TOPIC 01
Formulae, Equations and Amount of Substance
Chemistry uses formulae and equations to represent substances and chemical reactions. Quantitative chemistry allows the amounts of reactants and products to be calculated.
Relative Atomic Mass
Relative atomic mass, Ar, is the weighted average mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12.
Relative Formula Mass
Relative formula mass, Mr, is obtained by adding the relative atomic masses of all the atoms represented in a formula.
The Mole
The mole is the amount of substance containing the Avogadro constant number of specified particles.
Amount of Substance
where n is amount in mol, m is mass in g and M is molar mass in g mol−1.
Number of Particles
Concentration
For concentration in mol dm−3, volume must be expressed in dm3.
Empirical Formula
The empirical formula gives the simplest whole-number ratio of atoms in a compound.
Molecular Formula
The molecular formula gives the actual number of atoms of each element in one molecule.
WORKED EXAMPLE
A compound contains 24.0 g of carbon and 4.0 g of hydrogen. Determine its empirical formula.
Carbon: 24.0 / 12.0 = 2.00 mol
Hydrogen: 4.0 / 1.0 = 4.00 mol
Divide by the smallest amount:
C : H = 1 : 2
Empirical formula = CH2
Percentage Yield
Atom Economy
Writing Equations
Chemical equations must conserve atoms and charge. Equations should be balanced using the smallest whole-number coefficients.
Always check that every element has the same number of atoms on both sides of an equation.
TOPIC 02
Atomic Structure and the Periodic Table
The arrangement of electrons in atoms explains many of the patterns observed in the Periodic Table.
Subatomic Particles
- Protons have a relative charge of +1.
- Neutrons have a relative charge of 0.
- Electrons have a relative charge of −1.
Atomic Number and Mass Number
Atomic number is the number of protons in an atom. Mass number is the total number of protons and neutrons.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Electron Configuration
Electrons occupy shells and orbitals around the nucleus. The orbitals relevant to Unit 1 include s, p and d orbitals.
- s subshell — one orbital
- p subshell — three orbitals
- d subshell — five orbitals
Electron Capacity
Each orbital can contain a maximum of two electrons. Electrons in the same orbital have opposite spins.
Ionisation Energy
First ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous singly charged ions.
- Distance of the outer electron from the nucleus.
- Nuclear charge.
- Shielding by inner electrons.
- Subshell and electron pairing effects.
Trends Across a Period
Across a period, nuclear charge generally increases while electrons are added to the same main shell. This produces characteristic trends in atomic properties.
Periodic Table Blocks
- s-block elements have their outer electrons in an s subshell.
- p-block elements have their outer electrons in a p subshell.
- d-block elements involve occupation of d subshells.
Mass Spectrometry
Mass spectrometry can be used to determine isotopic masses and relative abundances.
A mass spectrum contains peaks corresponding to ions with particular mass-to-charge ratios.
WORKED EXAMPLE
An element has two isotopes with masses 10 and 11. Their abundances are 20% and 80% respectively. Find its relative atomic mass.
Ar = (10 × 0.20) + (11 × 0.80)
Ar = 2.00 + 8.80
Ar = 10.80
TOPIC 03
Bonding and Structure
The properties of substances depend strongly on the type of bonding and structure present.
Ionic Bonding
Ionic bonding is the strong electrostatic attraction between oppositely charged ions.
Ions are formed when atoms gain or lose electrons.
Ionic Structures
Ionic compounds form giant lattices containing positive and negative ions arranged in a regular structure.
- High melting and boiling points.
- Usually solid at room temperature.
- Conduct electricity when molten or dissolved in water.
- Do not conduct electricity when solid.
Covalent Bonding
A covalent bond is a shared pair of electrons between atoms.
Electronegativity and Bond Polarity
Electronegativity describes the tendency of an atom to attract the bonding pair of electrons towards itself.
A difference in electronegativity can produce a polar covalent bond.
Shapes of Molecules
Electron pairs repel each other and arrange themselves as far apart as possible. Lone pairs repel more strongly than bonding pairs.
lone pair–lone pair > lone pair–bond pair > bond pair–bond pair
Common Molecular Shapes
- Two electron pairs → linear, approximately 180°.
- Three bonding pairs → trigonal planar, approximately 120°.
- Four bonding pairs → tetrahedral, approximately 109.5°.
- Three bonding pairs and one lone pair → trigonal pyramidal.
- Two bonding pairs and two lone pairs → bent / V-shaped.
Metallic Bonding
Metallic bonding consists of positive metal ions surrounded by delocalised electrons.
Intermolecular Forces
Molecules can attract one another through intermolecular forces. These are weaker than the covalent bonds within a molecule.
Van der Waals Forces
Temporary fluctuations in electron distribution can produce temporary dipoles and attractions between molecules.
Permanent Dipole-Dipole Attractions
Polar molecules can attract one another because of permanent partial charges.
Hydrogen Bonding
Hydrogen bonding is a particularly strong intermolecular attraction involving hydrogen bonded to a highly electronegative atom and a lone pair on another electronegative atom.
Structure and Physical Properties
The type of structure affects melting point, boiling point, electrical conductivity and solubility.
EXAM COMPARISON
A giant ionic compound generally has a high melting point because strong electrostatic attractions act throughout its lattice.
A simple molecular substance usually has a much lower melting point because only intermolecular forces need to be overcome during melting.
TOPIC 04
Introductory Organic Chemistry and Alkanes
Organic chemistry is the study of carbon-containing compounds. Unit 1 introduces organic structures, naming, isomerism and the chemistry of alkanes.
Homologous Series
A homologous series is a family of compounds with the same functional group and general formula, showing similar chemical properties. Successive members differ by CH2.
Alkanes
Alkanes are saturated hydrocarbons containing only carbon-carbon single bonds.
Names of Simple Alkanes
- Methane — CH4
- Ethane — C2H6
- Propane — C3H8
- Butane — C4H10
- Pentane — C5H12
- Hexane — C6H14
Structural Formulae
Organic compounds can be represented using displayed, structural, skeletal and molecular formulae.
Structural Isomerism
Structural isomers have the same molecular formula but different structural arrangements.
Combustion
Alkanes undergo combustion in oxygen.
Complete combustion requires sufficient oxygen. Incomplete combustion can produce carbon monoxide and/or carbon.
Cracking
Cracking breaks larger hydrocarbon molecules into smaller, more useful molecules.
Cracking can produce both alkanes and alkenes.
Free-Radical Substitution
Alkanes can react with halogens in the presence of ultraviolet radiation.
Initiation
Ultraviolet radiation causes a halogen molecule to undergo homolytic fission, producing radicals.
Propagation
Radicals react with molecules to produce new radicals, allowing the chain reaction to continue.
Termination
Two radicals combine, removing radicals from the reaction chain.
Bond breaking in which one electron from the bonding pair goes to each atom, producing radicals.
Petroleum and Fractional Distillation
Crude oil is a mixture of hydrocarbons. Fractional distillation separates hydrocarbons according to differences in boiling point.
Smaller molecules generally have lower boiling points and are more volatile than larger molecules.
TOPIC 05
Alkenes
Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond.
The C=C Double Bond
A carbon-carbon double bond consists of a sigma bond and a pi bond. The presence of the pi bond makes alkenes more reactive than alkanes.
Naming Alkenes
The longest carbon chain containing the double bond is selected. The chain is numbered so that the double bond receives the lowest possible number.
Electrophilic Addition
Alkenes undergo addition reactions because the electron-rich double bond attracts electrophiles.
Addition of Hydrogen
Hydrogen can add across the double bond in the presence of a suitable catalyst.
Addition of Halogens
Alkenes react with halogens by addition across the carbon-carbon double bond.
Addition of Hydrogen Halides
Hydrogen halides can add across an alkene double bond to form a haloalkane.
Hydration of Alkenes
Alkenes can react with steam to produce alcohols under suitable industrial conditions.
Test for an Alkene
Bromine water can be used to test for carbon-carbon double bonds. An alkene rapidly decolourises bromine water.
Polymerisation
Addition polymerisation occurs when many alkene molecules join together to form a polymer.
The C=C bond opens during addition polymerisation and forms new carbon-carbon single bonds linking monomer units.
WORKED EXAMPLE
Propene undergoes addition polymerisation. Identify the repeating unit of the polymer.
The carbon-carbon double bond opens and the carbon chain becomes linked to neighbouring monomer units.
The repeating unit is based on:
PRACTICE
Unit 1 Mixed Practice
These questions combine the five Unit 1 specification areas. Try them without looking at the answers first.
Show answer
Mr(NaCl) = 23.0 + 35.5 = 58.5
n = m / M = 5.85 / 58.5
n = 0.100 mol
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17 protons means the element is chlorine.
18 electrons means it has gained one electron, so the ion is Cl−.
Mass number = 17 + 18 = 35.
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Magnesium oxide has a giant ionic lattice with strong electrostatic attractions between oppositely charged ions. A large amount of energy is required to overcome these attractions.
Methane is a simple molecular substance with relatively weak intermolecular forces.
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Alkane: CnH2n+2
Alkene: CnH2n
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Alkenes contain a carbon-carbon double bond containing a pi bond. The pi bond provides an electron-rich region that can attract electrophiles and undergo addition reactions.
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Ar = (35 × 0.60) + (37 × 0.40)
Ar = 21.0 + 14.8
Ar = 35.8
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Carbon dioxide and water.
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Addition across the carbon-carbon double bond.
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Structural isomers have the same molecular formula but different structural arrangements of atoms.
Show answer
When solid, the ions are fixed in position and cannot move to carry charge. When molten, the ions are free to move and can carry electric charge.
QUICK REVIEW
Unit 1 Key Ideas
- n = m / M
- N = nNA
- c = n / V
- Percentage yield = actual yield / theoretical yield × 100
- Atom economy = desired product Mr / total reactant Mr × 100
- Isotopes have the same number of protons but different numbers of neutrons.
- Ionic, covalent and metallic bonding are different forms of strong chemical bonding.
- Alkanes are saturated hydrocarbons.
- Alkenes contain a carbon-carbon double bond.
- Alkenes commonly undergo addition reactions.
- Alkanes can undergo free-radical substitution under suitable conditions.