Final answer:
The decomposition of mercury(II) oxide is a chemically spontaneous process above 500°C as observed by the color change from red to silver. To determine the spontaneity at standard conditions, thermodynamic calculations involving the free energy change are needed. However, due to toxicity, mercury compounds are unsuitable as thruster fuels for space vehicles.
Step-by-step explanation:
The decomposition of red mercury(II) oxide (HgO) under standard state conditions is a reaction that can be analyzed by using thermodynamics to determine if it is spontaneous. Part (a) of your question requires considering the free energy change (ΔG°) under standard conditions.
A spontaneous reaction occurs when ΔG° is negative. For part (b), one must use the Gibbs free energy equation ΔG° = ΔH° - TΔS° and determine the temperature at which the sign of ΔG° becomes negative.
Mercury(II) oxide decomposes into liquid mercury and oxygen gas when heated. To evaluate if the reaction is spontaneous above a certain temperature, we would refer to standard free energies of formation and calculate the ΔG°.
Mercury(II) oxide is noted to decompose spontaneously above 500°C, with the observable red-to-silver color change indicating the reaction's progress.
In the context of space vehicle control thrusters, an ideal fuel requires spontaneity and exothermicity upon catalysis. While mercury compounds commonly decompose upon heating, their toxicity makes them less ideal for such applications. Care must be taken in their synthesis and application due to the hazardous nature of mercury compounds.