Calculating with Bond Energy - A

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Background

The energy consumed or liberated by a chemical reaction represents a deficit or an excess of energy that is a simple arithmetic sum of the reactant and product bond energies. This problem follows bond energy in a manner that is similar to balancing a checkbook. Textbook Discussion: Sections 7.12, pages 232. See Also: Example XII and P.O. 7.10, pages 234, and 248.

Problem

In the problem below, X, Y, and Z represent the symbols of three unknown elements. The following list gives the bond strengths of three diatomic molecules associated with the unknown elements X, Y, and Z at 25 degrees C:

X₂ = 51.3 kcal/mole      Y₂ = 20.3 kcal/mole      XY = 78.5 kcal/mole.

Based on this bond strength information, calculate the value of Q in the following thermochemical equation:

X₂ (g) + Y₂ (g) ---> 2 XY Q = ? kcal @ 25 degrees C