General Topics

1.0Introduction

Chemistry is the study of matter and the changes it undergoes. Known as the central science, it connects fields like biology, physics, and environmental science.

Modern chemistry began in the 19th century, allowing scientists to understand substances at the atomic and molecular level. Since atoms and molecules are extremely small, we use the mole as a counting unit, just like "dozen" in daily life.

The mole concept helps us measure specific numbers of particles by weighing. In chemical reactions, substances combine in fixed ratios, and we study these quantitative relationships through stoichiometry, which is essential for lab work and industrial processes.

2.0Dalton’s Atomic Theory 

John Dalton (1766–1844) laid the foundation of modern atomic theory in 1803 by explaining chemical laws like the Law of Conservation of Mass, Law of Constant Proportions, and Law of Multiple Proportions.

Postulates of Dalton’s Atomic Theory:

1. Matter is made of indivisible atoms.
2. Atoms of the same element are identical in mass and properties.
3. Atoms of different elements differ in mass and properties.
4. Atoms cannot be created or destroyed in chemical reactions (supports Law of Conservation of Mass).
5. Atoms combine in small whole-number ratios to form compounds (supports Law of Definite Proportions).

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 a simple whole number ratio.
• Example:
  CO (Carbon Monoxide): 1.3321 g O per 1.000 g C
  CO₂ (Carbon Dioxide): 2.6642 g O per 1.000 g C
  ⇒ Ratio: 2:1
  This supported the idea that compounds are made by combining discrete atoms in simple ratios, strengthening belief in atomic theory.

3.0Chemical Formula 

A chemical formula symbolically shows the composition of a compound, indicating the elements involved and their ratios. For example, H₂O means each water molecule has 2 hydrogen atoms and 1 oxygen atom.

Importance of Chemical Formulae

• Show the elements and their ratios in a compound
• Essential for writing chemical equations
• Can represent molecules, ions, and radicals

Types of Chemical Formulae

• Molecular Formula: Shows the actual number of atoms of each element (e.g., glucose = C₆H₁₂O₆)
• Empirical Formula: Shows the simplest whole-number ratio (e.g., glucose = CH₂O)
• Structural Formula: Shows how atoms are arranged in a molecule

Molecular Formula

The molecular formula, also known as the true formula, shows the exact number of atoms of each element in a molecule. Subscripts after each symbol indicate the total number of atoms. It also identifies the type of atoms present in the compound.

Example:
Molecular formula of glucose: C₆H₁₂O₆
It shows that one molecule of glucose has 6 carbon, 12 hydrogen, and 6 oxygen atoms.

Empirical Formula

The empirical formula gives the simplest whole-number ratio of atoms in a compound. It is often obtained from experimental data. It simplifies the molecular formula without changing the ratio of atoms.

Example:

Empirical formula of glucose: CH₂O
This shows the simplest ratio of C, H, and O in glucose.

Structural Formula:

A structural formula shows how atoms are bonded and arranged in a molecule. It helps visualize which atoms are connected and their spatial arrangement. For glucose, its structure reveals how C, H, and O atoms are linked.

How to Write Chemical Formulas:

Chemical formulas are written using the element names and valency rules. Binary compounds (made of two elements) follow certain steps:

1. Binary Compounds:
   Most compounds are binary, meaning they are made up of two different elements. Compounds with more than two elements are called ternary or polyatomic compounds.
2. Cations and Anions:

• A cation is an ion with a positive charge (usually a metal).
• An anion is an ion with a negative charge (usually a non-metal).

1. Naming Metal-Non-metal Compounds:
   In a compound containing a metal and a non-metal, the metal (cation) is written first, followed by the non-metal (anion).
   Example: NaCl → Na⁺ (sodium) and Cl⁻ (chloride)
2. Suffix ‘-ide’ for Anions:
   Anions with a -1 charge usually end in “-ide.”
   Example: F⁻ → Fluoride
3. Suffix ‘-ate’ for Oxyanions:
   Anions made of oxygen plus another element (called oxyanions) usually end in “-ate.” Example: SO₄²⁻ → Sulfate
4. Prefix ‘Bi-’ or ‘Hydrogen’ for H-containing Anions:
   If a polyatomic ion includes a hydrogen ion (H⁺), the name often begins with “bi-” or includes the word “hydrogen.”
   Example: HCO₃⁻ → Bicarbonate or Hydrogen carbonate

4.0Chemical Equation

A chemical equation is a symbolic way of showing a chemical reaction using chemical formulas and symbols (like →, +, (g), (s), (l), (aq)).

A balanced chemical equation has the same number of atoms of each element on both the reactant and product sides. This ensures the equation follows the Law of Conservation of Mass and the Law of Constant Proportions.

How to Balance a Chemical Equation:

• Count the atoms of each element on both sides.
• Multiply the coefficient in front of each compound by the subscript of the element in the formula.
• If an element appears in more than one compound, add up all the atoms from those compounds.
• The total number of atoms for each element must be equal on both sides of the equation.

5.0Mole Concept and Stoichiometry 

In chemistry, the mole concept and stoichiometry are used to calculate the quantities of reactants and products in chemical reactions. By applying balanced chemical equations and the mole concept, we can solve practical problems related to mass, volume, and concentration in these reactions.

This is especially useful in:

1. Oxidation-Reduction (Redox) Reactions: These involve the transfer of electrons. Using mole ratios from the balanced equation, we can find how much of one substance is oxidized or reduced.

Oxidation: Loss of electrons.

Reduction: Gain of electrons.

In stoichiometric calculations, we balance both mass and charge.
Example:
Fe²⁺+Cl₂→Fe³⁺+2Cl⁻

2. Neutralisation Reactions: These occur between acids and bases to form salt and water. Stoichiometry helps calculate how much acid is needed to neutralize a base and vice versa.

Occur between an acid and a base to form salt and water.

Moles of H⁺ from acid = Moles of OH⁻ from base.

Example:
HCl+NaOH→NaCl+H₂O

3. Displacement Reactions: In these, a more reactive element displaces a less reactive one. Mole ratios help determine the amount of product formed or reactant required.

A more reactive element displaces a less reactive one.
Use mole ratios to determine amounts.

• Example:
  Zn + CuSO₄ → ZnSO₄ + Cu

6.0Concentration of Solutions 

In chemistry, "concentration" refers to how much solute is present in a given amount of solution or solvent. Unlike casual phrases like "too salty," chemistry uses specific methods to express concentration accurately. Below are key ways to describe it:

1. Molarity (M)

• Moles of solute per litre of solution.
• Formula:
  M = moles of solute / volume of solution (in L)
• Temperature dependent.

2. Molality (m)

• Moles of solute per kg of solvent.
• Formula:
  m = moles of solute / mass of solvent (in kg)
• Temperature independent and ideal for calculations involving temperature changes.

3. Mole Fraction (X)

• Ratio of moles of one component to total moles of all components.
• Formula (for solute A and solvent B):

X_A = a / (a + b),

X_B = b / (a + b),  

X_A + X_B = 1

• Independent of temperature.

4. Normality (N)

• Number of gram equivalents of solute per litre of solution.
• Formula:
  N = number of equivalents / volume of solution (in L)
• Used especially in acid-base and redox reactions.

Each method has its advantages:

• Molarity is common but varies with temperature.
• Molality and mole fraction are more stable as they don’t depend on temperature.
• Normality is useful in reactions involving equivalents like titrations.

7.0Solved Example

25ml of a solution of barium hydroxide on titration with a 0.1molar solution of hydrochloric acid gave a litre value of 35 ml. The molarity of the barium hydroxide solution was… [AIEEE 2003]

Solution:        

Write the balanced chemical equation

Ba(OH)₂+2HCl→BaCl₂+2H₂O

From the equation:

• 1 mole of Ba(OH)₂ reacts with 2 moles of HCl.

Use the normality formula

N₁V₁=N₂V₂

Where:

• N₁​ = normality of Ba(OH)₂
• V₁=25 mL (volume of Ba(OH)₂)
• N₂=0.1 N (normality of HCl)
• V₂= 35 mL (volume of HCl)

N₁× 25 = 0.1 × 35

N₁​= 0.1×3525 = 0.14 

Convert normality to molarity

Barium hydroxide gives 2 OH⁻ ions, so:

M= N2 = 0.142 = 0.07 M

Ans. 0.07 M