13. a) The enthalpy of solution for KBr in water is 19.9 kJ/mol. Draw a reaction coordinate
diagram (vertical axis should be H) for the dissolution process, breaking it down into its
individual components.
b) The entropy of solution for KBr in water is 89.0 J/KCmol. Use words (not numbers) to
describe the components that make up this value.
H 2O
Equation for the dissolution of KBr in H2O:
a) In this context H (and
therefore ΔH) relates to bond
breaking and formation. The
forces holding the KBr(s)
together are ionic bonds,
which are usually quite
strong. The forces holding
the H2O(l) are Hydrogen
Bonds, which are usually a
good bit weaker than ionic
bonds (hence, the smaller
arrow). The bonds that form
are the dipole-ionic
interactions associated with
the hydration spheres.
Because ΔH = +19.9 kJ/mol,
the product plateau must be
19.9 kJ/mol higher than the
reactant plateau.
KBr(s) → K+(aq) + Br-1(aq)
b) Entropy relates to disorder. Larger (+) values indicate more disorder. The value +89.0 J'KCmol
indicates that the entropy in the products is greater than that in the reactants. Although the water
molecules are ordered around the ions when the dipole interactions form, the positive value of ÄS
indicates that the products are still considerably less ordered that the KBr (particularly highly ordered if a
crystalline solid) and the liquid water (which is relatively ordered for a liquid).