Key term
WJEC A-Level Chemistry 1410QS
Formulae and Equations
Basic Ideas About Atoms
Topic overviewRadioactive decay and nuclear equationsAlpha, beta and gamma radiationHalf-life, dating and the uses of radioisotopes: half-life of radioactive decayHalf-life, dating and the uses of radioisotopes: adverse consequences for living cellsIonisation energies and electronic structureOrbitals and the filling of sub-shellsAtomic spectra and the energy of light: atomic emission spectrum of the hydrogen atomAtomic spectra and the energy of light: relationship between energy and frequencyAtomic spectra and the energy of light: significance of the frequency
Chemical Calculations
Topic overviewRelative masses and the mass spectrometer: relative atomicRelative masses and the mass spectrometer: how a mass spectrometer worksRelative masses and the mass spectrometer: molecular ionThe mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectraThe mole, the Avogadro constant and molar mass: moles and percentage composition by massEmpirical and molecular formulaeConcentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3Concentration, solubility and titration calculations: stoichiometry and the limiting reagentConcentration, solubility and titration calculations: titration calculations and estimating a titreGas volumes and the ideal gas equationYield, atom economy and experimental uncertainty: concepts of atom economy and percentage yieldYield, atom economy and experimental uncertainty: how to estimate the percentage error
Bonding
Topic overviewIonic bonds and covalent bonds: ionic bonding in terms of ion formationIonic bonds and covalent bonds: covalent bonding and forcesElectronegativity, polarity and in-between bonds: concepts of electronegativity and bond polarityElectronegativity, polarity and in-between bonds: forcesForces between moleculesHydrogen bondingShapes of molecules and ions: VSEPR principle and its useShapes of molecules and ions: bond angles associated with linear
Solid Structures
The Periodic Table
Topic overviewHow the Periodic Table is organisedOxidation and reduction as electron transferTrends across periods and down groupsGroup 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acidGroup 2: reactions, trends and the reasons behind them — trends down the groupGroup 2: reactions, trends and the reasons behind them — thermal stability of the carbonatesGroup 2: reactions, trends and the reasons behind them — oxides and hydroxidesIdentifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxideIdentifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acidGroup 7: the halogens — physical properties and the trend in volatilityGroup 7: the halogens — reactions of the halogens with metalsGroup 7: the halogens — displacement reactions and the trendGroup 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium
Simple Equilibria and Acid-Base Reactions
Thermochemistry
Rates of Reaction
The Wider Impact of Chemistry
Organic Compounds
Hydrocarbons
Topic overviewAlkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefitsAlkane bonding, combustion and fossil fuels: combustion calculations and acid rainRadical substitution of alkanesThe C=C bond and why alkenes are reactiveE–Z isomerism in alkenesElectrophilic addition, Markovnikov's rule and testing for alkenes: mechanism of electrophilic additionElectrophilic addition, Markovnikov's rule and testing for alkenes: Markovnikov's rule and the bromine water testElectrophilic addition, Markovnikov's rule and testing for alkenes: testing for a C=C bondHydrogenation and addition polymers: conditions requiredHydrogenation and addition polymers: nature of addition polymerisation
Halogenoalkanes
Topic overviewNucleophilic substitution by hydroxide ionsElimination to form alkenesReactivity of the carbon–halogen bond and the silver nitrate testUses of halogenoalkanes, CFCs and the ozone layer: uses of halogenoalkanes and CFCsUses of halogenoalkanes, CFCs and the ozone layer: radical mechanismUses of halogenoalkanes, CFCs and the ozone layer: replacements for CFCsHeating under reflux in the laboratory
Alcohols and Carboxylic Acids
Topic overviewTwo routes to ethanol: hydration and fermentation — hydration of etheneTwo routes to ethanol: hydration and fermentation — fermentationTwo routes to ethanol: hydration and fermentation — comparing the two routes on rateClassifying alcohols and dehydrating themOxidising alcohols to aldehydes, ketones and acidsChemical tests for alcohols and carboxylic acidsCarboxylic acids, bases and saltsMaking an ester and separating it by distillation: esterificationMaking an ester and separating it by distillation: carrying out the preparationMaking an ester and separating it by distillation: separating and purifyingMaking an ester and separating it by distillation: yield
Instrumental Analysis
Redox and Standard Electrode Potential
Redox Reactions
Chemistry of the P-Block
Topic overviewAmphoteric behaviour of aluminium and leadStability of oxidation states down Groups 3, 4 and 5: increasing stability of the inert pair cationsStability of oxidation states down Groups 3, 4 and 5: change in relative stabilityValence shells that break the octet rule: how some Group 3 elements can form compoundsValence shells that break the octet rule: structure and bonding in Al2Cl6 and formationBoron nitride: two structures, two sets of propertiesGroup 4 oxides and chlorides, and lead in solution: structureGroup 4 oxides and chlorides, and lead in solution: hydrolysis of the Group 4 chloridesGroup 4 oxides and chlorides, and lead in solution: chloride hydrolysis and lead(II) precipitatesGroup 4 oxides and chlorides, and lead in solution: distinguishing lead from silver ionsChlorine, chlorate and the sodium halides: chlorine with water and with cold and hot alkaliChlorine, chlorate and the sodium halides: bleaching and bactericidal action of Cl2Chlorine, chlorate and the sodium halides: differences in behaviour of NaCl
Chemistry of the D-Block Transition Metals
Topic overviewVariable oxidation states and the colours of their solutions: how the d-block elements attain variousVariable oxidation states and the colours of their solutions: most important oxidation states of CrComplex ions: ligands, coordination number and shapeWhy transition metal complexes are colouredLigand exchange and changes in coordination numberTransition metals as catalysts: why transition metalsTransition metals as catalysts: variable oxidation stateTransition metals as catalysts: heterogeneous catalysisTransition metals as catalysts: surface areaTransition metal ions with aqueous hydroxide
Chemical Kinetics
Enthalpy Changes for Solids and Solutions
Entropy and Feasibility of Reactions
Equilibrium Constants
Acid-Base Equilibria
Topic overviewProton transfer and conjugate pairsStrong and weak acids: Ka and pKaWater, Kw and the pH scalepH calculations for acids and alkalis: how to use pHpH calculations for acids and alkalis: strong bases, dilution and mixturesBuffers: how they hold pH steady and why they matterTitrations: curve shapes, salt pH and choosing an indicator — pH curve shapes for strong and weak acids and basesTitrations: curve shapes, salt pH and choosing an indicatorTitrations: curve shapes, salt pH and choosing an indicator — how an indicator works and choosing oneTitrations: curve shapes, salt pH and choosing an indicator — using a pH probe
Stereoisomerism
Aromaticity
Alcohols and Phenols
Aldehydes and Ketones
Carboxylic Acids and Their Derivatives
Topic overviewComparing the acidity of carboxylic acids, phenols, alcohols and waterMaking carboxylic acids by oxidationReduction and decarboxylation of carboxylic acids: reduction of carboxylic acids using LiAlH₄Reduction and decarboxylation of carboxylic acids: decarboxylation of carboxylic acidsEsters and acid chloridesTurning acids and halogenoalkanes into amides and nitrilesHydrolysis and reduction of nitriles and amides
Amines
Topic overviewPreparing aliphatic and aromatic aminesThe basicity of aminesEthanoylation of primary aminesPrimary amines and cold nitric(III) acidCoupling reactions and azo dyes: coupling of benzenediazonium salts with phenolsCoupling reactions and azo dyes: role of the —N=N— chromophore in azo dyesThe origin of colour
Amino Acids, Peptides and Proteins
Organic Synthesis and Analysis
Topic overviewPlanning a synthesis as a sequence of reactionsWorking up a preparation: separation and purificationMelting temperature as a test of purityAddition and condensation polymerisation: polyesters and polyamidesPolymers: hydrolysis, properties and identifying repeat unitsHigh-resolution ¹H NMR and the elucidation of structure: low- and high-resolution proton NMRHigh-resolution ¹H NMR and the elucidation of structure: chemical shift, TMS, solvents and D₂O exchangeHigh-resolution ¹H NMR and the elucidation of structure: combining mass spectra and NMR to identify compoundsHigh-resolution ¹H NMR and the elucidation of structure: combining NMR with mass spectrometryChromatography and the composition of mixtures
Practical Skills and Techniques
Topic overviewMeasuring mass, volume, time and temperatureStandard solutions and titration techniqueIndicators and acid–base titration curvesHeating, refluxing, distilling and purifying: HeatingHeating, refluxing, distilling and purifying: purifyingMelting temperature, chromatography and pH measurementElectrochemical cells, rate methods and working safely: Electrochemical cellsElectrochemical cells, rate methods and working safely: rate methods and working safely
Planning, Analysis and Evaluation
Core Experiments and Their Chemistry
Topic overviewGravimetric and acid–base analysisEnthalpy and rate experiments: EnthalpyEnthalpy and rate experiments: rate experimentsRedox titrations and electrochemical cellsEquilibrium constants and following a titration with a pH probeOrganic synthesis, separation and identification: Organic synthesisOrganic synthesis, separation and identification: identification tests and chromatography
Mathematical Skills in Chemistry
Topic overviewUnits, standard form and proportion in chemical calculation: volume, equilibrium, rate and energy unitsUnits, standard form and proportion in chemical calculation: standard formUnits, standard form and proportion in chemical calculation: proportionUnits, standard form and proportion in chemical calculation: checking answers, buffers, rate constants and unitsSignificant figures, means and uncertainty in experimental data: significant figures in multiplicationSignificant figures, means and uncertainty in experimental data: relative atomic mass as a weighted meanSignificant figures, means and uncertainty in experimental data: absolute and percentage uncertaintySymbols, rearranging equations and logarithms: chemical symbols and what each one assertsSymbols, rearranging equations and logarithms: rearranging an equationSymbols, rearranging equations and logarithms: substituting into equations for Kc, enthalpy and pHSymbols, rearranging equations and logarithms: logarithms, the Arrhenius equation and rearranging formulaeGraphs: plotting, gradients, intercepts and tangents — reading a spectrumGraphs: plotting, gradients, intercepts and tangents — reaction order from data and plotting good graphsGraphs: plotting, gradients, intercepts and tangents — Arrhenius plots, zero-order graphs and tangentsGraphs: plotting, gradients, intercepts and tangents — initial rates from tangents and interpreting graphsShape, symmetry and three-dimensional structure: ShapeShape, symmetry and three-dimensional structure: symmetry
Organic Synthesis and Analysis
Chromatography and the composition of mixtures
- 3 things to know
- A2 Unit 4
Overview
Recall questions on Rf value and retention time in gas chromatography.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Key term
Why is the starting line on a chromatogram drawn in pencil, and why must it lie above the level of the solvent?
Key term
Why is a lid kept on the chromatography tank while the plate is running?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What two phases does every chromatographic separation involve, and what makes the components separate?
Model answer
There is a stationary phase, which stays fixed in place, and a mobile phase, which moves through or over it carrying the mixture. Each component of the mixture divides itself between the two phases according to how strongly it is attracted to each. A component strongly attracted to the mobile phase spends more time moving and travels further or emerges sooner, while one strongly attracted to the stationary phase is held back.
- 2
Identify the stationary and mobile phases in thin-layer chromatography.
Model answer
The stationary phase is a thin, uniform layer of a solid such as silica or alumina spread on a glass or plastic plate. The mobile phase is a liquid solvent, which rises up the layer by capillary action carrying the components of the mixture with it.
- 3
In paper chromatography, what actually acts as the stationary phase?
Model answer
It is not the cellulose of the paper itself but the water molecules held within the cellulose fibres. The components partition between this trapped water and the moving solvent, so paper chromatography separates chiefly by partition between two liquids.
- 4
Why is the starting line on a chromatogram drawn in pencil, and why must it lie above the level of the solvent?
Model answer
Pencil is graphite, which is insoluble in the solvent, so the line stays exactly where it was drawn; an ink line would dissolve and run up the plate, and its own components would separate and confuse the result. The line must be above the solvent level so that the spots are carried up by solvent rising through the stationary phase; if they were below it, the samples would simply dissolve off into the solvent in the tank.
- 5
Why is a lid kept on the chromatography tank while the plate is running?
Model answer
The lid allows the air in the tank to become saturated with solvent vapour. Without it, solvent evaporates from the surface of the plate as it rises, so the solvent front moves unevenly and slowly and the spots become distorted. A saturated atmosphere gives a straight, evenly advancing solvent front and reproducible results.
- 6
Define the Rf value.
Model answer
The Rf value is the distance travelled by the centre of a spot from the starting line, divided by the distance travelled by the solvent front from the same line, measured on the same chromatogram.
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