Q43 Ans : option b How Q43 Which of the following - Chemistry -

Question

Q43
Ans : option b
How?

Q43 Which of the following formula has involved all the energy
terms used to calculated ∆ H ƒ °   of   Na
2 O s ( ∆ Hsb: Sublimation energy ; LE1
: First ionisation energy; LE2 : Second ionisation
energy ; B D E, ; Bond dissosiation energy ; E G E1:
First electron gain enthalpy ; E G E2 : Second electron
gain enthalpy ; U : Lattice energy)

(A) + 2 ∆ Hsb + LE1 + LE2 +
B D E 2 +   E G E 1 + E G E 2 + U

(B) + 2 ∆ Hsb +2LE1 + B D E 2 +  
E G E 1 + E G E 2 + U

(C) + 2 ∆ Hsb +2LE1 + + B D E 2 + 2 E G
E 1 + U

(D) + 2 ∆ Hsb + LE1 + B D E 2 +  
E G E 1 + E G E 2 + U

Shashwat Kumar · Accepted Answer

Reaction of formation: 2Na(g) + 0 5O2(g) →
Na2O(s)
Note that 0 5O2 represents half mole of O2,
and not half molecule, wihch is meaningless

Now, Na, which is a solid has to be sublimated for the reaction to
take place; there are 2 moles of Na: 2∆HSb
Now, Na has to be ionized once to form Na+; there are 2
moles of Na: 2IE1
Now, O2 has to be broken into 2O, that is, the O=O bond
has to be broken (dissociated); there are 0 5 moles of
O2: (1/2) BDE
Note that when 0 5 moles O2 is dissociated, we will
have 0 5 * 2= 1 mole of O
Now, 2 electrons have to be added successively (one after the
other) to O to form O2-; there is 1 mole of O:
1(EGE1 + EGE2)
Now, Na2O, which is formed in gaseuos phase has to be
convereted into a solid lattice; there is 1 mole of
Na2O: 1(U)

Hence, total ∆Hf= 2∆Hsb +
2IE1 + (1/2)BDE + EGE1
+ EGE2 + U
Hence, correct option is (B)