Chemical Reactions and Equations Revision Notes Class 10

Chemical Reactions

A Chemical Reaction is a process that involves the breaking of old chemical bonds and the rearrangement of atomic structures to form new substances with entirely different properties.

• Reactants: The initial chemical substances that enter into and undergo change during a chemical reaction (written on the left side).
• Products: The new chemical substances formed as a outcome of the chemical reaction (written on the right side).
• Characteristics of Chemical Reactions: A chemical change can generally be identified or confirmed by any of the following observable changes:
  • Change in State
  • Change in Color
  • Evolution of a Gas
  • Change in Temperature (Evolution or absorption of heat)
  • Formation of a Precipitate

Chemical Equations

A Chemical Equation is the shorthand, symbolic representation of a chemical reaction using the chemical formulas of the reactants and products.

Physical State Symbols:

To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their formulas:

• Solid state is denoted by (s)
• Liquid state is denoted by (l)
• Gaseous state is denoted by (g)
• Aqueous solution (substance dissolved in water) is denoted by (aq)
• Precipitate formation is marked by a downward arrow ↓
• Gas evolution is marked by an upward arrow ↑

Note: Reaction conditions such as temperature, pressure, or a catalyst are written above and/or below the reaction arrow.

Balanced Chemical Equations

Law of Conservation of Mass:

According to this law, mass can neither be created nor destroyed in a chemical reaction. Consequently, the total mass of the elements present in the products must equal the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains identical before and after the reaction.

Balancing Equations:

• Equations are balanced using the Hit-and-Trial method.
• We adjust the stoichiometric coefficients (the numbers written in front of chemical formulas).
• CRITICAL RULE: Never alter the subscripts within a chemical formula to balance an equation (e.g., changing H₂O to H₂O₂ is incorrect).

Types of Chemical Reactions

1. Combination Reaction

A reaction in which two or more reactants combine together to synthesize a single, new product.

General Form: A + B → AB

• Key Example (Slaking of Lime): When Quicklime reacts vigorously with water, it produces Slaked lime, releasing a massive amount of heat.

CaO(s) [Quicklime] + H₂O(l) → Ca(OH)₂(aq) [Slaked Lime] + Heat

2. Decomposition Reaction

A reaction in which a single reactant breaks down into two or more simpler, individual products. This reaction is the exact opposite of a combination reaction and requires an input of energy to break chemical bonds.

General Form: AB → A + B

Based on the form of energy supplied, decomposition reactions are split into three types:

• A. Thermal Decomposition: Carried out by applying heat energy.
  CaCO₃(s) [Limestone] —(Heat)→ CaO(s) [Quicklime] + CO₂(g)
  2FeSO₄(s) [Green crystals] —(Heat)→ Fe₂O₃(s) [Brown] + SO₂(g) + SO₃(g)
• B. Electrolytic Decomposition (Electrolysis): Carried out by passing an electric current.
  2H₂O(l) —(Electric Current)→ 2H₂(g) + O₂(g)
• C. Photolytic Decomposition (Photolysis): Carried out by using light energy.
  2AgCl(s) [White] —(Sunlight)→ 2Ag(s) [Grey] + Cl₂(g)

  Application: The photolysis of Silver Chloride (AgCl) and Silver Bromide (AgBr) is utilized in black and white photography.

3. Displacement Reaction

A chemical reaction where a more reactive element displaces a less reactive element from its corresponding salt solution.

General Form: A + BC → AC + B   (Where metal A is more reactive than metal B)

• Key Example: When an iron nail is dipped into a blue Copper Sulfate solution, the blue color fades away to light green because iron displaces copper.

Fe(s) [Grey] + CuSO₄(aq) [Blue] → FeSO₄(aq) [Light Green] + Cu(s) [Brown deposit]

4. Double Displacement Reaction

A chemical reaction where two ionic compounds react by mutually exchanging their ions to form two entirely new compounds.

General Form: AB + CD → AD + CB

• Precipitation Reaction: Many double displacement reactions yield an insoluble solid product that separates out from the solution, known as a precipitate.
• Key Example: Mixing Sodium Sulfate and Barium Chloride forms a white, insoluble precipitate of Barium Sulfate.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ [White Precipitate] + 2NaCl(aq)

Exothermic and Endothermic Reactions

Exothermic Reactions:

Reactions accompanied by the evolution/release of heat energy into the surrounding environment. (Surrounding temperature increases).

• Example (Burning of Natural Gas):
  CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Energy
• Respiration: Respiration is considered an exothermic process because glucose breaks down inside our cells and combines with oxygen to release vital energy.

Endothermic Reactions:

Reactions that absorb heat or energy from the surroundings to proceed. (Surrounding temperature drops).

• Example (Photosynthesis):
  6CO₂(g) + 6H₂O(l) —(Sunlight/Chlorophyll)→ C₆H₁₂O₆(aq) + 6O₂(g)

Oxidation and Reduction (Redox Reactions)

Oxidation and Reduction always take place concurrently in a chemical reaction. Such mutual processes are designated as Redox Reactions.

Oxidation involves:

• The gain of Oxygen to a substance.
• The loss of Hydrogen from a substance.
• The loss of electrons.

Reduction involves:

• The loss of Oxygen from a substance.
• The gain of Hydrogen to a substance.
• The gain of electrons.

Key Definitions:

• Oxidizing Agent: The substance that gives oxygen or removes hydrogen (it gets reduced itself).
• Reducing Agent: The substance that removes oxygen or gives hydrogen (it gets oxidized itself).

Example:

CuO(s) + H₂(g) —(Heat)→ Cu(s) + H₂O(l)

• CuO loses oxygen → It is Reduced (Oxidizing Agent).
• H₂ gains oxygen → It is Oxidized (Reducing Agent).

Effects of Oxidation in Everyday Life

1. Corrosion

The slow, gradual deterioration and eating up of metals due to the chemical action of atmospheric air, moisture, and gases on their surface.

• Rusting of Iron: Iron reacts with oxygen and water to form a flaky, brown hydrated substance.
  4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s) [Rust]
• Other Examples: Green coating on Copper (Copper Carbonate), Black coating on Silver (Silver Sulfide).
• Prevention Methods: Barrier protection by painting, oiling, or greasing; Galvanization (coating iron with sacrificial Zinc); Chrome Plating; and turning metals into Alloys.

2. Rancidity

The aerial oxidation of fats, lipids, and oils present in food items when left exposed to air for a long duration. It results in stale chemical variations causing highly unpleasant odors and tastes.

• Prevention Methods:
  • Flushing food packaging packs with inert Nitrogen gas to displace oxygen (e.g., potato chip bags).
  • Storing food in strict airtight containers to minimize exposure.
  • Mixing chemical Antioxidants (such as BHA or BHT) into food formulations.
  • Refrigeration of food items at low temperatures to slow down oxidation rates.