Does a balanced chemical equation tell you which reagent is in excess?

Question

Naomi Aronson · Accepted Answer

No, but it does establish stoichiometry.

A balanced chemical equation establishes stoichiometry. It tells you the molar quantities required for complete reaction. Typically one reagent will be excess, and one will be in deficiency. For combustion reactions, typically the oxidant will be in excess, i.e. if I had 16 grams of methane, which I burned in air, I could clearly designate methane as the reagent in deficiency: #CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(g)# This balanced equation not only tells me how much heat I could potentially obtain from a reaction with a few more parameters, but it also clearly states that 16 g of methane needs 32 g of dioxygen for complete combustion. This is the reason chemistry teachers go to great lengths to establish mass equivalency and stoichiometry; stoichiometry cannot be established without a balanced chemical equation.

Owen Ambrose · Answer

undefined No, a balanced chemical equation does not directly tell you which reagent is in excess. To determine which reagent is in excess, you need to compare the stoichiometry of the balanced equation with the actual amounts of reactants used in the reaction. This can be done by calculating the moles of each reactant and comparing their ratios based on the coefficients in the balanced equation. The reactant that is not completely consumed and still remains after the reaction is the one in excess.