Question

Since, the enthalpy of elements in their standard states is taken to be zero, the heat of formation $\left( \vartriangle {{\text{H}}_{\text{f}}} \right)$ of compounds:
A. is always negative
B. is always positive
C. is zero
D. may be positive or negative

Answer 1

The standard enthalpy of formation for any element in its standard state is zero. It means that elements in their standard state are not formed, they just are. A substance at any temperature is said to be in its standard state when its activity (thermodynamic corrected pressure or temperature) is equal to 1.

Complete answer:
When an element can exist in more than one allotropic form under $\text{2}{{\text{5}}^{\text{o}}}\text{C}$, the form most stable at 1 atm pressure and $\text{2}{{\text{5}}^{\text{o}}}\text{C}$is chosen to have zero enthalpy.
Let us discuss the definition of enthalpy of formation:
Enthalpy of formation: It is defined as the enthalpy change $\left( \vartriangle \text{H} \right)$ accompanying the formation of one mole of compound from its elements. Like the enthalpy of formation of $\text{C}{{\text{O}}_{2}}$ is -393.5 KJ for the reaction $\text{C}\left( \text{s} \right)+{{\text{O}}_{2}}\left( \text{g} \right)\to \text{C}{{\text{O}}_{2}}\left( \text{g} \right)$.
Negative enthalpy of formation represents that the formation of a compound is exothermic when the amount of energy it takes to break bonds is less than the amount of energy released while making the bonds. Example is enthalpy of formation of $\text{C}{{\text{O}}_{2}}$ is -393.5 KJ.
A positive enthalpy of formation represents that the formation of a compound is endothermic when the amount of energy it takes to break bonds is greater than the amount of energy released while making the bonds. Example is enthalpy of formation of $\text{HI}$ is +25.9 KJ.
The correct answer to this question is option ‘d’ which is the heat of formation $\left( \vartriangle {{\text{H}}_{\text{f}}} \right)$ of compounds may be positive or negative.

Additional Information: Like enthalpy of formation there is standard enthalpy of formation also, it is the enthalpy change accompanying the formation of one mole of a compound from its elements, when all substances involved in the reaction are each at unit activity or their standard states. Represented by $\vartriangle \text{H}_{\text{f}}^{\text{o}}$ and the conditions are 1 atm and ${{25}^{\text{o}}}\text{C}$.

Note: The enthalpy of elements is assumed to be zero as it is difficult to define absolute enthalpies of elements. Do not think that standard enthalpy will be zero, at every state, it is just taken to be zero at standard state or ${{25}^{\text{o}}}\text{C}$. Not at every temperature, the enthalpy of elements is zero.