Final answer:
Without the specific reaction steps and their associated enthalpy changes, we cannot calculate the ΔH for the reaction Mg(NO₃)₂(s) + H₂(g) → 2HNO₃ + Mg directly. Hess's law is typically used to sum the enthalpy changes from known reactions to find the total enthalpy change for a reaction sequence.
Step-by-step explanation:
To calculate the enthalpy change (ΔH) for the reaction Mg(NO₃)₂(s) + H₂(g) → 2HNO₃ + Mg, it is important to understand Hess's law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction. However, since the reaction itself and the enthalpies for its individual steps haven't been provided explicitly in the question, we are unable to directly calculate the enthalpy change for this specific reaction.
To apply Hess's law, you would typically use known enthalpy changes of formation or given reaction steps to sum up to the total enthalpy change. For example, if we had the reaction 2NO₂(g) forming 2HNO₃(aq) + NO(g), the enthalpy change (ΔH) for that process would be taken into account along with other relevant reactions. From the given data, we know that summing different reaction enthalpies such as +285.8 kJ, -414.8 kJ, and others resulted in a total ΔH of -138.4 kJ for the reaction steps provided.
It's also essential to note reactions such as Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H2O(l) alongside those implying that a salt is produced, such as magnesium chloride, do not immediately contribute to the calculation of ΔH for the original reaction unless they are part of the reaction sequence outlined by Hess's law.