Chromate ions reactions

Self-Test 12.2A An alkaline (basic) solution of hypochlorite ions reacts with solid chromium(III) hydroxide to produce aqueous chromate ions and chloride ions. Write the net ionic equation for the reaction. 

The presence of the denominator term in the rate equation (17) suggests that the equilibrium (18) precedes the oxidation step. Two sequences of reactions are proposed (see below), depending on whether the sulphite radical ion dimerises (20) or attacks further acid chromate ion (21). It should be noted that of the species prevalent in dilute aqueous chromic acid, namely CrOj , Cr207, HCrO and H2Cr04, only the last is regarded as possessing oxidising powers. This fact, noted by Westheimer , is tacitly assumed in all recent discussion of... 

The observation of induced reactions involving chromate almost coincided with the discovery of the phenomenon of chemical induction itself. According to the the role of chromate ions in these reactions, two groups can be distinguished (i) Chromium(VI) plays the role of actor, whose reaction with various inductors listed in Table 1 results in the oxidation of several acceptor ions or molecules. 

Sometimes anodic protection is used, in which case the metal s potential is made more positive. The rate of spontaneous dissolution will strongly decrease, rather than increase, when the metal s passivation potential is attained under these conditions. To make the potential more positive, one must only accelerate a coupled cathodic reaction, which can be done by adding to the solution oxidizing agents readily undergoing cathodic reduction (e.g., chromate ions). The rate of cathodic hydrogen evolution can also be accelerated when minute amounts of platinum metals, which have a stroug catalytic effect, are iucorporated iuto the metaf s surface fayer (Tomashov, 1955). 

HCrO ] has been shown to be the photo-active species in the photochemical reaction of dilute solutions of chromium(vi) oxyacids with alcoholic reducing agents. The association of chromate with Np , Th, and Fe" in perchlorate solution has been studied spectrophotometrically and the respective formation constants, 63.6,4.70, and 1.93, have been determined. The greater stability of the Np complex has been interpreted in terms of the limited donation of 5/-electron density from Np to the d-orbitals of chromium in the chromate ion. ... 

In the above reaction the equilibrium, however, hes far to the left. Therefore the chromium(VI) oxide solution also contains trace amounts of chromate ion,... 

To describe the process of the formation of such structures, it is necessary to write down the equations for a component that may be composed of several species and consider reactions among the species (Fisher and Lasaga, 1981). For example, for the case of diffusion of silver ions into a gel containing chromate ions, there are two species of Ag one is Ag , which diffuses by interdiffusion with Na, and the other is Ag2Cr04 precipitation. The diffusivity of precipitated Ag2Cr04 is negligible. Therefore,... 

Similar mechanisms were postulated for the oxidation of glycols by periodate (32) and Ce(IV) (33, 34), and for the oxidation of glycerol by Ce(IV) (44). In these cases the existence of intermediate complexes was demonstrated. The oxidation of formaldehyde by Ce(IV) was also claimed to involve a pre-equilibrium of a Ce(IV)-formaldehyde complex (51). A similar complex was postulated in the formalde-hyde-Mn04 reaction (49, 87). The oxidation of isopropyl alcohol by chromate ions follows a similar mechanism, and a chromate ester was formed as intermediate (94). 

The effect (upon the overall rate of conversion of dichromate to chromate) of changing the chromate ion concentration was studied. The rate was inversely proportional to the square of the chromate concentration, as well as proportional to the dichromate concentration. Since oxygen and nitrogen dioxide had no effect on the rate, the nitryl ion, N(>2+ was postulated as intermediate. However, the equilibrium constant for the reaction could not be determined because too little N02+ was formed. 

The bromyl ion decomposes spontaneously. Chloryl and iodyl ions were studied through their attack on chloride and bromide ions respectively. By determining the rate of disappearance of dichromate and using an analysis similar to the pyrosulfate-nitrate case, the following equilibrium constants were found for the dichromate-halate reactions which produce halyl and chromate ions (not calculated as BaCrC>4, but as [Cr04 2] in solution) (2). 

None of these things happened when we tried this reaction. What did happen —and it took some puzzling to figure it out—was that it reacted with a nitrate ion as the solvent to give two chromate ions plus nitryl ion, and the nitryl ion made nitryl bromide which decomposes to bromine and N02. Incidentally, we found that... 

In the +6 oxidation state, the most important solution species are the yellow chromate ion (Cr042-) and the orange dichromate ion (Cr2072-). These ions are interconverted by the rapid equilibrium reaction... 

Ag+ preferentially reacts with the analyte to form a soluble salt or complex. During this addition, Ag+ reacts with the analyte only, and not the indicator. But when all the analyte is completely consumed by Ag+ and no more of it is left in the solution, addition of an excess drop of silver nitrate titrant produces an instant change in color because of its reaction with the silver-sensitive indicator. Some of the indicators used in the argentometric titrations are potassium chromate or dichlorofluorescein in chloride analysis and p -dime thy la m i nobe nzalrho da n i nc in cyanide analysis. Silver nitrate reacts with potassium chromate to form red silver chromate at the end point. This is an example of precipitation indicator, where the first excess of silver ion combines with the indicator chromate ion to form a bright red solid. This is also known as Mohr method. 

What is the volume in mL of 0.25 M AgN03 required to precipitate all the chromate ion from 20mL of a solution containing 100 g of Na2Cr04 per liter The reaction is... 

The reaction is reversible. In neutral (or alkaline) solutions the chromate ion is stable, while if acidified, dichromate ions will be predominant. Chromate and dichromate ions are strong oxidizing agents. Their reactions will be dealt with among the reactions of anions. 

After decomposing the excess of hydrogen peroxide by boiling, chromate ions may be identified in the solution by one of its reactions (cf. Section IV.33). 

Identification of chromium after oxidation to chromate. Having carried out the oxidation with one of the methods described under reaction 7, chromate ions may be identified with one of the following tests ... 

Write a balanced equation for the reaction of chromate ion, Cr04—, with stannite ion, SnfOH) —, in basic solution, to give stannate ion, Sn(OH)p—, and chromite ion, CrfOH) -. ... 

In basic solution, hypochlorite ions, CIO, oxidize chromite ions, Cr02, to chromate ions, Cr04, and are reduced to chloride ions. Write the balanced net ionic equation for this reaction. 

Indicators used in precipitation reactions. In the titration of sodium chloride with silver nitrate, a small quantity of K2Cr04 solution is added to serve as indicator. At the end point the chromate ion combines with Ag+ ions to form sparingly soluble AggCrO (brick red colour). Both AgCl and AggCrO are insoluble, but they are... 

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