Understanding Lattice Enthalpy in Chapter 22 - OCR A-Level Chemistry

Lattice Enthalpy - The Stability Secret

Think of ionic compounds as incredibly strong fortresses - they're almost impossible to break down because of the powerful electrostatic bonds holding everything together.

Lattice enthalpy is the energy change when one mole of an ionic compound forms from its gaseous ions under standard conditions. Picture potassium and chloride ions floating around as gases, then suddenly snapping together to form solid KCl - that releases a whopping 711 kJ/mol of energy!

The key thing to remember is that lattice enthalpy values are always negative because forming ionic compounds is exothermic. Energy gets released as those oppositely charged ions attract each other with tremendous force.

💡 Quick Tip: The more negative the lattice enthalpy, the more stable the ionic compound. It's like measuring how tightly the ions are hugging each other!

Born-Haber Cycles - The Indirect Route

Here's the thing about lattice enthalpy - you can't measure it directly in a lab, which is where Born-Haber cycles become your best friend. They're like taking a scenic route to reach the same destination.

Enthalpy change of formation is when one mole of a compound forms from its elements in their standard states. For example, solid magnesium plus solid iodine creating magnesium iodide. This process is typically exothermic.

Enthalpy change of atomisation breaks elements down into individual gaseous atoms. Think of it as completely vaporising and separating everything - like turning solid sodium into floating sodium atoms in the gas phase.

💡 Memory Hook: Formation brings things together (usually exo), while atomisation breaks them apart (always endo). Easy!

Ionisation Energy and Electron Affinity

Now we're getting to the real action - creating those charged ions that make ionic compounds possible.

First ionisation energy is the energy needed to rip one electron away from each atom in a mole of gaseous atoms, creating 1+ ions. This always requires energy input (endothermic) because you're fighting against the nucleus's attraction to its electrons.

First electron affinity works in the opposite direction - it's the energy change when gaseous atoms each grab an extra electron to form 1- ions. This typically releases energy (exothermic) because atoms often want that extra electron.

The beauty is in the balance: metals lose electrons relatively easily (lower ionisation energies), while non-metals love gaining electrons (more negative electron affinities).

💡 Exam Insight: Always check the direction of your arrows in equations - they tell you whether energy is being absorbed or released!

Multiple Charges - Going Further

Real chemistry gets more complex when ions carry charges greater than 1+ or 1-. That's where successive ionisation energies and electron affinities come into play.

For metals forming 2+ ions (like magnesium), you'll need both first and second ionisation energies. The second one is always much larger because you're removing an electron from an already positive ion - imagine trying to take something away from someone who's already holding on tighter!

For non-metals forming 2- ions (like oxygen), the second electron affinity is actually endothermic. Adding that second electron to an already negative ion requires energy input because like charges repel each other.

Understanding these successive values helps explain why some ionic compounds are more common than others - it's all about the energy costs involved.

💡 Pattern Spotter: Notice how successive ionisation energies increase dramatically, while successive electron affinities can flip from exo to endo!