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Stoichiometry describes the quantitative relationship between reactants and products in a balanced chemical equation. For example, in hydrogen combustion, two molecules of H2 react with one molecule of O2 to produce two molecules of H2O. Stoichiometry helps predict how much reactant is needed to produce a desired amount of product or how much product forms from a specific reactant amount.

The limiting reactant is the substance completely consumed first, restricting the amount of product formed. To identify it, compute the molar amounts of each reactant and compare them to stoichiometric amounts in the balanced equation. The reactant present in less-than-stoichiometric quantity is limiting, while excess reactant remains unconsumed after the reaction completes.

Reaction rate measures the speed at which a chemical reaction progresses, quantifying either the disappearance of reactants or the appearance of products per unit time. It is expressed in units of molarity per second. As a reaction proceeds, reactant concentrations decrease while product concentrations increase, changes that can be plotted on a graph as a function of time.

A reversible chemical reaction proceeds simultaneously in both forward and reverse directions, indicated by a double arrow in the equation. Chemical equilibrium is achieved when the rate of product formation equals the rate of reactant formation, causing reactant and product concentrations to remain constant over time. At this point, the system shows no net change in composition.

The equilibrium constant (Kc) is calculated from molar concentrations of products and reactants at equilibrium, each raised to their stoichiometric coefficients. The reaction quotient (Qc) uses the same expression form but applies to any concentration, not just equilibrium. Comparing Qc to Kc indicates whether a reaction will shift forward or reverse to reach equilibrium.

Excess reactant is deliberately added to ensure complete conversion of the limiting reactant into product. By providing more than the stoichiometric amount of one reactant, chemists maximize product yield from the other reactant. This strategy is especially useful when one reactant is expensive or when complete consumption of a specific reactant is critical to the reaction's success.

Concentration changes over time are plotted on a graph with time on the x-axis and molarity on the y-axis. As the reaction progresses, the reactant concentration curve decreases while the product concentration curve increases. This graphical representation clearly shows reaction rate, the point where equilibrium is reached, and the relative amounts of reactants and products throughout the reaction.