To calculate the enthalpy change (ΔH) for the reaction C+O2→CO2 using bond energies, we use the formula:
ΔH=∑(bondenergiesofbondsbroken)−∑(bondenergiesofbondsformed)
Let's identify the bonds broken in the reactants and bonds formed in the products.
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Reactants:
- C (graphite): The energy given for C(graphite) (717 kJ/mol) represents the energy required to atomize one mole of solid carbon into gaseous carbon atoms. This is an energy input, similar to breaking bonds.
- O2: This molecule contains one O=O double bond. The energy required to break this bond is 498 kJ/mol.
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Products:
- CO2: This molecule has a linear structure with two C=O double bonds. The energy released when these bonds are formed is 2×805 kJ/mol.
Step 1: Calculate the total energy required to break bonds (reactants).
Energy to atomize C(graphite) = 717 kJ/mol
Energy to break 1 mole of O=O bonds = 498 kJ/mol
Total energy for bond breaking = 717kJ/mol+498kJ/mol=1215 kJ/mol
Step 2: Calculate the total energy released when bonds are formed (products).
Energy released from forming 2 moles of C=O bonds = 2×805kJ/mol=1610 kJ/mol
Step 3: Calculate the enthalpy change (ΔH).
ΔH=(Energyforbondbreaking)−(Energyforbondformation)
ΔH=1215kJ/mol−1610kJ/mol
ΔH=−395kJ/mol
Comparing this value to the given options, -394 kJ/mol is the closest.
The final answer is A.−394kJ/mol.