Final answer:
The concentrations of reactants and products at equilibrium can vary; they are not necessarily equal. The equilibrium constant (K) informs us about their ratio at equilibrium, thus indicating which side of the reaction is favored, the correct option is 4).
Step-by-step explanation:
What can be said about the concentrations of reactants relative to the concentrations of products at equilibrium? The answer is that the concentrations of reactants and products can vary at equilibrium. At equilibrium, a chemical reaction has reached a state where the rates of the forward and reverse reactions are equal, leading to constant concentrations of reactants and products over time. However, this does not necessarily mean that the concentrations of reactants and products are the same. It is their rates of formation and consumption that are balanced.
The equilibrium constant, K, is the ratio of the concentrations of the products to the reactants, raised to the power of their stoichiometric coefficients in the balanced chemical equation. In cases where K is small, the concentration of reactants is higher than that of products. Conversely, when K is large, it indicates that the concentration of products is higher than that of reactants. In a situation where the concentrations of reactants and products are equal, K would be equal to 1.
The actual values of concentrations at equilibrium can be calculated using the equilibrium constant along with the initial concentrations of reactants and the stoichiometry of the reaction. Therefore, within different equilibrium systems, we may observe varying relationships between the concentrations of reactants and products, but in each system, those concentrations at equilibrium will be constant unless environmental conditions (e.g., pressure, temperature) change.