Final answer:
The confusion between the reactivity series and the electrochemical series can be resolved by understanding that for single replacement reactions, the reactivity series is the tool to use, which accurately predicts that sodium can reduce calcium ions but not vice versa.
Step-by-step explanation:
The question deals with a common confusion between the reactivity series of metals and the electrochemical series, especially in the context of predicting reactions such as the reduction of metal ions. The reactivity series, which lists metals in order of their ability to displace other metals in aqueous solutions, predicts that sodium would be able to reduce calcium ions because sodium is more reactive than calcium. However, if we consider the reduction potential from the electrochemical series, calcium appears to have a lower reduction potential than sodium, which may suggest that calcium could reduce sodium ions.
It is important to note that the reactivity series is a more practical tool for predicting single replacement reactions. In the case of sodium and calcium, the reactivity series correctly implies that sodium, being higher on the series, will not be reduced by calcium. The seemingly contradictory information from the electrochemical series in reality complements the reactivity series, as it considers different conditions, such as standard states and concentrations, where direct metal displacement may not be the sole reaction occurring. Both series are valuable, but for single replacement reactions, the activity series is the more directly applicable tool to predict whether a reaction will occur.