Key term
OCR GCSE Chemistry A J248
Chemical Reactions
Topic overviewWriting formulae of elements and compoundsBalanced equations, and deducing formulae from ionsState symbols in equationsConservation of mass, and changes of mass in open systemsExothermic and endothermic reactions, and reaction profilesActivation energy and why reactions need itOxidation and reduction in terms of oxygenAcids, alkalis and neutralisationThe ionic equation for neutralisation, and reactions of acidsThe pH scale, acidity and alkalinityMeasuring pH with indicators and pH metersProducts at the electrodes, and molten ionic compoundsElectrolysis of aqueous solutionsInert and non-inert electrodes
Elements, Compounds and Mixtures
Topic overviewPurity, and melting point dataRelative formula mass and empirical formulaeFormulations, and separating mixturesPaper and thin layer chromatographyInterpreting chromatograms and Rf valuesChoosing a purification methodMetals and non-metals in the Periodic TableElectron arrangement and position in the Periodic TableIonic, covalent and metallic bonding comparedDot and cross diagrams, and the limits of modelsReactions, electron arrangement and Mendeleev's tableCarbon's four bonds, and the variety of organic compoundsDiamond, graphite, fullerenes and grapheneBond strength, intermolecular forces and predicting stateBulk properties of ionic, molecular and giant structuresNanoparticles: size and surface area to volumeUses and risks of nanoparticulate materials
Global Challenges
Topic overviewExtracting metals by reduction and by electrolysisThe Haber process and fertiliser productionNitrogen, phosphorus and potassium in agricultureLife cycle assessmentRecycling, and the decisions behind itAlloys, and the corrosion of metalsComparing glass, ceramics, polymers, composites and metalsFunctional groups and homologous seriesPredicting the products of organic reactionsAddition polymerisationDNA and other natural polymersFractional distillation of crude oilThe fractions, and crude oil as a feedstockDependence on hydrocarbons, and crackingChemical cells and fuel cellsThe Earth's early atmosphere, and how oxygen developedThe greenhouse effect, and evidence for climate changeEffects of carbon dioxide and methane, and air pollutantsProducing potable water
Monitoring and Controlling Chemical Reactions
Topic overviewCarrying out an acid–alkali titrationTheoretical mass and percentage yieldAtom economy and its calculationMeasuring the rate of a reaction, and rate graphsFactors affecting rate, and collision theorySurface area and rateCatalysts and how they workReversible reactions and dynamic equilibriumActivation energy, and enzymes as catalysts
Particles
Predicting and Identifying Reactions
Topic overviewGroups 1, 7 and 0, and why they behave as they doTransition metalsPredicting reactivity from the Periodic TableReactivity of metals with water and acidsTests for oxygen, hydrogen, carbon dioxide and chlorineTests for aqueous cations and anionsFlame tests, and identifying metal ionsIdentifying a species from test resultsInstrumental methods of analysis
Elements, Compounds and Mixtures
Paper and thin layer chromatography
- 3 things to know
- Paper 1
Overview
How a paper chromatogram is set up with a pencil baseline above the solvent, what a thin layer plate is made of and its advantage over paper, choosing a solvent, revealing colourless spots with a locating agent or ultraviolet light, the stationary and mobile phases in paper and thin layer chromatography, and why substances separate.
What you need to know
Answer each one before you reveal it.
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Key term
Describe how a paper chromatogram is set up.
Key term
Why is the baseline drawn in pencil rather than in ink?
Questions from this subtopic
Each one with the model answer underneath.
- 1
Describe how a paper chromatogram is set up.
Model answer
A baseline is drawn near the bottom of the paper, spots of the mixture are placed on it, and the paper is stood in solvent so that the solvent level is below the baseline and rises through the paper.
- 2
Why is the baseline drawn in pencil rather than in ink?
Model answer
Because pencil is insoluble in the solvent, so it will not travel up the paper or add spots of its own.
- 3
Why must the solvent level start below the baseline?
Model answer
So the spots dissolve into the solvent as it rises past them, instead of washing straight off the paper into the solvent below.
- 4
What is the plate made from in thin layer chromatography?
Model answer
It is a sheet of glass or plastic coated with a thin, even layer of silica or alumina.
- 5
Give an advantage of thin layer chromatography over paper chromatography.
Model answer
It is faster and gives sharper separation, because the coating is finer and more uniform than paper fibres.
- 6
When would a non-aqueous solvent such as ethanol be chosen instead of water?
Model answer
When the substances being separated do not dissolve in water, so they need an organic solvent to carry them.
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