Dichromate ions reactions

A chromatizing treatment consists of immersing the object to be treated into an electrol3de that contains chromate or dichromate ions. Reaction (12.5) shows a simphfied stoichiometry for the reaction of a bivalent metal with chromates taking place under these conditions. 

A voltaic cell consists of two half-cells. One of the half-cells contains a platinum electrode surrounded by chromium(III) and dichromate ions. The other half-cell contains a platinum electrode surrounded by bromate ions and liquid bromine. Assume that the cell reaction, which produces a positive voltage, involves both chromium(III) and bromate ions. The cell is at 25°C. Information for the bromate reduction half reaction is as follows ... 

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... 

C19-0083. Dichromate ions, C r2 0-j, oxidize acetaldehyde, CH3 CHO, to acetic acid, CH3 CO2 H, and are reduced to Cr . The reaction takes place in acidic solution. Balance the redox reaction and determine how many moles of electrons are required to oxidize 1.00 g of acetaldehyde. What mass of sodium dichromate would be required to deliver this many electrons ... 

In the case of high concentrations of chromate and low concentrations of arsenous acid, we must take into account that dichromate ions are also involved in reaction. Thus... 

Figure 4 is a good example of satellite structure associated with a surface species. The chromium 2p /2,l/2 spectrum results from the reaction of the dichromate ion,, with galena to yield both... 

The reaction of alcohol with orange dichromate ions to produce blue-green chromium(III) ions is used in the Breathalyzer test, a test that measures the presence of alcohol in a person s breath. How could a colorimeter be used in this analysis ... 

In the Breathalyzer test, the subject blows into a tube connected to a vial. The exhaled air collects in the vial, which already contains a mixture of sulfuric acid, potassium dichromate, water, and the catalyst silver nitrate. The alcohol reacts with the dichromate ion in the following redox reaction. 

This reaction is accompanied by a visible colour change, as orange dichromate ions become green chromium(lll) ions. The concentration of alcohol in the blood is determined by measuring the intensity of the final colour. 

Probably the most detailed study of the acid(HA)- and base(B)-catalyzed reactions of complex ions concerns the hydrolysis of the dichromate ion ... 

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). 

In the reactions of each of the oxalato complexes with cerium(IV) it is clear that no dichromate is formed until all the oxalate has been consumed. This observation is reasonable, for while the cerium(IV)-Cr(OH2)6+3 reaction is moderately rapid and while dichromate ion reacts sluggishly with even free oxalic acid, chromium(IV), a probable intermediate in the cerium(IV)-chromium(III) reaction, might be expected to behave very much like cerium(IV) toward the oxalato complexes. 

Although coordination compounds have sometimes been used as titrants, e.g. ferrocyanide ions for the titration of zinc(II), and permanganate or dichromate ions in redox titrations, probably the only outstanding reaction in classical titrimetry that involves the reversible formation of a coordination compound is the Liebig-Deniges reaction. 

Fig. 3. Chemistry of dichromated poly(vinyl alcohol) resist. Initially the dichromate ion absorbs light the light-activated Cr6+ species undergoes an electron-transfer reaction with the PVA matrix to form a polymer radical. These undergo further reactions to form Cr3+ and a cross-linked matrix (13).

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... 

Oxidation-Reduction in Blood Analysis Demonstrating the Reaction in a Breathalyzer," J. Chem. Educ., Vol. 67,1990, 263. The oxidation of a primary alcohol by the orange dichromate ion is shown to first form an aldehyde, then a carboxylic acid, and green chromium(III) ion. The use of this reaction, principles of spectrometry, and gas laws in a commercial device for measuring blood-alcohol content are discussed. 

A DIE of k 2o/k )2o = 3.2 has been found in the hydrolysis of a-benzoyloxystyrenes, 382, in concentrated perchloric acid solution450. The rate of hydrolysis was found to be linear with the acidity function, Ho- A small inverse deuterium KIE, A h /I d = 0.78, at the -carbon has been found in the oxidation of cinnamic and crotonic acids451 by quinolinium dichromate. The reaction involves electrophilic attack on the double bond and a carbonium-ion intermediate (p = —4.0). A small inverse D KIE (k /k ->) = 0.80, p = —4.0] has been also observed452 in the acid cleavage of substituted styrenes by quinolinium dichromate in DMF in the presence of acid452. 

An aliquot of sample is refluxed in a strong acid solution of a known excess of potassium dichromate (K2Cr207) in the presence of Ag2S04 and HgS04. Cr6 (dichromate ion) is reduced to Cr3 during oxidation. The net ionic reaction in the acid medium is as follows ... 

The mechanism starts with the formation of HCr04 ions, that is, Cr(VI), from dichromate ion in solution. In acid, these form chromate esters with alcohols. The esters (boxed in black) decompose by elimination of the Cr(IV) HCrO, which subsequently reacts with a Cr(VI) species to yield 2 x Cr(V). These Cr(V) species can oxidize alcohols in the same way, and are thereby reduced to Cr(III) (the final metal-containing by-product). Cr(VI) is orange and Cr(III) is green, so the progress of the reaction is easy to follow by colour change. 

Example 29 Glycerol CH2(OH)—CH(OH)—CH2(OH) or simply C3H803 can slowly be oxidized by dichromate ions in hot acid solution. Express the equation of the reaction. 

The acidified solution turns to orange because of the formation of dichromate ions in reaction (a). 

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. 

Balance the equation for the reaction of dichromate ion with tin(ll) ion to produce chromium(lll) ion, Sn02(s), and other products. 

Practice Problem 16.14 Complete and balance the equation for the reaction of methyl alcohol (CH3OH) with dichromate ion in acid solution... 

The reactions of alkenes with chromate or dichromate ions usually leads to an array of products arising from oxidative attack at the double bond and the allylic positions. Only in special cases where the double bond bears one or more phenyl" or alkoxy" substituents have good yields of the corresponding carbonyl compounds been reported. 

This reaction involves the reduction of the dichromate ion by ammonium ion. Chromic oxide is also made by heating sodium dichro-mate with sulfur, and leaching out the sodium sulfate with water ... 

As previously mentioned, aluminum oxide is used as our primary collector, but other media have been used for special situations. For example, when the Hanford reactors were operating, quantities of hexa-valent Cr were discharged to the Columbia River and subsequently to the ocean, and we were interested in studying the dispersion rate of the Columbia River plume. Alumina did not collect the dichromate ion eflBciently. By using alumina saturated with stannous chloride, the chromium was reduced oa contact to the trivalent state, and this was very eflBciently retained on the bed. We also found that by saturating alumina with barium sulfate, we could collect radium isotopes, presumably by a replacement reaction with the barium on the matrix. For... 

By using the half-cell potentials in Appendix E, calculate the equilibrium constant at 25°C for the reaction in problem 33. Dichromate ion (Cr207 ) is orange, and Cr is light green in aqueous solution. If 2.00 L of 1.00 M HClOy solution is added to 2.00 L of 0.50 M Cr(N03)3 solution, what color will the resulting solution have ... 

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