how will we draw a born haber cycle for MgCl2 ?

MgCl2 is formed from Mg and Cl.So we can write the steps for the formation of MgCl2

1)Mg metal exist in solid will convert into gas.

2)Mg(g) will loose 2 electrons to form Mg 2+(this can be written in two steps )

3)Chlorine gas is in form of diatomic molecule will break up into Chlorine atom 2chlorine atoms.

4)Chlorine atom will gain electron to form 2Cl-(g)

5)Mg2+ (g) and Cl-(g) will combine to form MgCl2

here diagram is showing for MgCl you can take a hint for MgCl2

  • 0

(1). Mg2+ + 2Cl- ==== MgCl2

[ ΔH formation ]

------------------

-----------------

(2). Mg (s) + Cl2 (g) === Mg (g) + Cl2 (g)

[ ΔHatomisation of Mg ]

---

(3). Mg (g) + Cl2 (g) === Mg+ (g) + e + Cl2 (g)

[ΔH 1st ionisation of Mg ]

---

(4). Mg+ (g) + e + Cl2 (g) === Mg2+(g) + 2e + Cl2 (g)

[ΔH 2nd ionisation of Mg ]

----

(5). Mg2+(g) + 2e + Cl2 (g) === Mg2+(g) + 2e + 2Cl (g)

[ ΔH bond energy of chlorine ]

-----

(6). Mg2+(g) + 2e + 2Cl (g) === Mg2+(g) + 2 Cl-

[ 2 ΔH 1st electron affinity of chlorine ]

----

(7). Mg2+(g) + 2 Cl- === MgCl2[ΔH lattice energy ]

----------

Apply Hesss Law to calculate ΔHLattice Energy for MgCl2 setting out the values in a methodical manner:

(1) = (2) + (3) + (4) + (5) + (6) + (7)

----

-641 = +148 + 738 + 1451 + 242 + (2x-364) + ΔHLE

----

ΔHLE = - 1851 - 641 = -2492 KJ mol-1

  • 3

A correction in the first step.

(1). Mg (s)+ Cl2 (g) ==== MgCl2 (s)

[ ΔH formation ]

------------------

-----------------

(2). Mg (s) + Cl2 (g) === Mg (g) + Cl2 (g)

[ ΔHatomisation of Mg ]

---

(3). Mg (g) + Cl2 (g) === Mg+ (g) + e + Cl2 (g)

[ΔH 1st ionisation of Mg ]

---

(4). Mg+ (g) + e + Cl2 (g) === Mg2+(g) + 2e + Cl2 (g)

[ΔH 2nd ionisation of Mg ]

----

(5). Mg2+(g) + 2e + Cl2 (g) === Mg2+(g) + 2e + 2Cl (g)

[ ΔH bond energy of chlorine ]

-----

(6). Mg2+(g) + 2e + 2Cl (g) === Mg2+(g) + 2 Cl- (g)

[ 2 ΔH 1st electron affinity of chlorine ]

----

(7). Mg2+(g) + 2 Cl- === MgCl2 (s) [ΔH lattice energy ]

----------

Apply Hesss Law to calculate ΔHLattice Energy for MgCl2 setting out the values in a methodical manner:

(1) = (2) + (3) + (4) + (5) + (6) + (7)

----

-641 = +148 + 738 + 1451 + 242 + (2x-364) + ΔHLE

----

ΔHLE = - 1851 - 641 = -2492 KJ mol-1

  • 2
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