Tiber Tutor

Home
Blog
IB Chemistry Lattice Enthalpy: Definition, Sign and Born-Haber

IB Chemistry Lattice Enthalpy: Definition, Sign and Born-Haber

12 min readOliver Kidd (Co-founder)23 Oct 2026

Lattice enthalpy is the energy needed to separate one mole of an ionic solid into its gaseous ions. In IB Chemistry it is a positive, endothermic value, and it measures how strongly the ions are held together. Get the definition and the sign right, and most Born-Haber questions become a matter of careful addition.

This guide walks you through the definition, the sign convention, a step-by-step Born-Haber cycle, how to rank lattice enthalpies, and the mistakes examiners see most often. Take it at your own pace. Each part builds on the one before.

Tibertutor
Build confidence in IB Chemistry
Practise lattice enthalpy and Born-Haber questions with exam-style resources, topic-specific tests and progress tracking for IB science students.
Explore IB Chemistry resources

TL;DR

  • IB lattice enthalpy is the enthalpy change when one mole of an ionic solid is separated into its gaseous ions. It is endothermic, so the value is positive.
  • Some textbooks define it in the opposite direction (formation, exothermic, negative). Always check the direction before you use a number.
  • A Born-Haber cycle is Hess’s law applied to an ionic compound. Every step is a named enthalpy change, and the lattice step closes the loop.
  • Higher ion charges and smaller ionic radii give a larger lattice enthalpy.
  • A gap between theoretical and experimental values points to covalent character in the bonding.

Table of Contents

What lattice enthalpy means in IB Chemistry

An ionic compound is held together by electrostatic attraction between positive and negative ions. Those forces are only fully removed when the ions are gaseous and so far apart that the attraction is negligible. Lattice enthalpy puts a number on that attraction.

The Chemistry LibreTexts page on lattice enthalpies/Crystal_Lattices/Thermodynamics_of_Lattices/Lattice_Enthalpies_and_Born_Haber_Cycles) describes it as a measure of the strength of the forces between the ions in an ionic solid. The greater the lattice enthalpy, the stronger the forces.

The IB convention, supported by the Data Booklet, uses the dissociation direction. In symbols:

MX(s) → M⁺(g) + X⁻(g), ΔH⦵latt > 0

That is why IB values are positive. Say “one mole”, “solid” and “gaseous ions” in your definition. Each of those words earns or loses a mark.

Why the sign causes so many lost marks

The same quantity is described in two directions across the wider chemistry world. Chemguide shows it for sodium chloride: forming the lattice from gaseous ions is -787 kJ mol⁻¹, while breaking it up is +787 kJ mol⁻¹.

So the same compound has two correct numbers, depending on the direction. Only one matches the IB definition.

Here is a habit that keeps you safe:

  1. Write the equation for the lattice step before you write the number.
  2. If the solid is on the left and gaseous ions are on the right, the value is positive.
  3. If you borrow data from a university textbook or a video, check which direction it uses.
  4. Use the phrase “lattice dissociation” or “lattice formation” in your own notes until the IB wording is automatic.

This matters beyond the exam. Students who learn the distinction early find first-year university thermodynamics much less confusing.

How to build a Born-Haber cycle

You cannot measure lattice enthalpy directly in the lab. It is hard to produce enough gaseous ions to measure the heat change. Instead, chemists use a thermochemical cycle: the sum of known steps equals the enthalpy of formation, and the one unknown is the lattice step. The University of Rhode Island lecture notes set out the same idea.

Take sodium chloride as the model. Work through the cycle in this order.

The steps, in order

  1. Atomisation of the metal: Na(s) → Na(g). Endothermic.
  2. First ionisation energy of the metal: Na(g) → Na⁺(g) + e⁻. Endothermic.
  3. Atomisation of the non-metal: ½Cl₂(g) → Cl(g). Endothermic. Note the half, because you only need one mole of chlorine atoms.
  4. Electron affinity of the non-metal: Cl(g) + e⁻ → Cl⁻(g). Exothermic for the first electron.
  5. Lattice formation: Na⁺(g) + Cl⁻(g) → NaCl(s). Strongly exothermic.
  6. Enthalpy of formation: Na(s) + ½Cl₂(g) → NaCl(s). This is the direct route.

By Hess’s law, the enthalpy of formation equals the sum of steps 1 to 5. Rearrange to find the unknown. If you are asked for the IB lattice enthalpy, remember to flip the sign of the formation step so the final answer is positive.

Drawing it cleanly

Put the elements in their standard states at the bottom or on one side. Climb upwards for endothermic steps and drop down for exothermic ones. Label every arrow with a name, not just a number. Examiners can give credit for a correct method even when the arithmetic slips, but only if they can see your method.

Check the stoichiometry for 2+ and 2- ions

For compounds such as MgO or CaF₂, count carefully. CaF₂ needs two moles of fluorine atoms, two electron affinities, and a second ionisation energy for calcium. Most Born-Haber errors come from missed multipliers, not from misunderstanding the concept.

Comparing lattice enthalpies

Bond-strength questions are a staple of bonding papers. Two factors control the size of the lattice enthalpy:

Factor Effect on lattice enthalpy Reason
Higher ion charge Larger Stronger attraction between ions
Smaller ionic radius Larger Ions can approach more closely

Charge usually matters more than size. Magnesium oxide, with 2+ and 2- ions, has a far larger lattice enthalpy than sodium chloride, with 1+ and 1- ions.

When you write an answer, name both factors and state the direction of the effect. A strong answer for a comparison such as LiF against CaF₂ would say that Ca²⁺ carries a higher charge than Li⁺, so the electrostatic attraction is stronger, so more energy is needed to separate the ions. Then comment on size if it is relevant.

When theory and experiment disagree

A purely ionic model treats ions as perfect point charges. Calculated values from that model can differ from experimental Born-Haber values. The LibreTexts page notes that for silver chloride, theoretical values can sit well below the Born-Haber figure.

The gap is not a mistake. It tells you the bonding has some covalent character, because the cation distorts the electron cloud of the anion. This is a handy idea for evaluation questions and for internal assessments that touch on the bonding continuum. The IBO publishes teacher support material on a Kapustinskii equation investigation that shows how lattice energies can be estimated from ion charges and sizes. It is worth a look if you want an IA with real analytical depth.

Common mistakes examiners see

  • Writing the definition with the wrong direction, or leaving out “gaseous”.
  • Forgetting the ½ in the atomisation of diatomic elements.
  • Using the second ionisation energy for a 1+ ion, or leaving it out for a 2+ ion.
  • Treating the electron affinity as always exothermic. The second electron affinity of oxygen is endothermic.
  • Giving a final answer with the wrong sign and no units. Use kJ mol⁻¹.

If you take one habit from this list, make it labelling every step. It stops most of these errors before they reach the final line.

Our view: why this topic rewards structured practice

We write our chemistry content with IB examiners, and lattice enthalpy is a topic where we see the same pattern. Students understand the idea quickly. They lose marks on direction, signs and multipliers, and those are habits, not gaps in intelligence.

Our view: why this topic rewards structured practice — overview diagram

That is good news. Habits improve with deliberate practice. We recommend doing a handful of Born-Haber questions, marking them against an examiner mark schemes line by line, and then logging exactly where each mark slipped. Over a few sessions, the same slip stops recurring. Confidence follows from that, and it is far steadier than confidence built on re-reading notes.

We also encourage students to link this topic to its neighbours: enthalpy of formation, Hess’s law, ionic bonding and the properties of ionic solids. When those connections are clear, a question that mixes them stops feeling like a surprise.

— Oliver

Practise it the way the exam asks

Reading about Born-Haber cycles helps. Answering exam-style questions under the right conditions is what makes them stick.

On Tiber Tutor, you can work through R1.2 energy cycle questions and R1.1 enthalpy questions, then check your bonding knowledge with the Topic 2 models of bonding questions. Our energetics formulas guide covers the equations that win marks in this unit. When you feel ready, sit a full paper in the IB Chemistry mock exams.

What sets the platform apart is that everything is connected. Questions, examiner mark schemes, notes, animated videos, flashcards and cram sheets sit together, so a mistake in a Born-Haber question leads you straight to the right explanation. Your progress tracking and performance analytics then show which topics and question types need more work. We are not aware of another IB STEM site that combines integrated tracking, student-led answer review against mark schemes, and content written by actual IB examiners.

That combination is why we believe it is the best value for money in IB science revision. The All-Access Plan is 19 $ per month, and the Per-Subject Plan is 9 $ per month, both after a free 7-day trial. See the plans on our pricing page.

Practise it the way the exam asks — overview diagram

FAQ

Is lattice enthalpy positive or negative in IB?

It is positive. The IB defines it as the enthalpy change when one mole of an ionic solid is separated into gaseous ions, which is an endothermic process. If a source gives a negative value, it is using the formation direction.

What is the formula for lattice enthalpy?

There is no single formula for the IB course. You calculate it from a Born-Haber cycle using Hess’s law: the enthalpy of formation equals the sum of atomisation, ionisation, electron affinity and lattice formation steps. Rearrange for the lattice term, then flip its sign if you need the dissociation value.

What is the order of lattice enthalpy for different compounds?

Compare ion charge first, then ionic radius. Higher charges and smaller ions give a larger lattice enthalpy. So MgO is larger than NaCl, and within a group of halides, the fluoride is larger than the iodide for the same cation.

Why do theoretical and experimental values differ?

The theoretical model assumes perfectly ionic bonding with point charges. Real compounds show some covalent character from polarisation, so the experimental Born-Haber value is usually higher. The size of the gap helps you judge how ionic the compound really is.

Sources