Overview
This lecture introduces stoichiometry—the quantitative relationships between reactants and products in chemical reactions—covering the concepts of mole ratios, balanced equations, limiting reactants, and percent yield.
Introduction to Stoichiometry
- Stoichiometry predicts quantities in chemical reactions using ratios from balanced equations.
- Coefficients in balanced equations function like ingredient ratios in recipes.
- Conversion factors between substances are based on these coefficients.
Balanced Equations and Mole Ratios
- A balanced chemical reaction has equal numbers and types of atoms on both sides, adhering to the law of conservation of mass.
- Coefficients represent mole ratios (e.g., 3 H₂ : 1 N₂ : 2 NH₃).
- Calculations use these mole ratios to predict the amount of products or reactants.
Converting Between Mass and Moles
- Use molar mass (from the periodic table) to convert between grams and moles.
- Grams → Moles: divide mass by molar mass.
- Moles → Grams: multiply moles by molar mass.
Types of Stoichiometry Problems
- Mole-to-mole: Convert moles of one substance to moles of another using ratio from the equation.
- Mass-to-mass: Convert grams to moles, use stoichiometry, then back to grams.
- Check for a balanced equation and quantities expressed in moles before solving.
Limiting Reactant and Theoretical Yield
- The limiting reactant is the substance that runs out first and determines the maximum amount of product.
- Find the limiting reactant by calculating the product formed from each reactant; the smallest result is the theoretical yield.
- Excess reactants are leftover materials after the reaction completes.
Percent Yield Calculation
- Percent yield = (actual yield / theoretical yield) × 100%.
- Actual yield is the amount actually obtained; theoretical yield is the amount predicted.
Problem-Solving Steps
- Ensure the equation is balanced.
- Convert given masses to moles.
- Use stoichiometry (mole ratios) to relate substances.
- Convert moles to desired units (grams or moles).
- In limiting reactant problems, repeat calculations for each reactant and choose the smallest predicted product.
Key Terms & Definitions
- Stoichiometry — Calculation of reactant and product quantities in chemical reactions using balanced equations.
- Mole — Unit representing 6.022 × 10²³ particles (atoms, molecules).
- Molar Mass — Mass (g) of one mole of a substance.
- Limiting Reactant (Reagent) — The reactant that is completely consumed first, limiting the amount of product formed.
- Excess Reactant — Reactant(s) remaining after the limiting reactant is used up.
- Theoretical Yield — The maximum product amount predicted from the limiting reactant.
- Actual Yield — The measured amount of product actually obtained.
- Percent Yield — Percentage ratio of actual yield to theoretical yield.
Action Items / Next Steps
- Practice stoichiometry problems, especially balancing equations and conversions between grams and moles.
- Review tutorial videos and web resources assigned for this chapter.
- Seek help if you struggle with balancing or conversion steps; mastery is essential for success.