Understanding the Conservation of Mass in Chemical Reactions

What is the Conservation of Mass?

• The law states that mass is never created or destroyed during a chemical reaction; only the arrangement of atoms changes.
• It applies whether you count atoms or measure mass in grams.

Balancing an Equation: Sodium + Chlorine → Sodium Chloride

• Reaction: 2 Na + Cl₂ → 2 NaCl
• Reason for the coefficients:
• Each Cl₂ molecule contains two chlorine atoms, so two NaCl molecules are needed on the product side.
• Two sodium atoms are required on the reactant side to match the two NaCl units.

Checking Mass Balance with Relative Formula Masses

• Left side: 2 × 23 (g Na) + 2 × 35.5 (g Cl) = 117 g
• Right side: 2 × 23 (g Na) + 35.5 (g Cl) = 117 g
• The totals match, confirming mass conservation.

Practical Experiment Using a Scale

• Example: React 2.3 g Na with 3.5 g Cl₂.
• If the reaction goes to completion, the product mass = 2.3 g + 3.5 g = 5.8 g NaCl.
• A scale will show the same total mass before and after the reaction.

Apparent Exceptions When Gases Are Involved

• Mass increase illusion: When a solid reacts with a gaseous reactant taken from the air (e.g., Mg + O₂ → MgO), the product appears heavier because the oxygen was not weighed initially.
• 1 g Mg → ~1.6 g MgO (extra mass comes from atmospheric O₂).
• Mass decrease illusion: When a product is a gas that escapes (e.g., CaCO₃ → CaO + CO₂), the measured mass drops as CO₂ leaves the system.

Why a Sealed Container Solves the Problem

• In a closed system, all gases are trapped, so the initial mass measurement includes the gaseous reactants, and the final measurement includes the gaseous products.
• Under these conditions, the total mass remains exactly the same, satisfying the conservation law.

Quick Recap

• Core principle: Mass is conserved in every chemical reaction.
• Apparent changes occur only when gases are not accounted for in the weighing process.
• Increase → missing gaseous reactant.
• Decrease → escaping gaseous product.
• Solution: Perform experiments in sealed containers to capture all gases.

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Mass never disappears or appears in a chemical reaction; any seeming change is due to unmeasured gases, which can be eliminated by using a sealed container.