Chemical Reactions

Consider a balanced chemical reaction, such as the combustion of hydrogen gas. Here, the quantitative relationship between the reactants and products — H₂, O₂, and H₂O — is that two molecules of H₂ react with one molecule of O₂ to produce two molecules of H₂O. This quantitative relationship is known as stoichiometry.

In a chemical reaction, reactants interact with each other to form products. The reactant that is completely consumed is the limiting reactant, and the reactant present in a quantity greater than necessary to react completely with the limiting reactant is the excess reactant.

A chemical reaction involves the chemical transformation of reactants into products. As the reaction progresses, the concentration of reactants decreases while the concentration of products increases. This variance in concentrations of reactants and products can be plotted in a graph as a function of time.

The speed at which a reaction progresses is called the reaction rate. It measures the rate of reactants’ disappearance or the rate of products’ appearance and is expressed in units of molarity-per-second.

A reversible chemical reaction is one in which the conversion of reactants into products and the conversion of products back into reactants occur simultaneously. A double arrow between the reactants and products signifies its reversible nature.

When the rate of formation of products, or the rate of the forward reaction, equals the rate of formation of reactants, or the rate of the reverse reaction, chemical equilibrium is achieved.

The equilibrium constant expression is written as the molar concentrations of the products, C and D, over the reactants, A and B, at equilibrium, each raised to their respective stoichiometric coefficients. When solved, the expression is equal to the equilibrium constant, K_c.

An expression in the same form can also be written for the reactants and products at any concentration, and the calculated quantity is known as the reaction quotient, Q_c.