METALS: CHROMIUM Chromium, a relatively rare element in the Earth's crust (0.01%), occurs mainly in the ore chromite (impure FeCr2O4). This transition metal, which has a high melting point (1857°C) and a high density (8.89 g cm-³), forms compounds in several oxidation states [e.g., (red) chromium(II) ethanoate, (green) chromium(III) sulfate, (brown-black) chromium(IV) oxide, and (orange) potassium dichromate(VI)]. [.. K > Ca > Na > Mg > Al > Zn > Cr > Fe > Pb > (H) > Cu > Hg > Ag ..]
1. Chromium is produced in two forms: ferrochrome, a carbon-containing alloy used to manufacture stainless and hard 'chromium' steels; and the pure metal, which is used to electroplate steel and to manufacture non-ferrous alloys (e.g., Nichrome). (a) Ferrochrome is formed when a mixture of chromite, coke, and limestone is heated in a furnace. Complete the symbol equation below, so as to summarize the conditions used to obtain this alloy:
 (b) The extraction of chromium from chromite is complex, but the final process involves chemical reduction of (green) chromium(III) oxide with aluminium. Construct the symbol equation for this redox reaction. _____ _______________________________________________________________________  The metal is electrolytically purified using impure chromium as the anode, pure chromium as the cathode, and an aqueous chromium salt as the electrolyte. State the energy change which occurs at the cathode. _______________________________________________________________________  (c) Shown below is a diagram of an electrolytic cell used to chromium- plate a toy-car bumper.
[Q = n × z × F and Q = I × t, where: Q, measured in coulombs (C), is the quantity of electricity; n is the number of moles of substance evolved at the electrode; z is the charge on the ion; F is a constant, with a value of 96500 C mol-¹; I, measured in amps (A), is the current; and t, measured in seconds (s), is the time.]
The mass (m) of the toy-car bumper increased by 0.0793 g when a direct current of 1.0 A flowed in the above circuit for 900 s. Calculate the: Number of moles (n) of chromium deposited at the cathode. _____________ _______________________________________________________________________ Quantity of electricity (Q) which flowed through this circuit. ________ _______________________________________________________________________ Charge (z) on the chromium ion of the electrolyte. ____________________ _______________________________________________________________________  (d) To prevent rusting, iron objects are sometimes plated with chromium rather than galvanized with zinc. Suggest one reason why chromium is used, despite zinc being cheaper and more effective as a sacrificial anode. ________________________________________________________________ 
2. Naturally occurring chromium contains four isotopes: Cr-49 (2.3%), Cr-50 (4.3%), Cr-52 (83.8%), and Cr-53 (9.6%). Calculate the exact relative atomic mass of chromium. _____________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________  3. Suggest what observations should be made, and construct the symbol equation, for the reaction of powdered chromium with each of these reactants [consider chromium(III) to be the preferred oxidation state]. Steam _________________________________________________________________ _______________________________________________________________________  Aqueous copper(II) sulfate ____________________________________________ _______________________________________________________________________ _______________________________________________________________________  4. Chromium(III) oxide, an amphoteric substance, is used as a green pigment in paints and as a catalyst for the polymerization of alkenes. The pure oxide can be obtained by the thermal decomposition of several chromium salts; e.g., as can be vividly demonstrated in the 'volcano' experiment, ammonium dichromate(VI) thermally decomposes as follows:
(a) State briefly the meaning of the term 'amphoteric oxide'. _________ _______________________________________________________________________  (b) Fairly strong heating of anhydrous chromium(III) nitrate results in the formation of a green solid, a brown gas which turns damp pH paper red, and a colourless gas which relights a glowing spill. Construct the symbol equation for the thermal decomposition of chromium(III) nitrate. _______________________________________________________________________  5. Dichromates are used extensively as oxidizing agents in industry and in laboratories; e.g., acidified sodium dichromate(VI) oxidizes ethanol to ethanoic acid. Complete this scheme to show the structural formula of the carboxylic acid obtained by oxidizing hexan-1-ol.
 Hexanoic acid, together with C7, C8, C9, C10, and C12 straight-chain carboxylic acids, is a component of the trail pheromone of at least one species of ant (Lasius fuliginosus). Speculate on the purpose of the 'trail'. ______________________________________________________________  6. Chromium ions are potential pollutants, because the amounts used industrially are large compared to the ambient concentrations tolerated by living organisms. These ions are certainly toxic and may also be carcinogenic: nevertheless, in trace quantities, Cr(III) ions have been shown to be essential in mammals; e.g., together with the hormones insulin and glucagon, they help maintain the homeostatic concentration of glucose in the blood. (a) In which organ is insulin biosynthesized? _________________________ (b) In which organ does insulin stimulate the conversion of soluble glucose to insoluble glycogen? ________________________________________ (c) Name one (non-infectious) disease caused by a deficiency of insulin or chromium(III) ions. ________________________________________________ 
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