Redox reactions voltaic cells

Recall that the voltaic cells convert chemical energy to electrical energy as a result of a spontaneous redox reaction. Electrolytic cells do just the opposite they use electrical energy to drive a nonspontaneous reaction. A common example is the electrolysis of water. In this case, an electric current decomposes water into hydrogen and oxygen. 

When Zn metal is placed into a Cu solution, Zn is oxidized and Cu is reduced—electrons are transferred directly from the Zn to the Cu . Suppose we separate the reactants and force the electrons to travel through a wire to get from the Zn to the Cu . The flowing electrons constitute an electrical current and can be used to do electrical work. This process is normally carried out in an electrochemical cell, a device that creates electrical current from a spontaneous redox reaction (or that uses electrical current to drive a nonspontaneous redox reaction). Electrochemical cells that create electrical current from spontaneous reactions are called voltaic cells or galvanic cells. A battery is a voltaic cell that (usually) has been designed for portability. 

Electricity interacts with matter because electrons are part of matter and form the chemical bonds. When electrons are transferred from one molecule to the other we call it a redox reaction. Since electric current is the movement of electrons, micro electric currents then exist in the solution where redox reactions take place. If all these micro-currents were made to flow in one direction we should be able to measure them as one macro electric current. Batteries (which are also called galvanic cells or voltaic piles ) are devices which do exactly this they produce electric current by making redox reactions take place at electrodes, i.e. at the metal solution boundary. The metal can be either the source or the sink for electrons. Thus electric current is made to flow from the metal into the solution or from the solution into the metal. Can one do the reverse Can one induce redox reactions by passing through the solution current from a source The answer is definitely yes . The instrument by which such changes are produced is an electrolytic cell . A simple cell can be constructed from two pieces of dissimilar metals dipping into a solution of some electrolyte in a beaker. The metal pieces are now the electrodes. This book is concerned with chemical reactions produced by electric current or electric current produced by chemical reactions at electrodes. It is concerned with redox reactions in cells. 

A voltaic cell produces electrical energy through spontaneous redox chemical reactions. When zinc metal is placed in a solution of copper sulfate, an electron transfer takes place between the zinc metal and copper ions. The driving force for the reaction is the greater attraction of the copper ions for electrons ... 

In principle at least, any spontaneous redox reaction can serve as a source of energy in a voltaic cell. The cell must be designed in such a way that oxidation occurs at one electrode (anode) with reduction at the other electrode (cathode). The electrons produced at the anode must be transferred to the cathode, where they are consumed. To do this, the electrons move through an external circuit, where they do electrical work. 

The driving force behind the spontaneous reaction in a voltaic cell is measured by the cell voltage, which is an intensive property, independent of the number of electrons passing through the cell. Cell voltage depends on the nature of the redox reaction and the concentrations of the species involved for the moment, we ll concentrate on the first of these factors. 

When a voltaic cell operates, supplying electrical energy, the concentration of reactants decreases and that of the products increases. As time passes, the voltage drops steadily. Eventually it becomes zero, and we say that the cell is dead. At that point, the redox reaction taking place within the cell is at equilibrium, and there is no driving force to produce a voltage. 

The tarnish on silver, Ag2S, can be removed by boiling the silverware in slightly salty water (to improve the water s conductivity) in an aluminum pan. The reaction is an oxidation-reduction reaction that occurs spontaneously, similar to the redox reaction occurring in a voltaic cell. The Ag in Ag2S is reduced back to silver, while the A1 in the pan is oxidized to Al3+. 

Voltaic (chemical) cell — electrical energy is produced by a spontaneous redox reaction. Anode is (-) terminal cathode is (+) terminal. 

Galvanic (voltaic) cells produce electricity by using a redox reaction. Let s take that zinc/copper redox reaction that we studied before (the direct electron transfer one) and make it a galvanic cell by separating the oxidation and reduction half-reactions. 

Galvanic (voltaic) cells produce electricity through the use of a redox reaction. 

A galvanic cell, also called a voltaic cell, is a device that converts chemical energy to electrical energy. The key to this invention is to prevent the reactants in a redox reaction from coming into direct contact with each other. Instead, electrons flow from one reactant to the other through an external circuit, which is a circuit outside the reaction vessel. This flow of electrons through the external circuit is an electric current. 

Electrons created in the oxidation reaction at the anode of a voltaic cell flow along an external circuit to the cathode, where they fuel the reduction reaction taking place there. We use the spontaneous reaction between zinc and copper as an example of a voltaic cell here, but you should realize that many powerful redox reactions power many types of batteries, so they re not limited to reactions between copper and zinc. 

These voltaic cells can t run forever, however. The loss of mass at the zinc anode will eventually exhaust the supply of zinc, and the redox reaction won t be able to continue. This phenomenon is why most batteries run out over time. Rechargeable batteries take advan-tc e of a reverse reaction to resupply the anode, but many redox reactions don t allow for this, so rechargeable batteries must be made of very specific reactants. 

A voltaic cell (also known as a galvanic cell) is a device that allows for the transfer of electrons (in a redox reaction) to be completed in a separate pathway from the reaction mixtures. In a voltaic cell, the two half-reactions are physically separated from each other by placing them into two separate reaction vessels. The electrons are transferred from one vessel to the other by a connecting wire (see Figure 18.1). In voltaic cells, the reactions in each vessel must be spontaneous. In figure 18.1, in the reaction on the left, a zinc strip is placed in a zinc sulfate solution, where zinc from the strip replaces zinc in solution (Zn —> Zn2+ + 2 c ). In the reaction vessel on the left, the zinc strip will lose mass over time. Electrons create an electric potential difference across the wire, which is also known as a voltage. The voltage across the wire will allow electrons to be forced from the zinc strip, across the wire, to the copper strip. However, an electric current cannot be established until the circuit is completed. 

Because redox reactions can make electrons move from one substance to another it is possible to create a setup so that the electrical energy produced in a redox reaction can be channeled to do work. There is a way to harvest the electrons produced by a redox reaction. Today these devices are called batteries. The first device that could do this was called a voltaic cell. In a voltaic cell a redox reaction occurs spontaneously so that the electrons can be used to do work. A typical voltaic cell is shown in Figure 10.1. 

Vaporization Voltaic Cell Process by which a liquid enters the gas phase. A setup that allows a redox reaction to occur spontaneously so that the electrons can be used to do work. 

There is another way in which electrons can be rearranged in a chemical reaction, and that is through a wire. Electrochemistry is redox chemistry wherein the site for oxidation is separated from the site for reduction. Electrochemical setups basically come in two flavors electrolytic and voltaic (also known as galvanic) cells. Voltaic cells are cells that produce electricity, so a battery would be classed as a voltaic cell. The principles that drive voltaic cells are the same that drive all other chemical reactions, except the electrons are exchanged though a wire rather than direct contact. The reactions are redox reactions (which is why they produce an electron current) the reactions obey the laws of thermodynamics and move toward equilibrium (which is why batteries run down) and the reactions have defined rates (which is why some batteries have to be warmed to room temperature before they produce optimum output). 

The completed device shown in Figure 21-Ic is an electrochemical cell. An electrochemical cell is an apparatus that uses a redox reaction to produce electrical energy or uses electrical energy to cause a chemical reaction. A voltaic cell is a type of electrochemical cell that converts chemical energy to electrical energy by a spontaneous redox reaction. Figure 21-2 shows a version of the original voltaic cell as devised by its inventor Alessandro Volta. 

In a voltaic cell, the oxidation and reduction halfreactions of a redox reaction are separated and ions flow through a salt-bridge conductor. 

How can the spontaneous redox reaction of a voltaic cell be reversed (21.3)... 

The basic components of a voltaic cell are a wire, two electrodes and two partially-separated solutions. When the electrodes are placed in their respective solutions and the wire is used to connect them, a spontaneous flow of electrons occurs in the wire from one electrode to the other. The impetus for current flow comes from the difference between the oxidation potentials of the electrodes and the solutions, or between the electrodes themselves or between the two solutions in which the electrodes are immersed. A chemical redox reaction occurs between these separated species such that the oxidation half of the reaction occurs in one solution and the reduction half occurs in the other. The partial separation of the solution can be accomplished by a membrane or a salt bridge, which allows an electrolytic connection but does not allow a general mixing of the two solutions. Within the cell, electrical current moves in the form of free electrons in the wire and as ions in the electrolyte. 

An electrolytic cell is similar to a voltaic cell except the electrochemical reactions involved do not occur spontaneously but require the input of current from an external source. Wires connected to each end of a battery and submerged in a suitable electrolyte can represent an electrochemical cell. As with voltaic cells, the creation and/or removal of ions at the electrodes facilitates the transfer of current into and out of solution. If the electrolytes in solution are redox-inert within the stability field of water (e.g., Na and Cf) and the voltage is over 1.2 volts, the hydrolysis of water may transfer current at the electrodes ... 

Assume that a voltaic cell utilizes the redox reaction 2Al(s) + 3Cu2+(aq)... 

Assume that a voltaic cell, proposed as a method for the purification of uranium, utilizes the redox reaction... 

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