Acidic solutions oxidation-reduction reactions

The group RuNO can occur in both anionic and cationic octahedral complexes in which it is remarkably stable, being able to persist through a variety of substitution and oxidation-reduction reactions. Ruthenium solutions or compounds that have at any time been treated with nitric acid can be suspected of containing nitric oxide bound to the metal. The presence of NO may be detected by infrared absorption ca. 1930-1845 cm-1. 

Reduction. Quinoline may be reduced rather selectively, depending on the reaction conditions. Raney nickel at 70—100°C and 6—7 MPa (60—70 atm) results in a 70% yield of 1,2,3,4-tetrahydroquinoline (32). Temperatures of 210—270°C produce only a slightly lower yield of decahydroquinoline [2051-28-7]. Catalytic reduction with platinum oxide in strongly acidic solution at ambient temperature and moderate pressure also gives a 70% yield of 5,6,7,8-tetrahydroquinoline [10500-57-9] (33). Further reduction of this material with sodium—ethanol produces 90% of /ra/ j -decahydroquinoline [767-92-0] (34). Reductions of the quinoline heterocycHc ring accompanied by alkylation have been reported (35). Yields vary widely sodium borohydride—acetic acid gives 17% of l,2,3,4-tetrahydro-l-(trifluoromethyl)quinoline [57928-03-7] and 79% of 1,2,3,4-tetrahydro-l-isopropylquinoline [21863-25-2]. This latter compound is obtained in the presence of acetone the use of cyanoborohydride reduces the pyridine ring without alkylation. 

NH2OH can exist as 2 configurational isomers (cis and trans) and in numerous intermediate gauche conformations as shown in Fig. 11.7. In the crystalline form, H bonding appears to favour packing in the trans conformation. The N-O distance is 147 pm consistent with its formulation as a single bond. Above room temperature the compound decomposes (sometimes explosively) by internal oxidation-reduction reactions into a complex mixture of N2, NH3, N2O and H2O. Aqueous solutions are much more stable, particularly acid solutions in which the compound... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Oxidation-reduction reactions involving perchlorates have been mentioned in several of the preceding sections and the reactivity of aqueous solutions is similar to that of aqueous solutions of perchloric acid. 

Many oxidation-reduction reactions (nicknamed redox reactions) take place in aqueous solution. One of these was mentioned in Section 11-2.1 when we characterized acids ... 

In titrimetric analysis (often termed volumetric analysis in certain books), the substance to be determined is allowed to react with an appropriate reagent added as a standard solution, and the volume of solution needed for complete reaction is determined. The common types of reaction which are used in titrimetry are (a) neutralisation (acid-base) reactions (b) complex-forming reactions (c) precipitation reactions (d) oxidation-reduction reactions. 

In an electrochemical cell, electrical work is obtained from an oxidation-reduction reaction. For example, consider the process that occurs during the discharge of the lead storage battery (cell). Figure 9.3 shows a schematic drawing of this cell. One of the electrodes (anode)q is Pb metal and the other (cathode) is Pb02 coated on a conducting metal (Pb is usually used). The two electrodes are immersed in an aqueous sulfuric acid solution. 

Practically in every general chemistry textbook, one can find a table presenting the Standard (Reduction) Potentials in aqueous solution at 25 °C, sometimes in two parts, indicating the reaction condition acidic solution and basic solution. In most cases, there is another table titled Standard Chemical Thermodynamic Properties (or Selected Thermodynamic Values). The former table is referred to in a chapter devoted to Electrochemistry (or Oxidation - Reduction Reactions), while a reference to the latter one can be found in a chapter dealing with Chemical Thermodynamics (or Chemical Equilibria). It is seldom indicated that the two types of tables contain redundant information since the standard potential values of a cell reaction ( n) can be calculated from the standard molar free (Gibbs) energy change (AG" for the same reaction with a simple relationship... 

Consequently, reduction of cobalt(III) ammines in basic solution is not favorable. A variety of reducing agents has been used to effect reaction (11). The fortunate coincidences that cobalt(III) complexes are substitution inert while cobalt(II) systems are labile and that cobalt(II) is resistant to oxidation or further reduction in acid solution offer many advantages in the study of redox processes. Not surprisingly, work with cobalt(III) complexes forms the basis for much of the present understanding of oxidation-reduction reactions. 

As we have covered, titration is used to determine the concentration of an unknown substance in acid-base reactions through the use of a known concentration in a solution. It can also be used in oxidation-reduction reactions for the same analytical... 

Vanadium(II) compounds are usually prepared by reduction of acidic solutions of higher oxidation states. The usual geometry is octahedral and the electronic spectra often show three d-d spin-allowed bands. These spectra, as well as those of tetrahedral and linear complexes, have been reviewed.58 For this cr configuration, a spin-only magnetic moment of 3.87 BM is expected. Slow substitution can be predicted for vanadium(II) complexes but such reactions are not as slow as those for the isoelectronic Crm ions. 

Perhaps even more important is the effect of hydrogen ion concentration on the emf of a half-reaction of a particular species. Consider the permanganate ion as an oxidizing agent in acid solution (as it often is). From the Latimer diagram above we can readily see that the reduction emf is 1.51 V when all species have unit activity. What is not shown is the complete equation ... 

Write balanced ionic half-reaction equations for the reduction of each of the following oxidizing agents in acid solution. 

Hydrazotc acid, HN,. ply.. = 4.72, and most of its covalent compounds (including its heavy metal salts) are explosive. It is formed (1) in 90% yield by reaction of sodium amide with nitrous oxide, (2) by reaction of hydraztntum ion with nitrous acid, (3) by oxidation of hydrazimum salts, (4) by reactio n of hydt azinium hydrate with nitrogen trichloride tin benzene solution). Hvdrazoic acid forms metal azides with the corresponding hydroxides and carbonates. It reacts with HC1 to give ammonium chlonde and nitrogen, with H2SO4 to form hydrazinium acid solfate, with benzene to form aniline, and it enters into a number of oxidation-reduction reactions. 

Quinoline may be reduced rather selectively, depending on the reaction conditions. Catalytic reduction with platinum oxide in strongly acidic solution at ambient temperature and moderate pressure gives a 70% yield of 5,6,7,8-tetrahydroquinoline. Further reduction of this material with sodium-ethanol produces 90% of fratrr-decahydroquinoline. 

One can draw a useful analogy between acid-base and oxidation-reduction reactions. Both involve the transfer of a species from a source, the donor, to a sink, the acceptor. The source and sink nomenclature implies that the tendency of the proton (in the case of acids) or of the electron (for reducing agents) to undergo transfer is proportional to the fall in free energy. From the relation AG° = RTIn Ka. you can see that the acid dissociation constant is a measure of the fall in free energy of the proton when it is transferred from a donor HA to the solvent H2O, which represents the reference (zero) free energy level of the proton in aqueous solution. 

CARCINOGENIC.) In this test use is made of the fact that benzidine, which is unaffected by normal molybdates and by free molybdic acid, is oxidized in acetic acid solution by phosphomolybdic acid or by its insoluble ammonium salt (see reaction 4 above). This reaction is extremely sensitive two coloured products are formed, viz. the blue reduction product of molybdenum compounds ( molybdenum blue ) and the blue oxidation product of benzidine ( benzidine blue ). Moreover, solutions of phosphates which are too dilute to show a visible precipitate with the ammonium molybdate reagent will react with the molybdate reagent and benzidine to give a blue colouration. 

Experimental Observations. The nearest we have come to demonstrating the existence of N—F ions is in the electrochemical oxidation-reduction reactions of HNF2. The oxidation has been carried out in water and in various polar organic solvents under acid conditions (15). The first step of this reaction is formation of the NF2 radical. The NF2 radical undergoes combination processes on the surface of the electrode rather than diffusing into the body of the solution before being involved in further reactions. The combination process on the electrode surface has been used to prepare various NF2 compounds by simultaneously generating other radical species—e.g.,... 

Most aqueous reaction equations can be balanced by trial and error. Oxidation-reduction reactions require a more systematic approach to balancing equations using either an acidic or basic solution. 

Balancing oxidation-reduction reactions depends on whether the solution is acidic or basic. The method for balancing redox reactions in an acidic solution is as follows ... 

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. 

Balance the following oxidation-reduction reactions, which occur in acidic solution, using the half-reaction method. 

Cupric bromide, CiiBr., is a black solid obtained by reaction of copper and bromine or by solution of cupric oxide, CuO, in hydro bromic acid. It is interesting that cupric iodide, Culo, does not exist when a solution containing cupric ion is added to an iodide solution there occurs an oxidation-reduction reaction, with precipitation of (uprous iodide, Cul ... 

Oxidation-Reduction Reactions. The various oxidation states of neptunium in the KF solution were obtained by adding the neptunium, which had been adjusted to the desired valence state in dilute acid, to the KF solution. Observations were made of the eflFect of a number of potentially useful oxidizing and reducing agents upon the oxidation state of the neptunium. The valence states of the neptunium ions, IV, V, and VI, were determined by absorption spectra. During the reduction to the III state, the solution color disappeared, and a purple solid was produced. 

Oxidation-reduction reactions of inorganic species can be described in many different ways. For example, hydrogen exhibits oxidation states of -1, 0, and +1. In acidic aqueous solution, these oxidation states occur in the half-reactions... 

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