Use average bond energies to calculate delta Hrxn for C(s)+2H2O(g)-->2H2(g)+CO2(g)
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(sum bond energies reactants)-(sum bond energies products)
To calculate ΔHrxn (the enthalpy change of a reaction) using average bond energies, we can follow these steps:
1. Write the balanced chemical equation for the reaction:
C(s) + 2H2O(g) → 2H2(g) + CO2(g)
2. Identify the bonds broken and formed in the reaction. In this case:
- Bonds broken: C=O (double bond in CO2) and O-H (in H2O)
- Bonds formed: C-H (in CH4), O=O (in O2), and H-H (in H2)
3. Determine the average bond energies for the bonds involved. Here are the average bond energies (in kJ/mol) commonly used:
- C=O: 745 kJ/mol
- O-H: 464 kJ/mol
- C-H: 413 kJ/mol
- O=O: 498 kJ/mol
- H-H: 436 kJ/mol
4. Calculate the energy change (ΔH) for the bonds broken and formed:
Bonds Broken = (bonds broken) x (bond energy of each bond)
= (1 × C=O) + (2 × O-H)
= (1 × 745 kJ/mol) + (2 × 464 kJ/mol)
Bonds Formed = (bonds formed) x (bond energy of each bond)
= (2 × C-H) + (1 × O=O) + (2 × H-H)
= (2 × 413 kJ/mol) + (1 × 498 kJ/mol) + (2 × 436 kJ/mol)
5. Calculate ΔHrxn by finding the difference between the energy change of the bonds broken and the energy change of the bonds formed:
ΔHrxn = (energy change of bonds broken) - (energy change of bonds formed)
= [ (1 × 745 kJ/mol) + (2 × 464 kJ/mol) ] - [ (2 × 413 kJ/mol) + (1 × 498 kJ/mol) + (2 × 436 kJ/mol) ]
6. Perform the calculation to determine the value of ΔHrxn in kJ/mol.
Please note that using average bond energies to calculate enthalpy changes is an approximation, and the actual values may differ.
To calculate the enthalpy change (ΔHrxn) for a reaction using average bond energies, you need to follow these steps:
Step 1: Write the balanced equation for the reaction:
C(s) + 2H2O(g) → 2H2(g) + CO2(g)
Step 2: Identify all the bonds broken and formed during the reaction. In this case, there are four bonds involved:
- Two O-H bonds are broken in water (H2O).
- One C=O bond is formed in carbon dioxide (CO2).
- Four H-H bonds are formed in hydrogen gas (H2).
Step 3: Look up the average bond energies for each bond involved in the reaction. These values can be found in reference books or online databases. Here are the approximate average bond energies in kilojoules per mole (kJ/mol):
- O-H bond: 463 kJ/mol
- C=O bond: 743 kJ/mol
- H-H bond: 436 kJ/mol
Step 4: Calculate the total energy required to break the bonds in the reactants. This is the sum of the bond energies of the bonds broken:
Energy required to break O-H bonds in 2H2O = 2 × 2 × 463 kJ/mol = 1852 kJ/mol
Energy required to break C=O bond in CO2 = 1 × 743 kJ/mol = 743 kJ/mol
Total energy required to break bonds in reactants = 1852 kJ/mol + 743 kJ/mol = 2595 kJ/mol
Step 5: Calculate the total energy released when the new bonds are formed. This is the sum of the bond energies of the bonds formed:
Energy released when H-H bonds are formed in 2H2 = 2 × 4 × 436 kJ/mol = 3488 kJ/mol
Step 6: Calculate ΔHrxn using the equation:
ΔHrxn = (Total energy required to break bonds) - (Total energy released when new bonds are formed)
ΔHrxn = 2595 kJ/mol - 3488 kJ/mol = -893 kJ/mol
Step 7: The negative sign indicates that the reaction is exothermic, meaning that it releases energy.
Therefore, the ΔHrxn for the reaction C(s) + 2H2O(g) → 2H2(g) + CO2(g) using average bond energies is approximately -893 kJ/mol.