Conservation of Mass and Measuring Mass Loss in Chemical Reactions

Key Concepts

• Conservation of Mass:

  • In any chemical reaction, the total number of atoms remains constant.
  • Total mass of reactants equals total mass of products, indicating mass conservation.

• Gas Loss in Reactions:

  • Some products may escape the reaction vessel as gases, causing apparent mass loss.
  • Example: Alka-Seltzer in water releases CO2, demonstrating effervescence.
  • Loss of liquid volume in reaction indicates mass loss due to escaping gas.

Importance of Measuring Mass Loss

• Reaction Rate:

  • Measuring mass loss helps determine the rate of reaction.
  • Important for industrial chemical production efficiency.

• Experiment Success and Purity Check:

  • Total mass loss measurement indicates experimental success.
  • Helps verify reactant purity and identify experimental issues.

Measuring Mass Loss: Example Reaction

• Marble Chips and Hydrochloric Acid Reaction:

  • Reactants: Calcium carbonate (marble chips) and hydrochloric acid.
  • Products: Calcium chloride, carbon dioxide (gas), and water.

• Practical Measurement:

  • Use an electronic scale to measure mass loss over time.
  • Initial mass includes beaker and reactants.
  • Record mass at intervals to observe decrease due to CO2 escape.
  • Plotting mass loss vs. time graph helps calculate reaction rate.

Calculation Challenge: Mass Lost in a Reaction

• Scenario:

  • 0.25 moles of calcium carbonate reacts with excess hydrochloric acid.
  • Determine total mass lost at reaction completion.

• Steps to Calculate:

  • Moles of CO2 Produced: Each mole of CaCO3 produces 1 mole of CO2.
  • Molar Mass of CO2: 44 grams/mole (calculated as 16+16+12).
  • Mass Formula: Mass of CO2 = moles of CO2 x molar mass of CO2.
  • Result: Mass of CO2 = 0.25 moles x 44 g/mole = 11 grams.

• Importance of Calculations:

  • Predictive calculations guide experimental expectations.
  • Discrepancies in results indicate potential experimental issues such as impurities or procedural errors.

Summary

• Mass loss in reactions involving gaseous products can be observed and measured using an electronic scale.
• This data is crucial for calculating reaction rates and verifying experimental integrity.