Chemical reactions often do not go to completion, meaning that reactants do not fully convert into products. Instead, they reach a state known as chemical equilibrium, where the forward and reverse reactions occur simultaneously. For example, when 3 moles of hydrogen gas (H2) react with 1 mole of nitrogen gas (N2) to produce 2 moles of ammonia (NH3), the initial state contains only reactants. Over time, these reactants break down to form products, and as the concentration of products increases, they begin to decompose back into reactants. This dynamic process is characterized by two rate constants: one for the forward reaction and one for the reverse reaction.
Initially, the forward rate is at its maximum since there are only reactants. However, as reactants are consumed, the forward rate decreases. Conversely, the reverse rate starts at zero and increases as products accumulate. Eventually, both rates reach a point of equilibrium where they are equal. At this stage, the equilibrium constant (K) can be defined as the ratio of the rate constant for the forward reaction (kf) to the rate constant for the reverse reaction (kr), expressed mathematically as:
K = \frac{k_{f}}{k_{r}}
Additionally, K can be represented in terms of concentrations of products and reactants, excluding solids and liquids, as follows:
K = \frac{[NH_{3}]^{2}}{[H_{2}]^{3} \cdot [N_{2}]}
By substituting actual concentrations into this expression, one can calculate the numerical value of K, which indicates the favorability of the reaction. If K > 1, the products are favored, suggesting that the reaction proceeds more towards the formation of products. Conversely, if K < 1, the reactants are favored, indicating a tendency to revert to the original reactants. When K = 1, the system is at ideal equilibrium, meaning neither side is favored, and the concentrations of reactants and products are balanced.
Understanding these principles of equilibrium is crucial, as they lay the foundation for further exploration of topics such as equilibrium concentrations, ICE (Initial, Change, Equilibrium) charts, and other related concepts. Ultimately, equilibrium is established when the rates of the forward and reverse reactions become equal, allowing for a stable concentration of reactants and products over time.