Dichromate half-reactions

The atoms in each half-reaction are then balanced. The zinc half-reaction is already balanced in this respect, so we begin by balancing chromium in the dichromate half-reaction. 

To construct the spontaneous cell reaction, combine the two half-reactions, leaving the permanganate half-reaction as a reduction and reversing the dichromate half-reaction. To match numbers of electrons, multiply the manganese half-reaction by 6 and the chromium half-reaction by 5 ... 

Chromium metal can be electroplated from an aqueous solution of potassium dichromate. The reduction half-reaction is... 

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... 

An alternative to the oxidation-number method for balancing redox reactions is the half-reaction method. The key to this method is to realize that the overall reaction can be broken into two parts, or half-reactions. One half-reaction describes the oxidation part of the process, and the other half-reaction describes the reduction part. Each half is balanced separately, and the two halves are then added to obtain the final equation. Let s look at the reaction of aqueous potassium dichromate (K2Cr2C>7) with aqueous NaCl to see how the method works. The reaction occurs in acidic solution according to the unbalanced net ionic equation... 

Potassium dichromate is another common oxidizing reagent employed in redox titrations. The titration is carried out in the presence of 1MHC1 or H2S04. The half-reaction is as follows ... 

Ammonium peroxydisulfate in acidic medium can oxidize Mn2+ to permanganate, Ce3+ to Ce4+ and Cr3+ to CY (dichromate). The half-reaction for hydrogen peroxide in acid medium is as follows ... 

Permanganate and dichromate are among our earliest titrimetric reagentspermanganate was introduced in 1846 by Margueritte, and dichromate independently by Schabus and by Penny in 1850. They were used for the titration of iron(II) produced by reduction with zinc. Both oxidants arc strong, E° values for the half-reactions in acid solution being ... 

Ammonium peroxydisulfate, (NH4)2S20h, is also a powerful oxidizing agent. In acidic solution, it converts chromium(lll) to dichromate, cerium(III) to cerium(IV), and manganese(II) to permanganate. The half-reaction is... 

Chromium plating is applied by electrolysis to objects suspended in a dichromate solution, according to the following (unbalanced) half-reaction ... 

In oxidation-rreduction reactions, we do not have to rely on oxidation numbers but can use the balanced half-reaction. For example, when dichromate is reduced to Cr, the half-reaction is... 

The standard dichromate (Cr207 ) ion/chromium(III) ion half-cell consists of a 1 M Pt/Cr2072-(i M), Cr +(1 M) concentration of each of the two ions in contact with an inert electrode. The balanced half-reaction in acidic solution (1.0 M H ) is... 

The redox reaction between dichromate ion and iodide ion in acid solution is shown in Figure 19.15. Use the half-reaction method to balance the equation for this redox reaction. 

The oxidation state of chromium changes from +6 in Cr207 to -(-3 in Cr ". The oxidation state of carbon changes from -2 in ethyl alcohol to -t 4 in CO2. Therefore dichromate is reduced, and carbon is oxidized. The atomically balanced half-reactions are... 

Potassium dichromate (or potassium dichromate(VI), K2Cr207) is also used as an oxidizing agent in similar reactions to those described for potassium permanganate. In acid solution, it reacts according to the following half-reaction ... 

For example, iron(II) can be determined by titration with dichromate, so combination of the appropriate half-reactions [VII] and [XIII], so as to achieve a charge and mass balance, gives the overall reaction [XVI] ... 

Chromium metal can be electroplated onto a copper electrode from an aqueous solution of H2Cr2C>7, as shown in the Electroplating movie (eChapter 20.9). (a) Write the half-reaction for the reduction of dichromate ion to chromium metal, (b) What current would have to be applied for 15 minutes in order to plate out 0.75 gram of chromium metal from a solution of H2Cr2C>7 ... 

The dichromate ion in acidic solution is a common oxidizing agent for organic compounds. Derive an expression for the potential of an electrode for which the half-reaction is the reduction of Cr20T ions to CH ions in acidic solution. 

In a 5-I. round-bottom flask, fitted with a mechanical stirrer, are placed 680 g. of sodium dichromate, 1500 cc. of water, and 230 g. of -nitrotoluene. Stirring is started, and 1700 g. of concentrated sulfuric acid are allowed to flow in during about thirty minutes. The heat of dilution of the sulfuric acid will cause the nitrotoluene to melt, and rapid oxidation will soon take place. The last half of the sulfuric add must be added gradually, in order to prevent too violent a reaction. Since a small amount of nitrotoluene is volatilized, it is advisable to carry on this work under a hood. 

Preparation of Chromium Potassium Alum. Pour 25 ml of water into a 50-ml beaker and dissolve 2.5 g of potassium dichromate in it. Add a concentrated sulphuric acid solution to the mixture (one-and-a-half the stoichiometric amount). First cool the mixture to room temperature, and then put it into water with ice and add ethanol dropwise from a dropping funnel until the solution acquires a violet colour (sulphur dioxide can also be used as the reducing agent). Keep the temperature below 40 °C (why ). After adding the ethanol, let the solution stand to your next lesson. Write the equation of the reaction. 

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