Concept explainers
Estimate ΔH for the following reactions using bond energies given in Table 8.5.
3CH2=CH2(g) + 3H2(g) → 3CH2–CH3(g)
The enthalpies of formation for C6H6(g) and C6H12 (g) are 82.9 and −90.3 kJ/mol. respectively. Calculate ΔH° for the two reactions using standard enthalpies of formation from Appendix 4. Account for any differences between the results obtained from the two methods.
Interpretation:
The estimation has to be done using bond energy values and standard enthalpy of formation values.
Concept Introduction:
Enthalpy is heat content of the system. The value of enthalpy does not depend on the path of a reaction but depend on state of the system. It has a unique value for each state of the system. Thus, enthalpy is a state function. Enthalpy is represented as,
Enthalpy change, denoted by
Where
Energy required to break the existing bonds carries positive sign as energy is supplied to the system and the energy released in formation of new bonds carries negative as energy is removed from the system.
Answer to Problem 158CP
Explanation of Solution
The given reaction is,
In the above reaction, Hydrogen atoms are added to the
Energy required to break the existing bonds carries positive sign as energy is supplied to the system and the energy released in formation of new bonds carries negative as energy is removed from the system.
Accordingly, the enthalpy change for the above reaction is calculated as –
Enthalpy of formation of ethylene, hydrogen and ethane are
The term “bond energy” and “enthalpy of formation” are not similar. Bond energy refers to energy released or absorbed when isolated elements combine to form a bond. Enthalpy of formation refers to the energy released or absorbed during formation of a compound from other compounds.
Thus, enthalpy of formation of each of the compound on the reactant and product side is widely different from bond energy of the same compounds. Hence enthalpy of reaction calculated using these two different parameters is not same.
The another reaction is,
In the above reaction, Hydrogen atoms are added to the
Accordingly, the enthalpy change for the above reaction is calculated as –
Enthalpy of formation of benzene, hydrogen and cyclohexane are
As we discussed earlier, enthalpy of formation of each of the compound on the reactant and product side is widely different from bond energy of the same compounds. Hence enthalpy of reaction calculated using these two different parameters is not same.
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Chapter 22 Solutions
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