Question

All elements have a ΔHₐ of ______ kJ/mole at standard temperature and pressure.

a) O
b) 1
c) 4.184
d) 273

Answer 1

The standard enthalpy of formation (ΔHₐ) for any element in its most stable form is defined as 0 kJ/mole at standard temperature and pressure.

Step-by-step explanation:

All elements have a ΔHₐ of 0 kJ/mole at standard temperature and pressure. The correct answer is b) 1, which means the standard enthalpy of formation of any element in its most stable form is zero by definition. It is important to recognize that the enthalpy change of a reaction, often represented as ΔH, depends on the physical states of the reactants and products, which must be shown in the thermochemical equation. For instance, the reaction involving hydrogen gas and oxygen gas to form liquid water releases 286 kJ/mol of heat:

H₂(g) + ½ O₂(g) → H₂O(l), ΔH = -286 kJ/mol

If instead gaseous water forms, the heat released is less, with only 242 kJ/mol of heat being released.