Using The Born Haber Cycle To Determine H For An Ionic Compound printable pdf download

Using The Born Haber Cycle To Determine H For An Ionic Compound

Using The Born Haber cycle to determine ∆H for an ionic compound

Values you need to “look up” are on your Heat of formation chart (∆H

° chart) and the back of

your VSEPR chart.

Na(s) + 1/2 Cl

(g) –––> NaCl(s)

Step 1: Make gaseous atoms from “standard state” elements. Both processes are Endothermic

(Breaking bonds always absorbs energy)

Na(s) –––> Na(g)

∆H = ∆H

° for Na(g) = + 108 kJ

1/2 Cl

(g) –––> Cl(g)

∆H = ∆H

° for Cl(g) = + 122 kJ

(you could also use Bond energy for Cl

. Break 1/2 mol of Cl–Cl bonds = 242 kJ/2

= + 121 kJ or basically same energy)

Step 2: Make gaseous ions and transfer the electrons

Na loses an electron:

1st ionization energy

(Na absorbs energy to remove an electron,

Na(g) –––> Na

(g) + e

–

∆H = +496 kJ

Endothermic)

Cl gains an electron:

Electron affinity

(Cl releases energy to add an electron,

Cl(g) + e

–

–––> Cl

–

(g)

∆H = –349 kJ

Exothermic)

Step 3: Bring gaseous ions together to make an ionic solid = Lattice energy.

(g) + Cl

–

(g) –––> NaCl(s)

∆H = – 788 kJ

Lattice energy is the energy required to separate 1 mol of an ionic compound into gaseous ions.

The value is +, endothermic, to separate the ions (add energy to break bonds) or

–, exothermic, to bring the ions together (release energy when bonds are made)

When a compound is made, the ions are coming together, so the negative lattice energy is the last

step in the Born- Haber process. The large amount of energy released as the ions come together is

the factor that makes the formation of stable ionic compounds Exothermic.

To get overall ∆H, add up ∆H for each step, or Hess’s Law.

+108 + 122 + 496 – 349 – 788 kJ = –411 kJ which is very close to ∆H

° for NaCl(s)

Exothermic (∆H = – 411 kJ) so the compound is stable (you would have to add energy to break up

the compound).

Using The Born Haber Cycle To Determine H For An Ionic Compound

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