Which of the following equations represents a reaction for which the delta H can be called a standard enthalpy of formation, delta Hf?
a) FeO(s) + CO2(g) -> FeCO3(s)
b) 1/2 N2(g) + 2 O(g) -> NO2(g)
c) 2 Na(s) + Cl2(g) -> 2 NaCl(s)
d) Ba(s) + N2(g) + 3 O2(g) -> Ba(NO3)2(s)
e) more than one of the four equations above are standard enthalpies of formation.
I chose answer e.
is it correct?
No.
a is out because it contains compounds on the left.
b is out because O is not the standard state.
c is out because coefficient NaCl is 2.
e. can't be right since I've listed all but d.
The correct answer is option e) more than one of the four equations above are standard enthalpies of formation.
In a standard enthalpy of formation reaction, all reactants are in their standard states and the products are formed from their elements also in their standard states. The enthalpy change for such a reaction corresponds to the enthalpy of formation of the product.
In this case, options a), c), and d) all represent reactions where the reactants are in their standard states and the products are formed from their elements in their standard states. Therefore, the enthalpies of these reactions can be called standard enthalpies of formation.
To determine which equation represents a reaction for which the ΔH can be called a standard enthalpy of formation (ΔHf), we need to understand the concept of standard enthalpy of formation.
The standard enthalpy of formation is the enthalpy change when one mole of a substance is formed from its constituent elements in their standard states, with everything at standard conditions (25°C and 1 atm pressure). The standard enthalpy of formation is typically denoted with the symbol ΔHf.
In each of the given reactions, we need to check if all the reactants are in their standard states and the products are formed from those elements. Let's analyze each option:
a) FeO(s) + CO2(g) -> FeCO3(s)
In this reaction, FeO(s) is not formed from its elements and is not in its standard state. Therefore, this reaction does not represent the standard enthalpy of formation.
b) 1/2 N2(g) + 2 O(g) -> NO2(g)
In this reaction, NO2(g) is formed from its elements, and all the reactants are in their standard states. Therefore, this reaction could represent the standard enthalpy of formation.
c) 2 Na(s) + Cl2(g) -> 2 NaCl(s)
In this reaction, NaCl(s) is formed from its elements, and all the reactants are in their standard states. Therefore, this reaction could represent the standard enthalpy of formation.
d) Ba(s) + N2(g) + 3 O2(g) -> Ba(NO3)2(s)
In this reaction, Ba(NO3)2(s) is not formed from its elements and is not in its standard state. Therefore, this reaction does not represent the standard enthalpy of formation.
Based on the analysis, both options b and c represent reactions that can be called standard enthalpies of formation. Therefore, your answer of e) more than one of the four equations above are standard enthalpies of formation is correct.