The species which by definition has zero standard molar enthalpy of fo...
Standard molal enthalpy of formation is zero when an element is in its standard state ..hence its chlorine;
The species which by definition has zero standard molar enthalpy of fo...
The Standard Molar Enthalpy of Formation
The standard molar enthalpy of formation (ΔHf°) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. It is measured at a temperature of 298 K and a pressure of 1 bar.
Zero Standard Molar Enthalpy of Formation
A species that has a zero standard molar enthalpy of formation at 298 K means that no energy is released or absorbed when one mole of the compound is formed from its elements in their standard states.
Options:
Let's examine each option to determine which one has a zero standard molar enthalpy of formation at 298 K.
a) Br2(g): The standard molar enthalpy of formation of Br2(g) is -30.91 kJ/mol. This means that energy is released when one mole of Br2(g) is formed from its elements. Therefore, this option is not correct.
b) Cl2(g): The standard molar enthalpy of formation of Cl2(g) is 0 kJ/mol. This means that no energy is released or absorbed when one mole of Cl2(g) is formed from its elements. Therefore, this option is correct.
c) H2O(g): The standard molar enthalpy of formation of H2O(g) is -241.83 kJ/mol. This means that energy is released when one mole of H2O(g) is formed from its elements. Therefore, this option is not correct.
d) CH4(g): The standard molar enthalpy of formation of CH4(g) is -74.87 kJ/mol. This means that energy is released when one mole of CH4(g) is formed from its elements. Therefore, this option is not correct.
Therefore, the correct answer is option 'B' which is Cl2(g). No energy is released or absorbed when one mole of Cl2(g) is formed from its elements in their standard states at 298 K.