Open Question
Hydrogen, a potential future fuel, can be produced from carbon (from coal) and steam by this reaction: C(s) + 2 H2O(g) -> H2(g) + CO2(g). Use average bond energies to calculate ΔHrxn for the reaction.
11. Bonding & Molecular Structure
Bond Energy
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Calculate the enthalpy of the following reaction based on average bond enthalpies.
A
−99 kJ
B
−830 kJ
C
−1266 kJ
D
101 kJ
E
337 kJ
Verified step by step guidance1
Identify the bonds broken and formed in the reaction. In the reactants, ammonia (NH₃) has three N-H bonds per molecule, and there are two NH₃ molecules, so a total of six N-H bonds are broken. In the products, nitrogen gas (N₂) has one N≡N triple bond, and hydrogen gas (H₂) has two H-H bonds.
Use average bond enthalpies to calculate the energy required to break the bonds in the reactants. Look up the bond enthalpy values for N-H bonds and calculate the total energy needed to break six N-H bonds.
Calculate the energy released when new bonds are formed in the products. Look up the bond enthalpy values for N≡N and H-H bonds, and calculate the total energy released when one N≡N bond and two H-H bonds are formed.
Determine the enthalpy change of the reaction by subtracting the total energy released from the total energy required. The formula is: ΔH = Σ(bond enthalpies of bonds broken) - Σ(bond enthalpies of bonds formed).
Interpret the sign of the enthalpy change. If the result is negative, the reaction is exothermic, meaning it releases energy. If positive, the reaction is endothermic, meaning it absorbs energy.
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