Thermochemistry

Step 1: Standard Enthalpy of Formation.
The standard enthalpy of formation of an element in its most stable form at 1 bar and a specified temperature is defined as zero. This means elements in their standard states have a standard enthalpy of formation of zero.

Step 2: Analyzing the options.
- (1) S (Monoclinic): Sulfur in its monoclinic form is its most stable allotrope at 1 bar, so its enthalpy of formation is zero. - (2) P (White): White phosphorus is not its most stable allotrope at standard conditions, so its enthalpy of formation is nonzero. - (3) Br (liquid): Bromine is a liquid at 1 bar and standard temperature, and it is its most stable form, so its enthalpy of formation is zero. - (4) O (g): Oxygen gas is the most stable form of oxygen at standard conditions, and its enthalpy of formation is zero.

Final Answer: