A σ-bond is a covalent bond formed by the end-on overlap of two orbitals along the line joining the two nuclei, so the electron density is concentrated directly between the two atoms.

Why are all the bonds in an alkane described as σ-bonds?

Every carbon in an alkane forms four single bonds, and each of those C–C and C–H bonds arises from head-on overlap of orbitals along the internuclear axis, which is exactly what defines a σ-bond.

Why can the two halves of an ethane molecule rotate freely about the C–C bond?

The electron density of a σ-bond is symmetrical about the internuclear axis, so turning one carbon relative to the other does not reduce the orbital overlap and costs almost no energy.

What is the H–C–H bond angle in methane, and why?

It is 109.5°, because the carbon has four bonding pairs of electrons and no lone pairs, and four regions of electron density repel each other to the maximum extent in a tetrahedral arrangement.

Write the balanced equation for the complete combustion of methane.

The equation is CH₄ + 2O₂ → CO₂ + 2H₂O.

Write the balanced equation for the complete combustion of propane.

The equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

WJEC AS-Level Chemistry 1410QS

Formulae and Equations

Topic overview Charges on common ions Building formulae for ionic and molecular compounds Assigning oxidation numbers Using oxidation numbers to follow redox changes Balancing equations and using state symbols Ionic and half equations

Basic Ideas About Atoms

Topic overview Radioactive decay and nuclear equations Alpha, beta and gamma radiation Half-life, dating and the uses of radioisotopes: half-life of radioactive decay Half-life, dating and the uses of radioisotopes: adverse consequences for living cells Ionisation energies and electronic structure Orbitals and the filling of sub-shells Atomic spectra and the energy of light: atomic emission spectrum of the hydrogen atom Atomic spectra and the energy of light: relationship between energy and frequency Atomic spectra and the energy of light: significance of the frequency

Chemical Calculations

Topic overview Relative masses and the mass spectrometer: relative atomic Relative masses and the mass spectrometer: how a mass spectrometer works Relative masses and the mass spectrometer: molecular ion The mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectra The mole, the Avogadro constant and molar mass: moles and percentage composition by mass Empirical and molecular formulae Concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3 Concentration, solubility and titration calculations: stoichiometry and the limiting reagent Concentration, solubility and titration calculations: titration calculations and estimating a titre Gas volumes and the ideal gas equation Yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yield Yield, atom economy and experimental uncertainty: how to estimate the percentage error

Bonding

Topic overview Ionic bonds and covalent bonds: ionic bonding in terms of ion formation Ionic bonds and covalent bonds: covalent bonding and forces Electronegativity, polarity and in-between bonds: concepts of electronegativity and bond polarity Electronegativity, polarity and in-between bonds: forces Forces between molecules Hydrogen bonding Shapes of molecules and ions: VSEPR principle and its use Shapes of molecules and ions: bond angles associated with linear

Solid Structures

Topic overview Ionic crystals: sodium chloride and caesium chloride Giant covalent structures: diamond and graphite Simple molecular solids: iodine and ice Metals and the electron sea Using structure to explain physical properties

The Periodic Table

Topic overview How the Periodic Table is organised Oxidation and reduction as electron transfer Trends across periods and down groups Group 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acid Group 2: reactions, trends and the reasons behind them — trends down the group Group 2: reactions, trends and the reasons behind them — thermal stability of the carbonates Group 2: reactions, trends and the reasons behind them — oxides and hydroxides Identifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxide Identifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acid Group 7: the halogens — physical properties and the trend in volatility Group 7: the halogens — reactions of the halogens with metals Group 7: the halogens — displacement reactions and the trend Group 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium

Simple Equilibria and Acid-Base Reactions

Topic overview Reversible reactions and dynamic equilibrium Le Chatelier's principle and shifting the position of equilibrium The equilibrium constant Kc Acids, bases, salts and neutralisation pH, strong acids and weak acids Acid-base titrations and their calculations

Thermochemistry

Topic overview Enthalpy changes and their definitions Calculating enthalpy changes from experimental measurements Determining enthalpy changes in the laboratory Hess's law and energy cycles Average bond enthalpies

Rates of Reaction

Topic overview Collision theory and the conditions that change rate Measuring rate and relating it to concentration Energy profiles and activation energy Temperature and the Boltzmann distribution Catalysts and how they work Following reactions in the laboratory

The Wider Impact of Chemistry

Topic overview Social and economic consequences of chemical manufacture and energy supply Environmental consequences of chemical processes and energy production Green chemistry and sustainable practice

Organic Compounds

Topic overview Drawing and reading organic structures Naming organic compounds Chain length, functional groups and physical properties Structural isomerism Electrophiles, nucleophiles, radicals and bond fission

Hydrocarbons

Topic overview Alkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefits Alkane bonding, combustion and fossil fuels: combustion calculations and acid rain Radical substitution of alkanes The C=C bond and why alkenes are reactive E–Z isomerism in alkenes Electrophilic addition, Markovnikov's rule and testing for alkenes: mechanism of electrophilic addition Electrophilic addition, Markovnikov's rule and testing for alkenes: Markovnikov's rule and the bromine water test Electrophilic addition, Markovnikov's rule and testing for alkenes: testing for a C=C bond Hydrogenation and addition polymers: conditions required Hydrogenation and addition polymers: nature of addition polymerisation

Halogenoalkanes

Topic overview Nucleophilic substitution by hydroxide ions Elimination to form alkenes Reactivity of the carbon–halogen bond and the silver nitrate test Uses of halogenoalkanes, CFCs and the ozone layer: uses of halogenoalkanes and CFCs Uses of halogenoalkanes, CFCs and the ozone layer: radical mechanism Uses of halogenoalkanes, CFCs and the ozone layer: replacements for CFCs Heating under reflux in the laboratory

Alcohols and Carboxylic Acids

Topic overview Two routes to ethanol: hydration and fermentation — hydration of ethene Two routes to ethanol: hydration and fermentation — fermentation Two routes to ethanol: hydration and fermentation — comparing the two routes on rate Classifying alcohols and dehydrating them Oxidising alcohols to aldehydes, ketones and acids Chemical tests for alcohols and carboxylic acids Carboxylic acids, bases and salts Making an ester and separating it by distillation: esterification Making an ester and separating it by distillation: carrying out the preparation Making an ester and separating it by distillation: separating and purifying Making an ester and separating it by distillation: yield

Instrumental Analysis

Topic overview Reading a mass spectrum Infrared spectra and functional groups NMR spectra and atomic environments Putting the three techniques together

Practical Skills and Techniques

Topic overview Measuring mass, volume, time and temperature Standard solutions and titration technique Indicators and acid–base titration curves Heating, refluxing, distilling and purifying: Heating Heating, refluxing, distilling and purifying: purifying Melting temperature, chromatography and pH measurement Electrochemical cells, rate methods and working safely: Electrochemical cells Electrochemical cells, rate methods and working safely: rate methods and working safely

Planning, Analysis and Evaluation

Topic overview Planning an investigation and controlling variables Recording observations and presenting data Graphs, gradients and processing data Errors, uncertainty, accuracy and precision Evaluating a method and suggesting improvements Research and referencing

Core Experiments and Their Chemistry

Topic overview Gravimetric and acid–base analysis Enthalpy and rate experiments: Enthalpy Enthalpy and rate experiments: rate experiments Redox titrations and electrochemical cells Equilibrium constants and following a titration with a pH probe Organic synthesis, separation and identification: Organic synthesis Organic synthesis, separation and identification: identification tests and chromatography

Mathematical Skills in Chemistry

Topic overview Units, standard form and proportion in chemical calculation: volume, equilibrium, rate and energy units Units, standard form and proportion in chemical calculation: standard form Units, standard form and proportion in chemical calculation: proportion Units, standard form and proportion in chemical calculation: checking answers, buffers, rate constants and units Significant figures, means and uncertainty in experimental data: significant figures in multiplication Significant figures, means and uncertainty in experimental data: relative atomic mass as a weighted mean Significant figures, means and uncertainty in experimental data: absolute and percentage uncertainty Symbols, rearranging equations and logarithms: chemical symbols and what each one asserts Symbols, rearranging equations and logarithms: rearranging an equation Symbols, rearranging equations and logarithms: substituting into equations for Kc, enthalpy and pH Symbols, rearranging equations and logarithms: logarithms, the Arrhenius equation and rearranging formulae Graphs: plotting, gradients, intercepts and tangents — reading a spectrum Graphs: plotting, gradients, intercepts and tangents — reaction order from data and plotting good graphs Graphs: plotting, gradients, intercepts and tangents — Arrhenius plots, zero-order graphs and tangents Graphs: plotting, gradients, intercepts and tangents — initial rates from tangents and interpreting graphs Shape, symmetry and three-dimensional structure: Shape Shape, symmetry and three-dimensional structure: symmetry

Hydrocarbons

10 subtopics
29 things to know
AS Unit 2

What Hydrocarbons covers

Hydrocarbons is examined on AS Unit 2 of WJEC Chemistry, Energy, Rate and Chemistry of Carbon Compounds. 157 recall questions across 10 subtopics.

Where it is assessed

AS Unit 2

Energy, Rate and Chemistry of Carbon Compounds.

Check the WJEC specification ↗

The 10 subtopics

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

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What is a σ-bond?

Key term

Nitrogen is unreactive, so why do oxides of nitrogen come out of a car exhaust?

Key term

How is sulfur dioxide removed from the flue gases of a power station?

Key term

Why does acid rain damage limestone buildings?

Key term

Why is chloromethane never the only organic product when methane and chlorine react?

Key term

Why are all six atoms of ethene in the same plane?

Key term

Why is a C=C bond a region of high electron density?

Key term

Why does the double bond prevent rotation and so allow E–Z isomers to exist?

Key term

But-2-ene exists as two isomers but but-1-ene does not. Explain.

Key term

Does 3-methylpent-2-ene show E–Z isomerism? Explain which group takes priority on each carbon.

Numbers

Write the equation for the reaction of but-2-ene with bromine, and state the polarity of hydrogen bromide that lets it add to ethene.

Key term

Why is a secondary carbocation more stable than a primary one?

Numbers

Identify the major products when hydrogen bromide adds to propene and to 2-methylbut-2-ene, and explain why the more stable carbocation gives the major product.

Key term

Why does the addition of hydrogen bromide to ethene give no major and minor product, and what is the major product of propene with hydrogen chloride?

Key term

Describe the test for a carbon–carbon double bond using bromine water, including what you would see.

Numbers

Why is bromine water decolourised by an alkene?

Key term

Why is 2-bromopropane called the major rather than the only product of the reaction between propene and HBr?

Key term

How would you show experimentally that a colourless liquid is an alkene rather than an alkane?

Key term

What happens to the π-bond of each monomer during addition polymerisation?

Key term

Give the repeat units of poly(propene) and poly(chloroethene), and explain why addition polymerisation has an atom economy of 100%.

Key term

How do you work out the monomer from a given repeat unit?

Process

Give three possible termination steps for the chlorination of methane.

Numbers

What conditions are used for the catalytic hydrogenation of ethene?

Key term

Why is a blue flame on a gas burner a sign of better combustion than a yellow one?

Key term

Write the equation for the hydrogenation of ethene and name the product.

Process

Write the two propagation steps for the chlorination of methane.

Key term

What is a π-bond?

Key term

Chlorine also adds to ethene. Write the equation and name the product.

Key term

Why is a catalyst needed for the addition of hydrogen to ethene?

Take Hydrocarbons further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

A structural isomer vs A stereoisomer
A structural isomer: a compound of the same molecular formula with the atoms joined in a different order. A stereoisomer: a compound with the same order of bonding but a different arrangement in space.

Questions from this topic

Each one with the model answer underneath.

1
What is a σ-bond?
Model answer
A σ-bond is a covalent bond formed by the end-on overlap of two orbitals along the line joining the two nuclei, so the electron density is concentrated directly between the two atoms.
2
Why are all the bonds in an alkane described as σ-bonds?
Model answer
Every carbon in an alkane forms four single bonds, and each of those C–C and C–H bonds arises from head-on overlap of orbitals along the internuclear axis, which is exactly what defines a σ-bond.
3
Why can the two halves of an ethane molecule rotate freely about the C–C bond?
Model answer
The electron density of a σ-bond is symmetrical about the internuclear axis, so turning one carbon relative to the other does not reduce the orbital overlap and costs almost no energy.
4
What is the H–C–H bond angle in methane, and why?
Model answer
It is 109.5°, because the carbon has four bonding pairs of electrons and no lone pairs, and four regions of electron density repel each other to the maximum extent in a tetrahedral arrangement.
5
Write the balanced equation for the complete combustion of methane.
Model answer
The equation is CH₄ + 2O₂ → CO₂ + 2H₂O.
6
Write the balanced equation for the complete combustion of propane.
Model answer
The equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

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Formulae and Equations

Basic Ideas About Atoms

Chemical Calculations

Bonding

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An element is zero.

uncombinedionisedoxidised

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AVO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O.

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COne electron is gained.

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Hydrocarbons revision questions

Is Hydrocarbons examined at AS in WJEC Chemistry?: Yes. It sits in AS Unit 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 Hydrocarbons cover in WJEC AS-Level Chemistry?: 10 subtopics: alkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefits, alkane bonding, combustion and fossil fuels: combustion calculations and acid rain, radical substitution of alkanes, the c=c bond and why alkenes are reactive, e–z isomerism in alkenes, electrophilic addition, markovnikov's rule and testing for alkenes: mechanism of electrophilic addition, electrophilic addition, markovnikov's rule and testing for alkenes: markovnikov's rule and the bromine water test, electrophilic addition, markovnikov's rule and testing for alkenes: testing for a c=c bond, hydrogenation and addition polymers: conditions required and hydrogenation and addition polymers: nature of addition polymerisation. Each one has its own page with questions and model answers.
How is Hydrocarbons organised?: By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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