For which of the following reactions is the value of Delta H rxn equal to Delta H of formation?
I. 2Ca(s) + O2(g) --> 2CaO(s)
II. C2H2(g) + H2(g) --> C2H4(g)
III. S(s) + O3(g) --> SO3(g)
IV. 3Mg(s) + N2(g) --> Mg3N2(s).
According to my answer key, the only one is IV. I cannot figure out how to tell the difference. Is there a trend that I am missing?
The heat of formation must be an equation for the formation of the compound (1 mol) on the right from elements on the left.
I is OK BUT it is for two moles.
II is no go since C2H2 is not an element.
III is no go since O3 is not a standard state of oxygen (O2 is the standard state).
IV is from elements on the left, in their standard states, and forms 1 mol of the product in its standard state on the right..
N2 + C2H2 = 2HCN
To determine whether the value of Delta H rxn is equal to Delta H of formation for a given reaction, you need to consider the relationship between these two values.
Delta H rxn represents the change in enthalpy (heat) of a reaction, while Delta H of formation represents the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states.
The key relationship to consider is that the Delta H rxn of a reaction is equal to the sum of the Delta H of formation of the products minus the sum of the Delta H of formation of the reactants. This can be expressed as:
Delta H rxn = Σ(ΔHf products) - Σ(ΔHf reactants)
Now let's examine the given reactions:
I. 2Ca(s) + O2(g) --> 2CaO(s)
II. C2H2(g) + H2(g) --> C2H4(g)
III. S(s) + O3(g) --> SO3(g)
IV. 3Mg(s) + N2(g) --> Mg3N2(s)
For reaction I, you would need the values of Delta Hf for CaO, Ca, and O2 to calculate Delta H rxn. Since only one product (CaO) is given, it is not possible to compare Delta H rxn and Delta H of formation.
For reaction II, you would need the values of Delta Hf for C2H4, C2H2, and H2 to calculate Delta H rxn. Again, only one product (C2H4) is given, so it cannot be compared.
For reaction III, you would need the values of Delta Hf for SO3, S, and O3 to calculate Delta H rxn. Similar to the previous reactions, only one product (SO3) is given, making it impossible to compare the values.
Finally, for reaction IV, you would need the values of Delta Hf for Mg3N2, Mg, and N2 to calculate Delta H rxn. In this case, all the values are given, so you can calculate both Delta H rxn and compare it with Delta H of formation.
Since only reaction IV allows for comparison, it is the only reaction for which the value of Delta H rxn is equal to Delta H of formation. There is no specific trend that can be used in this case, as it depends on the reactants and products of each reaction and the availability of the necessary data for calculation.
To determine which of the given reactions has a value of ΔHrxn equal to ΔH of formation, we need to understand the concept of ΔH of formation and its relationship to ΔHrxn.
ΔH of formation (or ΔHf) is the enthalpy change that occurs when one mole of a compound is formed from its elements in their standard states. It is typically measured at a constant temperature and pressure.
ΔHrxn, on the other hand, is the enthalpy change for a given chemical reaction, usually measured as the difference between the enthalpy of the products and the enthalpy of the reactants.
If the reaction can be expressed as the summation of the formation reactions for the products minus the formation reactions for the reactants, then ΔHrxn will be equal to ΔH of formation.
Let's analyze each given reaction:
I. 2Ca(s) + O2(g) --> 2CaO(s)
In this reaction, we have only one compound (CaO) on the product side, which means we can calculate ΔH of formation for CaO. However, we also have elements (Ca and O2) on the reactant side, not compounds. Thus, we cannot directly calculate ΔH of formation for the reactants. Therefore, this reaction does not have ΔHrxn equal to ΔH of formation.
II. C2H2(g) + H2(g) --> C2H4(g)
Once again, we have compounds on both the reactant and product sides. However, we have more than one compound on each side of the equation, making it impossible to express the reaction as a summation of ΔH of formation values. Hence, this reaction does not have ΔHrxn equal to ΔH of formation.
III. S(s) + O3(g) --> SO3(g)
In this reaction, we have only one compound (SO3) on the product side, which means we can calculate ΔH of formation for SO3. However, we have an element (S) on the reactant side, not a compound. Therefore, this reaction does not have ΔHrxn equal to ΔH of formation.
IV. 3Mg(s) + N2(g) --> Mg3N2(s)
This reaction involves compounds on both sides of the equation. Additionally, by using appropriate stoichiometric coefficients, we can see that this reaction can be expressed as a summation of the formation reactions for the products minus the formation reactions for the reactants:
3 Mg(s) + N2(g) --> Mg3N2(s)
Formation of Mg3N2: ΔHf(Mg3N2)
Therefore, this reaction (IV) has ΔHrxn equal to ΔHf(Mg3N2).
In conclusion, based on the analysis above, the only reaction that has the value of ΔHrxn equal to ΔH of formation is reaction IV: 3Mg(s) + N2(g) --> Mg3N2(s).