A chemical equation is the symbolic representation of a chemical reaction using the formulae and symbols of the reactant and product species. It provides a concise, quantitative, and qualitative description of a chemical change, indicating the exact ratio of atoms and molecules participating in the process.
Anatomy of a Chemical Equation
A chemical equation consists of reactants written on the left-hand side (LHS) and products written on the right-hand side (RHS), connected by an arrow (→) pointing from reactants to products to show the direction of the reaction.
- Reactants: The initial substances that undergo a chemical change during the reaction.
- Products: The new substances formed as a result of the chemical reaction.
- Coefficients: The numbers placed in front of chemical formulas representing the number of moles or molecules involved (e.g., 2H2).
- Subscripts: The small numbers within a chemical formula indicating the number of atoms of that element in a single molecule (e.g., the 2 in H2O). Subscripts cannot be changed when balancing equations.
The Law of Conservation of Mass
The fundamental principle governing chemical equations is the Law of Conservation of Mass, formulated by Antoine Lavoisier. The law states that mass can neither be created nor destroyed in a chemical reaction.
- Implication for Equations: The total mass of the reactants must equal the total mass of the products.
- Atomic Balance: The total number of atoms of each element on the reactant side (LHS) must exactly equal the total number of atoms of that same element on the product side (RHS).
- Skeletal Equation: An unbalanced chemical equation where the number of atoms on both sides is not equal is termed a skeletal chemical equation.
Step-by-Step Balancing of Chemical Equations (Hit-and-Trial Method)
Balancing a chemical equation involves adjusting coefficients, never the subscripts, to equalize the number of atoms on both sides of the arrow.
Step 1: List the Unbalanced Equation
Write down the correct chemical formulas for the reactants and products.
- Example: Fe + H2O → Fe3O4 + H2
Step 2: Count the Atoms of Each Element
Create an inventory of the number of atoms of each element present on both sides of the equation.
| Element | Number of Atoms in Reactants (LHS) | Number of Atoms in Products (RHS) |
| Iron (Fe) | 1 | 3 |
| Hydrogen (H) | 2 | 2 |
| Oxygen (O) | 1 | 4 |
Step 3: Balance the Element with the Maximum Number of Atoms
Select the compound containing the maximum number of atoms to start balancing—in this case, Fe3O4, which contains 4 oxygen atoms. Multiply H2O on the LHS by 4 to balance oxygen atoms.
- Equation becomes: Fe + 4H2O → Fe3O4 + H2
Step 4: Balance the Remaining Elements
Now, hydrogen atoms have become 8 on the LHS (4 × 2), while there are only 2 on the RHS. Multiply H2 on the RHS by 4 to equalize.
- Equation becomes: Fe + 4H2O → Fe3O4 + 4H2
Next, look at Iron (Fe). There are 3 atoms on the RHS but only 1 on the LHS. Multiply Fe on the LHS by 3.
- Equation becomes: 3Fe + 4H2O → Fe3O4 + 4H2
Step 5: Verify the Balance
Recount the atoms on both sides to confirm that the equation is balanced.
- LHS: 3Fe, 8H, 4O
- RHS: 3Fe, 8H, 4O
- The chemical equation is now fully balanced.
Making Chemical Equations More Informative
To convey complete descriptive data, specific symbols are integrated into chemical equations to clarify the physical states and environmental conditions required for the reaction.
1. Notation for Physical States
The physical states of the reactants and products are mentioned along with their chemical formulas in parentheses:
- Solid state: Denoted by (s)
- Liquid state: Denoted by (l)
- Gaseous state: Denoted by (g)
- Aqueous solution: Denoted by (aq) (used when the substance is dissolved as a solute in water).
2. Notation for Directional Manifestations
- Gas Evolution: An upward arrow (↑) placed next to a product indicates the liberation of a gas.
- Precipitate Formation: A downward arrow (↓) placed next to a product indicates the formation of an insoluble solid precipitate.
3. Specification of Reaction Conditions
Crucial parameters like temperature, pressure, or catalysts required for the reaction to progress are written above or below the reaction arrow (→).
- Heat Input: The symbol delta (Δ) is written above the arrow to indicate that heat is being supplied to the system (Endothermic Reaction).
- Atmospheric Pressure: Precise pressure limits are noted (e.g., CO(g) + 2H2(g) 340 atm→ CH3OH(l)).
- Photosynthetic Conditions: Essential natural catalysts are designated around the arrow:6CO2(aq) + 12H2O(l) Sunlight→{Chlorophyll} C6H12O6(aq) + 6O2(aq) + 6H2O(l)
Key Insights and Trivia for Prelims
- Stoichiometry: The quantitative study of the relative amounts of reactants and products in a chemical reaction, directly derived from balanced chemical equations.
- Reversible Reactions: Represented by double arrows (⇌), indicating that the products can react to reform the original reactants (e.g., the synthesis of ammonia via Haber’s Process).
- Catalysts: Substances that alter the rate of a chemical reaction without undergoing any permanent chemical change themselves. They are always written over the arrow, never as a reactant or product.