Which substance has [tex]$\Delta H f$[/tex] defined as [tex]$0 kJ / mol$[/tex] ?
[tex]$H _2 O$[/tex] (s)
Ne (i)
[tex]$F _2(g)$[/tex]
[tex]$CO _2(g)$[/tex]
Answer (1)
Standard enthalpy of formation ( Δ H f ) is 0 kJ/mol for an element in its standard state.
Examine each substance to determine if it's an element in its standard state.
F 2 ( g ) is fluorine in its standard state (diatomic gas).
Therefore, the substance with Δ H f defined as 0 kJ/mol is F 2 ( g ) .
Explanation
Problem Analysis The question asks us to identify the substance from the given list that has a standard enthalpy of formation ( Δ H f ) defined as 0 kJ/mol.
Key Concept Recall that the standard enthalpy of formation of an element in its standard state is defined as 0 kJ/mol. This means we need to find an element in its most stable form under standard conditions (298 K and 1 atm).
Substance Examination Let's examine each substance:
H 2 O ( s ) : This is water in its solid state (ice). Water is a compound, not an element, so its standard enthalpy of formation is not necessarily 0 kJ/mol.
N e ( i ) : This is neon in its liquid state. Neon is an element, but its standard state is a gas, N e ( g ) , not a liquid.
F 2 ( g ) : This is fluorine in its gaseous state. Fluorine is an element, and its standard state is a diatomic gas, F 2 ( g ) .
C O 2 ( g ) : This is carbon dioxide in its gaseous state. Carbon dioxide is a compound, not an element, so its standard enthalpy of formation is not necessarily 0 kJ/mol.
Conclusion Since the standard enthalpy of formation of an element in its standard state is 0 kJ/mol, and F 2 ( g ) is fluorine in its standard state (diatomic gas), the substance with Δ H f defined as 0 kJ/mol is F 2 ( g ) .
Examples
Understanding standard enthalpies of formation is crucial in chemical engineering for designing and optimizing chemical reactions. For instance, when designing a process to produce ammonia ( N H 3 ) from nitrogen and hydrogen, knowing the standard enthalpies of formation of reactants and products allows engineers to calculate the overall enthalpy change of the reaction. This helps in determining the energy requirements for the process, such as heating or cooling, and ensures the process is both energetically feasible and economically viable. This concept is also used in material science to predict the stability of different materials under various conditions.
