Class 9 Chemistry Notes - AKUEB

Chemistry is the scientific study of matter, its properties, composition, structure, behavior, and the changes it undergoes during chemical reactions.

• Organic Chemistry: Study of carbon-containing compounds, especially those found in living organisms and synthetic materials like plastics and pharmaceuticals.
• Inorganic Chemistry: Focuses on substances that do not contain carbon-hydrogen bonds, such as salts, metals, and minerals.
• Physical Chemistry: Applies physics concepts to understand chemical systems, including reaction mechanisms, thermodynamics, and quantum chemistry.
• Analytical Chemistry: Involves identifying substances and measuring their quantities, used in quality control, forensic science, and environmental testing.
• Nuclear Chemistry: Studies atomic nuclei, radioactive decay, and nuclear reactions used in medicine and energy production.
• Biochemistry: Explores the chemical processes in living organisms, focusing on biomolecules like enzymes, DNA, and proteins.
• Industrial Chemistry: Applies chemical knowledge to develop and improve industrial products and processes, such as in food production, cleaning agents, and polymers.
• Environmental Chemistry: Examines chemical interactions in the environment, studying pollution, waste management, and sustainable practices.

• Organic Chemistry: Essential in developing medicines, plastics, fuels, and synthetic fibers that impact daily life and industry.
• Inorganic Chemistry: Important for materials science, catalysis, and chemical engineering processes used in construction and electronics.
• Physical Chemistry: Helps us understand how and why chemical reactions occur, aiding in the development of new technologies and energy sources.
• Analytical Chemistry: Plays a key role in monitoring product quality, crime scene investigation, environmental safety, and health diagnostics.
• Nuclear Chemistry: Vital for producing nuclear energy, developing radiation-based medical treatments, and understanding radioactive substances.
• Biochemistry: Crucial for studying diseases, genetics, nutrition, and creating life-saving drugs and vaccines.
• Industrial Chemistry: Powers manufacturing and large-scale production, making essential products more accessible and affordable.
• Environmental Chemistry: Aids in solving global challenges like pollution and climate change, ensuring sustainability and protecting public health.

Atoms: The smallest unit of an element that maintains its chemical identity.
Elements: Substances made of only one type of atom, identified by their atomic number.
Compounds: Substances formed by the chemical combination of two or more elements in a fixed ratio.
Mixtures: Combinations of two or more substances that are not chemically bonded and can be separated physically.
Molecules: Groups of two or more atoms bonded together, representing the smallest unit of a compound.

Elements: Contain only one type of atom and cannot be broken down into simpler substances.
Compounds: Made of different types of atoms chemically bonded together in a specific ratio.
Mixtures: Physical blends of elements or compounds that retain their individual properties and are not chemically combined.

Atoms and Molecules: Atoms are single particles of an element, while molecules consist of two or more atoms chemically bonded.
Atoms and Ions: Atoms are electrically neutral, while ions are charged due to the loss or gain of electrons.
Molecules and Molecular Ions: Molecules are neutral compounds of bonded atoms, whereas molecular ions are similar groups that carry an electric charge.
Ions and Free Radicals: Ions have a positive or negative charge, while free radicals are uncharged but have unpaired electrons, making them highly reactive.

Atomic Number: The number of protons in the nucleus of an atom.
Mass Number: The total number of protons and neutrons in the nucleus.
Isotopes: Variants of the same element with the same number of protons but different numbers of neutrons.
Atomic Mass: The average mass of an element’s atoms, weighted according to the abundance of each isotope.
Atomic Mass Unit (amu): A standard unit of mass equal to one-twelfth the mass of a carbon-12 atom.

Relative Atomic Mass: It is calculated by taking the sum of the products of each isotope’s mass and its fractional abundance.
Formula: Relative Atomic Mass = ∑(Isotopic Mass × Fractional Abundance)

Relative Atomic Mass: It is the average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom.

Molecular Mass:
The total mass of a molecule, calculated by adding the atomic masses of all atoms present in that molecule.
Formula Mass:
The total mass of a formula unit of an ionic compound, calculated by summing the atomic masses of all ions in the formula.

Elements:
O₂, He
Mixtures:
Air, Saltwater
Compounds:
H₂O, CO₂
Ions:
Na⁺, Cl⁻
Molecular Ions:
NH₄⁺, SO₄²⁻
Free Radicals:
OH•, NO•

Valency:
The combining power of an element, representing the number of hydrogen atoms it can combine with or displace in a chemical reaction.

Sodium (Na): 1
Oxygen (O): 2
Sulfate (SO₄²⁻): 2

Use the crisscross method to balance the charges between ions.
For example, if sodium (Na⁺) combines with chloride (Cl⁻), the charges are balanced directly, so the formula is NaCl.
For calcium (Ca²⁺) and chloride (Cl⁻), crisscrossing the charges gives CaCl₂.

Gram Atomic Mass: Mass of one mole of atoms of an element in grams.
Gram Molecular Mass: Mass of one mole of molecules in grams.
Gram Formula Mass: Mass of one mole of formula units in grams.
Formula Unit: The lowest whole-number ratio of ions represented in an ionic compound.
Mole: A unit representing 6.022 × 10²³ particles (atoms, molecules, etc.) of a substance.
Avogadro’s Number: 6.022 × 10²³, the number of atoms, ions, or molecules in one mole of a substance.

1. Gram Atomic Mass
Definition: The mass of one mole of atoms of an element, expressed in grams.
Relation to Mole: It is numerically equal to the atomic mass of the element but in grams. For example, carbon has an atomic mass of 12 amu, so its gram atomic mass is 12 g/mol.
Relation to Avogadro’s Number: One mole of an element contains 6.022 × 10²³ atoms. Therefore, 12 grams of carbon contains Avogadro’s number of atoms.

2. Gram Molecular Mass
Definition: The mass of one mole of molecules of a substance, expressed in grams.
Relation to Mole: It is numerically equal to the molecular mass in amu but expressed in grams. For example, water (H₂O) has a molecular mass of ~18 amu, so its gram molecular mass is 18 g/mol.
Relation to Avogadro’s Number: One mole of a substance contains Avogadro’s number of molecules. So, 18 grams of water contains 6.022 × 10²³ water molecules.

3. Gram Formula Mass
Definition: The mass of one mole of formula units of an ionic compound, in grams.
Relation to Mole: It equals the formula mass of the compound in amu, but expressed in grams. For example, NaCl has a formula mass of 58.5 amu, so its gram formula mass is 58.5 g/mol.
Relation to Avogadro’s Number: One mole of NaCl contains Avogadro’s number of formula units. So, 58.5 grams of NaCl = 6.022 × 10²³ NaCl units.

Number of Moles (n) = Mass of substance (g) ÷ Molar mass (g/mol)
Number of Atoms or Molecules = n × Avogadro’s number (6.022 × 10²³)
These formulas are used to convert mass to moles, and moles to number of particles (atoms, molecules, or formula units).

Empirical Formula: The simplest whole-number ratio of atoms in a compound.
Molecular Formula: The actual number of atoms of each element in a molecule.

Step 1: Assume the total mass of the compound is 100g, so each percentage becomes mass in grams.
Step 2: Convert the mass of each element to moles by dividing by its atomic mass.
Step 3: Divide all mole values by the smallest number of moles calculated.
Step 4: Multiply by a suitable factor if needed to get whole numbers.
Step 5: Write the empirical formula using the resulting mole ratio.

Step 1: Find the empirical formula and calculate its empirical formula mass.
Step 2: Divide the molecular mass by the empirical formula mass to get a whole number (n).
Step 3: Multiply the subscripts in the empirical formula by n to get the molecular formula.

Chemical Reaction: A process where reactants transform into products through bond breaking and forming.
Chemical Equation: A symbolic representation of a chemical reaction showing reactants and products.

Formation: Chemical equations represent the transformation of reactants into products using chemical symbols.
Characteristics: A balanced chemical equation follows the Law of Conservation of Mass and maintains correct stoichiometric relationships between reactants and products.

Displacement Reaction: A + BC → AC + B
Decomposition Reaction: AB → A + B
Addition/Synthesis Reaction: A + B → AB
Combustion Reaction: Hydrocarbon + O₂ → CO₂ + H₂O
Double Displacement Reaction: AB + CD → AD + CB
Neutralization Reaction: Acid + Base → Salt + Water
Hydrolysis Reaction: AB + H₂O → AH + BOH

To construct a balanced chemical equation:
- Identify the reactants and products.
- Write the unbalanced chemical equation using correct formulas.
- Adjust coefficients to ensure the same number of each type of atom on both sides of the equation.

Balancing by inspection involves trial and error adjustment of coefficients to balance the number of atoms of each element on both sides of the equation. This method ensures mass conservation and correct stoichiometry.

In combustion analysis:
- Use the balanced equation of the combustion reaction.
- Determine the moles of CO₂ and H₂O produced.
- Calculate the moles and mass of carbon and hydrogen in the original compound.
- Use this data to determine the empirical or molecular formula of the compound.

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