Reaction Quantities and Limiting Reactants

Objective: Interpret coefficients as ratios and identify a simple limiting case.

Equation coefficients give stoichiometric ratios of reacting particles and, at bulk scale, moles. For 2H₂ + O₂ → 2H₂O, two moles of H₂ require one mole of O₂ and can yield two moles of H₂O under complete conversion. Starting with 4 moles H₂ and 1 mole O₂, oxygen limits the reaction to 2 moles H₂O while 2 moles H₂ remain, in the ideal model. A limiting reactant is consumed first according to the ratio, not necessarily the reactant with the smallest numerical mass. Actual yield may be less than theoretical due to incomplete reaction, competing processes, or losses. State whether numbers are particles, moles, or masses.

Objective: Apply reaction quantities and limiting reactants to calculations, evidence, and specified situations.

Stoichiometry uses balanced coefficients as mole ratios. Convert a measured mass to moles first when necessary, apply the coefficient ratio, and convert to the requested unit. With multiple reactants, compare amount divided by coefficient or compute each possible product yield; the smaller possible yield identifies the limiting reactant. Excess reactant does not permit production beyond that limit. Percent yield compares actual collected product with theoretical product, with the same units.