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Chemical Equations
Chemical reactions play a crucial role in our daily lives. From cooking food to rusting iron, chemical reactions are happening all around us. In CBSE Class 10 Science, the chapter "Chemical Reactions and Equations" introduces students to the fundamentals of how substances interact and transform.
This blog post provides detailed notes, explanations, and examples to help students understand the topic effectively and score full marks in their exams.
Activity 1.1
1. Mg Ribbon (3-4 cm) rubbed with sandpaper
2. Hold it with tongs.
3. Burn it using a burner and collect the ash.
Inference:
- Burning of Mg in air forms Magnesium oxide.
- Equation:
- 2Mg + O₂ → 2MgO
- (Reactants) → (Product)
A chemical reaction has taken place and resulted in the formation of MgO.
If there is:
- Change in state
- Change in colour
- Evolution of gas
- Change in temperature
Then a chemical reaction has taken place.
Chemical Equation:
- It represents a chemical reaction.
- LHS: Reactants
- RHS: Products
Balanced Equation:
- The number of atoms of each element should be equal on both sides.
- 2Mg + O₂ → 2MgO
Law of Conservation of Mass:
- Mass can neither be created nor destroyed.
- The total mass of elements present in the product of a chemical reaction has to be equal to the total mass of elements present in the reactants.
- In other words, the number of atoms of each element remains the same before and after the chemical reaction.
Example 1:
Zn + H₂SO₄ → ZnSO₄ + H₂↑
| Reactants | Products |
| Zn – 1 | Zn – 1 |
| H - 2 | H – 2 |
| S - 1 | S – 1 |
| O - 4 | O - 4 |
So this is a balanced chemical equation.
Example 2:
Fe + H₂O → Fe₃O₄ + H₂
| Reactants | Products |
| Fe – 1 | Fe – 3 |
| H – 2 | H - 2 |
| O - 1 | O - 4 |
Steps to balance the equation:
1. Fe + H₂O → Fe₃O₄ + H₂
2. 3Fe + H₂O → Fe₃O₄ + H₂
3. 3Fe + 4H₂O → Fe₃O₄ + H₂
4. 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Now, the equation is balanced.
Physical States in Chemical Equations:
To make the chemical equation more informative, the physical states of reactants and products are mentioned as:
- (s) = Solid
- (g) = Gaseous
- (aq) = Aqueous (Liquid)
- When (g) is used in H₂O, it indicates that a chemical reaction takes place using steam instead of liquid water.
Indicating Conditions in Reactions:
Sometimes, temperature, pressure, and catalysts are indicated above or below the arrow in the equation.
Example:
CO (g) + H₂ (g) → CH₃OH (at 340 atm)
Photosynthesis Reaction:
CO₂ (aq) + H₂O (aq) → C₆H₁₂O₆ (aq) + O₂ (g) + H₂O (aq)
(Occurs in sunlight with chlorophyll)
Catalyst in Chemical Reactions:
A catalyst is a chemical substance that speeds up a chemical reaction without reacting with the reactants or products.
Example:
Potassium permanganate (KMnO₄) is used in the decomposition of hydrogen peroxide into water and oxygen.
Reaction:
H₂O₂ (aq) → H₂O (l) + O₂ (g) (KMnO₄ as catalyst)
Types of Chemical Reactions
Combination Reactions:
When two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.
Example:
CaO (s) + H₂O (l) → Ca(OH)₂ (aq) + Heat
Quicklime + Water → Slaked Lime
C (s) + O₂ (g) → CO₂ (g)
2H₂ (g) + O₂ (g) → 2H₂O (g)
Exothermic Reactions:
Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
Burning of Natural Gas:
CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)
(Methane)
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