Electrochemistry is the branch of science dedicated to studying the relationship between chemical change and electrical energy. This field explores how electron movement can either generate electricity from a chemical reaction or use electricity to drive a chemical transformation. These processes involve the transfer of electrons between substances, which are known as redox reactions. Redox reactions occur at specialized conductive surfaces called electrodes and are fundamental to devices like batteries and industrial plating processes.
Defining Reduction and Oxidation
The term “redox” combines two complementary processes: reduction and oxidation. These two half-reactions always occur together, as electrons lost by one substance must be gained by another. Oxidation is defined as the loss of electrons by a chemical species, causing its oxidation state to increase. Conversely, reduction is the gain of electrons, resulting in a decrease in the oxidation state. A simple way to remember this distinction is the mnemonic “OIL RIG” (“Oxidation Is Loss, Reduction Is Gain”).
When a substance is oxidized, it acts as a reducing agent because it supplies electrons to facilitate the reduction of another substance. In the same way, the substance undergoing reduction is considered the oxidizing agent, as it accepts electrons. This flow of electrons dictates the chemical outcome and the electrical properties of the system.
The Cathode’s Role: Reduction
Reduction always occurs at the cathode. The cathode is the electrode where chemical species dissolved in the solution migrate to accept electrons, causing them to be reduced. This process holds true across all electrochemical cells, regardless of whether the cell is generating power or consuming it. To help remember this constant relationship, chemists often use the mnemonic “Red Cat” (Reduction at Cathode), alongside “An Ox” which confirms that oxidation occurs at the anode.
The function of the cathode is to supply electrons directly to the chemical species in the electrolyte solution. For instance, in a common zinc-copper cell, copper ions migrate toward the cathode and gain two electrons to become solid copper metal. This movement of electrons toward the cathode from the external circuit drives the reduction half-reaction. The fundamental definition of the cathode is based solely on the chemical process occurring there, which is the gain of electrons by a reactant.
How Cell Type Affects Cathode Charge
While reduction consistently takes place at the cathode, the electrical charge assigned to the cathode can change depending on the type of electrochemical cell. This is often a source of confusion for those new to the topic, as the sign convention is directly tied to the cell’s energy flow. Electrochemical cells are broadly categorized into two types: Galvanic (or Voltaic) cells and Electrolytic cells.
In a Galvanic cell, such as a standard battery, the redox reaction is spontaneous and naturally produces electrical energy. Here, the cathode is designated as the positive electrode. The positive sign indicates that the cathode has a lower potential energy than the anode, which encourages electrons to spontaneously flow toward it from the anode through the external circuit.
The situation is reversed in an Electrolytic cell, where an external power source must be used to force a non-spontaneous chemical reaction to occur, such as in electroplating or recharging a battery. The external source forces electrons onto the cathode, making it the negative electrode. Despite the negative sign, the chemical process remains the same: the cathode is still the site where a species gains electrons, confirming that the chemical definition of the cathode as the site of reduction is constant.