Answer:
B. ΔHreaction = ΔH°f reactants- ΔH°f products
Explanation:
<em>Using Hess's law, it is possible to sum ΔH of several related reactions to find ΔH of a particular reaction</em>.
Having in mind Hess's law, ΔH°f is defined as the change in enthalpy during the formation of 1 mole of substance from its constituent elements (That is, pure elements, mono or diatomics, that have a ΔH° = 0).
For example, in ΔH°f of H₂O, the equation is:
H₂(g) + 1/2O₂(g) → H₂O(g)
The constituent elements with ΔH°f = 0 are H₂(g) and O₂(g).
Now, using Hess's law, you can sum the ΔH°f of substance in a reaction as, for example:
NaOH + HCl → H₂O + NaCl. ΔHr
The ΔH°f for each substance in the reaction is:
NaOH: Na + 1/2H₂ + 1/2O₂ → NaOH <em>(1)</em>
HCl: 1/2H₂ + 1/2Cl₂ → HCl <em>(2)</em>
H₂O: H₂ + 1/2O₂ → H₂O <em>(3)</em>
NaCl: Na + 1/2Cl₂ → NaCl <em>(4)</em>
The algebraic sum of (3) + (4) is -(ΔH°f reactants):
H₂ + 1/2O₂ + Na + 1/2Cl₂ → NaCl + H₂O ΔH°f reactants
This reaction - {(1)+(2)} ΔH°f products
NaOH + HCl → H₂O + NaCl.
ΔHr = ΔH°f reactants- ΔH°f products
In the example, we obtain this relationship that can be expanded for all reactions. Thus, right answer is:
<h3>B. ΔHreaction = ΔH°f reactants- ΔH°f products</h3>
