Redox Reactions | ICSE Class 9 Chemistry

Redox Reactions

Chapter 2 | Section 4: Oxidation & Reduction

1. Classical Concept

Oxidation

• Addition of Oxygen.
• Removal of Hydrogen.

$2Mg + O_2 \rightarrow 2MgO$

Reduction

• Addition of Hydrogen.
• Removal of Oxygen.

$CuO + H_2 \rightarrow Cu + H_2O$

2. Electronic Concept

In modern chemistry, redox is explained by the transfer of electrons:

• Oxidation: Loss of electrons by an atom or ion.
  $Fe^{2+} \rightarrow Fe^{3+} + e^-$
• Reduction: Gain of electrons by an atom or ion.
  $Cu^{2+} + 2e^- \rightarrow Cu$

Remember: OIL RIG (Oxidation Is Loss, Reduction Is Gain)

3. Redox: A Simultaneous Process

Oxidation and reduction always occur together. If one substance loses oxygen (reduced), another must gain it (oxidized).

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

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Energy Changes in Chemical Reactions | ICSE Class 9 Chemistry

Energy Changes in Reactions

Chapter 2 | Section 3: Exothermic and Endothermic Changes

1. Energy in Chemical Reactions

In every chemical reaction, bonds in reactants are broken (requires energy) and new bonds in products are formed (releases energy). The difference determines the energy change.

Exothermic Reactions

Chemical reactions which proceed with the release of heat energy.

• Temperature of surroundings increases.
• Energy of Reactants > Energy of Products.

$A + B \rightarrow C + D + \text{Heat}$

Endothermic Reactions

Chemical reactions which proceed with the absorption of heat energy.

• Temperature of surroundings decreases.
• Energy of Products > Energy of Reactants.

$A + B + \text{Heat} \rightarrow C + D$

2. Key Examples

Exothermic Examples:

• Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$
• Burning of Coal: $C + O_2 \rightarrow CO_2 + \Delta$
• Reaction of Water with Quicklime: $CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$

Endothermic Examples:

• Photosynthesis: $6CO_2 + 6H_2O + \text{Light} \rightarrow C_6H_{12}O_6 + 6O_2$
• Formation of Nitric Oxide: $N_2 + O_2 \xrightarrow{\Delta} 2NO$
• Decomposition of Limestone: $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$

3. Activation Energy

The minimum amount of energy required to start a chemical reaction is called **Activation Energy**. Even exothermic reactions (like burning a matchstick) need a little "spark" to get started.

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Types of Chemical Reactions | ICSE Class 9 Chemistry

Types of Chemical Reactions

Chapter 2 | Section 2: Synthesis, Decomposition, Displacement & Double Decomposition

1. Direct Combination (Synthesis)

A reaction in which two or more substances (elements or compounds) combine to form a **single** new substance.

General Form: $A + B \rightarrow AB$
Example: $2Mg + O_2 \rightarrow 2MgO$ (Magnesium Oxide)

2. Decomposition Reaction

A reaction in which a single compound breaks down into two or more simpler substances. Usually requires energy in the form of heat, light, or electricity.

• Thermal Decomposition: $2KClO_3 \xrightarrow{\Delta} 2KCl + 3O_2 \uparrow$
• Electrolytic Decomposition: $2H_2O \xrightarrow{\text{elec.}} 2H_2 + O_2$
• Photochemical Decomposition: $2AgCl \xrightarrow{\text{light}} 2Ag + Cl_2$

3. Displacement Reaction

A reaction in which a more reactive element displaces a less reactive element from its compound.

General Form: $A + BC \rightarrow AC + B$
Example: $Fe + CuSO_4 \rightarrow FeSO_4 + Cu \downarrow$

4. Double Decomposition

A reaction in which two compounds react by an exchange of radicals to form two new compounds.

• Precipitation: An insoluble solid (precipitate) is formed.
  $AgNO_3 + NaCl \rightarrow AgCl \downarrow + NaNO_3$
• Neutralization: An acid reacts with a base to form salt and water.
  $NaOH + HCl \rightarrow NaCl + H_2O$

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Chemical Changes & Reactions | ICSE Class 9 Chemistry

Chemical Changes & Reactions

Chapter 2 | Section 1: Conditions & Characteristics

1. Conditions Necessary for Chemical Changes

A chemical reaction occurs only when reactants are in close contact or under specific conditions:

• Mixing/Contact: Reactions occur when substances are brought together. E.g., Iodine and Phosphorus react when mixed.
• Solution: Some reactions only occur in an aqueous state. E.g., $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl \downarrow + NaNO_3(aq)$.
• Heat: Many reactions require thermal energy to start. E.g., $2HgO \xrightarrow{\Delta} 2Hg + O_2 \uparrow$.
• Light (Photochemical Reactions): Reactions triggered by light. E.g., $H_2 + Cl_2 \xrightarrow{\text{Sunlight}} 2HCl$.
• Electricity (Electrochemical Reactions): Decomposition using electric current. E.g., $2H_2O \xrightarrow{\text{Elec.}} 2H_2 + O_2$.
• Pressure: Some gaseous reactions require high pressure. E.g., Haber's process for Ammonia.
• Catalyst: A substance that alters the rate of reaction without being consumed.

2. Characteristics of Chemical Reactions

A chemical reaction is often accompanied by observable changes:

• Evolution of Gas: E.g., Zinc reacting with dil. $H_2SO_4$ evolves Hydrogen gas ($H_2 \uparrow$).
• Change of Color: E.g., Heating Copper Carbonate (green) turns it into Copper Oxide (black).
• Formation of Precipitate: An insoluble solid settles out. E.g., White precipitate of $BaSO_4$.
• Change of State: Burning a candle (solid wax) produces $CO_2$ gas and water vapour.
• Energy Change: Exothermic (heat released) or Endothermic (heat absorbed).

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Atomic & Molecular Mass | ICSE Class 9 Chemistry

Atomic & Molecular Mass

The Language of Chemistry | Chapter 1, Section 4

1. Relative Atomic Mass (RAM)

Atoms are too small to weigh directly. Therefore, their mass is expressed relative to a standard. The current standard is the **Carbon-12 isotope**.

$\text{RAM} = \frac{\text{Mass of 1 atom of an element}}{\frac{1}{12} \times \text{Mass of 1 atom of Carbon-12}}$

RAM is a ratio and has **no units**. However, it is often expressed in **amu** (atomic mass units) or **u** (unified mass).

2. Relative Molecular Mass (RMM)

The RMM of a substance is the number of times one molecule of the substance is heavier than $1/12^{th}$ the mass of a Carbon-12 atom.

Calculation: It is the sum of the atomic masses of all atoms present in the molecule.

Example: $H_2O$
$(2 \times \text{Mass of H}) + (1 \times \text{Mass of O}) = (2 \times 1) + 16 = 18$

3. Percentage Composition

Percentage composition is the percentage by weight of each element present in a compound.

$\% \text{ of element} = \frac{\text{Total mass of the element in 1 molecule}}{\text{Relative Molecular Mass (RMM)}} \times 100$

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