Question

a. The ΔHºsoln (standard state) of KCl is +17.2 kJ/mole (Gas = 1 atm pressure, pure gas, Liquid = 1 mole pure liquid, Solid = 1 mole pure compounds, Solutes = 1 M dissolved solute, 25º C (298 K). What is ΔSºsurr ? If the molar solubility of KCl is 4.2 M, what does this tell you about the value and sign of ΔSºsys ?

b. Almost all the the salts with negative Sº_soln values share a common property; they all have at least one polyvalent ion, e.g SO₄^2- , CO₃^2- or PO₄^3- for anions, and Cu²⁺, Mg²⁺ or Fe³⁺ for cations. Suggest how the addition of these ions to a solution can cause the order to increase (i.e., number of available microstates to decrease)? And if Sº is negative, doesn't that mean there is an increase in order, which threatens the very fabric of the universe?

Answer 1

Solutions 1: entropy of system is decreases and entropy of surrounding increases. Thus the entropy of universe increases.

Entropy change of surrounding=-(-17.20)/298=0.057kj/mole/k

Sign of entropy change of system is -ve as entropy of system decreases.

Part 2 when there is addition of any ions there is more randomness and hence more collision due to which temperature is raised and thus entropy of surrounding and then entropy change of universe increases.

Again if entropy standard is -ve it's mean entropy of system is decreases and hence decrease in order was of the system