1. Y 2 (g), has a bond energy of 750 kJ/mol, and the molecular compound XY(g) ha
ID: 773581 • Letter: 1
Question
1.
Y2(g), has a bond energy of 750 kJ/mol, and the molecular compound XY(g) has a bond energy of 1500 kJ/mol. Using bond-energy information, calculate ?H for the following reaction:
X2(g) + Y2(g)? 2XY(g)
2.
Given the following information, as well as the information previously presented, predict whether or not the hypothetical ionic compound AX is likely to form. In this compound, A forms the A+ cation, and X forms the X?anion. Be sure to justify your answer.
Reaction: A(XAX(2ggs)())+?12
The first ionization energy of A(g) is 400 kJ/mol.
The electron affinity of X(g) is ?525 kJ/mol.The lattice energy of AX(s) is 100 kJ/mol.
3.
If you predicted that no ionic compound would form from the reaction in part e, what minimum amount of AX(s) lattice energy might lead to compound formation?
Explanation / Answer
X2 bond energy:
to form 1 mole of X2 molecules, 1 mole of X-X bonds are made, releasing a total energy of 500 kJ.
so the bond energy is 500 kJ/mol.
if energy per bond is required:
1 mole = Avogadro's number = 6 x 10^23.
so energy per X-X bond x 6 x 10^23 = 500 kJ = 5 x 10^5 J
so energy per X-X bond = (5/6) x 10^(5-23) = 0.833 x 10^-18 = 8.33 x 10^-19 J.
energy per bond is often quoted in eV instead of J, where 1 eV = 1.6 x 10^-19 J.
so energy per bond in eV = (8.33 x 10^-19)/(1.6 x 10^-19) = 5.2.
ie energy per bond for X-X = 5.2 eV.
DH for formation of 2XY:
X2 --> X + X DH = +500 kJ
Y2 --> Y + Y DH = +750 kJ
so
X2+Y2 --> 2X + 2Y DH = +1250 kJ
X + Y --> XY DH = -1500kJ
2X + 2Y --> 2XY DH = - 3000 kJ
therefore
X2 + Y2 --> 2X + 2Y --> 2XY DH =+1250 - 3000 kJ = -750 kJ.
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