What must happen to an ionic lattice before the solid can dissolve?

The electrostatic attractions holding the oppositely charged ions in the lattice must be broken, which requires an energy input equal in size to the lattice enthalpy.

What type of bond forms between a dissolved ion and the water molecules around it?

An ion–dipole bond, an electrostatic attraction between the charge on the ion and the oppositely charged end of the polar water molecule.

How do water molecules orientate themselves around a dissolved cation?

They turn their δ− oxygen atoms towards the cation, forming a shell of water around it.

How do water molecules orientate themselves around a dissolved anion?

They turn their δ+ hydrogen atoms towards the anion.

Why is water such an effective solvent for many ionic solids?

It is strongly polar, so it forms strong ion–dipole bonds whose energy release can compensate for the energy needed to pull the lattice apart.

OCR A-Level Chemistry B H433

Development of Practical Skills in Chemistry

Topic overview Planning experiments Implementing: apparatus, units and recording data Significant figures and titration calculations Graphs, gradients and drawing a conclusion Anomalous results, precision, accuracy and types of error Calculating uncertainties and improving a method Hazards, risk and control measures Recording, processing and judging data Measuring pH, mass and volume, and choosing an indicator Reflux, distillation, purification and the standard tests PAG1 to PAG6: moles, enthalpy, qualitative analysis and organic synthesis PAG7 to PAG12: electrochemistry, rates and pH

Elements of Life

Topic overview The mole, relative masses and empirical formulae Concentration, titration and writing balanced equations Orbitals, sub-shells and electron configurations Sub-atomic particles and the evidence for the model of the atom Fusion reactions and the origin of the elements Ionic, covalent, dative and metallic bonding and the four structures Melting point, conductivity and solubility of each structure Shapes of molecules and ions The periodic table, the blocks and trends in melting point Ion charges, the common polyatomic ions and writing formulae Group 2 chemistry and charge density Ionisation enthalpy and trends in reactivity Solubility rules and the tests for the common anions Hydroxide precipitates, ionic equations and identifying an unknown salt Acids, bases and preparing salts The electromagnetic spectrum and photon energy calculations Emission and absorption spectra, and flame tests Mass spectrometry and relative atomic mass

Developing Fuels

Topic overview Molar gas volume and the ideal gas equation Gas volume calculations and measuring a gas in the laboratory Bonding and shape in organic molecules Enthalpy changes: definitions and standard conditions Bond enthalpies q = mcΔT and measuring an enthalpy change in solution Burning a fuel, correcting for heat loss, and the assumptions made Hess' law and enthalpy cycles Catalysts, heterogeneous catalysis and catalyst poisons Cracking and how it is carried out The pollutants from burning fuels and their effects Catalytic converters and cutting emissions Aliphatic and aromatic compounds, functional groups and general formulae Deducing formulae and naming a branched compound Combustion of organic compounds Addition of bromine and hydrogen halides, and the test for unsaturation Hydrogenation and hydration of alkenes Addition polymerisation Electrophilic addition mechanism Displayed, shortened and skeletal formulae Chain, position and functional group isomerism Why alkenes show stereoisomerism, and the E/Z and cis/trans rules Naming and comparing the stereoisomers Sustainable fuels: benefits and risks

Elements From the Sea

Topic overview Atom economy Extraction of the halogens from sea minerals Electrolysis of molten salts and writing electrode half-equations Electrolysis of aqueous solutions and the electrolysis of brine Redox as electron transfer The rules for assigning oxidation states Oxidation state calculations, and defining oxidation and reduction Balancing a redox equation with oxidation states The iodine-thiosulfate titration and its calculations Systematic nomenclature of inorganic compounds Physical properties of the halogens Halogen reactivity and the displacement equations Observing a displacement, and using an organic solvent or starch Silver halide precipitation and tests Preparation of the hydrogen halides Properties of the hydrogen halides Chlorine: risks, benefits and uses Dynamic equilibrium Writing a Kc expression and calculating Kc What the magnitude of Kc tells you, and what changes it Using the Kc expression to explain a shift in position Temperature, pressure and catalysts on an equilibrium

The Ozone Story

Topic overview Electronegativity and bond polarity Intermolecular bonds Boiling points and intermolecular bonds Activation enthalpy and enthalpy profiles Collision theory and the Boltzmann distribution Following a reaction and finding the rate from a graph Catalysts and alternative reaction routes Homogeneous catalysis and intermediates Composition of gas mixtures by volume Recognising and naming haloalkanes Primary, secondary and tertiary haloalkanes, and the amines Properties and nucleophilic substitution of haloalkanes Substitution, nucleophiles and the SN2 mechanism Relative reactivity of the haloalkanes Homolytic and heterolytic fission, radicals and half curly arrows Initiation, propagation and termination in a radical chain reaction Ozone depletion by haloalkanes Ozone in the stratosphere and troposphere Radiation of the Earth and the Sun Effects of UV and visible radiation on molecules Calculations with frequency, wavelength and energy

What's in a Medicine?

Topic overview Formulae of oxygen-containing homologous series Primary, secondary and tertiary alcohols The acidity of phenol compared with alcohols and carboxylic acids The iron(III) chloride test and esterifying a phenol Esterification and oxidising primary, secondary and tertiary alcohols Dehydration, halogenation and planning a two-step synthesis Preparing aspirin, filtering under reduced pressure and recrystallisation Thin layer chromatography, Rf values and percentage yield Reflux and distillation in an organic preparation Washing, drying and redistilling a liquid product The aims of green chemistry and atom economy Catalysts, renewable feedstocks and measuring greenness Elimination reactions The molecular ion and fragment ions Identifying a compound from its fragments and the M+1 peak How infrared absorption works and the characteristic absorptions Reading a spectrum, the fingerprint region and combining techniques

The Chemical Industry

Topic overview Rate equations, order and the rate constant Deducing the units of k and rate calculations Half-lives Continuous monitoring, quenching and the initial rates method Reading order from concentration-time and rate-concentration graphs The Arrhenius equation and what each symbol means The ln k against 1/T plot and finding the activation enthalpy Rate-determining step and mechanism Effect of conditions on the magnitude of the equilibrium constant Economic operating conditions for an industrial process Writing a Kc expression and deducing its units ICE-table calculations and measuring Kc experimentally Chemical reactions in industrial processes Nitrogen, ammonia and the ammonium ion Oxidation states of nitrogen and the tests for nitrate and ammonium Costs, benefits and risks of industrial processes

Polymers and Life

Oceans

Topic overview Solubility in aqueous and non-aqueous solvents Lattice, hydration and solution enthalpies and how they relate Measuring an enthalpy change of solution Charge density and the size of enthalpy terms Entropy: qualitative treatment Total entropy change and the condition for feasibility The limiting temperature, and why a feasible reaction may not happen Calculating the entropy change of the system The solubility product expression, its units and calculations Predicting precipitation and the common ion effect Bronsted-Lowry theory of acids and bases Strong acids and strong bases Weak acids, Ka and pKa pH of strong acids and strong bases pH of weak acids, dilution and the limits of the approximation What a buffer is and how it resists a change in pH Buffer calculations, capacity and the natural buffers How the greenhouse effect works and which gases cause it Why a molecule absorbs, concentrations in ppm, and ocean chemistry

Developing Metals

Topic overview Manganate(VII) titrations and their calculations Thiosulfate titrations, recording results and good technique Shapes of complexes and coordination number Balancing redox equations involving acid-base reactions Simple electrochemical cells Action of an electrochemical cell Standard electrode potentials and calculating E°cell Rusting and how it is prevented Transition metals: definition, oxidation states and colours d-block electron configurations of the atoms and their ions Why transition metals show variable oxidation states Ligands and complex ions Coordinate bonding and what makes a ligand Bidentate and multidentate ligands, and finding a complex's formula Reactions of Fe2+, Fe3+ and Cu2+ with hydroxide and ammonia Catalytic activity of transition metals Origin of colour in transition metal complexes Colorimetry

Colour by Design

Topic overview How dyes attach to fibres Structure of a dye molecule Fats and oils Arenes and delocalisation Representations of benzene and enthalpy of hydrogenation Tests for the common functional groups Identifying a polyfunctional molecule from a set of test results Halogenation and nitration of benzene and the electrophiles involved The nitration mechanism, sulfonation and the Friedel-Crafts reactions Diazotisation and why it is done cold Coupling reactions, azo dyes and the route from benzene Oxidising an aldehyde and the three carbonyl tests Cyanohydrin formation and its use in synthesis Using the Data Sheet reactions to plan a route Choosing between routes on yield and atom economy Nucleophilic addition mechanism Classifying organic reactions and unfamiliar mechanisms Origins of colour in organic molecules How gas-liquid chromatography separates a mixture Retention time, peak area and the limits of the technique

Chemical Literacy

Topic overview Extracting and using data and information from a scientific text Written communication and comprehension

Mathematical Skills in Chemistry

Topic overview Units, standard form and the mole in calculations Yield, atom economy and logarithmic quantities Significant figures, means and handling data Symbols, and rearranging the standard equations Substituting into rearranged equations Reading spectra and kinetics graphs Gradients, calibration lines and the Arrhenius plot Shapes and bond angles from electron pair repulsion Chiral centres, E/Z isomerism and optical activity

Oceans

19 subtopics
43 things to know
Components 01, 02 and 03

What Oceans covers

The storyline that carries thermodynamics and acid-base equilibria: solubility and the enthalpies of lattice, hydration and solution, entropy and feasibility, the solubility product, strong and weak acids with pH and buffers, and the greenhouse effect and ocean acidification.

Where it is assessed

Components 01, 02 and 03

Storyline content, drawn on by all three written papers.

Check the OCR specification ↗

The 19 subtopics

Each one has its own page.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why will sodium chloride not dissolve in hexane, why does iodine prefer cyclohexane, and why does alcohol solubility fall with chain length?

Key term

Why do non-polar solutes dissolve so poorly in water?

Numbers

Why is the enthalpy change of hydration always negative?

Key term

When ammonium nitrate dissolves in water and the temperature falls, why is the enthalpy change of solution positive?

Numbers

Why is an expanded-polystyrene cup with a lid used rather than a glass beaker?

Process

Why does the lattice enthalpy appear with its sign reversed in an enthalpy cycle for dissolving?

Numbers

Why does greater charge density make lattice and hydration enthalpies more exothermic, and why is the lattice enthalpy of MgO far more exothermic than that of NaCl?

Key term

Why is the lattice enthalpy of sodium fluoride more exothermic than that of sodium iodide?

Numbers

Ca²⁺ has a radius of about 100 pm and Na⁺ about 102 pm. Which factor governs the difference in their hydration enthalpies?

Numbers

What are the units of entropy, how do the three states rank, and why does entropy rise on melting?

Key term

Why does the entropy of any substance increase as its temperature rises?

Key term

Why is counting the moles of gas on each side the quickest way to predict the sign of ΔsysS?

Key term

Why does mixing two gases increase the entropy?

Numbers

How is the entropy change of the surroundings related to ΔH, what does a total entropy change of zero indicate, and why does an exothermic reaction increase the entropy of the surroundings?

Numbers

How is the total entropy change worked out, and why must ΔH be in J mol⁻¹ and the temperature in kelvin?

Numbers

Why does the decomposition of calcium carbonate (ΔH +178 kJ mol⁻¹, ΔsysS +160.4 J K⁻¹ mol⁻¹) become feasible only at high temperature, where is forming N₂O₄ from NO₂ (ΔH −57.2 kJ mol⁻¹, ΔsysS −175.8 J K⁻¹ mol⁻¹) feasible, and why does a positive ΔtotS not guarantee a reaction is seen?

Numbers

How is the entropy change of the system found from standard entropies, why is each multiplied by its balancing number, and why are the standard entropies of elements not zero?

Key term

Why is the entropy change of the system for the vaporisation of water large and positive?

Key term

Why are standard entropies quoted at a stated temperature and pressure?

Numbers

What are the solubility product expressions for silver chloride and magnesium hydroxide, and why is the solid left out?

Numbers

Make s the subject of the expression Ksp = 4s³.

Numbers

Why must the concentrations be halved when equal volumes of two solutions are mixed?

Process

How does a fall in ocean pH threaten shells made of calcium carbonate?

Numbers

What are a Brønsted-Lowry acid and base, and which is the proton donor when hydrogen chloride dissolves?

Numbers

What are the conjugate base of hydrochloric acid and the conjugate acid of ammonia, and which species is the acid when ammonia dissolves?

Numbers

What happens when carbon dioxide dissolves in the ocean, and how does acid strength relate to conjugate base strength?

Numbers

In HCO₃⁻(aq) + OH⁻(aq) → CO₃²⁻(aq) + H₂O(l), which species is the base?

Numbers

What are the equations for nitric acid ionising, for the first ionisation of sulfuric acid, and for sodium hydroxide dissolving?

Numbers

What is the equation for barium hydroxide dissolving, what is the difference between a strong acid and a concentrated acid, and can an acid be both strong and dilute?

Numbers

Why does [H⁺] equal the acid concentration for a strong monoprotic acid?

Numbers

What is the equation for ethanoic acid ionising, why is an equilibrium arrow used, and what is the Ka expression?

Numbers

What is pKa, what is a Ka of 1.8 × 10⁻⁵ as a pKa, and which of methanoic and ethanoic acid is stronger?

Key term

Why is [H⁺] taken as equal to [A⁻] in a solution of a weak acid alone?

Numbers

What is pH, how is the hydrogen ion concentration recovered from it, and what is the ionic product of water?

Numbers

What expression gives [H⁺] for a weak acid, which two approximations does it rest on, and what are three strong acids?

Numbers

Write the equation for what happens in an ethanoate buffer when a little alkali is added.

Numbers

State the expression used to find [H⁺] in an acidic buffer.

Numbers

Why does diluting a buffer leave its pH almost unchanged?

Numbers

Why does the Earth radiate infrared, what happens inside a greenhouse gas molecule, and how does that warm the atmosphere?

Numbers

Methane is present at only about 1.9 ppm. Why does it still matter for the climate?

Process

How does rising atmospheric carbon dioxide change the chemistry of the oceans?

Key term

Why will sodium chloride not dissolve in hexane?

Numbers

How do you obtain the hydrogen ion concentration from a pH value?

Take Oceans further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

Lattice enthalpy vs Enthalpy change of hydration
Lattice enthalpy: the enthalpy change when one mole of an ionic solid forms from its gaseous ions. Enthalpy change of hydration: the enthalpy change when one mole of gaseous ions is dissolved in water.
Entropy of the system vs Entropy of the surroundings
Entropy of the system: the disorder of the reacting substances themselves. Entropy of the surroundings: the disorder change outside the reaction, found from the enthalpy change and temperature.
Strong acid vs Concentrated acid
Strong acid: an acid that is fully ionised in aqueous solution. Concentrated acid: an acid present at a high number of moles per unit volume.

Questions from this topic

Each one with the model answer underneath.

1
What must happen to an ionic lattice before the solid can dissolve?
Model answer
The electrostatic attractions holding the oppositely charged ions in the lattice must be broken, which requires an energy input equal in size to the lattice enthalpy.
2
What type of bond forms between a dissolved ion and the water molecules around it?
Model answer
An ion–dipole bond, an electrostatic attraction between the charge on the ion and the oppositely charged end of the polar water molecule.
3
How do water molecules orientate themselves around a dissolved cation?
Model answer
They turn their δ− oxygen atoms towards the cation, forming a shell of water around it.
4
How do water molecules orientate themselves around a dissolved anion?
Model answer
They turn their δ+ hydrogen atoms towards the anion.
5
Why is water such an effective solvent for many ionic solids?
Model answer
It is strongly polar, so it forms strong ion–dipole bonds whose energy release can compensate for the energy needed to pull the lattice apart.
6
Why will sodium chloride not dissolve in hexane?
Model answer
Hexane is non-polar and cannot form ion–dipole bonds, so only very weak induced-dipole attractions are available and these release far too little energy to break up the lattice.

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Development of Practical Skills in Chemistry

Elements of Life

Developing Fuels

Elements From the Sea

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A acid is formed.

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A3 ⁴₂He → ¹²₆C.

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Nitrogen is +2 in NO and +4 in NO₂.

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Oceans revision questions

What does Oceans cover in OCR A-Level Chemistry B?: 19 subtopics: solubility in aqueous and non-aqueous solvents, lattice, hydration and solution enthalpies and how they relate, measuring an enthalpy change of solution, charge density and the size of enthalpy terms, entropy: qualitative treatment, total entropy change and the condition for feasibility, the limiting temperature, and why a feasible reaction may not happen, calculating the entropy change of the system, the solubility product expression, its units and calculations, predicting precipitation and the common ion effect, bronsted-lowry theory of acids and bases, strong acids and strong bases, weak acids, ka and pka, ph of strong acids and strong bases, ph of weak acids, dilution and the limits of the approximation, what a buffer is and how it resists a change in ph, buffer calculations, capacity and the natural buffers, how the greenhouse effect works and which gases cause it and why a molecule absorbs, concentrations in ppm, and ocean chemistry. Each one has its own page with questions and model answers.
How is Oceans organised?: By the OCR H433 specification, subtopic by subtopic, so it matches the order you are taught it in.

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