Concerete is produced from a mixture of cement, water, sand and small stones. It consists primarily of calcium silicates and calcium aluminates formed by heating and grinding of clay and limestone. In later steps of cement production a small amount of gypsum, CaSO 4 .2H 2 O is added to improve subsequent hardening of concrete. The use of elevated temperatures during the final production may lead to formation of unwanted hemihydrate, CaSO 4 .1/2H 2 O. Consider the following reaction :
CaSO 4 .2H 2 O(s) CaSO 4 .H 2 O(s) + H 2 O (g)
The following thermodynamic data apply at 27ºC, standard pressure : 1 bar
R = 8.314 JK –1 mol –1
(i) Δ Hº for the formation of 1.45 kg of CaSO 4 . 
H 2 O(s) from CaSO 4 .2H 2 O(s) is
Text Solution
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(i)d Δ Hº = Δ H f º of 
+ Δ H f º of 
– Δ H f º of CaSO 4 .2H 2 O(s) = +83 kJ mol –1
(ii)c Δ Sº = 119.5 + (188 × 1.5) – 194 = 207.5 J/K mol
Δ Gº = Δ Hº – T Δ Sº = 19550 J mol –1
Δ Gº = –2.303 RT log K p = – 2.303 R × 300 log 
log 
= 
= – 2.26
= 4 × 10 –3 atm
(iii)a 
= 1, K p = 1
Δ Gº = 0 at eqm.
Δ Hº – T Δ Sº ⇒ T = 
= 
= 400 K = 127ºC
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