Chapter 15, Problem 89A

Interpretation Introduction

Interpretation:

Standard enthalpy of formation of a given compound is to be described.

Concept introduction:

Hess’s law : According to this law, total enthalpy change of a reaction is the sum of all the reactions regardless of the steps or stages of reaction.

Answer to Problem 89A

The standard enthalpy (heat) of formation ( ${\text{ΔH}}_{\text{f}}^{\text{°}}\text{=}\text{-}\text{396kJ}$ ) is defined as the change in enthalpy that accompanies the formation of one mole of the compound in its standard state from its elements in their standard states.

Explanation of Solution

Hess’s law allows to calculate the unknown $\text{ΔH}$ values using known reactions and their experimentally determined $\text{ΔH}$ values. The $\text{ΔH}$ value for such a reaction is called the standard enthalpy (heat) of formation of the compound. The standard enthalpy (heat) of formation ( ${\text{ΔH}}_{\text{f}}^{°}$ ) is defined as the change in enthalpy that accompanies the formation of one mole of the compound in its standard state from its elements in their standard states. A typical standard heat of formation reaction is the formation of one mole of ${\text{SO}}_{\text{3}}^{}$ from its elements

$\text{S(s)+}\frac{\text{3}}{\text{2}}{\text{O}}_{\text{2}}^{}\text{(g)}\to {\text{SO}}_{\text{3}}^{}\text{(g)}$. ${\text{ΔH}}_{\text{f}}^{\text{°}}\text{=}\text{-}\text{396kJ}$

The product of this equation is ${\text{SO}}_{\text{3}}^{}$, a suffocating gas that produces acid rain when mixed with moisture in the atmosphere.

Conclusion

Standard enthalpies of formation are stated based on the following assumption: Elements in their standard state have a ${\text{ΔH}}_{\text{f}}^{°}$ of 0.0 kJ.