---
title: "Lattice Enthalpy — AP Chem Definition & Exam Guide"
description: "Lattice enthalpy is the energy needed to pull one mole of an ionic solid apart into gaseous ions. Predict it with Coulomb's law: ion charge and size rule."
canonical: "https://fiveable.me/ap-chem/key-terms/lattice-enthalpy"
type: "key-term"
subject: "AP Chemistry"
unit: "Unit 2"
---

# Lattice Enthalpy — AP Chem Definition & Exam Guide

## Definition

Lattice enthalpy is the energy required to separate one mole of an ionic solid into its gaseous ions, and it measures how strongly the ions attract each other. On the AP Chem exam, you predict it qualitatively using Coulomb's law: bigger ion charges and smaller interionic distances mean a larger lattice enthalpy.

## What It Is

Lattice enthalpy is the energy it takes to rip one [mole](/ap-chem/key-terms/mole "fv-autolink") of an [ionic solid](/ap-chem/key-terms/ionic-solid "fv-autolink") apart into separate gaseous ions. Think of it as the price of breaking up the crystal. A huge lattice enthalpy means the ions are holding on tight; a smaller one means the crystal comes apart more easily.

The AP version of this concept is really just Coulomb's law applied to a whole crystal. An ionic solid is a repeating 3-D array of cations and anions arranged to maximize attractions and minimize repulsions (that's essential knowledge 2.3.A.1). The strength of those attractions depends on two things you can read off the periodic table: the charges on the ions and the distance between them (which comes from ionic radii). Higher charges and smaller ions pack more attraction into the [lattice](/ap-chem/unit-2/structure-metals-alloys/study-guide/yTmM9j22ZoTxtr6TCC3p "fv-autolink"), so MgO (2+ and 2-, small ions) has a much larger lattice enthalpy than NaCl (1+ and 1-, bigger ions). You don't calculate exact values on the exam. You rank and explain them.

## Why It Matters

Lattice enthalpy lives in Topic 2.3 (Structure of Ionic Solids) in [Unit 2](/ap-chem/unit-2 "fv-autolink"): Compound Structure and Properties. It directly supports learning objective [AP Chem](/ap-chem "fv-autolink") 2.3.A, which asks you to represent an ionic solid with a particulate model consistent with Coulomb's law and the properties of the ions. Lattice enthalpy is the energetic payoff of that model: it converts "these ions attract each other" into a number you can compare across compounds. It's also one of the most reusable ideas in the course. The same charge-and-distance logic explains melting points in Unit 2, dissolution energetics when ionic solids meet water, and thermochemistry problems in Unit 6. One important relief: the CED's exclusion statement says specific crystal structures (like face-centered cubic) are NOT assessed, so the focus is entirely on the Coulombic reasoning, not geometry trivia.

## Connections

### [Lattice Energy (Unit 2)](/ap-chem/key-terms/lattice-energy)

These two terms describe the same physical quantity, the energy tied to forming or breaking apart an ionic lattice. Textbooks sometimes flip the sign depending on direction (forming releases energy, separating requires it), but on the AP exam what matters is the magnitude and what controls it.

### [Coulomb's Law (Unit 2)](/ap-chem/key-terms/coulombs-law)

Lattice enthalpy is basically [Coulomb's law](/ap-chem/key-terms/coulombs-law "fv-autolink") scaled up to a mole of crystal. Energy is proportional to the product of the charges divided by the distance between ions, so a quick charge-and-radius comparison lets you rank lattice enthalpies without any math.

### [Melting Point (Unit 2)](/ap-chem/key-terms/melting-point)

[Melting](/ap-chem/key-terms/melting "fv-autolink") an ionic solid means partially overcoming the lattice attractions, so compounds with larger lattice enthalpies melt at higher temperatures. This is why MgO melts around 2800°C while NaCl melts around 800°C, and it's a classic AP explain-the-trend question.

### [Enthalpy of Reaction (Unit 6)](/ap-chem/key-terms/enthalpy-of-reaction)

When ionic solids form or react, lattice enthalpy is a big chunk of the overall energy change. The 2026 short FRQ on sodium reacting with oxygen to form Na2O(s), with ΔH°rxn = -828 kJ/mol, is exactly this kind of problem, where formation enthalpies of an ionic solid drive the [thermochemistry](/ap-chem/unit-6 "fv-autolink").

## On the AP Exam

Multiple-choice questions usually hand you two ionic compounds and ask which has the higher lattice enthalpy, melting point, or electrostatic potential energy, then make you justify it with charges and ionic radii. Fiveable practice questions in this style ask what single experimental value you'd need (the interionic distance) to calculate the potential energy between Mg²⁺ and O²⁻, or ask you to predict solubility differences between CaSO₄ and Na₂SO₄ using the higher charge density of Ca²⁺. On FRQs, lattice-related energetics show up inside thermochemistry problems, like the 2026 short FRQ where sodium and oxygen form the ionic solid Na₂O and you work with standard enthalpies of formation. Your job is always the same: identify the charges, compare the ion sizes, and write a Coulomb's law justification in words. Saying "MgO has stronger bonds" earns nothing; saying "MgO has 2+ and 2- ions at a smaller interionic distance, so the Coulombic attraction is greater" earns the point.

## Lattice enthalpy vs Lattice energy

Lattice enthalpy and lattice energy refer to the same quantity, and AP questions treat them interchangeably. The only wrinkle is sign convention: defined as separating the solid into gaseous ions, the value is positive (endothermic); defined as forming the solid from gaseous ions, it's negative (exothermic) with the same magnitude. Don't confuse either one with bond enthalpy, which applies to covalent bonds in molecules, not the collective electrostatic attractions in an ionic crystal.

## Key Takeaways

- Lattice enthalpy is the energy required to separate one mole of an ionic solid into gaseous ions, so a larger value means stronger ionic bonding.
- You predict lattice enthalpy with Coulomb's law: greater ion charges and smaller interionic distances both increase it.
- Charge matters more than size, which is why MgO (2+ and 2-) has a far larger lattice enthalpy than NaCl (1+ and 1-).
- Higher lattice enthalpy translates into higher melting points and often lower solubility, since more energy is needed to pull the ions apart.
- The AP exam never asks you to calculate an exact lattice enthalpy or memorize crystal structures; it asks you to rank compounds and justify the ranking with charge and distance.
- On FRQs, always justify comparisons explicitly with ion charges and ionic radii, not vague phrases like 'stronger bonds.'

## FAQs

### What is lattice enthalpy in AP Chem?

It's the energy needed to separate one mole of an ionic solid into its gaseous ions, which measures the strength of the electrostatic attractions in the crystal. It's covered in Topic 2.3 (Structure of Ionic Solids) and predicted using Coulomb's law.

### Is lattice enthalpy the same as lattice energy?

Yes, for AP purposes they're the same quantity, just sometimes defined in opposite directions. Separating the solid into ions is endothermic (positive), forming the solid from ions is exothermic (negative), and the magnitude is identical either way.

### Do I have to calculate lattice enthalpy or do Born-Haber cycles on the AP exam?

No. The AP exam tests qualitative reasoning, like ranking compounds by lattice enthalpy using ion charges and radii. The CED even explicitly excludes specific crystal structures from being assessed.

### Why does MgO have a higher lattice enthalpy than NaCl?

MgO's ions carry 2+ and 2- charges versus NaCl's 1+ and 1-, and Mg²⁺ and O²⁻ are smaller, so the ions sit closer together. By Coulomb's law, bigger charges at smaller distances mean a much stronger attraction, which is why MgO melts around 2800°C compared to NaCl's roughly 800°C.

### Is lattice enthalpy positive or negative?

It depends on the direction. Breaking the lattice apart into gaseous ions requires energy, so that process is positive (endothermic), while forming the lattice from gaseous ions releases the same amount of energy and is negative. On the AP exam, focus on comparing magnitudes.

## Related Study Guides

- [2.3 Structure of Ionic Solids](/ap-chem/unit-2/structure-ionic-solids/study-guide/3khaTI6A3tdnaMTMJy2u)

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