Atoms, Molecules and Chemical Arithmetic Class 12 Chemistry NIOS
Chapter 1: Atoms, Molecules and Chemical Arithmetic
Scope of Chemistry
Chemistry is called the central science because it connects physics, biology, medicine, agriculture, and environmental science.
Health and Medicine
- Vaccines, antibiotics, and anesthesia revolutionized healthcare.
- Gene therapy is a breakthrough that repairs defective genes.
- Chemists design drugs with fewer side effects.
Energy and Environment
- Fossil fuels are finite – chemists explore solar, nuclear, and hydrogen-based energy.
- Solar cells and fuel cells can replace pollution-heavy fuels.
- Chemists help reduce greenhouse gases and air pollution.
Materials and Technology
- Development of polymers, ceramics, liquid crystals, adhesives, etc.
- Superconductors can eliminate energy loss during electricity transfer.
Food and Agriculture
- Fertilizers, pesticides, and irrigation chemicals increase crop yields.
- Genetically modified seeds improve resistance to pests and climate conditions.
Particulate Nature of Matter
Matter can be divided into smaller particles up to a limit. These smallest indivisible particles are called atoms.
- Leucippus and Democritus proposed the atomic theory around 440 B.C.
- Maharshi Kanad in India called these particles Parmanu.
Laws of Chemical Combinations
Law of Conservation of Mass
Antoine Lavoisier showed that:
“Mass is neither created nor destroyed in a chemical reaction.”
Example: Decomposition of Mercury (II) oxide gives mercury and oxygen, and total mass remains the same.
Law of Definite Proportions
Joseph Proust showed that a compound always contains the same elements in the same proportion by mass.
Example: Water is always H:O = 1:8 by mass, regardless of source.
Law of Multiple Proportions
When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole number ratios.
Example: In CO and CO₂, oxygen combines with carbon in a 2:1 mass ratio.
Dalton’s Atomic Theory
- All matter is made of indivisible atoms.
- Atoms of a given element are identical in mass and properties.
- Atoms of different elements differ in mass and chemical behavior.
- Atoms combine in whole-number ratios to form compounds.
- Atoms are neither created nor destroyed during chemical changes.
What is an Atom?
The smallest particle of an element that retains its chemical properties.
What is a Molecule?
A molecule is a group of two or more atoms chemically bonded together.
Examples:
- Diatomic molecules: O2, H2, N2
- Polyatomic molecules: H2O, NH3, CH4
What is an Element?
A pure substance consisting of only one type of atom. Represented by symbols (H, O, Fe, etc.)
Definitions
- Atom: The smallest particle of an element with its chemical identity.
- Molecule: Group of atoms bonded together (e.g., H₂, CO₂).
- Element: Pure substance made of only one kind of atom.
SI Units
- Mass – kilogram (kg)
- Length – metre (m)
- Time – second (s)
- Temperature – kelvin (K)
- Amount of substance – mole (mol)
Mole Concept
To count particles at the atomic level, we use the concept of mole.
1 mole = 6.022 × 10²³ particles (atoms/molecules/ions)
Avogadro’s Constant
Avogadro’s constant (NA) = 6.022 × 10²³ mol⁻¹
- 1 mole of atoms = 6.022 × 10²³ atoms
- 1 mole of molecules = 6.022 × 10²³ molecules
Empirical and Molecular Formula
Empirical Formula
The simplest whole-number ratio of atoms of each element in a compound.
Example: CH is the empirical formula of benzene (C₆H₆).
Molecular Formula
The actual number of atoms of each element in a molecule.
Formula:
Molecular Formula = n × Empirical Formula
where n = (Molecular mass) / (Empirical formula mass)
Steps to Calculate Empirical Formula:
- Find % composition or mass of elements.
- Convert mass to moles (divide by atomic mass).
- Divide by smallest value to get simplest ratio.
- Write the empirical formula.
Molar Volume of a Gas
At standard temperature and pressure (STP = 0°C and 1 atm):
- 1 mole of any gas occupies 22.4 L.
- This is known as the molar volume.
Applications:
- To calculate volume from moles:
Volume = moles × 22.4 L - To calculate moles from volume:
Moles = volume / 22.4 L
Chemical Equations
What is a Chemical Equation?
A symbolic representation of a chemical reaction showing reactants and products.
Steps to Write & Balance an Equation:
- Write correct chemical formulas of all reactants and products.
- Balance atoms of each element on both sides.
- Use smallest whole-number coefficients.
Example:
Unbalanced: H₂ + O₂ → H₂O
Balanced: 2H₂ + O₂ → 2H₂O
⚖️Stoichiometry
It is the quantitative relationship between reactants and products in a chemical reaction.
Based on the balanced equation, it helps in calculating:
- Mass of reactants/products
- Volume of gases
- Number of moles or particles
Example:
From the equation:
2H₂ + O₂ → 2H₂O
- 2 moles H₂ react with 1 mole O₂ to give 2 moles H₂O
- 4 g H₂ reacts with 32 g O₂ to give 36 g H₂O
Limiting Reagent
In a chemical reaction, the reactant that is completely used up first and limits the amount of product formed is called the limiting reagent.
Steps to Identify Limiting Reagent:
- Convert given masses to moles.
- Divide by the respective stoichiometric coefficients.
- The smaller value identifies the limiting reagent.
Example:
If 4 g H₂ and 32 g O₂ are given:
2H₂ + O₂ → 2H₂O
- 4 g H₂ = 2 moles
- 32 g O₂ = 1 mole
- As per the equation: 2 mol H₂ reacts with 1 mol O₂
- Both are in exact proportion → No limiting reagent
Practice Questions
- Define empirical and molecular formula. How are they related?
- Calculate the number of molecules in 2 moles of CO₂.
- Write and balance the reaction between Na and H₂O.
- What is meant by limiting reagent? Explain with an example.
- What is the volume occupied by 3 moles of a gas at STP?