Consider the following reduction processes:
Zn2+ + 2e– → Zn(s) ; E° = – 0.76 V
Ca2+ + 2e– → Ca(s) ; E° = – 2.87 V
Mg2+ + 2e– → Mg(s) ; E° = – 2.36 V
Ni2+ + 2e– → Ni(s) ; E° = – 0.25 V
The reducing power of the metals increases in the order:
Greater SOP ⇒ More tendency to be oxidised
⇒ More reducing power
This question involves understanding the relationship between standard reduction potential (E°) and reducing power of metals. The standard reduction potential measures the tendency of a species to gain electrons and be reduced. A more negative E° value indicates a lower tendency to gain electrons and a higher tendency to lose electrons (i.e., act as a reducing agent).
The standard reduction potentials given are:
To find the order of increasing reducing power, we need to arrange the metals from weakest to strongest reducing agent. Since reducing power is inversely related to the reduction potential, the metal with the most negative E° value is the strongest reducing agent.
Let's list the E° values in increasing order (from most negative to least negative):
Ca: -2.87 V (most negative)
Mg: -2.36 V
Zn: -0.76 V
Ni: -0.25 V (least negative)
Therefore, the order of increasing reducing power (weakest to strongest) is:
Ni < Zn < Mg < Ca
This matches the third option: Ni < Zn < Mg < Ca.
Standard Reduction Potential (E°): The tendency of a chemical species to acquire electrons and thereby be reduced. It is measured under standard conditions: 1 M concentration for each ion, 1 atm pressure for gases, and 25°C.
Reducing Power: The ability of a substance to donate electrons. A stronger reducing agent has a more negative reduction potential.
Electrochemical Series: A series of elements arranged in order of their standard reduction potentials. Elements with more negative E° values are stronger reducing agents and are placed higher in the series.
Key Formula: The relationship is qualitative. A more negative E° means:
And the strength increases as E° becomes more negative.