An active (metal) electrode was found to lose mass as the oxidation-reduction reaction was allowed to proceed. Was the electrode an anode or a cathode? Explain.

Electrochemical reactions are fundamental processes involving the movement of electrons from one substance to another. An easy way to remember the difference between an anode and a cathode is through the acronym 'AN OX' and 'RED CAT'. This stands for 'Anode Oxidation' and 'Reduction Cathode'. During electrochemical reactions, the anode is the electrode where oxidation happens — meaning it loses electrons.

On the flip side, the cathode is where reduction takes place, and it gains electrons. Understanding this is crucial to grasp the behavior of electrodes in electrochemistry. For instance, when a metal electrode is part of a reaction and it gradually loses mass over time, this indicates that it is serving as the anode. That's because the metal atoms are giving up electrons and transforming into ions. These ions then dissociate into the solution, leading to a decrease in mass of the original electrode — a sure sign of oxidation.

Oxidation and reduction are chemical reactions foundational to electrochemistry and are collectively known as redox reactions. Oxidation is the process where an element loses electrons, often described as an increase in oxidation state. In contrast, reduction is the gain of electrons, which corresponds to a decrease in oxidation state.

Oxidation:

When referring to metal electrodes, oxidation implies that metal atoms are emitting electrons and converting them into metal ions. This is crucial for electrochemical cells as it maintains the flow of electrons through the circuit.

Reduction:

Conversely, when reduction occurs, ions in the solution acquire electrons at the cathode and become neutral atoms or molecules. Reduction is key to the deposition of material at the cathode. It's essential when understanding the loss or gain of material in electrochemical cells, just as in the exercise where the electrode's loss of mass signaled an oxidation process.

Metal electrodes are an integral part of electrochemical cells, acting as conduits for the flow of electrical charge in and out of the cell through redox reactions. The metals used as electrodes must be conductive to facilitate this electron transfer. During operation, the surface of the electrodes participates directly in the reaction, with oxidation occurring at the anode and reduction at the cathode.

Selecting a suitable metal for the electrode depends on the reaction specifics. Some metals are more prone to oxidation and are therefore better suited for use as anodes; others have a higher tendency to gain electrons and are preferred as cathodes. In advanced applications, electrodes can be coated with catalytic materials to improve their efficiency or to protect them from corrosion, which is the degradation of metal due to the redox processes.
Thus, when an electrode is observed losing mass, such as in the textbook exercise, it's a clear indication that it is the anode and is undergoing oxidation.