Thermodynamics

Thermodynamics

Question

If the bond dissociation energies of XY, X2 and Y2 (all diatomic molecules) are in the ratio of 1:1:0.5 and ∆f H for the formation of XY is -200 kJ mole-1. The bond dissociation energy of X2 will be

  • 100 kJ mol-1

  • 800 kJ mol-1

  • 300 kJ mol-1

  • 400 kJ mol-1

Answer

B.

800 kJ mol-1

XY→ X(g) + Y(g) ; ∆H +a kJ/ mole ............(i)
X2 → 2X; ∆H = +a kJ/mole........(ii)
Y2 → 2Y; ∆H =+0.5a kJ/mole.......(iii)
1 half space straight X subscript 2 space plus 1 half space straight Y subscript 2 space rightwards arrow space XY semicolon
increment straight H space equals space open parentheses plus straight a over 2 plus fraction numerator begin display style 0.5 end style over denominator 2 end fraction straight a minus straight a close parentheses kJ divided by mol
plus straight a over 2 space plus fraction numerator 0.5 straight a over denominator 2 end fraction minus straight a space equals space minus 200
straight a space equals space 800

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