Entropy counts how many ways the particles and energy of a system can be arranged — in short, how disordered it is.

What is the natural direction of change in terms of entropy?

Change runs naturally towards greater total entropy, so anything spontaneous has ΔS(total) above zero.

Why does entropy increase when a substance changes from solid to liquid to gas?

Particles become more disordered as they gain freedom of movement through each phase transition.

Explain why entropy increases when a solid ionic lattice dissolves in water.

The ions become dispersed throughout the solution, increasing disorder compared to the ordered lattice structure.

What happens to entropy when a reaction makes or consumes gas molecules?

Entropy changes occur because gases have high entropy; an increase in moles of gas increases entropy, while a decrease in moles of gas decreases entropy.

Edexcel A-Level Chemistry 9CH0

Atomic Structure and the Periodic Table

Topic overview Protons, neutrons, electrons and isotopes Relative atomic mass and the mass spectrum Ionisation energy and the evidence for sub-shells Orbitals and electron configuration Periodicity across a period and down a group

Bonding and Structure

Topic overview Ionic bonding Covalent bonding, dative bonding and the shapes of molecules Electronegativity, polarity and intermolecular forces Structure and properties

Redox I

Topic overview Oxidation numbers Oxidation, reduction, disproportionation and half-equations

Inorganic Chemistry and the Periodic Table

Topic overview Flame tests and how they work Thermal decomposition of nitrates and carbonates Carrying out flame tests and testing the gases Group 2 elements and their compounds Trends in Group 7 and predicting fluorine and astatine Halogen displacement reactions Disproportionation and the reactions of chlorine Halides with concentrated sulfuric acid, and the hydrogen halides Testing for halide ions with silver nitrate Testing for carbonate and hydrogencarbonate ions Testing for sulfate and ammonium ions

Formulae, Equations and Amounts of Substance

Topic overview The mole, molar mass and empirical formula The ideal gas equation and molar volume Equations, concentration and reacting quantities Standard solutions, titration and uncertainty Percentage yield, atom economy and acid reactions Core practicals 1 to 3 Empirical formula and water of crystallisation by experiment Apparatus, uncertainty and mathematical skills

Organic Chemistry I

Topic overview Formulae, homologous series and functional groups IUPAC nomenclature Types of organic reaction and structural isomerism Stereoisomerism and E/Z nomenclature Practical techniques in organic chemistry Alkanes, crude oil and combustion pollutants Radical substitution Alkenes, the double bond and hydrogenation Electrophilic addition, Markovnikov's rule and the test for an alkene Addition polymers and waste polymers Nucleophilic substitution of halogenoalkanes Rates of hydrolysis and the SN1 and SN2 mechanisms Classifying and oxidising alcohols Alcohols: practical techniques and the core practicals

Modern Analytical Techniques I

Topic overview Mass spectrometry Infrared spectroscopy and the characteristic absorptions Deducing functional groups from an infrared spectrum

Energetics I

Topic overview Enthalpy changes, standard definitions and calorimetry Hess's law and bond enthalpies

Kinetics I

Topic overview Collision theory and factors affecting rate Maxwell-Boltzmann distribution and temperature Catalysts, reaction profiles and measuring rate

Equilibrium I

Topic overview Dynamic equilibrium and Le Chatelier's principle The equilibrium constant Kc

Equilibrium II

Topic overview Partial pressures and Kp Calculating Kp and the effect of conditions

Acid-Base Equilibria

Topic overview Bronsted-Lowry acids and bases, and the pH scale Strong and weak acids, Ka and Kw Calculating pH, and titration curves Buffer solutions and the Henderson-Hasselbalch equation Buffers in blood, and the core practical

Energetics II

Topic overview Lattice energy Born-Haber cycles and electron affinity Entropy and the direction of change Gibbs free energy and thermodynamic feasibility

Redox II

Topic overview Standard electrode potentials and the hydrogen electrode Electrochemical cells, emf and cell diagrams Feasibility, the electrochemical series and disproportionation Storage cells, fuel cells and redox titrations

Transition Metals

Topic overview Electron configuration and what makes a transition metal Complex ions, ligands and the colour of transition metal ions Coordination number, monodentate ligands and the shapes of complexes Bidentate and multidentate ligands, and ligand exchange Vanadium and chromium chemistry Reactions with sodium hydroxide and ammonia Cobalt and copper complexes Ligand exchange, amphoteric behaviour and the chelate effect Transition metals as catalysts

Kinetics II

Topic overview Rate equations, orders and the rate constant Determining orders from experiments Rate-determining step and mechanisms Arrhenius equation and activation energy

Organic Chemistry II

Topic overview Chirality Aldehydes, ketones and their oxidation Reduction and nucleophilic addition of HCN Testing for a carbonyl group: 2,4-DNPH and the iodoform reaction Carboxylic acids and their reactions Esters and acyl chlorides Reflux, extraction and recrystallisation Distillation, melting temperature and working safely

Organic Chemistry III

Topic overview The structure of benzene and the evidence for it Electrophilic substitution of benzene Nitration, halogenation, Friedel-Crafts and phenol Amines: structure and basicity Reactions of amines and preparing them Amides and condensation polymers Amino acids, zwitterions and proteins Empirical and molecular formulae from analytical data Grignard reagents and multi-step synthesis Purification techniques and the aspirin preparation

Modern Analytical Techniques II

Topic overview Carbon-13 and proton NMR Splitting, integration and the reference standard Chromatography High-resolution mass spectrometry

Energetics II

Entropy and the direction of change

3 things to know
Papers 1 and 3

Overview

Recall questions on what entropy measures, on comparing the entropy of solids, liquids and gases, and on calculating the entropy change of a system and of its surroundings.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

State the equation that relates total entropy change to the entropy changes of the system and surroundings.

Key term

Give an example that shows enthalpy change alone cannot predict whether a reaction will occur.

Key term

Define entropy.

Questions from this subtopic

Each one with the model answer underneath.

1
Give an example that shows enthalpy change alone cannot predict whether a reaction will occur.
Model answer
Some endothermic reactions occur spontaneously at room temperature, demonstrating that a positive enthalpy change does not prevent a reaction from happening.
2
Define entropy.
Model answer
Entropy counts how many ways the particles and energy of a system can be arranged — in short, how disordered it is.
3
What is the natural direction of change in terms of entropy?
Model answer
Change runs naturally towards greater total entropy, so anything spontaneous has ΔS(total) above zero.
4
Why does entropy increase when a substance changes from solid to liquid to gas?
Model answer
Particles become more disordered as they gain freedom of movement through each phase transition.
5
Explain why entropy increases when a solid ionic lattice dissolves in water.
Model answer
The ions become dispersed throughout the solution, increasing disorder compared to the ordered lattice structure.
6
What happens to entropy when a reaction makes or consumes gas molecules?
Model answer
Entropy changes occur because gases have high entropy; an increase in moles of gas increases entropy, while a decrease in moles of gas decreases entropy.

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Atomic Structure and the Periodic Table

Bonding and Structure

Redox I

Inorganic Chemistry and the Periodic Table

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AAtomisation turns each element into gaseous atoms.

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