Which substance has [tex]$\Delta H f$[/tex] defined as [tex]$0 kJ / mol$[/tex] ?
[tex]$H _2 O ( s )$[/tex]
Ne (l)
[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.
Neon (Ne) as a liquid is not in its standard state.
Fluorine ( F 2 ) as a gas is in its standard state.
Therefore, F 2 ( g ) has Δ H f = 0 kJ/mol. F 2 ( g )
Explanation
Understanding the Problem The question asks which of the given substances has a standard enthalpy of formation ( Δ H f ) defined as 0 kJ/mol. The substances are: H 2 O ( s ) , Ne (l), F 2 ( g ) , C O 2 ( g ) .
Identifying the Standard State The standard enthalpy of formation of an element in its standard state is defined as 0 kJ/mol. We need to identify which of the given substances is an element in its standard state.
Analyzing Neon Neon (Ne) is a noble gas. Its standard state is a gas, but here it is given as a liquid (Ne (l)), so it is not in its standard state.
Analyzing Fluorine Fluorine ( F 2 ) is a diatomic halogen. Its standard state is a gas, and it is given as a gas ( F 2 ( g ) ), so it is in its standard state.
Conclusion Therefore, F 2 ( g ) has a standard enthalpy of formation of 0 kJ/mol.
Examples
Understanding standard enthalpy of formation is crucial in chemical engineering for designing and optimizing chemical reactions. For example, when designing a process to produce ammonia ( N H 3 ), engineers need to know the standard enthalpies of formation of nitrogen ( N 2 ), hydrogen ( H 2 ), and ammonia to calculate the overall enthalpy change of the reaction. Since N 2 and H 2 are elements in their standard states, their Δ H f values are 0 kJ/mol, simplifying the calculations and allowing for accurate energy balances in the process design.
