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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
Out of v to y the most endothermic is___.

A
v
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B
w
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C
x
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D
y
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Solution

The correct option is B w
The bond dissociation enthalpy ΔHdissociation, is the energy needed to break one mole of the bond to give separated atoms - everything being in the gas state. It is endothermic as heat is given to system. It is endothermic as heat is absorbed

The electron affinity ΔHelectronaffinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion. it is exothermic as heat energy is released from the system

The ionization energy ΔHionizationenergyis qualitatively defined as the amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to form a cation.it is endothermic as heat is absorbed.

Enthalpy of atomization ΔHatomisation is the amount of enthalpy change when a compound's bonds are broken and the component atoms are reduced to individual atoms.it is endothermic as heat is absorbed.

The lattice energyΔHlatticeenergyof a crystalline solid is usually defined as the energy of formation of the crystal from infinitely-separated ions and as such is invariably negative. It is exothermic as heat is released.

The energy required for dissociation is more than required for ionisation as we can see from given data. So w is not most endothermic. x is most endothermic.

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