Question

1. Thermodynamics and acid solutions a) The following reaction occurs when HCl is bubbled into water at 25°C and latm pressure: HCl(g) -> H+(aq) + Cl (aq) Calculate AS rxn for this reaction. b) Sketch two possible structures of the hydronium ion in water. Circle the hydronium ion in each case. Also sketch the local structures of water surrounding the hydronium and chloride ions in aqueous solution. Include the important water molecules in both sets of sketches. c) Draw a thermodynamic cycle that could be used to calculate the changes in state functions for the reaction in (a). Use the terms dissociation, ionization and (de-)solvation to label appropriate steps in the cycle. Indicate whether each step you identified will occur spontaneously or not by writing an inequality sign for AG for that step.

Answer 1

Reaction of HCl (g) bubbled through water can be written as the following:

(i)

HCI (g) → H(aq) + Cl" (aq)

Change in entropy of the this reaction is given by:

∆S=Sproduct-S reactant

∆S= SH++ SCl--SHCl

AS = SH+ + Sci- - Suci

The value of corresponding entropies are given in the question table.

∆S=0+57-187

∆S= -137 J/mol. K

(ii)

Two possible structures of hydronium ions in water:

a) H7O3 : H+(H20)3

IO

b) H9O4+ : H+(H2O)4

O - I-

Structure of Chloride ion in water

*Η

(iii) The thermodynamic cycle of HCl(g) bubbled through water is given by:

Solvation HCl (g) – HCI (1) Dissociation Ionisation ht(g) + Cl (g) Solvation → H+ aq) + Cl" (aq)

ΔG for:

Dissociation unfavourable ΔG > 0

Solvation favourable, ΔG < 0

Ionisation favourable, ΔG < 0