Table 4.

The standard Gibbs free energy ($\Delta G_f^0$) of formation of probable species containing calcium and sulfide in an aqueous solution.

Species	$\Delta G_f^0$ (kJ/mol)
${\text{Ca}}^{2+}$	−553.6
${\text{S}}^{2-}$	85.8
${\text{H}}^{+}$	0
CaS	−477.4
${\text{H}}_2\text{S}$	−33.6
Thermodynamics in water ($\Delta G_f$ (kJ/mol))
${\text{CaS}}_{(\text{aq})}\to {\text{Ca}}_{(\text{aq})}^{2+}+{\text{S}}_{(\text{aq})}^{2-}$	9.62
${\text{CaS}}_{(\text{s})}+2{\text{H}}^{+}\to {\text{Ca}}_{(\text{aq})}^{2+}+{\text{H}}_2{\text{S}}_{(\text{g})}$	−109.74