Redox And Primary Batteries

- Apr 26, 2018-

Redox and primary batteries:

Inserting the Zn piece into the CuSO4 solution spontaneously proceeds as follows:


At this time, it was observed that the Zn piece dissolved, Cu was continuously precipitated on the Zn piece, and the blue CuSO4 solution became lighter. Loss of electrons in Zn is oxidized and Cu2+ gets electrons reduced. Since the Zn flakes and the CuSO4 solution are in direct contact, the electrons are directly transferred from the surface of the Zn flakes to the Cu2+ in the solution, and the flow of the electrons is disordered and no directional electron flow can be generated. The chemical energy contained in the material cannot be converted into electrical energy and can only be released in the form of heat energy, raising the temperature of the reaction solution.

According to the principle of electricity, if a device can be used to convert the electrons in the redox reaction into an ordered electron flow, that is, not to directly transfer the electrons, the electrons lost by the reducing agent are transferred to the oxidant through the wires. You can get power. Daniel Battery is the device to realize this idea. In the Daniel battery, the left side is a ZnSO4 solution with a Zn piece inserted, and the right side is a CuSO4 solution with a Cu piece inserted. The two solutions are separated by a porous membrane that allows the free passage of ions. When the Zn plate and the Cu plate were connected by a series of wires with galvanometers, the galvanometer's hands deflected to prove that there was current flowing through the wires.

This device that uses an oxidation-reduction reaction to convert chemical energy into electrical energy is called a primary battery, or simply battery.

In the primary battery, it is stipulated that one of the electrons flows out of the negative electrode, and the electron flows into the positive electrode. In the Cu-Zn primary cell, electrons flow from the Zn plate to the Cu plate. One end of the Zn plate is a negative electrode, and one end of the Cu plate is a positive electrode. The Cu2+ in the CuSO4 solution gets electrons from the Cu wafer and is reduced to Cu deposited on the Cu wafer. Zn loses electrons to become Zn2+ into the ZnSO4 solution. On two electrodes

The responses are:

Negative electrode: Zn-2e-Zn2+ (oxidation reaction)

Positive electrode: Cu2++2e—Cu (reduction reaction)

The two electrodes are added together to form a battery reaction: Zn+Cu2+—Zn2++Cu.

Because the zinc (or copper) electrode only constitutes half of the primary cell, each electrode is also called a half cell. The oxidation (or reduction) reaction that occurs on each electrode is called a half cell reaction or a half reaction. Each half-reaction is composed of two different oxidation number materials of the same element, in which the high oxidation number is called an oxidized type, and the low oxidation number is called a reduced type, and they constitute a redox pair, which is abbreviated as an electricity pair. Symbolized as oxidized/reduced. Such as electricity on Zn2+/Zn, Cu2+/Cu.

In theory, any spontaneous redox reaction can be designed as a primary cell. In the redox reaction, the oxidant undergoes a reduction reaction with an electron, and the reducing agent loses electrons to undergo an oxidation reaction. Therefore, when the primary battery is constituted, the pair of oxidants and their reduction products constitutes a positive electrode, and the reducing agent and its oxidation product constitutes a pair of electrons. negative electrode.

In the Cu-Zn primary battery, as the battery reaction progresses, Zn2+ in the ZnSO4 solution continuously increases, so that the solution is positively charged, and the opposite electrode attracts the Zn so that it is difficult to lose electrons and prevent further Zn oxidation. At the same time, the CuSO4 solution deposited on the Cu wafer due to Cu2+ becomes Cu, and the SO42+ in the solution is excessively negatively charged. The same-sex repulsion prevents the electrons from flowing to the Cu wafer, preventing further Cu2+ reduction. Both sides of the solution are charged and will inevitably hinder the battery reaction. However, the porous membrane between the two solutions allows the ions to pass through, and the excess Zn2+ in the left solution diffuses to the right solution; the excess SO42+ in the right solution diffuses to the left, maintaining the neutrality of the solutions on both sides so that the battery reaction continues. Because the speed of Zn2+ and SO42+ diffusion through the contact interface of the two solutions is different, a potential difference, called the junction potential, will occur at the contact interface of the two solutions, and its presence will affect the accurate determination of the electromotive force of the battery. To eliminate the liquid junction potential, salt bridges are often used in place of porous separators.


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