May 2011 - Career Point

(1) C 2 H 6 (g) + 27O2 (g) → 2CO 2 (g) + 3H 2 O(1) :∆ C H = 3∆ f H(H 2 O, 1) + 2∆ f H(CO 2 , g) – ∆ f H(C 2 H 6 , g)Thus,∆ f H(C 2 H 6 ,g) = – ∆ C H + 3∆ f H(H 2 O, 1)+ 2∆ f H(CO 2 , g)= (1556.5 – 3 × 284.5 – 2 × 393.3) kJ mol –1= – 83.6 kJ mol –1(2) C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O(1)∆ C H = 3∆ f H(CO 2 , g)+ 4∆ f H(H 2 O), 1) – ∆ f H(C 3 H 8 , g)Thus∆ f H(C 3 H 8 , g) = –∆ C H + 3∆ f H(CO 2 , g) + 4∆ f H(H 2 O, 1)= (2217.5 – 3 × 393.5 – 4 × 284.5) kJ mol –1= –101.0 kJ mol –1To calculate the ε C–H and ε C–C , we carry out thefollowing manipulations.(i) 2C(graphite) + 3H 2 (g) → C 2 H 6 (g)∆H = – 83.6 kJ mol –12C(g) → 2C (graphite)∆H = –2 × 719.7 kJ mol –16H(g) → 3H 2 (g)∆H = –3 × 435.1 kJ mol –1Add2C(g) + 6H(g) → C 2 H 6 (g)∆H (i) = (–83.6 – 2 × 719.7 – 3 × 435.1) kJ mol –1= – 2828.3 kJ mol –1(ii) 3C(graphite) + 4H 2 (g) → C 3 H 8 (g)∆H = –101.0 kJ mol –13C(g) → 3C (graphite)∆H = –3 × 719.7 kJ mol –18H(g) → 4H 2 (g)∆H = – 4 × 435.1 kJ mol –1Add3C(g) + 8H(g) → C 3 H 8 (g)∆H (ii) = (– 101 – 3 × 719.7 – 4 × 435.1) kJ mol –1= – 4000.5 kJ mol –1Now,∆H (i) = ε C–C – 6ε C–H= –2828.3 kJ mol –1∆H (ii) = –2ε C–C – 8ε C–H= –4000.5 kJ mol –1Solving for ε C–C and ε C–H , we getε C–H = 414.0 kJ mol –1and ε C–H = 344.3 kJ mol –1GalenaGalena is the natural mineral form of lead sulfide. Itis the most important lead ore mineral.Galena is one of the most abundant and widelydistributed sulfide minerals. It crystallizes in thecubic crystal system often showing octahedralforms. It is often associated with the mineralssphalerite, calcite and fluorite.Galena deposits often contain significant amountsof silver as included silver sulfide mineral phases oras limited solid solution within the galena structure.These argentiferous galenas have long been themost important ore of silver in mining. In additionzinc, cadmium, antimony, arsenic and bismuth alsooccur in variable amounts in lead ores. Seleniumsubstitutes for sulfur in the structure constituting asolid solution series. The lead telluride mineralaltaite has the same crystal structure as galena.Within the weathering or oxidation zone galenaalters to anglesite (lead sulfate) or cerussite (leadcarbonate). Galena exposed to acid mine drainagecan be oxidized to anglesite by naturally occurringbacteria and archaea, in a process similar tobioleaching [3]Galena uses :One of the earliest uses of galena was as kohl,which in Ancient Egypt, was applied around theeyes to reduce the glare of the desert sun and torepel flies, which were a potential source ofdisease.[4]Galena is a semiconductor with a small bandgap ofabout 0.4 eV which found use in early wirelesscommunication systems. For example, it was usedas the crystal in crystal radio sets, in which it wasused as a point-contact diode to detect the radiosignals. The galena crystal was used with a safetypin or similar sharp wire, which was known as a"cat's whisker". Making such wireless sets was apopular home hobby in the North of England duringthe 1930s. Derbyshire was one of the main areaswhere Galena was mined. Scientists that werelinked to this application are Karl Ferdinand Braunand Sir Jagdish Bose. In modern wirelesscommunication systems, galena detectors have beenreplaced by more reliable semiconductor devices,though silicon point-contact microwave detectorsstill exist in the market.XtraEdge for IIT-JEE 37 MAY 2011