Hydrochloric acid reaction with chromium

Chromium corrodes in hydrochloric acid ofpH= 1 with activity ofCr of 10 M. The values of the cathodic (6J and anodic b Tafel slopes are —0.1 and 0.12 V per decade, respectively. The exchange current densities for the anodic and cathodic reactions are... 

Write balanced molecular and net ionic equations for the reactions of (a) manganese with dilute sulfuric acid, (b) chromium with hydrohromic acid, (c) tin with hydrochloric acid, (d) aluminum with formic acid, HCOOH. 

Pyridinium chiorochromate (PCC) is a popular reagent for the selective oxidation of primary alcohols to aldehydes and secondary alcohols to ketones. PCC is prepared commercially by the reaction of pyridine with hydrochloric acid in the presence of chromium trioxide (CrOg). The chromium atom in both CrOg and also in PCC is in the 6-I- oxidation state (orange color). 

The early research of RFB systems was mainly carried out in the United States and Japan. NASA built the first 1 kW true RFB system with an Fe/Cr redox couple in the 1970s [11]. In this system, an aqueous solution of ferric-ferrous is employed as the positive reactant redox couple, and the negative reactant is a solution of chromos-chromic couple, with hydrochloric acid as a supporting electrolyte in most cases. Because of the poor kinetics of the chromium redox reaction, a serious deterioration of RFBs was observed after a long period of time moreover, a relatively low open circuit potential was also obtained. These drawbacks limited its practical application. In the following years, several RFB systems were evaluated, but none of them was developed on a commercial scale until the bromine/polysulphide RFB and vanadium system was invented [4]. In this section, in addition to these two systems, we will also introduce new progress in tme RFB systems. 

In order to produce the aldehyde as the final product, it is necessary to use a more selective oxidizing reagent, one that will react with the alcohol but will not react with the aldehyde. Many such reagents are available, including pyridinium chlorochromate (PCC). PCC is formed from the reaction between pyridine, chromium trioxide, and hydrochloric acid. 

An autoclave (Note 1) is charged with 200 g. (1.28 moles) of 2,3-dimethylnaphthalene (Note 2), 940 g. (3.14 moles, 23% excess) of sodium dichromate dihydrate, and 1.8 1. of water. The autoclave is closed, heated to 250°, and shaken continuously at this temperature for 18 hours. The autoclave is cooled with continued agitation (Note 3), the pressure is released, and the autoclave is opened. The contents are transferred to a large vessel (Note 4). To effect complete transfer, the autoclave is rinsed with several 500-ml. portions of hot water. Green hydrated chromium oxide in the reaction mixture is separated on a large Buchner funnel and washed with warm water until the filtrate is colorless. The combined filtrates (7-8 1.) are acidified with 1.3 1. of 6N hydrochloric acid. The acidified mixture is allowed to... 

It has been claimed that chromium trioxide reacts with tetra-n-butylammonium chloride in water to produce tetra-n-butylammonium chromate, n-Bu4N+HCr04- [7], whereas benzyltriethylammonium dichromate is obtained from the closely analogous reaction of benzyltriethylammonium chloride with chromium trioxide in dilute hydrochloric acid [8]. 

Preparation of Chromium(II) Chloride. (Perform one of the following two experiments.) 1. Put several pieces of metallic chromium into a test tube, pour in 2-3 ml of a 20% hydrochloric acid solution, and insert a stopper with a gas-discharge tube. Connect to the latter a rubber tube with a slot closed by a glass rod (a Bunsen valve. Fig. 125). What is the role of the slot in the rubber tube Identify the substance in the solution. Write the equation of the reaction. 

Put several pieces of chromium into test tubes and test its reaction with water, alkalies, and also with dilute and with concentrated solutions of hydrochloric, sulphuric, and nitric acids (in the cold and with heating). Write the equations of the reactions. What position does chromium occupy in the electrochemical series of metals ... 

Complexes of 8-hydroxyquinoline and chromium(III) have been known for many years.1139 Complexes have been mercurated and the deuterated quinoline isolated.1139 The bromination of chelated 8-hydroxyquinoline proceeds about thirty-five times faster than that of the free ligand.1140 Tris(8-hydroxyquinolinato)chromium(III) absorbs large amounts of hydrochloric, hydrobromic and hydrofluoric acids. Chemical reaction with the complex was considered a more likely explanation than solid solution or clathrate formation, even though more than one mole of acid was absorbed per mole of complex.1141... 

A yellow coloration of the fused mass indicates chromium. The mass is taken up in the platinum dish with water and acetic acid then added a little at a time until the reaction is acid. In a small portion of the acetic acid solution the presence of chromium may be confirmed by means of lead acetate. The acetic add solution and any undissolved part of the mass are concentrated to a small volume in a porcelain dish over a naked flume and then taken to dryness on a water-bath. The residue is heated for some time in an air-oven at iio° and then treated with water and hydrochloric acid and filtered. The filtrate is tested for tin and copper by means of hydrogen sulphide. 

DMAP-HCr03Cl is obtained by reaction of 4-dimethylaminopyridine with chromium trioxide (1 equiv.) in aqueous hydrochloric acid. It is a yellow-orange, nonhygroscopic solid that is moderately sensitive to light. 

Three grams of tris(biguanidato)chromium(III) 1-hydrate is triturated in a mortar with 3 N hydrochloric acid in the cold (10°) until the mixture gives a slight acid reaction (8 ml. of acid is required). The red 1-hydrate is converted completely into yellow crystals of the hydrochloride which are filtered and washed with 3 to 5 ml. of cold water. The crystals are purified by recrystallizing from 38 ml. of water, filtering, and washing, first with 3 ml. of ice-cold water and then with 5 ml. of alcohol. The mother liquor is treated with a saturated solution of ammonium chloride to complete precipitation, and the second crop of crystals is treated as the first. The total yield is 2.45 g. (0.0053 mol or 66%). 

Very good reactions with yields of 95% and more are obtained only with very pure and absolutely dry chemicals. The aluminum chloride being used must necessarily be freshly sublimed it must be ground and filled in as fast as possible. The benzene used must be free of other aromatic impurities. To avoid already hydrolyzed salt, the chromium (III) chloride is treated with boiling 2 N hydrochloric acid and washed with water, methanol, and ether. The aluminum powder is purified from organic impurities by washing it with benzene. 

A further method consists in treating a solution of chromate udth an excess of methylene white (the leuco base of methylene blue) in presence of hydrochloric acid. The methylene blue produced is then titrated with standard titanous chloride, the temperature being kept above 40° C. in order to sharpen the end-point. The whole process should be performed in an atmosphere of carbon dioxide. In applying this method to ferrochrome or chromium steel, the iron and chromium may be estimated together, the iron then being determined separately by ordinary methods and the chromium obtained by difference. The reaction may also be used colorimetrieally. ... 

Poly(vinylpyridinium) chlorochromate (PVPCC) is obtained as a liright-orange solid by adding chromium trioxide and concentrated hydrochloric acid to an aqueous suspension of cross-linked poly(vinylpyridine) (PVP) resin (50-100 mesh). Oxidations of primary and secondary alcohols to carbonyl compounds are carried out on heating with a slight excess of the resin at 75-80 °C. The reaction is best carried out in cyclohexane [612]. 

Chromyl chloride, Cr202Cl2, a dark-red liquid (mp -96.5 °C, bp 117 °C, d 1.911), is prepared from chromium trioxide or sodium dichromate, hydrochloric acid, and sulfuric acid [665]. The reagent is used in solutions in carbon disulfide, dichloromethane, acetone, tert-butyl alcohol, and pyridine. Oxidations with chromyl chloride are often complicated by side reactions and do not always give satisfactory yields. The mechanism of the oxidation with chromyl chloride, the Etard reaction, is probably of free-radical nature [666]. Complexes of chromyl chloride with the compounds to be oxidized have been isolated [666, 667, 668]. 

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