HSLC Science in Assamese - Assam Board Chemistry

Chemical Reactions, Acids, Bases, Salts, and Periodic Classification of Elements

Chemical Reactions and Equations

A chemical reaction involves the transformation of one or more substances into new substances with different properties. Indicators of a chemical reaction include change in color, temperature, formation of a precipitate, or evolution of gas. Chemical equations represent these reactions symbolically, showing reactants on the left and products on the right, separated by an arrow.

• Reactants: Substances that undergo change.
• Products: New substances formed.
• Law of Conservation of Mass: The total mass of reactants equals the total mass of products; the…

Chemical Reactions, Acids, Bases, Salts, and Periodic Classification of Elements

Chemical Reactions and Equations

A chemical reaction involves the transformation of one or more substances into new substances with different properties. Indicators of a chemical reaction include change in color, temperature, formation of a precipitate, or evolution of gas. Chemical equations represent these reactions symbolically, showing reactants on the left and products on the right, separated by an arrow.

• Reactants: Substances that undergo change.
• Products: New substances formed.
• Law of Conservation of Mass: The total mass of reactants equals the total mass of products; the number of atoms of each element is the same on both sides of a balanced equation.
• Balancing Equations: Adjust coefficients to ensure equal numbers of each atom on both sides.

Types of Chemical Reactions

Chemical reactions are classified based on the nature of the transformation and the number of reactants and products.

• Combination (Synthesis) Reaction: Two or more reactants combine to form a single product.
  • Example: $CaO + H_2O \rightarrow Ca(OH)_2$
• Decomposition Reaction: A single reactant breaks down into two or more products.
  • Example: $2AgCl \xrightarrow{\text{sunlight}} 2Ag + Cl_2$
• Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
  • Example: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$
• Double Displacement Reaction: Exchange of ions between two compounds to form new compounds.
  • Example: $AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$
• Redox Reaction: Both oxidation and reduction occur simultaneously.
  • Example: $CuO + H_2 \rightarrow Cu + H_2O$
• Precipitation Reaction: Formation of an insoluble product (precipitate) from soluble reactants.

Balancing Chemical Equations

Balancing ensures the law of conservation of mass is obeyed. The process involves adjusting coefficients to equalize the number of atoms of each element on both sides.

• Write the unbalanced equation.
• Count atoms of each element on both sides.
• Adjust coefficients systematically.
• Re-check all elements and ensure the smallest whole-number coefficients.

Example: $Mg + HCl \rightarrow MgCl_2 + H_2$ Balanced: $Mg + 2HCl \rightarrow MgCl_2 + H_2$

Oxidation and Reduction (Redox) Reactions

Oxidation involves the gain of oxygen or loss of hydrogen, while reduction involves the loss of oxygen or gain of hydrogen. Both processes occur together in redox reactions.

• Oxidizing Agent: Substance that causes oxidation and is itself reduced.
• Reducing Agent: Substance that causes reduction and is itself oxidized.
• Example: $ZnO + C \rightarrow Zn + CO$ (ZnO is reduced, C is oxidized)

Corrosion and Rancidity

Corrosion is the deterioration of metals due to reaction with environmental elements like oxygen and moisture, forming oxides or other compounds.

• Rusting of Iron: $4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$
• Prevention: Painting, oiling, galvanization.
• Rancidity: Oxidation of fats and oils in food, leading to bad smell and taste. Prevented by antioxidants or storing in nitrogen.

Periodic Classification of Elements

Elements are arranged in the periodic table based on atomic number and recurring chemical properties. Early attempts included Dobereiner’s Triads and Newlands’ Law of Octaves, followed by Mendeleev’s and the Modern Periodic Table.

• Dobereiner’s Triads: Groups of three elements with similar properties; the atomic mass of the middle element is approximately the average of the other two.
• Newlands’ Law of Octaves: Every eighth element has similar properties when arranged by increasing atomic mass.
• Mendeleev’s Periodic Table: Elements arranged by increasing atomic mass; left gaps for undiscovered elements.
• Modern Periodic Law: Properties of elements are periodic functions of their atomic numbers.

Structure of the Modern Periodic Table

The modern periodic table is organized into periods (horizontal rows) and groups (vertical columns).

• Groups: 18 vertical columns; elements in the same group have the same number of valence electrons and similar chemical properties.
• Periods: 7 horizontal rows; elements in the same period have the same number of electron shells.
• Blocks: s-block, p-block, d-block, f-block, based on the subshell being filled.
• Metals, Non-metals, Metalloids: Metals are on the left, non-metals on the right, and metalloids along the zigzag line.

Trends in the Periodic Table

Periodic trends include changes in atomic size, metallic and non-metallic character, and valency.

• Atomic Size: Decreases across a period (left to right), increases down a group.
• Metallic Character: Decreases across a period, increases down a group.
• Non-metallic Character: Increases across a period, decreases down a group.
• Valency: Determined by the number of valence electrons; elements in the same group have the same valency.

Acids, Bases, and Salts

Acids are substances that produce hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$) in aqueous solution, while bases produce hydroxide ions ($OH^-$).

• Acids: Sour taste, turn blue litmus red, conduct electricity in solution.
• Bases: Bitter taste, slippery, turn red litmus blue, conduct electricity in solution.
• Salts: Formed by the neutralization reaction between acids and bases; properties depend on the strength of the acid and base used.

Key reactions:

• Acid + Metal $\rightarrow$ Salt + Hydrogen gas
• Acid + Metal Oxide $\rightarrow$ Salt + Water
• Acid + Base $\rightarrow$ Salt + Water (Neutralization)
• Acid + Metal Carbonate/Hydrogen Carbonate $\rightarrow$ Salt + Water + $CO_2$

pH Scale and Indicators

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or basicity.

• pH < 7: Acidic solution
• pH = 7: Neutral solution
• pH > 7: Basic solution
• Indicators: Substances that change color to indicate acidic or basic nature (e.g., litmus, phenolphthalein, methyl orange, turmeric).

Important Compounds and Their Uses

Several compounds are highlighted for their practical uses and preparation methods.

• Bleaching Powder ($CaOCl_2$): Produced by passing chlorine gas over dry slaked lime; used for bleaching and disinfection.
• Baking Soda ($NaHCO_3$): Used in baking, as an antacid, and in fire extinguishers; decomposes on heating to give $Na_2CO_3$, $CO_2$, and $H_2O$.
• Washing Soda ($Na_2CO_3 \cdot 10H_2O$): Used in cleaning and softening water.
• Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$): Made by heating gypsum ($CaSO_4 \cdot 2H_2O$) at 373 K; used in casts and construction.
• Brine: Aqueous solution of sodium chloride; electrolysis produces $NaOH$, $Cl_2$, and $H_2$ (Chlor-alkali process).

Exam-Focused Takeaways

• Always balance chemical equations to obey the law of conservation of mass.
• Identify reaction types by analyzing the number and nature of reactants and products.
• Recognize redox reactions by tracking oxidation and reduction changes.
• Understand periodic trends and their impact on element properties.
• Use indicators to distinguish between acids and bases.
• Remember the practical applications and preparation of key compounds like bleaching powder, baking soda, washing soda, and plaster of Paris.
• Practice writing and balancing equations for all reaction types mentioned.
• For periodic table questions, focus on group and period trends, block identification, and the significance of atomic number.
• For pH, recall that lower values indicate stronger acids, higher values indicate stronger bases, and 7 is neutral.

This set of points covers the essential facts, definitions, and exam-oriented details as explicitly stated in the content.