Chemistry Lecture: Conservation of Mass

Key Concepts

Conservation of Mass

• Principle: In a chemical reaction, mass is always conserved.
• No atoms are created or destroyed; only bonds between atoms change.
• Important for balancing chemical equations.

Example: Sodium and Chlorine Reaction

• Reaction: Sodium + Chlorine -> Sodium Chloride.
• Balancing:
  • 2 Chlorine atoms in Cl₂ molecule → 2 NaCl as product → 2 Na on reactant side.
• Mass Calculation:
  • Reactants: 2(23) + 2(35.5) = 117
  • Products: 2(23) + 35.5 = 117
  • Total mass balances on both sides of the equation.

Practical Mass Comparison

• Reacting 2.3 g Sodium with 3.5 g Chlorine gas results in 5.8 g of Sodium Chloride.
• Using scales, mass should remain constant during reaction.

Exceptions to Conservation of Mass

Reactions Involving Gases

• Mass might appear to change due to gas not being measured if it enters or exits the reaction space.

Example 1: Magnesium and Oxygen

• Reaction: Magnesium + Oxygen -> Magnesium Oxide.
• Mass observation:
  • 1 g Magnesium becomes ~1.6 g Magnesium Oxide.
  • Appears as increase because oxygen from the air isn’t initially measured.

Example 2: Decomposition of Calcium Carbonate

• Reaction: Calcium Carbonate -> Calcium Oxide + Carbon Dioxide.
• Mass observation:
  • CO₂ gas escapes, reducing apparent mass of products.

Sealed Container Solution

• Conducting reactions in a sealed container prevents gas from entering or leaving.
• Ensures mass of reactants and products is accurately measured and conserved.

Summary

• Conservation of mass: Mass conserved in chemical reactions, whether in grams or by counting atoms.
• Apparent mass change:
  • Increase due to gas reactants.
  • Decrease due to gaseous products.
• To prevent measurement errors, use sealed containers during experiments.

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• Conclusion: Always consider gas involvement in reactions for accurate mass measurements.