On what basis can the SRP value of different elements in the periodic table be explained? For example why does Cu have a greater tendency to accept an electron and Zn have a tendency to lose electrons in a Daniel cell?


There are many factors and the ideal way to understand this would be with the quantum mechanical description of atoms, which gets too complicated for answering such questions. But in general, following are few simple-to-understand face that helps in understanding the SRP values-

  1. Electronegativity and Electron gain enthalpy: Higher the electronegativity, greater is the tendency to accept electrons, be reduced and hence greater (more positive) is the Reduction potential. For example, reduction potential of fluorine is greater than chlorine and that is greater than bromine. As we move down the halogen group, electronegativity decreases.

  2. Ionization potential: This is a consideration for metals, which usually loose electrons and hence electronegativity is not the right quantity to consider. The lesser the IP, easier is the oxidation, and more negative (lower) is the SRP. In gas phase, the SRP of cesium is the most negative (in the group) and it becomes less negative as we move upwards.

  3. Hydration: This is an important fact to consider since most electrochemical reactions occur in aqueous phase and hence the commonly cited SRP values corresponds to the aqueous phase. Greater the hydration of the ion formed after oxidation/reduction, greater is the tendency to lose/gain electrons and hence lower/higher is the SRP values. Lithium ion has very high hydration enthalpy and hence lithium has a strong tendency to lose an electron to form an ion which can be excellently solvated. This lends it one of the most negative SRP in the periodic table. The same holds true for fluorine, which is excellently solvated in the ionic form by water and hence has a high SRP.

  4. Size, EC etc: Several factors like ionic size, it's electronic configuration, the inner orbital structure and the nuclear force of attraction determines how the incoming electron will be received and consequently the SRP values. Higher atomic size, higher inner electron repulsion, almost-stable configuration(just 1 electron more) and lower nuclear force, all favour lower SRP values.

All of these consideration, in varying proportions can be applied to the transition elements, but they act differently and to different extent in different compounds. Hence, predicting the SRP order without experimental data is very difficult and not trustworthy.


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