Half-reactions balancing

At the left electrode in Figure 19-5 the halfreaction occurring is Cl- —>- Cl4.g) + e, and at the right electrode the half-reaction is Na+ + er — - Naff). Which electrode is the anode and which is the cathode With these half-reactions, balance the net reaction occurring in the electrolysis cell. 

Balancing the chemical equation for a redox reaction by inspection can be a real challenge, especially for one taking place in aqueous solution, when water may participate and we must include HzO and either H+ or OH. In such cases, it is easier to simplify the equation by separating it into its reduction and oxidation half-reactions, balance the half-reactions separately, and then add them together to obtain the balanced equation for the overall reaction. When adding the equations for half-reactions, we match the number of electrons released by oxidation with the number used in reduction, because electrons are neither created nor destroyed in chemical reactions. The procedure is outlined in Toolbox 12.1 and illustrated in Examples 12.1 and 12.2. 

Divide the reaction into half-reactions, balance each using the stepwise procedure, combine the half-reactions, and then clean up the result to eliminate duplicated species. 

For each half-reaction, balance the number of atoms that are oxidized or reduced. 

Using half-reactions, balance the equation for the redox reaction when Cr207 (fl ) and Fe " (fl ) react to form Cr " (a ) and Fe " (fl ) in acidic solution. 

Cu + 2HNO3 Cu(N03)2 + 2H+ + 2e-This is the balanced oxidation half-reaction. Balance the reduction half-reaction. 

Oxidation half-reaction Balancing order a. elements (except H,0) b. oxygen (use H2O) c. hydrogen (use H ) Reduction half-reaction... 

Then to obtain balanced half-reactions, balance the atoms other than hydrogen and oxygen by multiplying the reactants or products by appropriate integers,... 

The reduction half-reaction balanced with respect to oxygen is... 

A few redox reactions have more than one oxidation half-reaction or more than one reduction half-reaction. Balancing the equations for these reactions is more complicated. However, the multiple half-reactions are often stoichiometricaUy hnked. Maintain the correct ratio of the elements and balance the electron transfer by multiplying them both by the same integer. Balance the following net ionic equation for a reaction in basic solution. OH" or HjO (but not H" ") may be added as necessary. 

Step 4. For each of the half-reactions, balance the electric charge by adding electrons to the right side of the oxidation half-reaction and to the left side of the reduction half-reaction. 

For each half-reaction, balance the elements other than H and 0. If necessary, balance 0 by adding H2O, and H by adding H+. 

Now, consider the electrolysis process shown in Figure 2.3b. In the water at the negatively charged cathode, a reduction reaction takes place, with electrons (e ) from the cathode being given to hydrogen cations to form hydrogen gas (the half-reaction balanced with acid) ... 

Separating a redox reaction into its half-reactions is useful if you need to balance the reaction. One method for balancing redox reactions is reviewed in Appendix 4. 

Balancing a redox reaction is often more challenging than balancing other types of reactions, because we must balance both electrons and elements. Perhaps the simplest way to balance a redox reaction is by the half-reaction method, which consists of the following steps. 

Balance all atoms in each half-reaction except for O and H. 

Balance the oxygen in each half-reaction by adding appropriate amounts of H2O. 

For acidic solutions, balance the hydrogen in each half-reaction by adding H3O+ and H2O to opposite sides of the reaction for basic solutions, add OH and H2O to opposite sides of the reaction. 

Balance the net charge in each half-reaction by adding electrons the electrons should be a reactant for the reduction half-reaction and a product for the oxidation half-reaction. 

The atoms in each half-reaction are then balanced. The zinc half-reaction is already balanced in this respect, so we begin by balancing chromium in the dichromate half-reaction. 

Both half-reactions are balanced with respect to charge by adding electrons as needed. 

Adding together the two half-reactions and simplifying gives us the final balanced redox reaction. 

When these half-reactions are summed, there is no net reaction. Thus the material balance of the cell is not altered by overcharge. At open circuit, equation 19 at the negative electrode is the sum of a two-step process, represented by equation 15 and... 

Oxidation-reduction reactions must be balanced if correct predictions are to be made. Just as in selecting a route for a trip from San Francisco to New York, there are several ways to reach the desired goal. Which route is best depends to some extent upon the likes and dislikes of the traveler. We will discuss two ways to balance oxidation-reduction reactions—first, using half-reactions and, next, using the oxidation numbers we have just introduced. 

Use of Half-Reactions for Balancing Oxidation-Reduction Reactions... 

For lithium metal in 1 Af HC1, the observed facts are that the metal dissolves spontaneously and a gas bubbles out of the solution. From Appendix 3 we select the two half-reactions (notice that the half-reactions are already balanced in both charge and number of atoms) ... 

Just as before, the first step in balancing a reaction must be to decide the products. Again, experiment provides the answer. Let us reconsider one of the same examples we balanced previously by the half-reaction method. For these we already know the products. 

Of course, the oxidation number method gives the same balanced equation as the half-reaction method. 

By use of half-reactions, give a balanced equation for each of the following reactions ... 

Since the dichromate ion on the left side of the equation has been reduced to chromic ion, Cr+ on the right side, the conversion of methanol to formaldehyde must involve oxidation. To show more clearly that methanol has been oxidized, let us balance this reaction by the method of half-reactions. We have encountered the halfreaction involving dichromate and chromic ions before (Problem 20b in Chapter 12). It is... 

Balance the half-reaction for the conversion of formaldehyde, HCHO, to formic acid, HCOOH. 

Balance the half-reaction involved in the oxidation of ethanol to acetic acid. Compare the number of electrons released per mole of ethanol with the number per mole of methanol in the equivalent reaction (73c). How many electrons would be released per mole of propanol in the oxidation to propionic acid ... 

Bacteria, 434 Balancing reactions, 42 by half-reactions, 218 by oxidation number, 219 oxidation-reduction reactions, 217, 219... 

H+], calculation of, 192, see also Hydrogen ion Haber, Fritz, 151 Haber process, 140, 150 Hafnium, oxidation number, 414 Haldane, J. B. S., 436 Half-cell potentials effect of concentration, 213 measuring, 210 standard, 210 table of, 211, 452 Half-cell reactions, 201 Half-life, 416 Half-reaction, 201 balancing, 218 potentials, 452 Halides... 

Oxidation-reduction reactions, 202 balancing with half-reactions, 217 balancing with oxidation numbers, 219 Oxides basic, 382... 

When balancing redox equations, we consider the gain of electrons (reduction) separately from the loss of electrons (oxidation), express each of these processes as a halfreaction, and then balance both atoms and charge in each of the two half-reactions. When we combine the halfreactions, the number of electrons released in the oxidation must equal the number used in the reduction. 

The general procedure for balancing the chemical equation for a redox reaction is first to balance the half-reactions separately. 

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