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.
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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. ___________
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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) ______________________________________________
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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. ______________________________________________________________
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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.
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Suggest one way aluminum hydroxide acts as a fire retardent. __________
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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. _______________________
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Suggest one reason why the quantity of calcium hydroxide must be very
carefully controlled, apart from preventing the aluminum hydroxide
redissolving. _________________________________________________________
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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. ____________________________________________________
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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. _________________________________
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