Use Hess's Law and a table of heats of formation to determine the enthalpy for the following reaction.
3SO2(g) + 3H2O(g) + 3/2O2(g) --->3H2SO4(l) ΔH = ?
dHrxn = (n*dHf products) - (n*dHf reactants)
dHformation can be found in yuor text, usually at the end of the book in the appendix.
To determine the enthalpy for the given reaction using Hess's Law and a table of heats of formation, you need to follow these steps:
Step 1: Write the balanced chemical equations for the reactions involved.
First, you need to recognize that the desired reaction is not directly given in the table. However, you can break it down into simpler reactions that are present in the table. The steps to break it down are as follows:
1) Multiply the equation for the formation of H2SO4 by 3:
3H2SO4(l) ↔ 3H2O(g) + 3/2O2(g) ΔH1
2) Look for reactions in the table that involve only the substances present in the reverse direction of ΔH1. In this case, we have:
3SO2(g) + 3/2O2(g) ↔ 3SO3(g) ΔH2
3) Write the reaction for the formation of H2O:
H2(g) + 1/2O2(g) ↔ H2O(g) ΔH3
4) Finally, write the reaction for the decomposition of SO3:
SO3(g) ↔ SO2(g) + 1/2O2(g) ΔH4
Step 2: Determine the known heats of formation.
Refer to a table of standard heats of formation to determine the values for ΔH for each of the individual reactions involved. The standard heat of formation (ΔHf) represents the enthalpy change when one mole of a compound is formed from its elements in their standard states (at 25°C and 1 atm).
From the table, considering the products minus reactants:
ΔH1 = [(3 moles of H2O x ΔHf(H2O))] - [(3 moles of H2SO4 x ΔHf(H2SO4))]
ΔH2 = [(3 moles of SO3 x ΔHf(SO3))] - [(3 moles of SO2 x ΔHf(SO2))]
ΔH3 = [(1 mole of H2O x ΔHf(H2O))] - [(1 mole of H2 x ΔHf(H2)) + (1/2 mole of O2 x ΔHf(O2))]
ΔH4 = [(1 mole of SO2 x ΔHf(SO2))] - [(1 mole of SO3 x ΔHf(SO3)) + (1/2 mole of O2 x ΔHf(O2))]
Step 3: Apply Hess's Law.
Hess's Law states that the overall enthalpy change of a reaction is equal to the sum of the enthalpy changes of the individual steps of the reaction. In this case, we can add up the enthalpy changes of the individual reactions to determine ΔH for the desired reaction.
ΔH(desired) = ΔH1 + ΔH2 + ΔH3 + ΔH4
Step 4: Calculate ΔH for the reaction.
Substitute the values of ΔH for each step into the equation above and calculate the overall enthalpy change (ΔH) for the reaction.
By following these steps, you can determine the enthalpy (ΔH) for the given reaction using Hess's Law and a table of heats of formation.