Entropy, S, is a measure of the dispersal or distribution of matter and/or energy in a system.

Whether you're an IB Chemistry student aiming to deepen your understanding of the syllabus or a prospective student exploring your science options, this post is here to guide you. We'll break down the IB Chemistry syllabus to give you a clear picture of what to expect in the course.

The IB Chemistry syllabus is made up of two overarching themes – Structure and Reactivity. Each of these themes consists of a variety of topics.

In the 2025 Chemistry syllabus, only one topic is exclusive to Higher Level (HL) students. Apart from that, HL and Standard Level (SL) students cover the same topics, with HL students delving into them in greater depth.

Chemistry

This topic focuses on how scientists represent the fundamental building blocks of matter and explain their interactions. It covers the states of matter, atomic structure based on the nuclear model, and how electron arrangements influence chemical behaviour. Students also learn about the mole as a way to measure numbers of particles, and examine gas behaviour using the ideal gas law to predict and calculate gas properties.

Models of the particulate nature of matter

Elements are the primary constituents of matter, which cannot be chemically broken down into simpler substances.

Introduction to the particulate nature of matter

Atoms contain a positively charged, dense nucleus composed of protons and neutrons (nucleons).

Isotopes are atoms of the same element with different numbers of neutrons.

Mass spectra are used to determine the relative atomic masses of elements from their isotopic composition.

The nuclear atom

Emission spectra are produced by atoms emitting photons when electrons in excited states return to lower energy levels.

The line emission spectrum of hydrogen provides evidence for the existence of electrons in discrete energy levels, which converge at higher energies.

The main energy level is given an integer number, n, and can hold a maximum of 2n2 electrons.

Electron configurations

The mole (mol) is the SI unit of amount of substance.

Masses of atoms are compared on a scale relative to 12C and are expressed as relative atomic mass Ar and relative formula mass Mr .

The empirical formula of a compound gives the simplest ratio of atoms of each element present in that compound.

The molar concentration is determined by the amount of solute and the volume of solution.

Avogadro’s law states that equal volumes of all gases measured under the same conditions of temperature and pressure contain equal numbers of molecules.

Counting particles by mass: The mole

An ideal gas consists of moving particles with negligible volume and no intermolecular forces.

Real gases deviate from the ideal gas model, particularly at low temperature and high pressure.

The molar volume of an ideal gas is a constant at a specific temperature and pressure.

The relationship between the pressure, volume, temperature and amount of an P1V1 P2V2 ideal gas is shown in the ideal gas equation PV = nRT and the combined gas law = .

Ideal gases

This section examines how atoms bond and how these bonds shape physical and chemical properties. You will study ionic, covalent, and metallic bonding and how each leads to different types of structures. The topic also looks at how molecular shape and intermolecular attractions affect boiling points, solubility, and chemical reactivity.

Models of bonding and structure

When metal atoms lose electrons, they form positive ions called cations.

The ionic bond is formed by electrostatic attractions between oppositely charged ions.

Ionic compounds exist as three-dimensional lattice structures, represented by empirical formulas.

The ionic model

A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei.

Single, double and triple bonds involve one, two and three shared pairs of electrons respectively.

The covalent model

A metallic bond is the electrostatic attraction between a lattice of cations and delocalized electrons.

The metallic model

Bonding is best described as a continuum between the ionic, covalent and metallic models, and can be represented by a bonding triangle.

The position of a compound in the bonding triangle is determined by the relative contributions of the three bonding types to the overall bond.

Alloys are mixtures of a metal and other metals or non-metals.

Polymers are large molecules, or macromolecules, made from repeating subunits called monomers.

Addition polymers form by the breaking of a double bond in each monomer.

Condensation polymers form by the reaction between functional groups in each monomer with the release of a small molecule.

From models to materials

This topic explains how substances are grouped according to their physical and chemical characteristics. You will learn how the periodic table is arranged and how repeating trends can be used to predict how elements behave. It also introduces organic functional groups and shows how their structures affect reactions and real-world uses.

Classification of matter

The periodic table consists of periods, groups and blocks.

The period number shows the outer energy level that is occupied by electrons.

Periodicity refers to trends in properties of elements across a period and down a group.

Trends in properties of elements down a group include the increasing metallic character of group 1 elements and decreasing non-metallic character of group 17 elements.

Metallic and non-metallic properties show a continuum.

The oxidation state is a number assigned to an atom to show the number of electrons transferred in forming a bond.

Discontinuities occur in the trend of increasing first ionization energy across a period.

Transition elements have incomplete d-sublevels that give them characteristic properties.

The formation of variable oxidation states in transition elements can be explained by the fact that their successive ionization energies are close in value.

Transition element complexes are coloured due to the absorption of light when an electron is promoted between the orbitals in the split d-sublevels.

The periodic table: Classification of elements

Organic compounds can be represented by different types of formulas.

Functional groups give characteristic physical and chemical properties to a compound.

A homologous series is a family of compounds in which successive members differ by a common structural unit, typically CH2.

Successive members of a homologous series show a trend in physical properties.

Functional groups: Classification of organic compounds

This section explores what drives chemical reactions and why they occur, with emphasis on energy changes and spontaneity. You will study how to calculate enthalpy changes, use Hess’s Law, and interpret energy diagrams to analyze reaction routes. It also covers topics such as enthalpy, entropy, spontaneity, and Gibbs free energy.

What drives chemical reactions?

Chemical reactions involve a transfer of energy between the system and the surroundings, while total energy is conserved.

Measuring enthalpy changes

Bond-breaking absorbs and bond-forming releases energy.

Hess’s law states that the enthalpy change for a reaction is independent of the pathway between the initial and final states.

Standard enthalpy changes of combustion, ΔHc⦵, and formation, ΔHf⦵, data are used in thermodynamic calculations.

An application of Hess’s law uses enthalpy of formation data or enthalpy of combustion data to calculate the enthalpy change of a reaction.

A Born–Haber cycle is an application of Hess’s law, used to show energy changes in the formation of an ionic compound.

Energy cycles in reactions

Reactive metals, non-metals and organic compounds undergo combustion reactions when heated in oxygen.

Energy from fuels

Change in Gibbs energy, ΔG, relates the energy that can be obtained from a chemical reaction to the change in enthalpy, ΔH, change in entropy, ΔS, and absolute temperature, T.

At constant pressure, a change is spontaneous if the change in Gibbs energy, ΔG, is negative.

As a reaction approaches equilibrium, ΔG becomes less negative and finally reaches zero.

Entropy and spontaneity (Additional higher level)

This topic focuses on the quantity of product produced, the speed of a reaction, and the extent to which a reaction continues before equilibrium is reached. You will use stoichiometry and moles to measure chemical amounts, apply rate laws to analyze reaction speed, and use equilibrium constants to study reversible reactions.

How much, how fast and how far?

Chemical equations show the ratio of reactants and products in a reaction.

The mole ratio of an equation can be used to determine: • the masses and/or volumes of reactants and products • the concentrations of reactants and products for reactions occurring in solution.

The limiting reactant determines the theoretical yield.

The percentage yield is calculated from the ratio of experimental yield to theoretical yield.

The atom economy is a measure of efficiency in green chemistry.

How much? The amount of chemical change

The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

Species react as a result of collisions of sufficient energy and proper orientation.

How fast? The rate of chemical change

A state of dynamic equilibrium is reached in a closed system when the rates of forward and backward reactions are equal.

The equilibrium law describes how the equilibrium constant, K , can be determined from the stoichiometry of a reaction.

The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent.

Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium.

The reaction quotient, Q, is calculated using the equilibrium expression with non- equilibrium concentrations of reactants and products.

The equilibrium law is the basis for quantifying the composition of an equilibrium mixture.

The equilibrium constant and Gibbs energy change, ΔG, can both be used to measure the position of an equilibrium reaction.

How far? The extent of chemical change

This topic examines the detailed stages of various chemical reactions. You will learn about acid-base and redox processes, as well as organic reaction mechanisms, focusing on how electrons are transferred or shared during reactions. Understanding these pathways helps explain how reactions proceed in various fields of chemistry.

What are the mechanisms of chemical change?

Brønsted–Lowry acid is a proton donor and a Brønsted–Lowry base is a proton acceptor.

A pair of species differing by a single proton is called a conjugate acid–base pair.

Proton transfer reactions

Oxidation and reduction can be described in terms of electron transfer, change in oxidation state, oxygen gain/loss or hydrogen loss/gain.

Half-equations separate the processes of oxidation and reduction, showing the loss or gain of electrons.

The relative ease of oxidation and reduction of an element in a group can be predicted from its position in the periodic table.

Acids react with reactive metals to release hydrogen.

Oxidation occurs at the anode and reduction occurs at the cathode in electrochemical cells.

A primary (voltaic) cell is an electrochemical cell that converts energy from spontaneous redox reactions to electrical energy.

Secondary (rechargeable) cells involve redox reactions that can be reversed using electrical energy.

An electrolytic cell is an electrochemical cell that converts electrical energy to chemical energy by bringing about non-spontaneous reactions.

Functional groups in organic compounds may undergo oxidation.

Functional groups in organic compounds may undergo reduction.

Reduction of unsaturated compounds by the addition of hydrogen lowers the degree of unsaturation.

Electron transfer reactions

A radical is a molecular entity that has an unpaired electron.

Electron sharing reactions

A nucleophile is a reactant that forms a bond to its reaction partner (the electrophile) by donating both bonding electrons.

In a nucleophilic substitution reaction, a nucleophile donates an electron pair to form a new bond, as another bond breaks producing a leaving group.

Heterolytic fission is the breakage of a covalent bond when both bonding electrons remain with one of the two fragments formed.

An electrophile is a reactant that forms a bond to its reaction partner (the nucleophile) by accepting both bonding electrons from that reaction partner.

Alkenes are susceptible to electrophilic attack because of the high electron density of the carbon–carbon double bond.

Collections

Physics

Biology

Chemistry

Entropy and spontaneity (Additional higher level)

Syllabus concepts to know

Study & practice R1.4 Entropy and spontaneity (Additional higher level)

Practice R1.4 in Question Bank

Read Notes for R1.4

Review R1.4 Flashcards