Answer:
HgS upon roasting with excess of
oxygen forms HgO. Since the \[\Delta {{G}^{o}}\] for the formation of HgO is
positive (see Ellingham diagram 6.6), it is unstable and decomposes of its own
under the reaction conditions to form metallic mercury
\[2HgS\,+3{{O}_{2}}\xrightarrow{\text{Heat}}\,2HgO+2S{{O}_{2}}\]
\[2\,HgO\,\xrightarrow{\text{Heat}}\,\,2Hg+{{O}_{2}}\]
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