concentration of H+ ions is 1 x 10^-14 mol/L.
Sorensen introduced a convenient new scale of expressing H+ ion concentration which is found to be very useful in chemical and biochemical processes.
It is the pH scale (pH : power of hydrogen).
The pH scale extends from 0 to 14.
According to this scale pure water has a pH of 7
The pH of an acidic solution is less than 7 and that of a basic solution is greater than 7.

Universal indicators :

In the pH scale, the pH of solutions varies from 0 to 14 in accordance with the strength of the acid or base.
To show these variations in pH, a universal indicator is used.
A universal indicator shows different colours at different values of pH.
Made by mixing several synthetic indicators in specific proportions.
The pH of a solution can be determined by means of a universal indicator solution or the pH paper made from it.
However, the most accurate method of measuring the pH of a solution is to use an electrical instrument called pH meter.
In this method, pH is measured by dipping electrodes into the solution.

Reactions of acids and bases

1. Neutralization :

In the neutralization reaction, an acid reacts with a base to form a salt and water.
When enough NaOH is added to HCl, the resulting aqueous solution contains only Na+ and Cl- ions, that is, NaCl, a salt, and the solvent water.
The only source of H+ and OH- ions in this solution is dissociation of water. Therefore, this reaction is called the neutralization reaction.
The neutralization reaction is also represented by the following simple equation.

2. Reaction of acids with metals :

The reaction of acids with metals is determined by the

strength
the concentration of the acid
reactivity of the metal and
the temperature.

It is easy to bring about the reaction of a dilute solution of strong acids with moderately reactive metals at normal temperature.
Magnesium metal reacts with dilute hydrochloric acid and an inflammable gas, hydrogen, is formed.

Mg + HCl ------> MgCl2 + H2

During this reaction, the reactive metal displaces hydrogen from the acid to release hydrogen gas.
At the same time, the metal is converted into basic radical which combines with the acidic radical from the acid to form the salt.

3. Reaction of acids with oxides of metals :

The chemical formula of red oxide is Fe₂O₃ .
The water-insoluble red oxide reacts with HCl to produce a water soluble salt FeCl₃ .
This gives a yellowish colour to the water.

4. Reaction of bases with oxides of non-metals :

Bases react with oxides of non-metals/ Acid to form a salt and water.
Hence, oxides of non-metals are said to be acidic in nature.

Sometimes the oxides of non-metals themselves are said to be examples of acids.

Zinc oxide reacts with sodium hydroxide to form sodium zincate (Na2ZnO2 ) and water.
Aluminium oxide reacts with sodium hydroxide to form sodium aluminate (NaAlO2 ) and water.

5. Reaction of acids with carbonates and bicarbonates of metals :

When limewater comes in contact with the CO2 gas released in the form of an effervescence, it turns milky.
This is a chemical test for carbon dioxide gas.
When lime water turns milky, we infer that the effervescence is of carbon dioxide gas.
This gas is produced on reaction of acids with carbonate and bicarbonate salts of metals.
A precipitate of CaCO3 is produced on its reaction with the limewater Ca(OH)2 . This reaction can be represented by the following chemical equation.

Types of salts : acidic, basic and neutral salts
1. Neutral Salt :

A neutral salt is formed by neutralization of a strong acid by a strong base.
The aqueous solution of a neutral salt has pH equal to 7.

2. Acidic Salt :

An acidic salt is formed by the neutralization reaction between a strong acid and a weak base.
The pH of the aqueous solution of an acidic salt is less than 7.

3. Basic Salt :

On the contrary, a basic salt is formed by a neutralization reaction between a weak acid and a strong base.
The pH of an aqueous solution of such a basic salt is greater than 7.

Water of Crystallisaton :

On heating, the crystalline structure of blue vitriol broke down to form a colourless powder and water came out.
This water was part of the crystal structure of blue vitriol. It is called water of crystallisation.
On adding water to the white powder a solution was formed which had the same colour as the solution in the first test tube.
No chemical change has occurred in the crystals of blue vitriol due to heating.
Losing water on heating blue vitriol, breaking down of the crystal structure, losing blue colour and regaining blue colour on adding water are all physical changes.

Ionic compounds are crystalline in nature.

These crystals are formed as a result of definite arrangement of ions.
In the crystals of some compounds water molecules are also included in this arrangement. That is the water of crystallisation.
The water of crystallisation is present in a definite proportion of the chemical formula of the compound.

Ionic compounds and electrical conductivity :

Aqueous solutions of NaCl, CuSO4 , H2SO4 and NaOH are conductors of electricity.
Electrons conduct electricity through electrical wires; and ions conduct electricity through a liquid or a solution.
Electrons leave the battery at the negative terminal, complete the electric circuit and enter the battery at the positive terminal.
When there is a liquid or a solution in the circuit, two rods, wires or plates are immersed in it. These are called electrodes.
Electrodes are usually made of a conducting solid.
The electrode connected to the negative terminal of a battery by means of a conducting wire is called a cathode and the electrode connected to the positive terminal of a battery is called an anode.

Dissociation of ions and electrical conductivity :

Aqueous solutions of the compounds NaCl, CuSO4 , H2SO4 and NaOH are electricity conductors.

NaCl and CuSO4 are salts,
H2SO4 is a strong acid and
NaOH is a strong base.

Strong acids and strong bases dissociate almost completely in their aqueous solutions. Therefore the aqueous solutions of all these three contain large numbers of cations and anions.
A characteristic of the liquid state is the mobility of its particles.
Due to this mobility, the positively charged cations in the solution are attracted towards the negative electrode or cathode and move towards the cathode
Negatively charged anions move in the direction of the anode.
The movement of ions in the solution towards the respective electrodes amounts to the conduction of electricity through the solution.
Those liquids or solutions which contain a large number of dissociated ions conduct electricity.

Electrolysis:

When an electric current started flowing through the circuit, the cations, that is, Cu++ ions in the solution got attracted towards the cathode.
Cu atoms are formed when electrons coming out from the cathode combine with the Cu++ ions.
A deposit of the copper appeared on the cathode.
Even though the Cu++ ions in the solution were used up in this manner, the colour of the solution remained the same.
Because, while electric current was on, electrons were removed from the Cu atoms of the anode and sent to the battery through the electric wire.
The Cu++ ions formed in this manner, entered the solution.
In this way decomposition of the solute in the solution took place due to the electric current. This is called electrolysis.
There are two parts in the electrolysis process.

Cathode reaction
Anode reaction.

If pure water is used in an electrolytic cell, current does not flow even on putting on the switch.
Pure water is a bad conductor of electricity.
The concentration of H+ and OH- ions formed by dissociation of water is very low, only 1 x 10^-7 mol/L for each ion.
However, the electrical conductivity of water increases on mixing a small amount of salt or a strong acid/base in it due to their dissociation and electrolysis of water takes place

+ point :

Substances which undergo dissociation to great extent in the liquid state or a solution are called strong electrolytes.
Salts, strong acids and strong bases are strong electrolytes. Their solutions have high electrical conductivity.
In other words strong electrolytes are good conductors of electricity in their liquid or solution state.
Weak acids and weak bases are weak electrolytes.
An assembly that consists of a container electrolyte and the electrodes dipped in it, is called an electrolytic cell

Electrolysis of water :

It is found in the above activity that the volume of the gas formed near the cathode is double that of the gas formed near the anode.
Scientists have shown that hydrogen gas is formed near the cathode and oxygen gas near the anode.
From this, it is clear that electrolysis of water has taken place and its constituent elements have been released. The concerned electrode reactions are as follows.