The electrode through which electrons enter the electrolytic solution is — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Understand the Setup and Components of an Electrolytic Cell An **electrolytic cell** is an electrochemical system that uses external electrical energy (supplied by a direct current, or DC, source like a battery) to drive a non-spontaneous chemical redox reaction. The cell consists of two metal or graphite plates, known as electrodes, immersed in an electrolytic solution containing free cations and anions. The electrodes are defined as follows: * **Anode:** The electrode connected to the positive ($+$) terminal of the external power source. * **Cathode:** The electrode connected to the negative ($-$) terminal of the external power source. Step 2 - Analyze Electron Flow in the External Circuit The external battery or DC power supply acts as an electron pump: 1. It pulls electrons away from the anode, causing the anode to acquire a positive charge. 2. It pushes (pumps) these electrons through the external wiring into the cathode, causing the cathode to acquire a negative charge. Therefore, in the external metallic circuit, electrons flow from the anode to the positive terminal of the power source, and from the negative terminal of the power source to the cathode. Step 3 - Analyze Electron Entry into the Solution (Cathodic Reaction) Since the cathode is negatively charged and accumulates a high density of electrons on its surface, cations ($\ce{M^{n+}}$) in the electrolyte migrate toward the cathode. At the cathode-solution interface, these cations accept the electrons from the cathode surface and undergo **reduction**: $$\ce{M^{n+}(aq) + n e^- -> M(s)}$$ Because the electrons are transferred from the physical cathode plate to the ionic species dissolved in the medium, the **cathode** is the electrode through which electrons physically **enter** the electrolytic solution. Step 4 - Analyze Electron Leaving the Solution (Anodic Reaction) Conversely, at the anode-solution interface, anions ($\ce{A^{m-}}$) in the electrolyte migrate toward the positively charged anode. They release electrons onto the anode surface and undergo **oxidation**: $$\ce{A^{m-}(aq) -> A(s) + m e^-}$$ These released electrons travel up the anode and return to the external circuit. Thus, the anode is the electrode through which electrons **leave** the electrolytic solution. Step 5 - Evaluate the Options * **Option (A) is correct:** As shown in Step 3, electrons pumped from the external battery build up on the cathode and enter the solution during the reduction of cations. * **Option (B) is incorrect:** The anode is the electrode through which electrons leave the solution and return to the external circuit. * **Option (C) is incorrect:** The direction of electron flow is fixed by the polarity of the external DC power source; electrons can only enter the solution through the cathode. * **Option (D) is incorrect:** Electrons enter at one electrode (cathode) and leave at the other (anode) to maintain a complete electrical circuit. They cannot enter through both. $$\text{Correct Option: } \boxed{\text{A}}$$