METALS: ALUMINUM & ZINC COMPOUNDS Both aluminum and zinc are used extensively in industry, either as the pure metals or in alloys. By contrast, compounds of these metals have found considerably fewer commercial applications, but some are used; e.g., as rodenticides, in paint pigments, as fire retardents, and in water purification [zinc phosphide, zinc oxide, aluminum hydroxide, and aluminum sulfate, respectively.] Aluminum and zinc form compounds with the metal ions in different oxidation states [Al(III) and Zn(II)]. This difference may partially explain why enzymes containing zinc - but not aluminum - are essential to all biological species; particularly ubiquitous are the carbonic anhydrases and the carboxypeptidases, which are involved in respiration and protein hydrolysis, respectively. Nevertheless, clearly reflecting their near-adjacent positions in the reactivity series, the chemistry of these metals and their compounds show noteworthy similarities - as these schemes perhaps over-emphasize?
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. Aluminum and zinc are both used to test for the presence of nitrate ions. Thus, heating an alkaline solution containing nitrate ions with either powdered aluminum or zinc results in the evolution of an alkaline gas (ammonia); the symbol equations for such reactions are:
Suggest why the nitrate ion is considered to be reduced in this test. _______________________________________________________________________ [1] 2. Aluminum chloride and zinc phosphide are both prepared directly by heating their respective elements. Construct the symbol equation for the synthesis of each of these covalently bonded compounds. ___________ _______________________________________________________________________ _______________________________________________________________________ [2] 3. Zinc phosphide hydrolyzes in water to give the extremely toxic gas phosphine:
A number of aluminum compounds are hydrolyzed in a similar manner to zinc phosphide. Predict the name and formula of the gas or solution produced when each of the following are added separately to water. Aluminum chloride (Al2Cl6) ____________________________________________ Aluminum nitride (AlN) ________________________________________________ Aluminum carbide (Al4C3) ______________________________________________ [6]
4. Zinc oxide is thermochromic; i.e., it changes color with changes in temperature:
Suggest one reason why both processes should be considered as chemical changes. ______________________________________________________________ [1] 5. Flammable polymers, such as poly(ethene) and poly(chloroethene), are used extensively in furnishings; to reduce their potential as fire hazards, flame retardents are often added to these materials. Construct the symbol equation, complete with a qualitative indication of the heat energy change, for the thermal decomposition of aluminum hydroxide. _______________________________________________________________________ [2] Suggest one way aluminum hydroxide acts as a fire retardent. __________ _______________________________________________________________________ [1] 6. As the reaction schemes show, the oxides and hydroxides of zinc and aluminum react with both acids and bases; i.e., they are amphoteric (cf. amphibians). Typical reactions which exemplify the amphoteric character of aluminum oxide are summarised by this pair of equations:
Complete the following symbol equations, which will summarize reactions exemplifying the similarly amphoteric character of zinc oxide:
7. Aluminum hydroxide is used as a coagulating agent in helping to remove suspended solids from domestic water supplies. Thus, the careful addition of calcium hydroxide to tanks of water containing aluminum sulfate results in the formation of aluminum hydroxide - which, as a gelatinous precipitate, is ideal for trapping suspended solids:
This symbol equation reveals two precipitation reactions. Construct a net ionic equation for each of these reactions. _______________________ _______________________________________________________________________ _______________________________________________________________________ [2] Suggest one reason why the quantity of calcium hydroxide must be very carefully controlled, apart from preventing the aluminum hydroxide redissolving. _________________________________________________________ _______________________________________________________________________ [1] In view of a possible connection between aluminum ions and Alzheimer's disease (which causes premature senile dementia), suggest one compound which might be a suitable alternative to aluminum hydroxide as the coagulating agent. ____________________________________________________ [1] 8. Aqueous ammonia is one reagent used to distinguish between aqueous solutions of aluminum and zinc ions. Thus, the dropwise addition of aqueous ammonia to a solution containing aluminum ions results in the formation of a white precipitate, Al(OH)3, which does not redissolve in excess reagent. However, although the dropwise addition of aqueous ammonia to a solution containing zinc ions similarly results in the formation of a white precipitate, Zn(OH)2, this does redissolve in excess reagent to form a solution containing a soluble tetraamminezinc ion, [Zn(H2O)2(NH3)4]2+(aq). Suggest one method of distinguishing between solid samples of aluminum oxide and zinc oxide, apart from their reactions with aqueous ammonia. _________________________________ _______________________________________________________________________ _______________________________________________________________________ [2]
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