Atoms and Molecules: Historical Background and the Law of Conservation of Mass

Learning Objectives

• Understand and apply the law of conservation of mass and the law of constant proportions.
• Explain Dalton's Atomic Theory with providing relevant examples.
• Learn the methodology to write chemical formulas for simple compounds.
• Identify and categorize ions into cations and anions.
• Understand and distinguish between atomic mass and molecular mass, including the calculation of these values.
• Distinguish between and define atomic number, mass number, isotopes, and isobars.

Brief History of the Atom

• Maharishi Kanad ($500\,BC$): An Indian philosopher who postulated that if matter ($\text{Padarth}$) is divided continuously, a point will be reached where the particles cannot be divided further. He named these indivisible particles $\text{Parmanu}$.
• Ancient Greek Philosophers: Democritus and Leucippus proposed similar theories, naming these indivisible particles "Atoms," which stems from a word meaning "uncuttable."

Definition and Characteristics of an Atom

• Definition: An atom is the smallest fundamental particle of an element that is capable of participating in a chemical reaction.
• Indivisibility: Under standard chemical methods, an atom is considered the limit of division and cannot be broken down further.
• Size: Atoms are extremely small in scale, with dimensions typically measured in nanometers ($\text{nm}$).
• Shape: For scientific modeling and calculations, atoms are considered to be nearly spherical in shape.

Laws of Chemical Combination

Atoms interact and combine according to specific set rules known as the Laws of Chemical Combination.

Law of Conservation of Mass

• Statement: This law states that mass can neither be created nor destroyed in a chemical reaction.
• Experimental Evidence (Activity 3.1): The reaction between Barium Chloride and Sodium Sulphate illustrates this principle.
  • Observation: When solutions of Barium Chloride and Sodium Sulphate are mixed, a white precipitate ($\text{ppt.}$) of Barium Sulphate is formed.
  • Conclusion: The sum of the masses of the reactants is equal to the sum of the masses of the products.
  • Chemical Equation:$BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$         (Note: $BaSO_4$ is the white precipitate).

Practical Applications and Problem Solving

Agreement with the Law of Conservation of Mass

Scenario 1: In a reaction, $5.3\,g$ of sodium carbonate reacted with $6\,g$ of ethanoic acid. The products formed were $2.2\,g$ of $CO_2$, $0.9\,g$ of $H_2O$, and $8.2\,g$ of sodium ethanoate.

• Reaction Equation: $\text{Sodium carbonate} + \text{Ethanoic acid} \rightarrow \text{Sodium ethanoate} + CO_2 + H_2O$
• Mass of Reactants: $5.3\,g + 6\,g = 11.3\,g$
• Mass of Products: $8.2\,g + 2.2\,g + 0.9\,g = 11.3\,g$
• Conclusion: Since the mass of reactants equals the mass of products, these observations agree with the law of conservation of mass.

Scenario 2:$15.9\,g$ of copper sulphate and $10.6\,g$ of sodium carbonate react together to give $14.2\,g$ of sodium sulphate and $12.3\,g$ of copper carbonate.

• Mass of Reactants: $15.9\,g + 10.6\,g = 26.5\,g$
• Mass of Products: $14.2\,g + 12.3\,g = 26.5\,g$
• Law Obeyed: The Law of Conservation of Mass is obeyed because the total mass remains constant throughout the chemical reaction.

Scenario 3: Carbon dioxide is added to $112\,g$ of calcium oxide. The product formed is $200\,g$ of calcium carbonate. Calculate the mass of carbon dioxide used.

• Reaction Flow: $CO_2 + CaO \rightarrow CaCO_3$
• Calculation:$\text{Mass of } CO_2 + \text{Mass of } CaO = \text{Mass of } CaCO_3$$\text{Mass of } CO_2 + 112\,g = 200\,g$$\text{Mass of } CO_2 = 200\,g - 112\,g$$\text{Mass of } CO_2 = 88\,g$
• Governing Law: The law of conservation of mass governs this calculation.