Final answer:
Zero-order reactions theoretically reach zero concentration at time t = Co/k, but real conditions may lead to deviations, where the concentration approaches but does not touch the x-axis.
Step-by-step explanation:
The question asked pertains to the completion of a zero-order reaction and why the concentration-time graphs do not always touch the x-axis. Based on the formula Ct = Co − kt, where Ct is the concentration at time t, Co is the initial concentration, and k is the rate constant, it's stated that the concentration should be zero when t = Co/k. However, as noticed in various sources, the graph between concentration and time for a zero-order reaction often shows the concentration approaching but not reaching zero.
This is because real-life reactions might not perfectly follow the ideal model due to various factors affecting the reaction. In zero-order kinetics, when the specific conditions for zero-order behavior are not met, reactions behave differently. The term pseudo-zero-order is used to describe this behavior. Therefore, while the theoretical model predicts the concentration reaching zero, practical conditions can lead to deviations from this ideal behavior.