Chemical Equations: Writing & Balancing | Science Class 10 PDF Download

Balancing a chemical equation involves adjusting the number of molecules or atoms of each substance so that the number of atoms of each element is equal on both sides of the equation.
This isn’t just a rule—it's a reflection of two fundamental scientific laws: the Law of Conservation of Mass (matter can’t be created or destroyed) and the Law of Constant Proportions (elements combine in fixed ratios). By adding the correct numbers, or coefficients, we ensure that the equation tells the true story of what’s happening in the reaction.

Chemical Equation and Rule for Writing Chemical Equations 

A chemical equation is a symbolic representation of a chemical reaction in which the reactants and products are denoted by their respective chemical formulae.

Example of a Chemical Equation

2H₂ + O₂ → 2H₂O 

This equation describes the reaction between hydrogen and oxygen to form water. 

Key Components of Chemical Equation

• Reactants: Substances that participate in a chemical reaction, written on the left side of the arrow (→).
• Products: New substances formed as a result of the reaction, written on the right side of the arrow.
• Skeletal Chemical Equation: A basic chemical equation showing only reactants and products without additional details like physical states.

Enhancing Chemical Equations

To make a chemical equation more informative, include:

1. Physical States: Indicate the state of each substance using standard symbols:

• (g): Gaseous state

• (l): Liquid state

• (s): Solid state

• (aq): Aqueous solution

2. Reaction Conditions: Specify conditions like temperature, pressure, or catalysts, typically written above or below the arrow.
Example: N₂ + 3H₂ 2NH₃ + 22400 calorie heat

3. Special Symbols:

• (↑): Indicates a gas evolved (e.g., H₂ (↑)).
  Example: 2Na(s) + 2H₂O(l) → 2 NaOH(aq) + H₂(g) or H₂ (↑). This shows sodium reacting with water to produce sodium hydroxide and hydrogen gas.

• (↓) : Indicates a precipitate formed (e.g., AgCl (↓)).
  Example: NaCl + AgNO₃ → NaNO₃ + AgCl (↓)

• (→): Represents an irreversible reaction.
  Example: 2H₂ + O₂ → 2H₂O 

• (⇌): Represents a reversible reaction.

In the above reaction, P₄O₁₀ & water are reactants and Phosphoric acid is the product.

Rules for Writing Informative Chemical Equations

1. Write the correct chemical formulas for reactants and products.

2. Place reactants on the left and products on the right, separated by an arrow (→ or ⇌).

3. Include physical states using (s), (l), (g), or (aq).

4. Indicate special outcomes like gas evolution (↑) or precipitate (↓).

5. Specify reaction conditions (e.g., temperature, pressure, catalyst) above or below the arrow.

6. Balance the chemical equation correctly.

Rules for Writing a Word Equation

A word equation is a way to describe a chemical reaction using the names of the substances involved instead of their chemical formulas. 

Here are the key rules for writing a word equation:

1. The substances involved in a chemical reaction (reactants) are always placed on the left side of the arrow.
2. The substances produced after the reaction (products) are always placed on the right side of the arrow.
3. A plus sign (+) is used between reactants or products if there are two or more.
4. An arrow (→) is positioned between the reactants and products; it indicates the direction of the reaction. This arrow is read as "to yield" or "to form".
5. Optionally include physical states (e.g., gas, liquid) in parentheses.
6. Keep it clear and accurate.

Example: Methane + Oxygen → Carbon Dioxide + Water

Steps to Balance Chemical Equations

A balanced chemical equation has the same number of atoms of each element on both sides. The process to achieve this is known as the "hit-and-trial method," where we try various coefficients to balance the equation using the smallest whole numbers. 

Here are the steps for balancing simple equations:

Step 1. Count the no. of atoms of various elements on both sides of the equation

Example: Fe + H₂O → Fe₃O₄ + H₂

Step 2. Start balancing with the compound which contains maximum number of atoms. It may be a reactant or product. In that compound select the element which has maximum number of atoms.
According to this rule Fe₃O₄ has maximum number of atoms & oxygen has 4 atoms so it is selected.

So the partly balanced equation is
Fe + 4H₂O → Fe₃O₄ + H₂

Step 3. Fe and H are not balanced in the above reaction so the above reaction repeat the above process for both i.e.

Now the equation becomes as
Fe + 4H₂O → Fe₃O₄ + 4H₂

Step 4. Balance the Iron atoms similarly.

Now the equation is
3Fe + 4H₂O → Fe₃O₄ + 4H₂

Step 5. Finally check the correctness of the balanced equation by counting the number of atoms on both sides of the equation.

Step 6. To make chemical equation more informative physical states of the reactants and products are mentioned as for solid (s), liquid (l), gas (g) and for aqueous solution of reactant or product (aq) is written.
Now the equation becomes as
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

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