Half-reactions basic equations

Sei f-Test 12.2B When iodide ions react with iodate ions in basic aqueous solution, triiodide ions, I,, are formed. Write the net ionic equation for the reaction. (Note that the same product is obtained in each half-reaction.)... 

Balance each of the following skeletal equations by using oxidation and reduction half-reactions. All the reactions take place in basic solution. Identify the oxidizing agent and reducing agent in each reaction. 

Iron(Il) hydrogen phosphite, FeHPO, is oxidized by hypochlorite ions in basic solution. The products are chloride ion, phosphate ion, and iron(lll) hydroxide. Write the balanced equation for each half-reaction and the overall equation for the reaction. 

The basic idea of this method is to split a complicated equation into two parts called half-reactions. These simpler parts are then balanced separately, and recombined to produce a balanced overall equation. The splitting is done so that one of the half-reactions deals only with the oxidation portion of the redox process, whereas the other deals only with the reduction portion. What ties the two halves together is the fact that the total electrons lost by the oxidation process MUST equal the total gained by the reduction process (step 6). 

The half-reaction method of balancing equations can be more complicated for reactions that take place under acidic or basic conditions. The overall approach, however, is the same. You need to balance the two half-reactions, find the LCM of the numbers of electrons, and then multiply by coefficients to equate the number of electrons lost and gained. Finally, add the halfreactions and simplify to give a balanced net ionic equation for the reaction. The ten steps listed above show this process in more detail. 

The Sample Problem on the next page illustrates the use of these steps for an acidic solution. To balance a net ionic equation for basic conditions by the half-reaction method, balance each half-reaction for acidic conditions, adjust for basic conditions, and then combine the half-reactions to obtain the balanced net ionic equation. The following Concept Organizer summarizes how to use the half-reaction method in both acidic and basic conditions. 

The following redox reactions occur in basic solution. Use the half-reaction method to balance the equations. 

According to reaction 15.47, for which the Nernst equation gives Eh = +0.401 V at pH 14.0, the oxidation of water is clearly favored by alkaline conditions (see Exercise 15.3). At the same time, however, many oxidation half-reactions also have lower Eh values in basic media. For example, Eh for the manganate/manganese dioxide couple... 

PROBLEM 4.20 Balance the following equation by the half-reaction method. The reaction takes place in basic solution. 

The general procedure is to balance the equations for the half-reactions separately and then to add them to obtain the overall balanced equation. The half-reaction method for balancing oxidation-reduction equations differs slightly depending on whether the reaction takes place in acidic or basic solution. 

Oxidation-reduction reactions can occur in basic as well as in acidic solutions. The half-reaction method for balancing equations is slightly different in such cases. 

Balancing Oxidation-Reduction Equations Occurring in Basic Solution by the Half-Reaction Method... 

Equation (4-7) represents an add-base half-reaction, which involves protons, analogous to an oxidation-reduction half-reaction (Chapter 15), which involves electrons. Protons, even less than electrons, do not exist in a free state to an appreciable extent. Therefore an add dissociates to yield protons only when a base is available to accept them that is, two conjugate pairs are necessary for an acid-base reaction. Several conjugate add-base pairs, arranged in order of decreasing acidity of HA and therefore increasing basicity of A , are listed in Table 4-1. 

Use the half-reaction method to balance these equations. Add water molecules and hydrogen ions (in acid solutions) or hydroxide ions (in basic solutions) as needed. Keep balanced equations in net ionic form. 

Frequently we need more oxygen or hydrogen to complete the mass balance for a reaction or half-reaction in aqueous solution. We must be careful, however, not to introduce other changes in oxidation number or to use species that could not actually be present in the solution. We cannot add H2 or O2 to equations because these species are not present in aqueous solutions. Acidic solutions do not contain significant concentrations of OH ions. Basic solutions do not contain significant concentrations of H+ ions. 

We are given the formulas for two reactants and two products we write as much of the equations as possible. The reaction occurs in basic solution we can add OH and H2O as needed. We construct and balance the appropriate half-reactions, equahze the electron transfer, add the half-reactions, and eliminate common terms. 

Add the two half-reactions together, canceling out species that appear on both sides of the reaction. Since the reaction occurs in basic solution and there are 4 H ions on the right side, 4OH need to be added to both sides. Combine the H and OH where appropriate to make water molecules to write the final balanced equation ... 

Summary. It is pointed out that, in order to avoid misconceptions, the introduction of ions is very important ions have been dealt with as basic particles of matter according to Dalton s atomic model (see Chap. 5). In order to understand the charges of ions and the change of ions and atoms by electron transfer, the differentiated atomic model with nucleus and electron shells should be introduced. With the assistance of a clear terminology, it is easy to formulate half-reaction for the oxidation and reduction steps, the number of electrons to be transferred can be clearly recognized. Finally, if mental models -for instance, from involved atoms or ions in Galvanic cells or in batteries - are relayed and drawn by the students themselves, then they could more easily see through the redox processes or even perhaps be able to repeat them independently. In all explanations, one should pay attention that the observations should be done at the substance level, but that the interpretations and discussions of reaction equations should consequently take place at the level of the smallest particles as atoms, ions and molecules. 

The Half-Reaction Method for Balancing Equations for Oxidation-Reduction Reactions Occurring in Basic Solution... 

In using the half-reaction method, we usually begin with a skeleton ionic equation showing only the substances undergoing oxidation and reduction. In such cases, we usually do not need to assign oxidation numbers unless we are unsure whether the reaction involves oxidation-reduction. We will find that H (for acidic solutions), OH (for basic solutions), and H2O are often involved as reaaants or products in redox reactions. Unless, ... 

Equation 9 can be simplified to two half-reaction, a 2-electron reduction of SbF (reaction 11) with the F ions being absorbed subsequently by other SbF5 molecules, and a 2-electron oxidation of the C—H bond (reaction 12). In the absence of any other base, attacks another alkane molecule to form RH2 and hydrogen is evolved. If a more basic molecule like acetone is present, then the proton adds to this base and no further reaction occurs. 

If a redox reaction occurs in basic solution, the equation must be balanced by using OH and H2O rather than and H2O. Because the water molecule and the hydroxide ion both contain hydrogen, this approach can take more moving back and forth from one side of the equation to the other to arrive at the appropriate half-reaction. An alternate approach is to first balance the half-reactions as if they occurred in acidic solution, count the number of in each half-reaction, and then add the same number of OH to each side of the half-reaction. This way, the reaction is mass-balanced because you are adding the same thing to both sides. In essence, what you are doing is neutralizing the protons... 

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