Multiple Choice
What is the standard enthalpy change (ΔH°) for the reaction: 2 NO(g) + O2(g) → 2 NO2(g)?
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8. Thermochemistry
Thermochemical Equations
Multiple Choice
Using the appropriate bond energies, calculate the enthalpy change (ΔH) for the reaction: H_2(g) + Cl_2(g) → 2HCl(g). Given the following average bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol.
A
-92 kJ
B
-184 kJ
C
+184 kJ
D
+92 kJ
Verified step by step guidance1
Identify the bonds broken and bonds formed in the reaction. In the reaction H_2(g) + Cl_2(g) → 2HCl(g), the bonds broken are one H–H bond and one Cl–Cl bond, and the bonds formed are two H–Cl bonds.
Write down the bond energies given: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, and H–Cl = 431 kJ/mol.
Calculate the total energy required to break the bonds (bond breaking is endothermic, so energy is absorbed): \(\n\)\(\text{Energy to break bonds} = D_{\text{H–H}} + D_{\text{Cl–Cl}} = 436 + 242\) kJ/mol.
Calculate the total energy released when new bonds are formed (bond formation is exothermic, so energy is released): \(\n\)\(\text{Energy released forming bonds} = 2 \times D_{\text{H–Cl}} = 2 \times 431\) kJ/mol.
Determine the enthalpy change (\(\Delta H\)) for the reaction using the formula: \(\n\)\(\Delta H = \text{Energy to break bonds} - \text{Energy released forming bonds}\). A negative value indicates an exothermic reaction.
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