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Question

The Born Haber cycle below represents the energy changes occurring at 298K when KH is formed from its elements
v : ΔHatomisation K=90kJ/mol
w : ΔHionisation K=418kJ/mol
x : ΔHdissociation H=436kJ/mol
y : ΔHelectronaffinity H=78kJ/mol
z : ΔHlattice KH=710kJ/mol
Calculate the value of ΔH showing all your working.

A
124 kJ/mol
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B
-124 kJ/mol
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C
124 J/mol
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D
None of these
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Solution

The correct option is A -124 kJ/mol

Born-Haber Cycle

It is a series of steps (chemical processes) used to calculate the lattice energy of ionic solids, which is difficult to determine experimentally. You can think of BH cycle as a special case of Hess's law which states that the overall energy change in a chemical process can be calculated by breaking down the process into several steps and adding the energy change from each step.

ΔHr=2 × 90 + 2 × 418 + 436 - 2 × 78 - 2 × 710
ΔHr= 124kJ/mole

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