  Based on the linear mixing approach, we calculate the latent heat for
crystallizing fully-ionized $^{12}$C/$^{16}$O and $^{16}$O/$^{20}$Ne mixtures
in white dwarf (WD) cores for two different parametrizations of the corrections
to the linear-mixing energies and with account of ion quantum effects. We
report noticeable composition-dependent deviations of the excess entropy in
both directions from the standard value of 0.77 per ion. Within the same
framework, we evaluate the excess entropy and released or absorbed heat
accompanying the exsolution process in solidified WD layers. The inclusion of
this effect is shown to be important for reliable interpretation of WD cooling
data. We also analyze the latent heat of crystallizing eutectic
$^{12}$C/$^{22}$Ne mixture, where we find a qualitative dependence of both the
phase diagram and the latent heat behaviour on ion quantum effects. This may be
important for the model with $^{22}$Ne distillation in cooling C/O/$^{22}$Ne WD
proposed as a solution for the ultramassive WD multi-Gyr cooling anomaly.
Astrophysical implications of our findings for crystallizing WD are discussed.
