Final answer:
The decomposition of red mercury(II) oxide becomes spontaneous at high temperatures, above 500°C, as sufficient thermal energy is needed to initiate the decomposition into mercury and oxygen gas. The correct option is B.
Step-by-step explanation:
When considering the decomposition of red mercury(II) oxide, spontaneity refers to whether the reaction occurs without external input of energy. Per the provided question, the reaction under investigation is:
2HgO(s, red) → 2Hg(l) + O₂(g)
Regarding whether this decomposition is spontaneous under standard state conditions, the reaction is not spontaneous at low temperatures, as indicated by the need to heat the substance above 500°C to initiate decomposition.
Given the thermodynamic principles where spontaneity is influenced by changes in enthalpy (∆H) and entropy (∆S) of a system, as summarized by the Gibbs free energy equation (∆G = ∆H - T∆S), as the temperature increases, the T∆S term becomes more significant.
If the entropy increase upon decomposition is favorable (∆S > 0), the reaction becomes spontaneous at higher temperatures where T∆S outweighs ∆H if it is positive, potentially making ∆G negative, hence spontaneous.
Therefore, the decomposition of red mercury(II) oxide becomes spontaneous only at high temperatures when sufficient thermal energy is provided to overcome the enthalpic and entropic barriers. The specific temperature at which this reaction becomes spontaneous can be determined through thermodynamic calculations involving the standard enthalpy and entropy changes for the reaction.