Voltaic cells spontaneous redox reactions

In an electrolytic cell, electrical energy from an external source causes nonspontaneous redox reactions. In a voltaic cell spontaneous redox reactions produce electricity. In an electrolytic cell, electrical energy turns to chemical energy in a voltaic cell, the reverse occurs. 

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. 

We have observed that voltaic cells use redox reactions that proceed spontaneously. Any reaction that can occur in a voltaic cell to produce a positive emf must be spontaneous. Consequently, it is possible to decide whether a redox reaction will be spontaneous by using half-cell potentials to calculate the emf associated with it. 

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. 

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

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. 

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. 

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

In a voltaic cell, a spontaneous redox reaction (AG < 0) is separated into an oxidation half-reaction (anode half-cell] and a reduction halfreaction (cathode half-celf). Electrons flow from anode to cathode through an external circuit, releasing electrical energy, and ions flow through a salt bridge to complete the circuit and balance the charge within the celi. 

Writing Spontaneous Redox Reactions Appendix D can be used to write spontaneous redox reactions, which is useful for constructing voltaic cells. 

Up to now, we ve been considering voltaic cells, those that generate electrical energy from a spontaneous redox reaction. The principle of an electrolytic cell is exactly the opposite electrical energy from an external source drives a nonspontaneous reaction. 

The five steps that follow summarize the procedure for calculating the potential of a voltaic cell in which a spontaneous redox reaction occurs. Suppose you must write the equation for and calculate the potential of a cell made up of these half-reactions ... 

We consider voltaic cells, which produce electricity from spontaneous redox reactions. Solid electrodes serve as the... 

The energy released in a spontaneous redox reaction can be used to perform electrical work. This task is accomplished through a voltaic (or galvanic) cell, a device in which the transfer of electrons takes place through an external pathway rather than directly between reactants present in the same reaction vesseL... 

We have observed that voltaic cells use spontaneous redox reactions to produce a positive cell potential. We can use this fact together with half-cell potentials to decide whether a given redox reaction is spontaneous. In doing so, it is useful to make Equation 20.8 more general so that we can see how it pertains to general redox reactions, not just... 

Although any spontaneous redox reaction can serve as the basis for a voltaic cell, making a commercial battery that has specific performance characteristics can require considerable ingenuity. The substances oxidized at the anode and reduced by the cathode determine the voltage, and the usable life of the battery depends on the quantities of these substances packaged in the battery. Usually a barrier analogous to the porous barrier of Figure 20.6 separates the anode and cathode half-cells. 

Voltaic cells are based on spontaneous redox reactions. It is also possible for nonspontaneous redox reactions to occur, however, by using electrical energy to drive them. For example, electricity can be used to decompose molten sodium chloride into its component elements Na and CI2. Such processes driven by an outside source of electrical energy are called electrolysis reactions and take place in electrolytic cells. 

VOLTAIC CELLS We consider voltaic cells, which produce electricity from spontaneous redox reactions. Solid electrodes serve as the surfaces at which oxidation and reduction take place. The electrode where oxidation occurs is the anode, and the electrode where reduction occurs is the cathode. 

This reaction is clearly a spontaneous redox reaction, but simply dipping a zinc rod into a copper(II) sulfate solution will not produce useful electric current. However, when we carry out this reaction in the cell shown in Figure 17.5, an electric current is produced. The cell consists of a piece of zinc immersed in a zinc sulfate solution and connected by a wire through a voltmeter to a piece of copper immersed in cop-per(II) sulfate solution. The two solutions are connected by a salt bridge. Such a cell produces an electric current and a potential of about 1.1 volts when both solutions are 1.0 M in concentration. A cell that produces electric current from a spontaneous chemical reaction is called a voltaic cell. A voltaic cell is also known as a galvanic cell. 

To better understand the relationship between voltaic cells and spontaneous redox reactions, let s look at what happens at the atomic or molecular level. The actual processes involved in the transfer of electrons are quite complex nevertheless, we can learn much by examining these processes in a simplified way. 

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. 

In a voltaic cell, a spontaneous redox reaction is used to produce electrical current. In electrolysis, electrical current is used to drive an otherwise nonspontaneous redox reaction. An electrochemical cell used for electrolysis is an electrolytic cell. We saw that the reaction of hydrogen wifli oxygen to form water is spontaneous and can be used to produce an electrical current in a fuel cell. By providing electrical current, we can cause the reverse reaction to occur, breaking water into hydrogen and oxygen ( Figure 16.16). 

A voltaic cell, sometimes called a galvanic cell, uses a spontaneous redox reaction to produce electrical energy. Examples of voltaic cells are batteries... 

In a voltaic cell, a spontaneous redox reaction produces electrical current. In an electrolytic cell, electrical current drives an otherwise nonspontaneous redox reaction through a process called electrolysis. We have seen that the reaction of hydrogen with... 

A voltaic electrochemical cell separates the reactants of a spontaneous redox reaction into two half-cells that are connected by a wire and a means to exchange ions so that electricity is generated. 

There are two types of electrochemical cells. A galvanic or voltaic cell produces electric current from a spontaneous reaction, whereas an electrolytic cell has an external current source, such as a battery, to drive a nonspontaneous reaction. Reactions in electrochemical cells are redox reactions since they involve a transfer of electrons from a reducing agent to an oxidizing agent. The reduction reaction withdraws electrons from one electrode, the positive electrode or cathode. The oxidation reaction supplies electrons to the other electrode, the negative electrode or anode. Current flows via an external circuit from the anode to the cathode. 

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 ... 

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. 

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