Hess's law: Determine the enthalpy change for the reaction: 2 CO + O2 → 2 CO2, given ΔHf°(CO) = -110.5 kJ/mol, ΔHf°(CO2) = -393.5 kJ/mol.

• A. -221 kJ
• B. -566 kJ
• C. -393.5 kJ
• D. -787 kJ

Answer: (B)

Hess's law states that the enthalpy change of a reaction is the difference between the sum of the formation enthalpies of the products and the sum for the reactants, scaled by their coefficients. Use standard formation enthalpies: CO2 (-393.5 kJ/mol) and CO (-110.5 kJ/mol); O2 is zero because it’s in its standard state.

For 2 CO2 forming from elements: 2 × (-393.5) = -787 kJ.

For 2 CO forming from elements: 2 × (-110.5) = -221 kJ.

O2 contributes 0.

ΔHrxn = (-787) − (-221) = -566 kJ.

The reaction releases 566 kJ of energy, so the correct enthalpy change is -566 kJ.