Overview
This lecture introduces stoichiometry, the calculation of reactant and product amounts in chemical reactions, emphasizing how balanced equations inform these calculations using moles and mass.
What is Stoichiometry?
- Stoichiometry involves calculations about the amounts of reactants and products in a chemical reaction.
- It uses balanced chemical equations to deduce quantitative relationships.
- The coefficients in a balanced chemical equation represent ratios in moles, not just molecules or atoms.
Using Balanced Equations for Moles Calculations
- A balanced equation tells the mole ratio of each reactant and product.
- Example: Combustion of propane (C₃H₈): 1 mol C₃H₈ reacts with 5 mol O₂ to produce 3 mol CO₂ and 4 mol H₂O.
- To find how many moles of a product are formed, multiply the moles of reactant by the ratio from the balanced equation.
Dimensional Analysis in Stoichiometry
- Use coefficients as conversion factors: e.g., 2.0 mol propane × (3 mol CO₂ / 1 mol propane) = 6.0 mol CO₂.
- For different starting substances, use the appropriate mole ratio (e.g., 2.0 mol O₂ × (3 mol CO₂ / 5 mol O₂) = 1.2 mol CO₂).
Gram-to-Gram Stoichiometry
- Because moles can't be measured directly, grams are often used.
- Converting grams to moles uses molar mass (grams per mole).
- Example: For octane (C₈H₁₈) combustion, 50 g C₈H₁₈ converts to moles, then to moles of CO₂, and finally to grams of CO₂.
- The overall process: grams (A) → moles (A) → moles (B) → grams (B).
General Stoichiometry Calculation Steps
- Always: Convert mass to moles, use balanced equation for mole ratios, then convert back to mass as needed.
- The calculation process is the same, regardless of whether reactants or products are involved.
Key Terms & Definitions
- Stoichiometry — The calculation of quantities of reactants and products in chemical reactions.
- Mole — Unit representing 6.022 × 10²³ particles (atoms, molecules, etc.).
- Molar Mass — The mass (in grams) of one mole of a substance.
- Balanced Equation — A chemical equation with equal numbers of atoms for each element on both sides.
- Dimensional Analysis — A method using conversion factors to move between units.
Action Items / Next Steps
- Read Chapter 5 of the textbook on stoichiometry.
- Practice converting between grams and moles for given reaction equations.
- Attempt sample stoichiometry problems using the “grams → moles → moles → grams” process.