METALS: SILVER
Silver, which is extremely rare in the Earth's crust (0.000007%), is
usually found in ores either as a sulfide or native. This element is a
typical transition metal, as evinced by its (fairly) high melting point 
(962°C), high density (10.49 g cm-³), variable oxidation states [e.g., 
(colourless) Ag(I) and (coloured) Ag(II)], and catalytic activity 
(e.g., it is used in the oxidation of ethene to ethane-1,2-diol).
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. One of several methods of extracting silver from low-grade ores involves heating silver(I) sulfide in air. Construct a symbol equation for this extractive method.
                                                                    [2]
2.  Silver is highly resistant to corrosion by atmospheric oxygen.
However, silver slowly 'tarnishes' in the presence of either sulfur or
compounds of sulfur; the corrosion product is known as silver tarnish 
(Ag2S). One method of removing this tarnish from a silver object is to 
rub its surface with aluminium (whose oxide layer has been removed by 
suspension in aqueous sodium chloride). Construct the symbol equation 
for this unusual application of a displacement reaction.
                                                                    [2]
3.  Silver's attractive appearance, high resistance to corrosion, and
high electrical conductivity has resulted in its extensive use as an 
electroplating metal. In silver-electroplating, the object to be plated
is made the cathode of an electrolysis cell which contains a silver
anode and an electrolyte of aqueous silver(I) nitrate.
Write an ionic equation for the reaction which occurs at the cathode.
                                                                    [1]
What energy change occurs in the endergonic process of electrolysis?
Electrical energy is transduced into chemical energy.
                                                                    [2]
4.  Silver(I) halides are photosensitive (a property which is exploited
in black-and-white photography and in photochromic lenses); e.g., the
exposure of silver(I) bromide to light energy results in photolysis:
What energy change occurs in the endergonic process of photolysis?  
Light energy is transduced into chemical energy.
                                                                    [2]
5.  White photographic paper, containing silver(I) bromide, darkens on
exposure to light as black metallic silver particles are formed. A 
chemist, who was researching new types of 'photo-paper', investigated
two related hypotheses: 'The speed (S) at which photographic paper "X"
darkens to standard grey colour decreases in linear proportion to the
distance (D) from a light source; i.e., S = k × D + c', and 'The speed
(S) at which photographic paper "X" darkens to standard grey colour
increases in linear proportion to the inverse square of the distance
(D-²) from a light source; i.e., S = k × D-² + c'; the Table shows a
summary of the chosen conditions and raw data.
Constants: photographic paper "X" (36 cm²); 30 W fluorescent light; standard grey colour (Munsell 6); ambient temperature (17°C); distance (D) measured from light bulb side-surface to paper.
Distance (D) / mm
  45 
  58 
  71 
  84 
  97 
  97 
  97 
Distance-² (D-²) / m-²
 494
 297
 198
 142
 107
 107
 107
Time (t) / s
 144 
 185 
 214 
 236 
 232 
 254 
 249 
Speed (S) / ms-¹
  69 
  54 
  47 
  42 
  43 
  39 
  40 


(a) Plot a graph, with distance (D) as the independent variable, and 
then draw a best curve through as many points as is sensible.
                                                                    [3]
Construct a precisely worded conclusion for distance (D) as the 
independent variable.  The speed (S) at which 36 cm² of photographic 
paper "X" darkened to standard grey colour (Munsell 6) at ambient 
temperature (17°C), using a 30 W fluorescent light, decreased as its 
distance (D) from the light source decreased (within the range 
45 to 97 mm). The curve indicates that these two variables are not in 
linear proportion to each other; i.e., S ¹ k × D + c.
                                                                    [4]
(b) Calculate the values for the inverse-square of the distance (D-²),
and insert these data in the Table.
                                                                    [3]
Plot a (second) graph, with the inverse-square of distance (D-²) as the
independent variable, and then draw a best straight line through as 
many points as is sensible.
                                                                    [3]
Determine the gradient of this second graph; this value, 'k', is the
proportionality constant in the linearly proportional relationship
S = k × D-² + c.
      (y2 - y1)      (62 - 42)     20
k  =  ————————  =  ——————————— = ————— = 0.077 ms-¹ m²
      (x2 - x1)     (400 - 140)    260
                                                                    [2]
Construct a precisely worded conclusion for the inverse-square of 
distance (D-²) as the independent variable.  The speed (S) at which 
36 cm² of photographic paper "X" darkened to standard grey colour 
(Munsell 6) at ambient temperature (17°C), using a 30 W fluorescent 
light, increased as the inverse-square of the distance (D-²) from the 
light source increased (within the range 100 to 500 m-²). The straight 
line indicates that these two variables are in linear proportion to 
each other; i.e., S = k × D-² + c, where k = 0.077 ms-¹ m².
                                                                    [5]

Dr. R. Peters Next Contents' List & Teacher's Notes