R1.2Reactivity 1 — What drives chemical reactions?

Energy cycles in reactions — IB Chemistry HL Revision Notes & Topical Questions

15 min read 29 parts Lesson 2 of 4 1 quiz

📚 IB Chemistry HL Hub / Topical Revision

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Lesson summary

R1.2 Energy cycles in reactions — IB CHEMISTRY HL notes: Use bond enthalpies, Hess's law, enthalpies of formation and combustion, and Born–Haber cycles to calculate enthalpy changes.Bond breaking — requires energy, so it is endothermic; the energy needed is the bond dissociation …

Bond forming
Exothermic overall
Endothermic overall
Definition
Why averaged
Draw structures

Part 1 of 10Bond breaking and bond making

Bond breaking

Bond breaking requires energy, so it is endothermic;
- the energy needed is the bond dissociation enthalpy (bond energy).

Bond forming

Bond forming releases energy, so it is exothermic, with the same magnitude but opposite sign.

Exothermic overall

Exothermic overall more energy is released forming bonds than is needed to break bonds;
- the products are more stable.

Endothermic overall

Endothermic overall more energy is needed to break bonds than is released forming bonds;
- the products are less stable.

Part 2 of 10Average bond enthalpy

Average bond enthalpy

Definition

Definition the energy needed to break one mole of bonds in a gaseous molecule, averaged over similar compounds.

Why averaged

Why averaged bond energies depend on the environment, and the successive C–H bonds in methane need different energies.

Use

Use it estimates ΔH_r when the value cannot be measured experimentally.

Equation

ΔH_r = Σ(bonds broken) + Σ(bonds formed), with bonds broken positive and bonds formed negative.

Draw structures

Draw structures draw full displayed structures so no bond is missed.

Coefficients

Coefficients multiply each bond enthalpy by the number of bonds, including equation coefficients.

Working

Working show each step, because bond enthalpy questions award marks for working.

Part 3 of 10Hess's law

1Hess's law

Hess's law the total enthalpy change of a reaction is independent of the route, provided the initial and final conditions are the same.

2Basis

Basis the conservation of energy.
- (the first law of thermodynamics)

3Use

Use to find enthalpy changes that cannot be measured by calorimetry, such as the formation of propane from carbon and hydrogen.

4Energy cycle

Energy cycle the direct route from elements to products equals the indirect route via the reactants.

5Relationship

ΔH₂ = ΔH₁ + ΔH_r, so ΔH_r = ΔH₂ − ΔH₁.

6Understanding

Understanding the principle matters more than the wording.

Part 4 of 10Solving Hess's law problems

1Cycles

Cycles add the value when following an arrow and subtract it when going against an arrow.

2Brackets

Brackets put values in brackets with the operator in front to avoid sign errors.

3Equations

Equations reversing an equation changes the sign of ΔH, and multiplying an equation multiplies ΔH.

4Combine

Combine add the adjusted equations and cancel species that appear on both sides.

5Either method

Either method know both cycles and equations.

Part 5 of 10Using enthalpies of formation (HL)

1Cycle

Cycle elements are placed at the bottom and the arrows point upwards, from elements to compounds.

2Equation

ΔH_r = ΣΔH_f°(products) − ΣΔH_f°(reactants).

3Elements

ΔH_f° of an element in its standard state is zero.

4Coefficients

Coefficients multiply each value by its coefficient.

5Data

Data values are in Section 13 of the data booklet.

Part 6 of 10Using enthalpies of combustion (HL)

1Cycle

Cycle combustion products are at the bottom and the arrows point downwards.

2Equation

ΔH_r = ΣΔH_c°(reactants) − ΣΔH_c°(products).

3Balance

Balance check that the atoms of each element balance in the cycle.

4Direction

Direction reverse the sign when the route goes against the arrow.

Part 7 of 10Born–Haber cycle definitions (HL)

1Born–Haber cycle

Born–Haber cycle a Hess's law cycle for ionic compounds that finds the lattice enthalpy, which cannot be measured directly.

2Lattice enthalpy

Lattice enthalpy the enthalpy change when one mole of an ionic compound separates into gaseous ions;
- endothermic, an upward arrow.

3Atomisation

Atomisation formation of one mole of gaseous atoms from the element in its standard state; endothermic.

4First and second ionisation energy

First and second ionisation energy both endothermic, with the second larger than the first.

5First electron affinity

First electron affinity can be exothermic, endothermic or neutral.

6Second electron affinity

Second electron affinity endothermic because the electron is repelled by the negative ion.

Part 8 of 10Drawing a Born–Haber cycle (HL)

Place the elements in their standard states with state symbols about one third up.

Make gaseous atoms with upward arrows for atomisation.

Make gaseous ions: ionisation upwards and electron affinity downwards if exothermic.

Add the enthalpy of formation downwards (or upwards if endothermic).

Join the solid to the gaseous ions with the lattice enthalpy.

1Direction matters

Direction matters up for endothermic and down for exothermic, but arrow length need not be to scale.

2Details

Details show state symbols and the electron in the ionisation step.

3Stoichiometry

Stoichiometry for MgCl₂, include both ionisation energies of Mg and double the atomisation and electron affinity of Cl.

4Exam

Exam completing a partly drawn cycle is more likely than drawing a whole one.

Part 9 of 10Born–Haber calculations (HL)

1Equation

ΔH_f = ΔH_at + ΔH_at + ΔH_ie + ΔH_ea − ΔH_latt.

2Simplified

ΔH_f = ΔH₁ − ΔH_latt, where ΔH₁ is the sum of the steps from elements to gaseous ions.

3Lattice enthalpy

ΔH_latt = −ΔH_f + ΔH₁.

4Any step

Any step the cycle can find any missing step if the others are known.

5Stoichiometry

Stoichiometry multiply values for the numbers of ions formed.

6Data

Data take values from the data booklet carefully.

Part 10 of 10More from the specification

Enthalpy of combustion per mole

Enthalpy of combustion per mole measuring heat liberated when burning stoichiometric moles of fuel provides a reliable means to determine bond energies.

Born-Haber cycle

An energy cycle applying Hess's Law to calculate experimental lattice enthalpy of an ionic solid from standard enthalpy terms.
- (such as sodium chloride)

Enthalpy of atomisation

The endothermic process required to produce one mole of separate gaseous atoms from an element in its standard state.

Ionisation energy and electron affinity

First ionisation energy is endothermic, while first electron affinity for non-metals like chlorine is predominantly exothermic.
- (removing electrons)

Hess's Law calculations

The total enthalpy change for a chemical reaction is independent of the pathway taken between initial and final states.

Born-Haber cycle

A Hess's law cycle linking formation enthalpy, lattice energy and other steps for an ionic compound.

Endothermic process

Change, such as dissolving or melting, that absorbs thermal energy.

Key terms

Term	Meaning
Bond enthalpy	Energy to break one mole of a bond in the gas phase.
Hess's law	Enthalpy change is independent of route.
Born–Haber cycle	A Hess cycle for forming an ionic compound from its elements.
Lattice enthalpy	Energy to separate one mole of an ionic solid into gaseous ions.
Enthalpy of atomisation	Energy to form one mole of gaseous atoms from an element.
Electron affinity	Enthalpy change when gaseous atoms gain electrons.

Avoid these mistakes

Giving bonds broken a negative sign and bonds formed a positive sign.

Forgetting to multiply bond enthalpies by the equation coefficients.

Forgetting to reverse the sign when going against an arrow in a cycle.

Forgetting to double the electron affinity and atomisation of chlorine for MgCl₂.

Last updated: 15 October 2026