Question

Predict the sign of delta S for the following reactions/processes, and explain your prediction.
a. Caffeine subliming
b. Mg (s) + 2 HCl (aq) → MgCl 2 (aq) + H 2 (g)
c. C (graphite) → C (diamond)
d. Pb(NO 3 ) 2 (aq) + K 2 CrO 4 (aq) → PbCrO 4 (s) + 2 KNO 3 (aq

Answer 1

disorder of physical state : Gases > liquid > solids.

a. Caffeine subliming , sublimation means

    Solid ---> gas   DS = +ve , (disorder increases)
b. Mg (s) + 2 HCl (aq) → MgCl 2 (aq) + H 2 (g)

   H2 gas is liberated. so that Ds = +ve
c. C (graphite) → C (diamond)

     DSrxn = S0diamond - S0graphite

                = 2.439 - (5.694) = -3.255

DSrxn = -ve
d. Pb(NO 3 ) 2 (aq) + K 2 CrO 4 (aq) → PbCrO 4 (s) + 2 KNO 3 (aq)

         DS = -ve , because here aqueous solution converts into solid, no of ions in solution decreass.