Process
OCR A-Level Chemistry H432
Foundations in Chemistry
Topic overviewAtomic structure, isotopes and counting sub-atomic particlesRelative isotopic, atomic, molecular and formula massPredicting ionic charges and the common ionsWriting formulae, balanced equations and ionic equationsThe mole, molar mass and the Avogadro constantEmpirical and molecular formulae, and hydrated saltsGas volumes, concentration and the ideal gas equationStoichiometry, percentage yield and atom economyApparatus, and why high atom economy mattersMathematical skills and measuring mass, volume and gasAcids, alkalis, dissociation and neutralisationPreparing a standard solutionCarrying out a titration and its calculationsPlanning an experiment: variables, apparatus and recording resultsPractical safety, heating and filtrationUncertainty, percentage error, precision and accuracyEvaluating results: anomalies, conclusions, limitations and improvementsAssigning oxidation numbersRoman numerals in names, and the chlorate and nitrate ionsOxidation, reduction and metals reacting with acidsRedox equations and identifying what is oxidisedShells, orbitals and sub-shellsThe order in which sub-shells fillWriting electron configurations for atoms and ions
Bonding and Structure
Topic overviewIonic bonding and the properties of ionic compoundsCovalent and coordinate bondingLone pairs, bonding pairs and average bond enthalpyElectron pair repulsion theory, linear and trigonal planar shapesTetrahedral, pyramidal and non-linear shapes and their bond anglesOctahedral shapes, ions, and drawing molecules in three dimensionsElectronegativity, polar bonds and polar moleculesPermanent and induced dipoles, and hydrogen bondingSimple molecular lattices and why they melt easilySolubility and conductivity of simple molecular compounds
Periodic Table and Energy
Topic overviewPeriodicity and the electron configurations of periods 2 and 3The blocks, and first and successive ionisation energiesMetallic and giant covalent structures and their propertiesGroup 2 reactions with oxygen, water and acidsIonisation energy down Group 2 and the alkalinity of the hydroxidesUses of Group 2 compounds in agriculture and medicineThe halogens: boiling points, London forces and reactivityHalogen displacement reactions and the colours seenDisproportionation and the chlorination of waterTesting for halide ions with silver nitrate and ammoniaTesting for carbonate, sulfate and halide ionsTesting for ammonium ions and the order of the testsConcentration, pressure and measuring reaction rateCatalysts and their industrial importanceThe Boltzmann distribution, temperature and catalysts
Enthalpy Changes
Topic overviewExothermic and endothermic reactions and enthalpy profilesStandard conditions and the standard enthalpy changesBond enthalpies, q = mcΔT and calorimetryCalculating enthalpy change from bond enthalpiesWhy average bond enthalpies give only an approximationHess' law and enthalpy cycles from combustion and formation dataWorking through an enthalpy cycle and the sign conventionsCalorimetry and correcting for heat loss
Chemical Equilibrium
Basic Concepts of Organic Chemistry
Topic overviewIUPAC nomenclature and the alkyl groupsGeneral, structural, displayed and skeletal formulaeHomologous series, functional groups and saturationStructural isomers, bond fission and curly arrowsChain, position and functional group isomerismCurly arrows, homolytic and heterolytic fissionNucleophiles, electrophiles, radicals and the mechanism steps
Core Organic Chemistry
Topic overviewAlkanes: bonding, shape and boiling pointsCombustion of alkanes and their reaction with halogensThe radical substitution mechanism and its limitationsAlcohols: polarity, solubility and classificationOxidation, dehydration and substitution reactions of alcoholsHydrolysis of haloalkanes and comparing the ratesNucleophilic substitution and carbon-halogen bond strengthCFCs, radicals and the catalytic destruction of ozoneCombining analytical techniques to identify a compound
Alkenes
Topic overviewAlkenes: the double bond, sigma and pi bonding, and shapeE/Z and cis-trans isomerism in alkenesHydrogenation, halogenation and hydration of alkenesElectrophilic addition, carbocation stability and Markownikoff's ruleAddition polymerisation and repeat unitsWaste polymers, and biodegradable and photodegradable alternatives
Organic Synthesis
How Fast?
Topic overviewRate equations, order and the rate constantDeducing orders and finding the units of kWhat the rate equation reveals about the mechanismConcentration-time graphs and continuous monitoringHalf-life and the rate constant of a first-order reactionRate-concentration graphs and the initial rates methodClock reactions, colorimetry and analysing kinetic graphsThe rate-determining step and testing a proposed mechanismTemperature, the rate constant and the Arrhenius equationThe ln k against 1/T graph and finding the activation energy
Physical Chemistry and Transition Elements
Topic overviewMole fraction, partial pressure and ICE tablesKc and Kp expressions, their units and calculationsWhat changes the equilibrium constant, and the reaction quotientLattice enthalpy and the Born-Haber cycleEnthalpy of solution and hydration, and the effect of ionic charge and sizeEntropy and calculating entropy changeGibbs free energy and thermodynamic feasibility
Acids, Bases and Buffers
Topic overviewBrønsted-Lowry acids and bases and their reactionsKa, pKa, pH and the ionic product of waterCalculating the pH of strong acids, strong bases and weak acidspH and hydrogen ion concentrationWhat a buffer is and how it resists a change in pHBuffer calculations and the buffer that controls blood pHTitration curves, choosing an indicator and using a pH meterpH titration curves for the four acid-base combinationsHow an indicator works and choosing the right one
Redox and Electrode Potentials
Topic overviewOxidising and reducing agents, and balancing half-equationsCombining half-equations and using oxidation numbersRedox titrations and their calculationsStandard electrode potentials and setting up half-cellsThe electrochemical series and predicting feasibilityStorage cells and fuel cells
Transition Elements
Topic overviewLigands, complex ions and coordination numberElectron configurations from Sc to Zn, and which are transition elementsVariable oxidation states, coloured ions and catalysisThe shapes of complexes, stereoisomerism and cisplatinLigand substitution, haemoglobin and the copper complexesHydroxide precipitates of iron, manganese and chromiumRedox interconversions of iron, chromium and copperWhy ligand substitution happens and the changes it causesPrecipitation reactions written as aqua complexesDissolving hydroxides in excess ammonia and distinguishing the ions
Aromatic Compounds
Topic overviewThe delocalised model of benzene and the evidence for itNitration, halogenation and Friedel-Crafts reactions of benzeneThe reactions of phenol, and directing effectsElectrophilic substitution and directing effectsPhenol: acidity, bromination and nitrationWhy phenol is more reactive than benzene, and planning a synthesis
Organic Chemistry and Analysis
Topic overviewOxidation and reduction of carbonyls, and the addition of HCNThe nucleophilic addition mechanism2,4-DNPH and Tollens' reagentCarboxylic acids: solubility and reactions with metals, carbonates and basesEsterification and the hydrolysis of estersAcyl chlorides and their reactions with alcohols, amines and ammoniaAmines as bases, and preparing aliphatic aminesSecondary and tertiary amines, and preparing aromatic aminesAmino acids, their reactions, and amidesOptical isomerism and identifying a chiral centreCondensation polymerisation and the ester and amide linkagesHydrolysing condensation polymers and telling them from addition polymersNitriles from haloalkanes and from carbonyl compoundsFriedel-Crafts reactions and lengthening the carbon chainThin-layer and gas chromatography, Rf values and retention timesTest-tube tests for the organic functional groupsTests for the inorganic ions and interpreting chromatograms
Spectroscopy
Topic overviewCarbon-13 and proton NMR: peaks, chemical shift and integrationSpin-spin splitting and predicting a spectrumTMS, deuterated solvents and the D₂O shakeCombining elemental analysis, mass spectrometry, IR and NMRMolecular formula from the molecular ion, and fragmentation lossesWhat IR and NMR each contribute to a structureInfrared absorption, global warming and breathalysersA systematic approach and the degree of unsaturation
Enthalpy Changes
- 8 subtopics
- 15 things to know
- Components 01 and 03
What Enthalpy Changes covers
Energy changes in reactions: exothermic and endothermic profiles, the standard enthalpy changes and how each is defined, calculating an enthalpy change from bond enthalpies, Hess' law and enthalpy cycles, and measuring an enthalpy change by calorimetry.
Where it is assessed
Components 01 and 03
Periodic Table, Elements and Physical Chemistry, then the unified paper.
Check the OCR specification ↗The 8 subtopics
Each one has its own page.
- Exothermic and endothermic reactions and enthalpy profilesRecall questions on the sign of ΔH for each type, what the axes of an enthalpy profile show, where the products sit, and how activation energy appears.Open →
- Standard conditions and the standard enthalpy changesRecall questions on standard conditions and standard states, and on the definitions of the standard enthalpy changes of reaction, formation, combustion and neutralisation.Open →
- Bond enthalpies, q = mcΔT and calorimetryRecall questions on average bond enthalpy, why bond breaking is endothermic and bond making exothermic, q = mcΔT, and what calorimetry measures.Open →
- Calculating enthalpy change from bond enthalpiesRecall questions on ΔH = Σ(bonds broken) − Σ(bonds formed), identifying which bonds break and form, and working the sum through.Open →
- Why average bond enthalpies give only an approximationRecall questions on why a bond's strength varies between molecules, why the values are averages, and the gaseous-state assumption behind them.Open →
- Hess' law and enthalpy cycles from combustion and formation dataRecall questions on the statement of Hess' law and why it holds, the two cycle constructions, and the formulae that follow from each.Open →
- Working through an enthalpy cycle and the sign conventionsRecall questions on reversing a reaction's sign, treating stoichiometric coefficients, and the order of steps in solving an unfamiliar cycle.Open →
- Calorimetry and correcting for heat lossRecall questions on q = mcΔT in practice, why a polystyrene cup and a lid are used, why stirring matters, and correcting for heat loss by extrapolation.Open →
What you need to know
Answer each one before you reveal it.
Your recallNot started
Numbers
What is activation energy, how is it shown on an enthalpy profile, and what are standard conditions as used in thermochemistry?
Numbers
What are standard states, and how are the standard enthalpy changes of reaction and of formation defined?
Numbers
How are the standard enthalpy changes of combustion and of neutralisation defined, and what equation gives the heat energy transferred?
Key term
Is breaking chemical bonds an endothermic or exothermic process?
Key term
Is making chemical bonds an endothermic or exothermic process?
Numbers
Why is energy released when bonds form, and what formula gives ΔH from bond enthalpies?
Evaluation
What are average bond enthalpies, why might a real C–H bond in methane differ from the tabulated value, and why are calculations using them only approximate?
Numbers
Calculate the enthalpy change when sum of bond enthalpies broken = 2648 kJ mol⁻¹, sum of bond enthalpies formed = 3462 kJ mol⁻¹.
Numbers
What does Hess' law state, why is it valid, and what formula applies it to enthalpy changes of combustion?
Numbers
What formula gives ΔrH from enthalpies of formation, what value does an element in its standard state take, and why is ΔfH of graphite zero?
Process
What happens to the sign of ΔH when a reaction is reversed, what must be accounted for when summing along an alternative route, and what is the first step in an unfamiliar enthalpy cycle?
Numbers
How is an unknown enthalpy change found from an alternative route, how can enthalpy changes be determined directly, and what equation gives the heat transferred?
Numbers
State the formula for calculating the enthalpy change of reaction using combustion data, including stoichiometric coefficients.
Key term
What does graphical extrapolation allow you to determine in a calorimetry experiment?
Take Enthalpy Changes further
The same content, four other ways.
Commonly confused
Pairs that lose marks when they are blurred together.
Exothermic change vs Endothermic change
Exothermic change: a process transferring heat from the system to the surroundings. Endothermic change: a process transferring heat from the surroundings to the system.
Enthalpy of formation vs Enthalpy of combustion
Enthalpy of formation: the enthalpy change when one mole of a compound forms from its elements. Enthalpy of combustion: the enthalpy change when one mole of a substance burns completely in oxygen.
Mean bond enthalpy vs Reaction enthalpy
Mean bond enthalpy: average energy needed to break a specified gaseous covalent bond. Reaction enthalpy: heat change accompanying a reaction under stated conditions.
Questions from this topic
Each one with the model answer underneath.
- 1
What defines an exothermic reaction in terms of enthalpy transfer?
Model answer
An exothermic reaction is one in which enthalpy is transferred from the system to the surroundings, resulting in a negative enthalpy change (ΔH < 0).
- 2
If a reaction has ΔH = −285 kJ mol⁻¹, is it exothermic or endothermic?
Model answer
Exothermic, because the enthalpy change is negative (ΔH < 0).
- 3
What defines an endothermic reaction in terms of enthalpy transfer?
Model answer
An endothermic reaction is one in which enthalpy is transferred from the surroundings to the system, resulting in a positive enthalpy change (ΔH > 0).
- 4
A reaction absorbs energy from its surroundings. What sign does its ΔH have?
Model answer
Positive (ΔH > 0), indicating an endothermic reaction.
- 5
On an enthalpy profile diagram, what quantities are represented on the y-axis and x-axis?
Model answer
The y-axis represents enthalpy and the x-axis represents the progress of the reaction.
- 6
On an enthalpy profile diagram for an exothermic reaction, how do the enthalpy levels of products compare to reactants?
Model answer
The products are at a lower enthalpy level than the reactants.
What is inside Levo
Six ways to work the same material.
Enthalpy Changes revision questions
- What does Enthalpy Changes cover in OCR A-Level Chemistry?
- 8 subtopics: exothermic and endothermic reactions and enthalpy profiles, standard conditions and the standard enthalpy changes, bond enthalpies, q = mcδt and calorimetry, calculating enthalpy change from bond enthalpies, why average bond enthalpies give only an approximation, hess' law and enthalpy cycles from combustion and formation data, working through an enthalpy cycle and the sign conventions and calorimetry and correcting for heat loss. Each one has its own page with questions and model answers.
- How is Enthalpy Changes organised?
- By the OCR H432 specification, subtopic by subtopic, so it matches the order you are taught it in.
Every question in Enthalpy Changes, on a schedule.
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