The correct option is **B. The concentrations of reactants and products remain constant over time.** ### Detailed Explanation: 1. **Understanding Equilibrium:** - In a chemical reaction, equilibrium is the state where the rate of the forward reaction (reactants forming products) equals the rate of the reverse reaction (products forming reactants). This means that the amounts of reactants and products remain constant over time, even though both reactions are still occurring. 2. **Why Option B is Correct:** - At equilibrium, the concentrations of reactants and products do not change because the forward and reverse reactions are happening at the same rate. This does not mean that the reactions have stopped; rather, they are continuously occurring, but the overall concentrations remain constant. This is a dynamic state, and it is crucial to understand that equilibrium does not imply that the amounts of reactants and products are equal, just that their ratios remain constant. 3. **Why the Other Options are Incorrect:** - **Option A: The concentrations of reactants are always greater than the concentrations of products.** - This statement is not universally true. The concentrations of reactants and products at equilibrium depend on the specific reaction and its equilibrium constant (K). In some reactions, products can be present in greater concentrations than reactants, and in others, it can be the opposite. Therefore, this statement is too absolute and does not hold true for all reactions. - **Option C: The concentrations of products are always greater than the concentrations of reactants.** - Similar to Option A, this statement is also incorrect. The relative concentrations of reactants and products at equilibrium depend on the specific reaction and its conditions. For example, in the reaction of hydrogen and iodine to form hydrogen iodide, at equilibrium, the concentrations of reactants and products can vary widely depending on the conditions (temperature, pressure, etc.). - **Option D: The reaction has stopped completely and no further changes occur.** - This statement is misleading. While it is true that the concentrations of reactants and products remain constant at equilibrium, it does not mean that the reactions have stopped. Both the forward and reverse reactions continue to occur, but they do so at equal rates, resulting in no net change in concentration. This is a common misconception about chemical equilibrium. ### Summary of Key Points: - At equilibrium, the concentrations of reactants and products remain constant over time due to equal rates of the forward and reverse reactions. - The relative concentrations of reactants and products can vary based on the specific reaction and its conditions. - Equilibrium is a dynamic state where reactions continue to occur, but there is no net change in concentrations. - Understanding the concept of equilibrium is crucial for predicting the behavior of chemical reactions under different conditions. ### Common Pitfalls: - Confusing equilibrium with completion of a reaction; remember that equilibrium means ongoing reactions at equal rates. - Assuming that reactants and products must be present in equal amounts at equilibrium; they can vary widely depending on the reaction specifics.