Chapter 6.3 - Salts formed from Neutralisation reaction

In the previous section, we saw the details about acidity and alkalinity. We also saw the pH scale. In this section we will see some practical situations where acidity or alkalinity will have to be considered. We will also learn more about salts.

In agriculture, soils with acidic nature are suitable for some crops. While soils with alkaline nature are suitable for some other crops. So it is important to test the soils before beginning the cultivation. Sample of soil is taken in a special manner, prescribed by the agricultural officer. This sample is mixed with distilled water. The mixture thus obtained is kept undisturbed for some time. The soil particles will settle down. The sample for testing is taken from the clear portion at the top. The pH value of this sample is determined. From the pH value, the acidity/alkalinity of the soil can be calculated. Based on this result, the officer will prescribe the suitable crop that can be planted in that soil. He can also determine the ‘quantity of acidity or alkalinity’. So he can prescribe whether any treatments have to be done to the soil, to make it suitable for cultivation.

Some times farmers spread powdered slaked lime. Slaked lime is Ca(OH)₂. We have seen it’s preparation when we studied the basics about alkalies at the beginning of a previous section here. It is used in soils which are highly acidic in nature. When the Ca(OH)₂ comes in contact with the rain water, (OH)^- ions will be released. These (OH)^- ions will neutralise H⁺ ions. Thus the acidity of the soil will be reduced.

Similarly, when the alkalinity of the soil is high, non-metallic oxides like SO₂ is added. These oxides produce acids when they come in contact with rain water. That means, H⁺ ions will be produced. They will neutralise the excess (OH)^- ions present in the soil.

When the acid level in the stomach increases, we feel acidity. Medicines used for reducing the acid level in stomach are called antacids. They are alkaline substances. They neutralise the excess acids.

Salts

• We have seen that, when an acid and an alkali react together, we get a salt and water. We have seen the example of the reaction between HCl and NaOH. The products are NaCl and H₂O. The equation is: NaOH + HCl → NaCl + H₂O

• We can write the above equation in terms of ions: Na⁺OH^- + H⁺Cl^- → Na⁺Cl^- + H⁺(OH)^-

We can note the following points:

1. The positive ion Na⁺ in the resulting salt, comes from the alkali.

2. The negative ion Cl^- in the resulting salt comes from the acid.

■ In fact, this is a common property in all neutralisation reactions. We can write it as:

• The positive ion in the resulting salt, comes from the alkali.

• The negative ion in the resulting salt comes from the acid.

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• Salts are usually ionic compounds. They dissociate into positive and negative ions when dissolved in water or on fusion (fusion is another term for ‘melting’).

• The positive ions are called cations, and the negative ions are called anions
The following table shows some cations and anions:

Name of Cation	Symbol		Name of Anion	Symbol
Potassium ion	K+		Hydroxide ion	OH-
Zinc ion	Zn2+		Carbonate ion	CO32-
Ferrous ion	Fe2+		Bicarbonate	HCO3-
Ferric ion	Fe3+		Nitrate ion	NO3-
Cuprus ion	Cu+		Sulphate ion	SO42-
Cupric ion	Cu2+		Bisulphate ion	HSO4-
Ammonium ion	NH4+		Phosphate ion	PO43-
Manganus ion	Mn2+		Dihydrogen phosphate ion	H2PO4-
Magnesium ion	Mg2+

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• If we know the reactants in a neutralisation reaction:

    ♦ We will be able to predict the name of the salt which will be formed

    ♦ We will also be able to write the chemical formula of the salt which will be formed

For doing the above two things, we must first do a careful analysis:
1. Our aim is to obtain the chemical formula of the salt
2. The data that we have is: Names of the reactants (acid and alkali)
3. We have seen that the cation in the final salt, comes from the alkali. So the first step is to split the alkali into cation and anion.
• From that, take out the cation
• Let the cation be represented by the letter 'C'. 
• A cation will be having positive charge.
• Let the number of positive charges be 'x'. 
• So our required cation is C^x+

4. We have seen that the anion in the final salt, comes from the acid. So the second step is to split the acid into cation and anion.
• From that, take out the anion
• Let the anion be represented by the letter 'A'. 
• An anion will be having negative charge. 
• Let the number of negative charges be 'y'. 
• So our required anion is A^y-.

5. Now assemble the cation and anion together. The cation should be written first. So we get:
C^x+   A^y-. 
6. We know that, the final salt is electrically neutral. That means, the net charge is zero. So x must be equal to y. 
7. This may not be always possible. 
For example, when we assemble Fe3+ and SO₄2-, x= 3 and y = 2. So the charges will not neutralise completely.
8. In such cases, we must assemble 'suitable numbers' of cations and anions.
Let 'm' be the number of cations required
Let 'n' be the number of anions required
Then (5) will become: C_m^x+   A_n^y- .   
9. Now total number of positive charges = mx, and total number of negative charges = ny
10. These must be equal. So we get mx = ny
11. To satisfy this equation, 
• m must be equal to y
• n must be equal to x
• Then we will get yx = xy, and thus, (10) will be satisfied
12. Thus we find that, there is a sort of 'interchanging'. 
• The number of cations required (m) is the number of charges in anion (y)
• The number of anions required (n) is the number of charges in cation (x)
■ When we give the above required number of ions, we will get a neutral salt. The solved example given below will demonstrate the procedure

Solved example 6.1
In the neutralisation reaction between the alkali Magnesium hydroxide (Mg(OH)₂) and the Hydrochloric acid (HCl), write the chemical formula of the salt formed. Also write the balanced equation of the neutralisation reaction.
Solution:
1. The cation in the final salt, comes from the alkali. So the first step is to split the alkali into cation and anion: Mg(OH)₂ → Mg²⁺ + 2(OH)^- .
2. The anion in the final salt, comes from the acid. So the second step is to split the acid into cation and anion: HCl → H⁺ + Cl^-.
3. Assemble the cation and anion with cation first: Mg²⁺  Cl^- .
4. Do the interchanging:
• Number of  Mg²⁺ ions = Number of charges in Cl^- ion = 1 
• Number of Cl^- ions = Number of charges in Mg²⁺ ion = 2
5. So chemical formula of the final salt is: MgCl₂.

Balanced equation for the neutralisation reaction:

Reactants:

    ♦ Magnesium hydroxide. One molecule is Mg(OH)₂. 

    ♦ Hydrochloric acid. One molecule is HCl.

Products:

    ♦ Magnesium chloride. One molecule is MgCl₂.

    ♦ Water. One molecule is H₂O

• So skeletal equation is:

Mg(OH)₂ + HCl → MgCl₂ + H₂O. This is not a balanced equation. The steps for writing the balanced equation are shown below:
Step 1: Mg(OH)₂ + HCl → MgCl₂ + H₂O
Step 2: Mg(OH)₂ + 2HCl → MgCl₂ + H₂O
Step 3: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

	Reactants					Products
	Mg	O	H	Cl		Mg	O	H	Cl
Step 1	1	2	3	1		1	1	2	2
Step 2	1	2	4	2		1	1	2	2
Step 3	1	2	4	2		1	2	4	2

So the balanced equation is: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Solved example 6.2

In the neutralisation reaction between the alkali Magnesium hydroxide (Mg(OH)₂) and the Sulphuric acid (H₂SO₄), write the chemical formula of the salt formed. Also write the balanced equation of the neutralisation reaction.
Solution:
1. The cation in the final salt, comes from the alkali. So the first step is to split the alkali into cation and anion: Mg(OH)₂ → Mg²⁺ + 2(OH)^- .
2. The anion in the final salt, comes from the acid. So the second step is to split the acid into cation and anion: H₂SO₄ → 2H⁺ + SO₄^2-.
3. Assemble the cation and anion with cation first: Mg²⁺  SO₄^2- .
4. Do the interchanging:
• Number of Mg²⁺ ions = Number of charges in SO₄^2- ion = 2 
• Number of SO₄^2- ions = Number of charges in Mg²⁺ ion = 2
5. So chemical formula of the final salt is: Mg₂(SO₄)₂.
6. Here, the subscripts (2 and 2) have a common factor. The common factor is '2'. In such cases we must divide the subscript by the common factor. We get 2 ÷ 2 = 1
7. We can write: Mg₁(SO₄)₁. When the subscript is '1', it is not usually written.
8. So final chemical formula is MgSO₄.

Balanced equation for the neutralisation reaction:

Reactants:

    ♦ Magnesium hydroxide. One molecule is Mg(OH)₂. 

    ♦ Sulphuric acid. One molecule is H₂SO₄.

Products:

    ♦ Magnesium sulphate. One molecule is MgSO₄.

    ♦ Water. One molecule is H₂O

• So skeletal equation is:

Mg(OH)₂ + H₂SO₄ → MgSO₄ + H₂O. This is not a balanced equation. The steps for writing the balanced equation are shown below:
Step 1: Mg(OH)₂ + H₂SO₄ → MgSO₄ + H₂O
Step 2: Mg(OH)₂ + H₂SO₄ → MgSO₄ + 2H₂O

	Reactants					Products
	Mg	O	H	S		Mg	O	H	S
Step 1	1	6	4	1		1	5	2	1
Step 2	1	6	4	1		1	6	4	1

So the balanced equation is:
Step 2: Mg(OH)₂ + H₂SO₄ → MgSO₄ + 2H₂O

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In the next section, we will see a few more solved examples. 

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Chapter 1.2 - Chemical Equations of Reactions

In the previous section, we saw the details about Chemical formula. In this section we will see Chemical Equations.
Consider the reaction between zinc and hydrochloric acid. As a result of the reaction, we will get two substances: zinc chloride and hydrogen. Substances taking part in a reaction are called Reactants. Substances formed as a result of the reaction are called products. In the above reaction:
• Reactants are zinc and hydrochloric acid
• Products are zinc chloride and hydrogen

We have seen that the molecules can be represented by chemical formulae. Using them, we can write 'equations to represent a reaction'. For the reaction between zinc and hydrochloric acid, the equation will be:
Zn + HCl → ZnCl₂ + H₂
Let us analyse the above equation:
• On the left side we have two items. Zn and HCl  
   ♦ Zn represents Zinc, and HCl represents Hydrochloric acid. These are the reactants
• On the right side we have two items. ZnCl₂ and H₂ 
   ♦ ZnCl₂ represents zinc chloride, and H₂ represents hydrogen. These are the products
• So a 'chemical equation' gives us an idea about 'what happens during a reaction'. But some details are missing:
   ♦ On the left side, there is only one chlorine atom. But on the right side, there are two chlorine atoms
   ♦ On the left side, there is only one hydrogen atom. But on the right side, there are two hydrogen atoms
• So the equation is not balanced. It is our job to balance a given equation

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• In the above equation, zinc chloride is present. It is a compound containing Zn (zinc) and Cl (chlorine). Why is zinc chloride represented as ZnCl₂ and not ZnCl ? 
• The reason is that, a molecule of zinc chloride will always consist of:
    ♦ One atom of zinc and
    ♦ Two atoms of chlorine
• So why is it that 'one molecule of  zinc chloride will always consist of One atom of zinc and Two atoms of chlorine'?

• Similarly, in the above equation, hydrochloric acid is present. It is a compound containing H₂ (hydrogen) and Cl (chlorine). Why is hydrochloric acid represented as HCl ? Why not HCl₂ or H₂Cl ?  
• The reason is that, a molecule of hydrochloric acid will always consist of:
    ♦ One atom of hydrogen and
    ♦ One atom of chlorine
• So why is it that 'one molecule of  hydrochloric acid will always consist of One atom of hydrogen and One atom of chlorine'?

Such a question can arise in the case of many compounds. We will learn the answers in later chapters. At present, all we need to learn is, how to balance a 'given equation'. Later, we will learn to write equations on our own, and then balance them.

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To get 2 hydrogen atoms and 2 chlorine atoms on the right side, can we write H₂Cl₂ (instead of HCl) on the left side ? No. because hydrochloric acid is HCl.  H₂Cl₂ represents something else. 
While balancing chemical equations, we must never alter the subscripts. Because altering them will alter the compound. We must alter only the coefficients.

We can follow a definite procedure to balance any chemical equation. We will discuss the procedure with the help of a few examples. First we will take the above equation:

• Write the 'number of atoms' of each element on the reactants side and products side. In the above table, in step 1, we have written the number of atoms according to the 'given equation'. These numbers must be same on both sides. Other wise the equation is not a balanced one. In our case, 
    ♦ Hydrogen has 1 on the reactants side, and 2 on the products side
    ♦ Chlorine has 1 on the reactants side and 2 on the products side
Such differences should be under lined. We do not have to pay attention to the numbers which are not underlined. Among those which are under lined, it is always better to consider elements other than hydrogen and oxygen first. 
• So we consider chlorine. It's number on the left side must become 2. 
• Remember that we can change only the coefficients. So, in the given equation, put 2 in front of HCl on the left side
• Now write the number of each element again in step 2. We find that hydrogen also becomes 2 on the left side
• There is nothing to underline. The equation is balanced

Another example:

• Write the 'number of atoms' of each element on the reactants side and products side. In the above table, in step 1, we have written the number of atoms according to the 'given equation'. These numbers must be same on both sides. Other wise the equation is not a balanced one. In our case, 
    ♦ Oxygen has 2 on the reactants side, and 1 on the products side
This difference is under lined. We do not have to pay attention to the numbers which are not underlined.  
• So we consider oxygen. It's number on the right side must become 2. 
• Remember that we can change only the coefficients. So, in the given equation, put 2 in front of H₂O on the right side
• Now write the number of each element again in step 2. We find that oxygen is balanced, but hydrogen is 2 on the left side, and 4 on the right side
• This 2 on the left side must become 4. So put 2 in front of H₂ on the left side
• Now write the numbers for each element in step 3. There is nothing to underline. The equation is balanced

Another example:

• Write the 'number of atoms' of each element on the reactants side and products side. In the above table, in step 1, we have written the number of atoms according to the 'given equation'. These numbers must be same on both sides. Other wise the equation is not a balanced one. In our case, 
    ♦ Oxygen has 2 on the reactants side, and 1 on the products side
This difference is under lined. We do not have to pay attention to the numbers which are not underlined.  
• So we consider oxygen. It's number on the right side must become 2. 
• Remember that we can change only the coefficients. So, in the given equation, put 2 in front of MgO on the right side
• Now write the number of each element again in step 2. We find that oxygen is balanced, but magnesium is 1 on the left side, and 2 on the right side
• This 1 on the left side must become 2. So put 2 in front of Mg on the left side
• Now write the numbers for each element in step 3. There is nothing to underline. The equation is balanced

Another example:

• Write the 'number of atoms' of each element on the reactants side and products side. In the above table, in step 1, we have written the number of atoms according to the 'given equation'. These numbers must be same on both sides. Other wise the equation is not a balanced one. In our case, 
    ♦ Carbon has 3 on the reactants side, and 1 on the products side
    ♦ Hydogen has 8 on the reactants side and 2 on the products side
    ♦ Oxygen has 2 on the reactants side, and 3 on the products side
Such differences should be under lined. We do not have to pay attention to the numbers which are not underlined. Among those which are under lined, it is always better to consider elements other than hydrogen and oxygen first. 
• So we consider carbon. It's number on the right side must become 3 
• Remember that we can change only the coefficients. So, in the given equation, put 3 in front of CO₂ on the right side
• Now write the number of each element again in line with step 2. We find that
    ♦ Carbon is balanced: 3,3
    ♦ Hydrogen remains in the same unbalanced state: 8,2
    ♦ Oxygen's numbers have changed. But it is still unbalanced: 2,7
• We kept hydrogen and oxygen for the last. So we took carbon first. Now carbon is balanced. Out of hydrogen and oxygen, we must take hydrogen first.
• For hydrogen, the present numbers are: 8,2. The right side also must become 4
• For that, we put 4 in front of H₂O on the right side
• Now write the number of each element again in step 3. We find that
    ♦ Carbon is balanced: 3,3
    ♦ Hydrogen is balanced: 8,8
    ♦ Oxygen's numbers have changed. But it is still unbalanced: 2,10
• For oxygrn, the 2 must become 10 on the left side
• For that, we put 5 in front of O₂ on the left side
• Now write the number of each element again in line with step 4. We find that
    ♦ Carbon is balanced: 3,3
    ♦ Hydrogen is balanced: 8,8
    ♦ Oxygen is balanced: 10,10
• There is nothing to underline. The equation is balanced.

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Some more solved examples can be seen here. In the next section we will see the 'Details of Atoms'.

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