Final answer:
A reaction achieves equilibrium on a concentration vs time graph when the lines representing reactant and product concentrations become flat, indicating constant concentrations due to equal forward and reverse reaction rates.
Step-by-step explanation:
When inspecting a concentration versus time graph for a chemical reaction, you can tell that the reaction has achieved equilibrium when the graph shows that concentrations of reactants and products become constant over time. This implies that the rates of the forward and reverse reactions have become equal, indicating a dynamic equilibrium. Initially, as the reaction proceeds toward equilibrium, reactant concentrations decrease and product concentrations increase, which means the reaction quotient Qc changes.
At equilibrium, however, this change ceases and Qc remains constant as the forward and reverse reaction rates balance each other. This state is visually represented on a graph by flat lines for both reactant and product concentrations, indicating no further change in their respective amounts. When conditions change, such as an increase or decrease in concentration of a reactant or product, we can use the graph to predict the direction in which the reaction will shift in order to re-establish equilibrium.