Nonspontaneous half-reactions

We must narrow the options. There will be only one cathode reaction and only one anode reaction. How do we pick the correct half-reactions If one of the half-reactions were spontaneous (positive), we would pick it for that electrode. (If more than were spontaneous, we would pick the largest positive value.) All four half-reactions in this case are nonspontaneous (negative). This is typical for electrolysis, because you are using electrical energy to force a nonspontaneous process to take place. 

Electrochemistry is the area of chemistry concerned with the interconversion of chemical and electrical energy. Chemical energy is converted to electrical energy in a galvanic cell, a device in which a spontaneous redox reaction is used to produce an electric current. Electrical energy is converted to chemical energy in an electrolytic cell, a cell in which an electric current drives a nonspontaneous reaction. It s convenient to separate cell reactions into half-reactions because oxidation and reduction occur at separate electrodes. The electrode at which oxidation occurs is called the anode, and the electrode at which reduction occurs is called the cathode. 

Because the reaction in an electrolytic cell is nonspontaneous, a certain amount of voltage must be applied across the circuit to cause the reaction to occur. The minimum voltage required to force the reaction is determined using Equation 18.2. For example, if an electrolytic cell contains molten MgCl2, we can determine the minimum emf for the cell. To begin, we need the half-reactions and reduction potentials of each substance ... 

Add the reduction and oxidation half-reactions, and add the reduction and oxidation potentials. will be positive for the resulting overall cell reaction. This indicates that the reaction as written is product-favored (spontaneous). A negative Eggu value would indicate that the reaction is reactant-favored (nonspontaneous). 

The Example Problems showed you how to use the data from Table 20.1 to calculate the standard potential (voltage) of voltaic cells. Another important use of standard reduction potentials is to determine if a proposed reaction under standard conditions will be spontaneous. How can standard reduction potentials indicate spontaneity Electrons in a voltaic cell always flow from the half-cell with the lower standard reduction potential to the half-cell with the higher reduction potential, giving a positive cell voltage. To predict whether any proposed redox reaction will occur spontaneously, simply write the process in the form of half-reactions and look up the reduction potential of each. Use the values to calculate the potential of a voltaic cell operating with these two half-cell reactions. If the calculated potential is positive, the reaction is spontaneous. If the value is negative, the reaction is not spontaneous. However, the reverse of a nonspontaneous reaction will occur because it will have a positive cell voltage, which means that the reverse reaction is spontaneous. 

Oxidation is the loss of electrons and results in a higher oxidation number reduction is the gain of electrons and results in a lower oxidation number. 21.2 No, one half-reaction cannot take place independently because there is a transfer of electrons from one substance to another. If one substance loses electrons, another substance must gain them. 21.3 Spontaneous reactions, ACjys < 0, take place in voltaic cells (also called galvanic cells). Nonspontaneous reactions, AC ys > 0, take place in electrolytic cells. 21.5(a) Cl (b)Mn04" (c) Mn04 (d) Cl... 

The stronger oxidizing agent is the one involved in the half-reaction with the more positive standard electrode potentiaL so Fe is the stronger oxidizing agent. The reaction is nonspontaneous as written. 

Everything is simply the reverse of the correct result. At this point, you realize your mistake, because the cell emf is found to be negative. A negative emf merely indicates that the cell reaction is nonspontaneous as written. To obtain the spontaneous reaction and a positive emf, you simply reverse both half-reactions and corresponding half-cell potentials. (You would also reverse the cell notation.) This changes the sign of the emf. 

The electrolysis of water, shown in Figure 3.4, leads to the cell reaction in which water is broken down into its elements, H2 and O2. Recall that hydrogen gas and oxygen gas combine spontaneously to form water and are used to power fuel cells, which produce electricity. Therefore, the reverse process (electrolysis of water) is nonspontaneous and requires electrical energy. The two half-reactions occur at the anode and cathode. 

The Electrolysis of Aqueous Sodium Chloride and Overvoltage An additional complication that we must consider when predicting the products of electrolysis is overvoltage—an additional voltage that must be applied in order to get some nonspontaneous reactions to occur. We can demonstrate this concept by considering the electrolysis of a sodium chloride solution, shown in Figure 18.25 t. In order to predict the product of the electrolysis, we consider the two possible oxidation half-reactions ... 

Faraday, -SS galvanic cell half-equation Nernst equation nonspontaneous reaction... 

In the section on voltaic cells, we saw that the anode lost mass over time (as the metals were oxidized and went into solution), while the cathode gained mass over time (as the cations were reduced and plated on the surface). The voltaic cell, however, requires spontaneous reactions in each half-cell, which limits the types of electrodes that can be used. In an electrolytic cell, because we are adding electric current to the cathode and the anode, we can force nonspontaneous reactions to occur. In some cases, this allows us to use electrolysis for purposes other than separating a molten compound or aqueous solution. One of the more common alternate uses is the purification of different metals. 

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