According to metal reactivity series, Na is more reactive than Ca. But according to the electrochemical series, Ca is a stronger reducing agent than Na. Why is that?
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5$\begingroup$ What is the definition and criterion of reactivity? It is undefined. $\endgroup$– ACRCommented Jun 2, 2022 at 15:53
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The electrochemistry series is based on the thermodynamic parameter - standard redox potential, directly linked to standard molar Gibbs energy of a formal reaction (See e.g. this libretexts doc):
\begin{align} \ce{Ox^{\mathit{x}} + \frac {n}{2} H2(g) &-> Red^{x-n} + n H+(aq)}\\ \Delta G^°_\mathrm{r}&=-nFE^° \end{align}
The latter equation is reformulation of the equation $\ce{W=q.U}$(non-V work, charge, voltage), known from electronics or electrostatics.
The reactivity series is vaguely defined and there must be specified scenario and criteria. The reactivity is primarily a kinetic parameter, that is loosely related to thermodynamics. But can be defined in thermodynamic context as well. Reactivity order may be different for different reaction scenarios, including also dry and hot scenarios with melted reagents.
Comparing metals from the same group, lithium and sodium, the former has lower standard redox potential = stronger reducing agent in water context. But, lithium is less reactive in contact with water, with slower reaction rate than sodium.
In an outdoorsy analogy, the difference between the trip start and destination altitudes (thermodynamics) and the altitude difference from the start to the saddle (=activation energy = kinetics) are mutually independent.
The "calcium trip" goes to a lower-altitude valley.
The "sodium trip" goes across a lower-altitude saddle.
