Reaction Stoichiometry Overview

Overview

This lecture covers reaction stoichiometry: balancing chemical equations, using mole ratios in calculations, identifying limiting reactants, calculating percent yield, and recognizing common types of chemical reactions.

Chemical Equations & Balancing

• Chemical equations represent reactions, showing reactants, products, and coefficients.
• Equations must be balanced so that the number of atoms of each element is equal on both sides.
• Only coefficients (not subscripts) can be changed to balance equations.
• Whole numbers or fractions can be used as coefficients, but whole numbers are often preferred.
• Practice is key to mastering equation balancing.

Mole Ratios & Stoichiometry

• Balanced equations provide quantitative relationships (mole ratios) between reactants and products.
• Mole ratios are used as conversion factors in stoichiometric calculations.
• To convert between mass and moles, use the compound's molar mass (molecular weight).

Solving Stoichiometry Problems

• Always convert given mass to moles before applying stoichiometry.
• Set up conversions: mass → moles (using molar mass) → moles of another substance (using mole ratio).
• For word problems, extract reactants and products to write and balance the chemical equation first.
• Use proper significant figures based on the given data.

Limiting Reactant Concept

• The limiting reactant is the one completely consumed first, determining the amount of product formed.
• To identify it, calculate product amounts using each reactant; the reactant giving the least product is limiting.
• The other reactant(s) are in excess.

Percent Yield

• Percent yield compares the actual yield (from experiment) to the theoretical yield (from stoichiometry).
• Formula: Percent Yield = (Actual Yield / Theoretical Yield) × 100%
• The actual yield cannot exceed the theoretical yield.

Types of Chemical Reactions

• Combination (Addition): Two or more reactants form one product (A + B → C).
• Decomposition: One reactant forms two or more products (A → B + C).
• Single Displacement: An element replaces another in a compound (A + BC → AC + B).
• Double Replacement: Two compounds swap partners (AB + CD → AD + CB).
• Use knowledge of cations/anions and the activity series to predict displacement reactions.

Key Terms & Definitions

• Stoichiometry — The quantitative relationship between reactants and products in a chemical reaction.
• Limiting Reactant — The substance that determines the maximum amount of product possible.
• Mole Ratio — The ratio of coefficients of substances in a balanced equation.
• Percent Yield — The ratio (as a percent) of actual product obtained to theoretical maximum.
• Molar Mass — Mass (g) of one mole of a substance, used for mass ↔ mole conversions.
• Diatomic Elements — Elements that exist naturally as molecules of two atoms (H2, N2, F2, O2, I2, Cl2, Br2).

Action Items / Next Steps

• Practice balancing chemical equations and solving stoichiometry problems.
• Review activity series and nomenclature for types of reactions.
• Check Blackboard for assigned activities and upcoming videos on solution and gas stoichiometry.