Content Strategy Manager | Updated On - Oct 15, 2024
Chemistry is all about chemical reactions. Balancing a chemical reaction is required in order to satisfy the law of conservation of mass. The law of conservation of mass states that, "The mass of any closed system can neither be created nor destroyed; it is conserved during chemical processes".
Therefore, for a balanced chemical equation, the total number of atoms present in the reaction must be equal on both sides of the reactant and product. However, the total mass remains the same but the bonded atoms get rearranged in the chemical reaction.
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Read Also: Class 10 Chemical Reactions
Before understanding the methods of balancing chemical equations, let us understand about the related terminology like chemical reaction and stoichiometric coefficients used in balancing chemical equations. This special part is a part of Unit 1 Chemical Substances-Nature and Behavior and has a total of 25 marks and 55 periods.
Chemical Equation and Balancing
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A chemical equation is an equation in which certain reactants react to form certain products. The reactants in the chemical equation are written on the left-hand side of the arrow, and the products are written on the right-hand side.
- The coefficients are written in front of the reactants and products which indicates the number of molecules or moles of a reactant or product.
- A chemical equation is balanced by arranging coefficients in front of reactants or products one at a time.
- Try starting with the most complex formula, so that both sides have the same number of atoms of each element.
Stoichometric Coefficients and Subscripts
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Two types of numbers that appear in a chemical equation are called coefficients and subscripts.
- Subscripts are part of the chemical formulas of the reactants and products
- Subscripts in a chemical formula can not be changed to balance a chemical equation, only the coefficients can be changed.
- Coefficients are placed in front of the reactants or products to indicate how many molecules of that substance are used or produced.
- The number of each substance involved in the reaction is indicated by the coefficient and may be changed to balance the equation.
Rules for Balancing a Chemical Equation
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Candidates can follow the method below which is an efficient approach to balance a chemical equation.
- Identify the most complex substance.
- Starting with that substance, select an element that appears in only one reactant and one product.
- Arrange the coefficients to get the same number of atoms of this element on both sides.
- Balance polyatomic ions as a unit.
- Balance the remaining atoms, ending with the least complex substance.
- Try to multiply both sides of the equation by the denominator to obtain whole numbers for the coefficients, if a fractional coefficient is used.
- Make sure that the atoms on both sides of the chemical reactions are balanced, this can be done by simply counting the number of atoms on both sides.
Procedure to Balance the equation
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Below are the steps that must be followed while balancing chemical equations.
Step 1
- Obtain the unbalanced equation from the chemical formula of the reactants and the products.
- The chemical formula of propane is C3H8. This burns with oxygen (O2) to form carbon dioxide (CO2) and water (H2O)
- The unbalanced chemical equation can be written as
C3H8 + O2 → CO2 + H2O
Step 2
Calculate the number of atoms of each element in the reaction (both sides) i. e. The reactant and product sides should be verified.
C3H8 + O2 → CO2 + H2O
| Reactant Side | Product Side |
|---|---|
| 3 Carbon atoms from C3H8 | 1 Carbon atom from CO2 |
| 8 Hydrogen atoms from C3H8 | 2 Hydrogen atoms from H2O |
| 2 Oxygen atoms from O2 | 3 Oxygen atoms, 2 atoms from CO2 , and 1 atom from H2O |
Step 3
- Now, coefficients are added to molecules containing an element that has a different number of atoms in the reactant side and the product side.
- The coefficient must balance the number of atoms on each side of the arrow.
- Now, the number of atoms of the elements on the reactant and product side must be altered.
- It is important to note that the number of atoms of an element in one species must be obtained by multiplying the coefficient with the total number of atoms of that element present in 1 molecule.
- Means when the 3 coefficient is assigned to the CO2 molecule, the total number of oxygen atoms in CO2 becomes 6.
- In this example, the coefficient is first assigned to carbon as tabulated below.
C3H8 + O2 → 3CO2 + H2O
| Reactant Side | Product Side |
|---|---|
| 3 Carbon atoms from C3H8 | 3 Carbon atoms from CO2 |
| 8 Hydrogen atoms from C3H8 | 2 Hydrogen atoms from H2O |
| 2 Oxygen atoms from O2 | 7 Oxygen atoms, 6 atoms from CO2 and 1 atom of Oxygen from H2O |
Step 4
Repeat Step 3 until all the number of atoms of the reacting elements are equal on the reactant and product side.
In this example, hydrogen is balanced next.
The chemical equation is transformed as follows.
C3H8 + O2 → 3CO2 + 4H2O
| Reactant Side | Product Side |
|---|---|
| 3 Carbon atoms from C3H8 | 3 Carbon atoms from CO2 |
| 8 Hydrogen atoms from C3H8 | 8 Hydrogen atoms from H2O |
| 2 Oxygen atoms from O2 | 10 Oxygen atoms of which 6atoms from CO2 and 4 atoms from H2O |
- Now, hydrogen atoms are balanced, the next element to be balanced is oxygen.
- There are 10 oxygen atoms on the product side, which means that the reactant side must also have 10 oxygen atoms.
- Each O2 molecule contains 2 oxygen atoms. Therefore, coefficient 5 must be assigned to the O2 molecule.
The updated chemical equation is tabulated below.
C3H8 + 5O2 → 3CO2 + 4H2O
| Reactant Side | Product Side |
|---|---|
| 3 Carbon atoms from C3H8 | 3 Carbon atoms from CO2 |
| 8 Hydrogen atoms from C3H8 | 8 Hydrogen atoms from H2O |
| 10 Oxygen atoms from O2 | 10 Oxygen atoms, 6 from CO2 and 4 from H2O |
Step 5
- Once all the individual elements are balanced, the total number of atoms of each element on the reactant and product side are compared once again to check if any inequalities are still present.
- If no inequalities are found, the chemical equation is said to be balanced. Therefore, the balanced chemical equation is
C3H8 + 5O2 → 3CO2 + 4H2O
Tips to Balance Equations:-
- Remember to write the seven diatomic elements namely,H2, N2, O2, F2, Cl2, Br2, I2 with the subscript2. Once they react, they will exist as individual atoms in a molecule.
- If the same polyatomic ion appears on both sides of the arrow, put a box around it and treat it as a single unit
- In some types of ionic reactions, it will help to write water as H–OH instead of H2
- Balance the elements in compounds first. Start with metals and then balance nonmetals.
- When the number of atoms of each element is the same before and after the arrow, the equation is balanced.
Sample Questions
Ques: Balance the chemical equation:
Al + O2 → Al2O3 (3 marks)
Sol:
| Reactant Side | Product Side |
| 1 Aluminium atom | 2 Aluminium Atoms |
| 2 Oxygen atoms | 3 Oxygen atoms |
First, the aluminium atoms are balanced. The equation becomes
2Al + O2 → Al2O3
Now, the oxygen atoms must be balanced, there are two oxygen atoms on the reactant side and 3 on the product side. Therefore, there must be 3 O2 molecules. The chemical equation is transformed into
2Al + 3O2 → 2Al2O3
Since the number of aluminium atoms on the product side has doubled, so should the number on the reactant side.
| Reactant Side | Product Side |
| 4 Aluminium Atoms | 4 Aluminium Atoms |
| 6 Oxygen Atoms | 6 Oxygen Atoms |
Since each element is balanced, the balanced chemical equation is
4Al + 3O2 → 2Al2O3
Ques: Write the balanced equations for the chemical reactions –
HNO3 + Ca(OH)2 → Ca(NO3)2+ H2O (1 mark)
Sol: 2HNO3 +Ca(OH)2 → Ca(NO3)2 + 2H2O
Ques: Write the balanced equations for the chemical reactions – Zinc + Silver nitrate → Zinc nitrate + Silver (1 mark)
Sol: Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
Ques: Balance : FeCl3 + MgO ---> Fe2O3 + MgCl2 (3 marks)
Sol:
Balance Cl:
2FeCl3 + MgO ---> Fe2O3 + 3MgCl2
Fe also gets balanced in this step.
Choose either the O or the Mg to balance now:
2FeCl3 + 3MgO ---> Fe2O3 + 3MgCl2
The other element (Mg or O) also gets balanced in this step.
Ques: ZnS + O2 ---> ZnO + SO2 (3 marks)
Sol: Balance the oxygen with a fractional coefficient:
ZnS + (3/2)O2 ---> ZnO + SO2
Multiply through to clear the fraction:
2ZnS + 3O2 ---> 2ZnO + 2SO2
Ques: Fe + HC2H3O2 ---> Fe(C2H3O2)3 + H2 (3 marks)
Sol:
Balance acetate:
Fe + 3HC2H3O2 ---> Fe(C2H3O2)3 + H2
Balance hydrogen:
Fe + 3HC2H3O2 ---> Fe(C2H3O2)3 + (3/2)H2
Clear the fraction:
2Fe + 6HC2H3O2 ---> 2Fe(C2H3O2)3 + 3H2
Ques: Fe2O3(s) + C(s) ---> Fe(s) + CO2(g) (3 marks)
Sol:
Balance iron:
Fe2O3(s) + C(s) ---> 2Fe(s) + CO2(g)
Balance oxygen:
Fe2O (s) + C(s) ---> 2Fe(s) + 3⁄2CO2(g)
Balance carbon:
Fe2O3(s) + 3⁄2C(s) ---> 2Fe(s) + 3⁄2CO2(g)
Note the 3⁄2 in front of the C and the CO2.
Multiply through by 2:
2Fe2O3(s) + 3C(s) ---> 4Fe(s) + 3CO2(g)
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