The Species with Zero Standard Molar Enthalpy of Formation at 298 K

To determine the species that has zero standard molar enthalpy of formation at 298 K, we need to understand the concept of standard molar enthalpy of formation and its relation to the given options.

Standard Molar Enthalpy of Formation
The standard molar enthalpy of formation (ΔHf°) is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements, all in their standard states, at a specified temperature and pressure (usually 298 K and 1 atm).

The standard molar enthalpy of formation is defined as zero for elements in their standard states because elements in their standard states are considered to have no energy associated with their formation. Therefore, the formation of elements in their standard states does not require any energy input or release.

Analysis of Options
Let's analyze each option to determine the species with zero standard molar enthalpy of formation at 298 K:

a) Br2(g)
b) Cl2(g)
c) H2O(g)
d) CH4(g)

Br2(g)
Bromine gas (Br2) is a diatomic molecule. Since it is an element in its standard state, the standard molar enthalpy of formation of Br2 at 298 K is zero.

Cl2(g)
Chlorine gas (Cl2) is also a diatomic molecule and is an element in its standard state. Therefore, the standard molar enthalpy of formation of Cl2 at 298 K is zero.

H2O(g)
Water gas (H2O) is a compound and not an element. The standard molar enthalpy of formation of H2O at 298 K is not zero because energy is required for the formation of water from its constituent elements, hydrogen and oxygen.

CH4(g)
Methane gas (CH4) is also a compound. The standard molar enthalpy of formation of CH4 at 298 K is not zero because energy is required for the formation of methane from its constituent elements, carbon and hydrogen.

Conclusion
From the analysis above, we can conclude that the species with zero standard molar enthalpy of formation at 298 K is Cl2(g). This is because chlorine gas is an element in its standard state, and the formation of chlorine gas from its constituent elements does not require any energy input or release.