Use standard enthalpies of formation to calculate ΔHºrxn for the following reaction: 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g). Which of the following is the correct value for ΔHºrxn?

• A. -640 kJ/mol
• B. -890 kJ/mol
• C. -1120 kJ/mol
• D. -450 kJ/mol

Answer: (C)

Verified step by step guidance

1. Identify the standard enthalpy of formation (ΔHºf) for each compound involved in the reaction. These values are typically found in tables in chemistry textbooks or reliable databases. For this reaction, you need the ΔHºf for H2S(g), O2(g), H2O(l), and SO2(g). Note that the ΔHºf for elements in their standard state, like O2(g), is zero.
2. Write the balanced chemical equation for the reaction: 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g). Ensure that the equation is balanced with respect to both mass and charge.
3. Apply the formula for calculating the standard enthalpy change of the reaction (ΔHºrxn) using the standard enthalpies of formation: ΔHºrxn = Σ(ΔHºf of products) - Σ(ΔHºf of reactants).
4. Substitute the ΔHºf values into the formula. Multiply the ΔHºf of each compound by its stoichiometric coefficient from the balanced equation. For example, for H2O(l), you would calculate 2 * ΔHºf(H2O(l)).
5. Calculate the sum of the enthalpies for the products and the reactants separately, then subtract the sum of the reactants from the sum of the products to find ΔHºrxn. This will give you the standard enthalpy change for the reaction.