Final answer:
To calculate the standard enthalpy change for a reaction, sum the standard enthalpies of formation of the products and subtract the sum of those of the reactants. The standard enthalpy of formation is the energy change when one mole of a substance is formed from its elements in their standard states. This follows from Hess's Law.
Step-by-step explanation:
To calculate the standard enthalpy change (ΔH°rxn) for the reactions given, we will use the standard enthalpies of formation (ΔH°f) for each substance involved in the reactions from Appendix G or the data provided. The standard enthalpy of formation is defined as the energy change when one mole of a substance is formed from its elements in their standard states. We apply Hess's Law which states that the total enthalpy change for a reaction is the same, regardless of the number of steps the reaction is carried out in. For a chemical reaction, the standard enthalpy change can be calculated using the following formula:
ΔH°rxn = Σ(ΔH°f products) - Σ(ΔH°f reactants)
Example Calculation
For the reaction 3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g), the standard enthalpy change of the reaction (ΔH°rxn) is calculated by summing the standard enthalpies of formation of the products and subtracting the sum of the standard enthalpies of formation of the reactants:
ΔH°rxn = [(2 x ΔH°f HNO3(aq)) + (ΔH°f NO(g))] - [(3 x ΔH°f NO2(g)) + (ΔH°f H2O(l))]
We then plug in the values from Appendix G or the data provided to get the final ΔH°rxn value. The same method applies to any other reaction where standard enthalpies of formation are used.