Complete Chemistry notes on Chemical Reactions and Equations for BPSC and Other Competitive Exams in 2025

Complete Chemistry notes on Chemical Reactions and Equations for BPSC and Other Competitive Exams in 2025

General Science- 10 (Chemistry)

Here’s a detailed explanation of Chemical Reactions and Equations.

Chemical Reactions and Equations

A chemical reaction is a process that involves the rearrangement of the atomic, molecular, or ionic structure of a substance, as opposed to a change in physical form. In a chemical reaction, new substances with different properties are formed.

1. What are Chemical Reactions?

Chemical reactions involve the breaking of existing chemical bonds and the formation of new chemical bonds. This results in the transformation of one or more substances (reactants) into different substances (products).

Characteristics/Observations that indicate a Chemical Reaction has occurred:

1. Change in State: Reactants might be liquids and products might include a gas or solid (e.g., combustion of a candle wax).
2. Change in Color: The color of the reaction mixture might change (e.g., rust formation, silver tarnishing).
3. Change in Temperature: The reaction might release heat (exothermic) or absorb heat (endothermic).
   • Exothermic Reaction: Release of heat (e.g., burning of fuel, quicklime in water).
   • Endothermic Reaction: Absorption of heat (e.g., photosynthesis, dissolving ammonium chloride in water).
4. Evolution of a Gas: Bubbles might be observed (e.g., acid reacting with metal).
5. Formation of a Precipitate: An insoluble solid might form from a solution (e.g., mixing lead nitrate and potassium iodide solutions).

2. Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction, showing the reactants and products involved.

• Reactants: The substances that undergo a chemical change are written on the left side of the equation.
• Products: The new substances formed as a result of the chemical change are written on the right side of the equation.
• Arrow (→): Separates reactants from products and indicates the direction of the reaction. For reversible reactions, a double arrow (⇌) is used.

Example:

A+B→C+D

Here, A and B are reactants, and C and D are products.

Indicating Physical States:

To make chemical equations more informative, the physical states of reactants and products are often mentioned:

• (s) for solid
• (l) for liquid
• (g) for gas
• (aq) for aqueous solution (dissolved in water)

Example:

2H2​(g)+O2​(g)→2H2​O(l) (Hydrogen gas reacts with Oxygen gas to form liquid water)

3. Balancing Chemical Equations

According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. This means the total mass of the reactants must be equal to the total mass of the products. Consequently, the number of atoms of each element must be the same on both sides of a chemical equation.

Balancing a chemical equation involves:

• Adjusting the coefficients (numbers placed in front of chemical formulas) to ensure the number of atoms of each element is equal on both sides.
• Never change the subscripts within a chemical formula, as that would change the identity of the compound.

Example of Balancing:

Consider the reaction of hydrogen and oxygen to form water:

Unbalanced equation: H2​(g)+O2​(g)→H2​O(l)

• On reactant side: H = 2, O = 2
• On product side: H = 2, O = 1

To balance oxygen, put a ‘2’ in front of H2​O:

H2​(g)+O2​(g)→2H2​O(l)

Now:

• On reactant side: H = 2, O = 2
• On product side: H = 4, O = 2

To balance hydrogen, put a ‘2’ in front of H2​:

Balanced equation: 252H2​(g)+O2​(g)→2H2​O(l)

Now:

• On reactant side: H = 4, O = 2
• On product side: H = 4, O = 2 (Balanced)

4. Types of Chemical Reactions

Chemical reactions are broadly classified into several types based on how atoms and molecules rearrange.

1. Combination (Synthesis) Reaction:
   • Two or more reactants combine to form a single product.
   • General Form: A+B→AB
   • Examples:
     • Formation of water: 2H2​(g)+O2​(g)→2H2​O(l)
     • Burning of carbon: C(s)+O2​(g)→CO2​(g)
     • Quicklime with water: CaO(s)+H2​O(l)→Ca(OH)2​(aq) (often exothermic)
2. Decomposition Reaction:
   • A single reactant breaks down into two or more simpler products. These reactions often require energy (heat, light, or electricity) to occur.
   • General Form: AB→A+B
   • Types of Decomposition:
     • Thermal Decomposition: By heating (e.g., CaCO3​(s)heat ​CaO(s)+CO2​(g))
     • Electrolytic Decomposition (Electrolysis): By passing electricity (e.g., 2H2​O(l)electricity ​2H2​(g)+O2​(g))
     • Photolytic Decomposition: By light energy (e.g., 2AgCl(s)sunlight ​2Ag(s)+Cl2​(g) – used in black and white photography)
3. Displacement (Single Displacement) Reaction:
   • A more reactive element displaces a less reactive element from its compound.
   • General Form: A+BC→AC+B (where A is more reactive than B)
   • Reactivity Series (Activity Series): Metals are arranged in order of their reactivity. A metal higher in the series can displace a metal lower in the series.
     • Common Series (most reactive to least reactive): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au
   • Examples:
     • Iron displacing copper: Fe(s)+CuSO4​(aq)→FeSO4​(aq)+Cu(s)
     • Zinc displacing hydrogen from acid: Zn(s)+2HCl(aq)→ZnCl2​(aq)+H2​(g)
4. Double Displacement Reaction:
   • Two compounds react by an exchange of ions to form two new compounds.
   • General Form: AB+CD→AD+CB
   • Types:
     • Precipitation Reactions: One of the products formed is an insoluble solid (precipitate).
       • Example: AgNO3​(aq)+NaCl(aq)→AgCl(s)↓+NaNO3​(aq) (AgCl is a white precipitate)
     • Neutralization Reactions: An acid reacts with a base to form salt and water. (This is a specific type of double displacement).
       • Example: HCl(aq)+NaOH(aq)→NaCl(aq)+H2​O(l)
5. Redox Reactions (Oxidation and Reduction):
   • Reactions that involve the transfer of electrons. Oxidation and reduction always occur simultaneously.
   • Oxidation:
     • Loss of electrons.
     • Gain of oxygen.
     • Loss of hydrogen.
   • Reduction:
     • Gain of electrons.
     • Loss of oxygen.
     • Gain of hydrogen.
   • Oxidizing Agent (Oxidant): The substance that causes oxidation (itself gets reduced).
   • Reducing Agent (Reductant): The substance that causes reduction (itself gets oxidized).
   • Example: CuO(s)+H2​(g)→Cu(s)+H2​O(l)
     • Here, CuO is losing oxygen, so it’s reduced (and is the oxidizing agent).
     • H2​ is gaining oxygen, so it’s oxidized (and is the reducing agent).

5. Corrosion and Rancidity

These are common everyday examples of oxidation reactions.

Corrosion:

• Definition: The process of slow eating away of the surface of a metal when exposed to air, moisture, or a chemical (like an acid) for a period of time. It’s essentially an oxidation reaction.
• Rusting: The corrosion of iron, specifically, is called rusting. Iron reacts with oxygen and moisture (water) in the air to form hydrated iron(III) oxide (Fe2​O3​⋅xH2​O), which is commonly known as rust.
  • Equation: 4Fe(s)+3O2​(g)+xH2​O(l)→2Fe2​O3​⋅xH2​O(s)
• Prevention of Corrosion/Rusting:
  • Painting/Oiling/Greasing: Forms a barrier between the metal surface and the environment.
  • Galvanization: Coating iron with a layer of zinc, which is more reactive and corrodes preferentially.
  • Electroplating: Coating a metal with a layer of another non-corrosive metal (e.g., chromium plating).
  • Alloying: Mixing metals to form alloys (e.g., stainless steel is an alloy of iron with chromium and nickel, which resists rusting).

Rancidity:

• Definition: The unpleasant smell and taste that develop in fatty and oily foods when they are left exposed to air for a long time. This is due to the oxidation of fats and oils present in the food.
• Prevention of Rancidity:
  • Adding Antioxidants: Substances that prevent oxidation (e.g., BHA, BHT are added to packaged foods).
  • Packaging in Nitrogen Gas: Flushing food packets with inert gases like nitrogen displaces oxygen, preventing oxidation (e.g., chips packets).
  • Refrigeration: Storing food in refrigerators slows down the oxidation process.
  • Storing in Air-tight Containers: Reduces exposure to oxygen.
  • Storing away from Light: Light can accelerate oxidation.

Practice Questions

1. Which of the following observations indicates that a chemical reaction has taken place?
   • A) Change in shape
   • B) Melting of ice
   • C) Boiling of water
   • D) Evolution of gas
   • Rationale: Evolution of gas (like bubbles forming) is a common sign of a chemical reaction, whereas melting and boiling are physical changes.
2. A reaction that absorbs heat from its surroundings is called a(n):
   • A) Exothermic reaction
   • B) Endothermic reaction
   • C) Neutralization reaction
   • D) Precipitation reaction
   • Rationale: Endothermic reactions absorb heat, causing the surroundings to cool down.

3. In a chemical equation, the substances written on the left side of the arrow are called:
   • A) Products
   • B) Reactants
   • C) Catalysts
   • D) Solvents
   • Rationale: Reactants are the starting materials that undergo a chemical change, written on the left.
4. What does the symbol ‘(aq)’ next to a chemical formula in an equation represent?
   • A) Solid state
   • B) Liquid state
   • C) Gaseous state
   • D) Aqueous solution
   • Rationale: (aq) stands for aqueous solution, meaning the substance is dissolved in water.

5. The balancing of a chemical equation is based on the Law of:
   • A) Conservation of Energy
   • B) Multiple Proportions
   • C) Conservation of Mass
   • D) Constant Proportions
   • Rationale: The Law of Conservation of Mass states that atoms are rearranged, not created or destroyed, during a chemical reaction, necessitating equal numbers of each type of atom on both sides.
6. In the balanced equation 2Al(s)+xFe2​O3​(s)→Al2​O3​(s)+2Fe(s), the coefficient ‘x’ is:
   • A) 1
   • B) 2
   • C) 3
   • D) 1
   • Rationale: To balance Iron (Fe), there are 2 Fe atoms on the product side (2Fe). On the reactant side, Fe2​O3​ already has 2 Fe atoms. So, x = 1. The equation is actually 2Al(s)+Fe2​O3​(s)→Al2​O3​(s)+2Fe(s).

7. The reaction where two or more reactants combine to form a single product is called a:
   • A) Decomposition reaction
   • B) Combination reaction
   • C) Displacement reaction
   • D) Double displacement reaction
   • Rationale: In a combination reaction, multiple simpler substances unite to form a more complex one.
8. Which type of reaction is represented by 2AgCl(s)sunlight​2Ag(s)+Cl2​(g)?
   • A) Combination reaction
   • B) Displacement reaction
   • C) Photolytic decomposition reaction
   • D) Double displacement reaction
   • Rationale: A single compound (AgCl) breaks down into simpler substances (Ag and Cl2​) under the influence of light (photo-), hence it’s photolytic decomposition.
9. In the reaction Fe(s)+CuSO4​(aq)→FeSO4​(aq)+Cu(s), which type of reaction is occurring?
   • A) Combination
   • B) Decomposition
   • C) Displacement
   • D) Double displacement
   • Rationale: Iron (Fe) is displacing Copper (Cu) from its compound, which is characteristic of a single displacement reaction.
10. What happens during oxidation?
    • A) Gain of electrons
    • B) Loss of oxygen
    • C) Loss of electrons
    • D) Gain of hydrogen
    • Rationale: Oxidation is fundamentally defined as the loss of electrons.

11. The process of rusting of iron is an example of:
    • A) Reduction
    • B) Oxidation
    • C) Neutralization
    • D) Decomposition
    • Rationale: Rusting involves iron reacting with oxygen (and moisture), which is a process of oxidation.
12. Which method is NOT commonly used to prevent rancidity in food?
    • A) Adding antioxidants
    • B) Packaging in nitrogen gas
    • C) Refrigeration
    • D) Adding salt in large quantities (as a primary method for preventing rancidity)
    • Rationale: While salt is a preservative, it primarily works by inhibiting microbial growth (osmosis), not directly by preventing the oxidation of fats and oils (rancidity). Antioxidants, nitrogen packing, and refrigeration are direct methods for preventing rancidity.