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29 September, 09:36

How does the A Hreaction relate to the A He of molecules involved in a reaction?

A. A Hreaction = A Hf, reactants

B. AHreaction = A Hf, products - AHf, reactants

C. A Hreaction = AHf, products

D. A Hreaction = A Ht, products + AHf, reactants

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  1. 29 September, 09:40
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    B. ΔHreaction = ΔH°f reactants - ΔH°f products

    Explanation:

    Using Hess's law, it is possible to sum ΔH of several related reactions to find ΔH of a particular reaction.

    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 (1)

    HCl: 1/2H₂ + 1/2Cl₂ → HCl (2)

    H₂O: H₂ + 1/2O₂ → H₂O (3)

    NaCl: Na + 1/2Cl₂ → NaCl (4)

    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:

    B. ΔHreaction = ΔH°f reactants - ΔH°f products
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