Why does a bond absorb only certain frequencies of infrared radiation?

Each bond vibrates at its own natural frequency, and it can only take up energy from radiation whose frequency matches that natural frequency exactly.

What two properties of a bond determine the natural frequency at which it vibrates?

The frequency is determined by the strength of the bond and by the masses of the two atoms that the bond joins.

How does the strength of a bond affect the wavenumber at which it absorbs?

A stronger bond vibrates more rapidly, so it absorbs radiation of higher frequency and therefore appears at a higher wavenumber.

How does the mass of the bonded atoms affect the wavenumber at which a bond absorbs?

Heavier atoms vibrate more slowly, so a bond joining heavy atoms absorbs at a lower frequency and therefore at a lower wavenumber.

CCEA AS-Level Chemistry 1110

Formulae, Equations and Amounts of Substance

Topic overview Ionic charges and the formulae of compound ions Writing and balancing full and ionic equations The mole, molar mass and Avogadro's constant Reacting masses and reactants in excess Hydrated salts and water of crystallisation

Atomic Structure

Topic overview Protons, neutrons and electrons Atomic number, mass number and the composition of ions Isotopes and relative isotopic mass Relative atomic, molecular and formula mass Mass spectra and relative atomic mass calculations Mass spectra of diatomic elements Electronic configuration and the building up principle Orbitals, electron spin and orbital shapes The s-, p-, d- and f-blocks of the Periodic Table Ionisation energies and their equations Trends in ionisation energy as evidence for shells and subshells

Bonding

Topic overview Ionic bonding and electron transfer Dot and cross diagrams for ionic compounds Covalent bonding and the shared pair Lone pairs and co-ordinate bonding Dot and cross diagrams for covalent molecules and ions The octet rule and its limitations Electronegativity and bond polarity Metallic bonding

Intermolecular Forces and Structure

Topic overview Van der Waals' forces and permanent dipole-dipole attractions Hydrogen bonding, boiling points and solubility Giant ionic, metallic and giant covalent structures Molecular covalent structures and melting points across Period 3

Shapes of Molecules and Ions

Topic overview Electron pair repulsion and VSEPR theory Shapes and bond angles up to four electron pairs Shapes with five and six electron pairs Polar bonds and polar molecules

Redox

Topic overview Rules for assigning oxidation states Oxidation states in peroxides, hydrides and Roman numeral names Oxidation, reduction and electron transfer Oxidising agents, reducing agents and disproportionation Half-equations and combining them into redox equations

The Halogens

Topic overview Colours, physical states and solubility of the halogens Chlorine with water and the treatment of drinking water Disproportionation of the halogens with sodium hydroxide Oxidising ability and displacement reactions Solid halides with concentrated sulfuric and phosphoric acid

Acid-Base Titrations

Topic overview Strong and weak acids and bases Concentration, molarity and units Preparing a standard solution Titration technique, mean titres and uncertainty Indicators and the end point Titration calculations

Qualitative Tests

Topic overview Chemical tests for the gases Flame tests for metal cations Testing for the ammonium ion, sulfate and carbonate Distinguishing the halides, and the starch test for iodine

Formulae and Amounts of a Substance

Topic overview Empirical and molecular formulae Calculating formulae from composition data Molar gas volume and reacting gas volumes Percentage yield Atom economy

Nomenclature and Isomerism in Organic Chemistry

Topic overview Homologous series and functional groups IUPAC rules for naming organic compounds Structural, displayed and skeletal formulae Structural isomerism Geometrical isomerism and E–Z notation

Alkanes

Topic overview Alkanes as saturated hydrocarbons Boiling points of alkanes and van der Waals' forces Boiling points of branched isomers Complete and incomplete combustion of alkanes Pollutants from alkane fuels and the atmosphere Catalytic converters Radicals, homolytic fission and heterolytic fission Free radical substitution of alkanes

Alkenes

Topic overview Unsaturated hydrocarbons and the bromine water test Sigma and pi bonds in the carbon–carbon double bond Electron density at C=C, reactivity and catalytic hydrogenation Addition of chlorine and bromine to alkenes Addition of hydrogen chloride and hydrogen bromide Electrophiles, heterolytic fission and the addition mechanism Carbocation stability and Markovnikov addition Addition polymerisation of alkenes

Halogenoalkanes

Topic overview Classification and boiling points of halogenoalkanes Laboratory preparation of a halogenoalkane from an alcohol Nucleophiles and hydrolysis with aqueous alkali Reactions with ammonia and with potassium cyanide Relative rates of hydrolysis and carbon–halogen bond enthalpy Elimination using ethanolic potassium hydroxide CFCs and the ozone layer

Alcohols

Topic overview Hydrogen bonding, boiling point and solubility of alcohols Classifying alcohols and preparing them from halogenoalkanes Combustion of alcohols and their use as fuels Reactions with sodium, hydrogen bromide and phosphorus pentachloride Oxidation with acidified potassium dichromate(VI)

Infrared Spectroscopy

Topic overview Molecular vibrations and the absorption of infrared radiation Characteristic absorptions and the wavenumber scale Identifying functional groups from infrared spectra Distinguishing compounds and following reactions by infrared

Energetics

Topic overview Exothermic and endothermic changes Standard conditions and the standard enthalpy change Enthalpy level diagrams Standard enthalpies of combustion and formation Standard enthalpy of neutralisation Measuring enthalpy changes by experiment Enthalpy calculations using q = mcΔT Hess's Law and enthalpy cycles Indirect enthalpy calculations using Hess's Law Average bond enthalpies and their limitations

Kinetics

Topic overview Reaction rate and the factors that change it Collision theory and activation energy The Maxwell–Boltzmann distribution of molecular energies Catalysts and the activation energy barrier

Equilibrium

Topic overview Reversible reactions and dynamic equilibrium Changing conditions and the position of equilibrium The equilibrium constant Kc and its units The magnitude of Kc and the extent of reaction Industrial processes and compromise conditions

Group II Elements and Their Compounds

Topic overview s-block character and electronic configuration Trends in atomic radius and first ionisation energy Reactions of the elements with oxygen, water and dilute acids The oxides and their reactions with water and dilute acids Thermal stability of the carbonates and hydroxides Uses of magnesium oxide, calcium carbonate and lime Solubility trends and solubility curves

Infrared Spectroscopy

4 subtopics
12 things to know
Unit AS 2

What Infrared Spectroscopy covers

Infrared Spectroscopy is examined in Unit AS 2, Further Physical and Inorganic Chemistry and an Introduction to Organic Chemistry, and drawn on again in Unit AS 3, Basic Practical Chemistry. It covers molecular vibrations and the absorption of infrared radiation, characteristic absorptions and the wavenumber scale, identifying functional groups from infrared spectra and distinguishing compounds and following reactions by infrared.

Where it is assessed

Unit AS 2

Further Physical and Inorganic Chemistry and an Introduction to Organic Chemistry, and again in Unit AS 3, Basic Practical Chemistry.

Check the CCEA specification ↗

The 4 subtopics

Each one has its own page. CCEA AS Chemistry students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

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Key term

What happens to a molecule that absorbs infrared radiation, and what fixes the wavenumber at which a bond absorbs?

Key term

Give the wavenumber ranges in which an alcohol O–H bond, an N–H bond and a nitrile C≡N bond absorb.

Numbers

A spectrum shows a strong sharp peak at 1715 cm⁻¹ and nothing at all above 3000 cm⁻¹. What can be concluded?

Numbers

How can infrared spectroscopy distinguish propan-2-ol from propanone?

Key term

What happens to a molecule when it absorbs infrared radiation?

Key term

What is a wavenumber and in what unit is it measured?

Key term

A spectrum shows absorption at 3400 cm⁻¹ but no carbonyl peak and no C–O peak. Which functional group is likely to be present?

Numbers

How can infrared spectroscopy be used to distinguish ethylamine from ethanol?

Key term

Name the two main kinds of vibration that a bond in a molecule can undergo.

Key term

How is wavenumber related to the energy of the radiation absorbed?

Key term

Why is the absence of an absorption just as informative as its presence?

Key term

How is infrared spectroscopy used to check the purity of a product?

Take Infrared Spectroscopy further

The same content, four other ways.

Questions from this topic

Each one with the model answer underneath.

1
What happens to a molecule when it absorbs infrared radiation?
Model answer
The bonds within the molecule vibrate more energetically, so the molecule is raised to a higher vibrational energy level.
2
Name the two main kinds of vibration that a bond in a molecule can undergo.
Model answer
The two kinds are stretching, in which the length of the bond changes, and bending, in which the angle between bonds changes.
3
Why does a bond absorb only certain frequencies of infrared radiation?
Model answer
Each bond vibrates at its own natural frequency, and it can only take up energy from radiation whose frequency matches that natural frequency exactly.
4
What two properties of a bond determine the natural frequency at which it vibrates?
Model answer
The frequency is determined by the strength of the bond and by the masses of the two atoms that the bond joins.
5
How does the strength of a bond affect the wavenumber at which it absorbs?
Model answer
A stronger bond vibrates more rapidly, so it absorbs radiation of higher frequency and therefore appears at a higher wavenumber.
6
How does the mass of the bonded atoms affect the wavenumber at which a bond absorbs?
Model answer
Heavier atoms vibrate more slowly, so a bond joining heavy atoms absorbs at a lower frequency and therefore at a lower wavenumber.

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Infrared Spectroscopy revision questions

Is Infrared Spectroscopy examined at AS in CCEA Chemistry?: Yes. It sits in Unit AS 2, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Infrared Spectroscopy cover in CCEA AS-Level Chemistry?: 4 subtopics: molecular vibrations and the absorption of infrared radiation, characteristic absorptions and the wavenumber scale, identifying functional groups from infrared spectra and distinguishing compounds and following reactions by infrared. Each one has its own page with questions and model answers.
How is Infrared Spectroscopy organised?: By the CCEA 1110 specification, subtopic by subtopic, so it matches the order you are taught it in.

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