3.6 Enthalpy changes for solids and solutions

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Last updated 4:27 PM on 4/9/26
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15 Terms

1
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Define lattice enthalpy, ΔHLE⦵

The enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions.

2
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What temperature change is lattice enthalpy always?

Exothermic, so negative. It’s influenced by charge densities, so when its greater, its more exothermic.

3
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5 steps of lattice enthalpy.

  1. Standard enthalpy change of formation (ΔHf⦵)

  2. Standard enthalpy change of atomisation (ΔHat⦵)

  3. Ionisation energy of the metal (ΔHI⦵)

  4. Electron affinity of the non-metal (ΔHEA⦵)

  5. Lattice enthalpy

4
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For KCl, what is the standard enthalpy change of formation?

K(s) +1/2Cl2(g) → KCl(s) ΔH = -ve

5
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For KCl, what is the standard enthalpy change of atomisation?

K(s) → K(g) ΔH = +ve

1/2Cl2(g) → Cl(g) ΔH = +ve

6
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For KCl, what is the ionisation energy.

First ionisation energy: K(g) → K+(g) ΔH = +ve

Second ionisation energy: K+(g) → K2+(g) ΔH = more +ve

Endothermic.

7
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For KCl, what is the electron affinity?

ΔHEA1 - Cl(g) + e- → Cl-(g) ΔH = -ve

ΔHEA2 - O-(g) + e- → O2-(g) ΔH = +ve

It’s endothermic as you have to overcome the negative charge on ΔHEA2

8
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For CsCl, what is the born-haber cycle?

Formation equation: Cs(s)+ ½ Cl2(g) → CsCl(s)

Haber cycle in photo.

<p>Formation equation: Cs<sub>(s)</sub>+ ½ Cl<sub>2(g)</sub> → CsCl<sub>(s)</sub></p><p>Haber cycle in photo.</p>
9
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How to calculate LE of a born haber cycle?

Anticlockwise = Clockwise, so the LE = ∑anticlockwise - ∑clockwise

10
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Whats the lattice enthalpy for Na2O.

Seen in photo.

<p>Seen in photo.</p>
11
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Whats the lattice enthalpy for CaCl2. Explain the difference.

Seen in photo.

The second ionisation energy of calcium is higher than the first because the atomic radius is smaller on Ca+, so there is less shielding and therefore a greater effective nuclear charge.

<p>Seen in photo.</p><p>The second ionisation energy of calcium is higher than the first because the atomic radius is smaller on Ca<sup>+</sup>, so there is less shielding and therefore a greater effective nuclear charge.</p>
12
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Whats the enthalpy change of solution?

The enthalpy change when 1 mol of a compound is completely dissolved under standard conditions ca be endothermic or exothermic.

13
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Enthalpy change of solution diagram for KCl

Seen in photo.

<p>Seen in photo. </p>
14
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With enthalpy changes, how can you tell if a substance is soluble.

If the hydration enthalpy (ΔHhyd) is more exothermic than the lattice enthalpy (ΔHLE), the substance will tend to be soluble. If the hydration enthalpy is more negative, it will be more exothermic.

15
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Whats the enthalpy change of hydration of F- and explain the difference.

The increase in entropy can outweigh hydration enthalpy causing it to be soluble due to increase in chaos if hydration enthalpy is small.

<p>The increase in entropy can outweigh hydration enthalpy causing it to be soluble due to increase in chaos if hydration enthalpy is small.</p>