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