A standard hydrogen electrode has zero electrode potential because — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Principle of Measuring Single Electrode Potentials In electrochemistry, it is physically impossible to measure the absolute potential of a single isolated electrode (half-cell). This is because an oxidation or reduction half-reaction cannot occur independently; a oxidation half-reaction at one electrode must always be coupled with a reduction half-reaction at another electrode to complete the electrical circuit. Consequently, only the potential difference (electromotive force, or EMF) between two coupled electrodes can be experimentally measured. Step 2 - The Need for a Reference Electrode To determine and compare the relative electrode potentials of various half-cells, scientists require a standardized benchmark or reference point. A reference electrode is an electrode that maintains a constant, stable, and reproducible potential. The Standard Hydrogen Electrode ($\text{SHE}$), consisting of a platinized platinum foil immersed in a $1\text{ M}$ $\ce{H^+(aq)}$ solution with pure hydrogen gas ($\ce{H2}$) bubbled at a pressure of $1\text{ atm}$, is chosen as the primary reference electrode: $$\ce{Pt(s) \mid H2(g, 1 atm) \mid H+(aq, 1 M)}$$ Step 3 - Arbitrary Assignment of the Reference Value By thermodynamic convention established by IUPAC, the standard electrode potential ($E^\circ$) of the Standard Hydrogen Electrode is arbitrarily defined as exactly $0.00\text{ V}$ at all temperatures: $$E^\circ_{\ce{H^+|H2}} = 0.00\text{ V}$$ This assigned value of zero is not a measured physical quantity, nor is it a consequence of any unique physical or chemical property of the hydrogen atom itself. It is a purely agreed-upon baseline, much like assigning an elevation of zero meters to sea level to measure the relative heights of mountains. Step 4 - Evaluation of the Options * **Option (A) is incorrect:** Hydrogen is not the easiest element to oxidize. Active metals such as lithium ($\ce{Li}$), potassium ($\ce{K}$), and sodium ($\ce{Na}$) have much more negative standard reduction potentials, meaning they lose electrons and oxidize far more readily than hydrogen. * **Option (B) is correct:** The potential of the standard hydrogen electrode is zero because it has been arbitrarily assumed and defined to be zero by scientific convention to act as a reference scale. * **Option (C) is incorrect:** While a hydrogen atom physically contains only one electron, this atomic property has no direct bearing on the thermodynamic decision to define the SHE potential as zero. * **Option (D) is incorrect:** Although hydrogen is the lightest element in the periodic table, this physical characteristic is not the thermodynamic reason why its standard electrode potential is defined as zero. $$\text{Correct Option: } \boxed{\text{B}}$$