Question

which of the following is the entropy change positive?
A. Thermal dissociation of ammonium chloride
B. Reaction between an acid and a base
C. Addition of concentrated acid to water
D. Dissolution of sodium metal in water?
please give the answer and explain.

Answer 1

Entropy change positive means endothermic, only one of these is endothermic, and it is a solid dissociating to a gas: positive entropy.

Answer 2

For this problem, thermal decomposition of the ammonium chloride would require an input of energy (endothermic) to dissociate the physical bond between HCl and Ammonia as an increase in disorder occurs with yield of more particles of product than the number of particles of reaction. However, saying that entropy means endothermic can be misleading in that there are thermochemical reactions that are exothermic with increasing entropy values. In general, combustion in presence of O2 is exothermic but is accompanied with an increase in disorder, positive entropy. Now, if this refers to requirements for achieving 'Activation Energy' for generating spontaneity, then yes, all reactions begin with an input of energy, but the net change in energy (enthalpy) may be exothermic with a positive entropy of reaction. :-)