(212) Stoichiometry Tutorial: Step by Step Video + review problems explained | Crash Chemistry Academy

Introduction to Stoichiometry

  • Stoichiometry is the study of the quantitative relationships between the reactants and products in a chemical reaction.

  • It allows us to understand how different components of a chemical equation relate mathematically.

Understanding Coefficients

  • Coefficients in a chemical equation represent:

    • Specific amounts of particles.

    • Amount of reactants and products.

    • Ratios of substances involved in the reaction.

Example Reaction: Hydrogen and Nitrogen

  • Reaction: H₂ + N₂ → NH₃

  • Initial reaction is unbalanced; coefficients need to be applied:

    • 3 H₂ + 1 N₂ → 2 NH₃.

  • This shows that:

    • 3 hydrogen molecules react with 1 nitrogen molecule, producing 2 ammonia molecules.

  • The constant ratio in reactions:

    • 3:1:2 (H:N:NH₃).

  • Example with large quantities:

    • 6 H₂ reacts with 2 N₂, yielding 4 NH₃.

    • 9 H₂ reacts with 3 N₂ yields 6 NH₃.

Molar Ratios from Coefficients

  • Coefficients also indicate molar ratios:

    • 3 moles of H₂ react with 1 mole of N₂ to produce 2 moles of NH₃.

  • Molar relationships assist in stoichiometric calculations.

Stoichiometric Calculations

  • Key focus in stoichiometry is mole-mole conversions:

    • From given amount (A) to wanted amount (B).

  • Example Problem:

    • If 7.5 moles of H₂ are present, how many moles of N₂ react?

    • Use ratio: 3 moles of H₂:1 mole of N₂.

    • Calculation: 7.5 moles H₂ x (1 mole N₂ / 3 moles H₂) = 2.5 moles N₂.

  • Another example:

    • Given 0.8 moles of NH₃, how much H₂ is needed?

    • Calculation: 0.8 moles NH₃ x (3 moles H₂ / 2 moles NH₃) = 1.2 moles H₂.

Converting from Mass to Moles

  • Direct mole counting is impractical; mass is easier to measure.

  • Conversion steps:

    • Given mass → convert to moles (using molar mass).

    • Then do mole-mole conversions.

    • Finally, convert moles back to mass (using again molar mass).

  • Example: How much NH₃ is produced from 42 grams of N₂?

    • Steps:

      1. Convert 42 g N₂ to moles using molar mass: 42 g N₂ x (1 mol N₂ / 28 g N₂) = 1.5 moles N₂.

      2. Convert moles N₂ to moles NH₃ using ratio: 1.5 moles N₂ x (2 moles NH₃ / 1 mole N₂) = 3 moles NH₃.

      3. Convert moles NH₃ to mass: 3 moles NH₃ x (17 g NH₃ / 1 mol NH₃) = 51 g NH₃.

Setting Up Stoichiometry Problems

  • Keep an organized 'map' of conversions:

    • Identify what is given and what is wanted.

    • Use proper ratios and molar masses to ensure correct calculations.

  • Example Problem:

    • Find the mass of O₂ needed to react with 5.95 grams of NH₃.

      • Perform conversions starting from g NH₃ to moles NH₃, then moles O₂, and finally to mass O₂.

      • Result: 19.6 grams of O₂ required.

Practice Example: Ethane Combustion

  • Involves similar stoichiometry steps for calculating the product mass.

  • Utilizing balanced chemical equations to derive relationships helps determine outcomes.

    • Example: Starting with 37.5 g of C₂H₆ produces 110 g of CO₂.

Conversions Beyond Mass

  • Stoichiometry is versatile beyond mass calculations:

    • Convert particles (atoms, molecules) to moles using Avogadro's number (6.02 x 10²³).

    • Volume of gases can be addressed by using 22.4 L for 1 mole at standard temperature and pressure (STP).

Conclusion

  • Stoichiometry is essential for predicting quantities in chemical reactions.

  • Mastery involves understanding how to set up conversions accurately and applying coefficient ratios effectively.