Learning Goal: To use the equilibrium constant and Le Châtelier's principle to determine how a reaction will respond to external factors. A reaction is at equilibrium when the concentrations of the reactants and products no longer change over time. This does not mean the reaction is over; rather, two competing reactions continue to occur simultaneously at equal rates. The two competing reactions are the forward reaction (reactants \( \rightarrow \) products) and the reverse reaction (products \( \rightarrow \) reactants). If a reaction at equilibrium is subjected to a stress, the concentrations of reactants and products adjust to reestablish equilibrium. This is called Le Châtelier's principle. A stress might be a change in the concentration of reactants or products, a change in the volume of the reaction container, a change in temperature, or the addition of a catalyst.

This passage explains some key ideas about chemical equilibrium and how systems respond to external changes. Here are the main points in simpler terms: 1. A reaction is at equilibrium when the rates of the forward and reverse reactions are equal. This means that even though the reactions are still happening, the concentrations of all the reactants and products stay constant over time. 2. Equilibrium does not mean that the reaction has “stopped” or that the reactants have been completely used up. Instead, it means that the system has reached a balance where the forward reaction (making products from reactants) occurs at the same rate as the reverse reaction (making reactants from products). 3. Le Châtelier's principle tells us that if a change (or “stress”) is applied to a system at equilibrium, the system will adjust in order to counteract that change and establish a new equilibrium. Common stresses include:   • Changing the concentrations of reactants or products (e.g., adding more of a reactant).   • Changing the volume of the reaction container (which affects pressure, especially in reactions involving gases).   • Changing the temperature.   • Adding a catalyst (although it's important to note that a catalyst speeds up both the forward and reverse reactions equally, so it doesn’t change the overall equilibrium composition). Understanding these concepts helps predict how a system at equilibrium will shift when any of these factors change.