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AS Chemistry Covalent bonding
AS Chemistry
Covalent bonding
Learning ObjectivesCandidates should be able to:
describe, including the use of ‘dot-and-cross’ diagrams, covalent bonding, as in hydrogen; oxygen; chlorine; hydrogen chloride; carbon dioxide; methane; ethene.
describe covalent bonding in terms of orbital overlap.
Starter Activity
Forming a bond
Shared pair of electrons
Electron density maps for a hydrogen molecule
Electron density map
This single σ covalent bond can be simply represented as:
or H – H
Representing a covalent bond
Overlap of atomic orbitals
Overlap of one s and one p orbital – e.g. HF
Overlap of two p-orbitals – e.g. F2
Dot-cross diagrams
Oxygen
EtheneHydrogen chloride
Carbon dioxide
MethaneChlorine
Unexpected structures !!
Breaking the octet rule
Bonding in CH4 – promotion of an electron
C 1s2 2s2 2p2
Bonding in CH4 – hybridisation
Hybridisation in PCl5
AS Chemistry
Co-ordinate bonding
Learning ObjectivesCandidates should be able to describe, including the use of ‘dot-and-cross diagrams, co-ordinate (dative covalent) bonding, as in the formation of the ammonium ion and in the Al2Cl6 molecule.
A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.
Starter Activity
Dot-cross diagrams
H2S SF6SiCl4
CH3OH CO
Reaction between NH3(g) and HCl(g)
Reaction between NH3(g) and HCl(g)
Reaction between H2O and HCl
Electron-deficient BF3
Aluminium chloride vapour
Aluminium chloride vapour
AS Chemistry
Electronegativity
Learning ObjectivesCandidates should be able to explain the origin of polar bonds, with reference to electronegativity differences between atoms.
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.
Electronegativity values
Starter Activity
Why are bonds like bears?
…’cos some of them are POLAR!
What happens if two atoms of equal electronegativity bond together?
What happens if B is slightly more electronegative than A?
Electronegativity differences
Representing polar bonds
Molecule Electronegativity difference
Dipole/debye
HCl 0.9 1.03
HBr 0.7 0.78
HI 0.4 0.38
Representing polar bonds
What happens if B is a lot more electronegative than A?
An ionic bond is formed!!!
Large electronegativity difference
Type of bond
Type of bond Electronegativity difference
Non-polar Covalent 0.5Polar covalent Between 0.5 and 1.7Ionic 1.7
As a rough guide:
Positive cation Negative anion
Polarisation of Anions
Truly ionic
Ionic with some covalent character
Property Cation is the most powerful polarising
agent when....
Anode is most easily polarised
when...Charge
RadiusHigh
Small
High
Large
Polarisation of Anions
Do polar bonds make polar molecules?
CCl4 molecule is tetrahedral - the partial negative charges on the Cl atoms are distributed pretty symmetrically around the molecule. The partial positive charge on the C is buried in the center of the molecule.
The most electronegative element is fluorine.If you remember that fact, everything becomes easy, because electronegativity must always increase towards fluorine in the Periodic Table.
Trends in electronegativity
AS Chemistry
Ionic bonding
Learning Objectives
Candidates should be able to describe ionic (electrovalent) bonding, as in sodium chloride and magnesium oxide, including the use of ‘dot-and-cross’ diagrams.
“The name’s Bond, Ionic Bond – taken, not shared!!!
Starter Activity
Approaching atoms
Ionic bonding
Ionic bonding
MgO CaCl2
Al2O3K2O
Why CaCl2 and not CaCl or CaCl3?
Why CaCl2 and not CaCl or CaCl3?
Positive cation Negative anion
Polarisation of Anions
Truly ionic
Ionic with some covalent character
Property Cation is the most powerful polarising
agent when....
Anode is most easily polarised
when...Charge
RadiusHigh
Small
High
Large
Polarisation of Anions
AS Chemistry
Shapes of molecules
Learning ObjectivesCandidates should be able to explain the shapes of, and bond angles in, molecules by using the model of electron-pair repulsion.
Starter Activity
Balloon molecules
2 Balloons give a linear geometry3 Balloons give a trigonal planar geometry4 Balloons give a tetrahedral geometry5 Balloons give a trigonal bipyramidal geometry6 Balloons give an octahedral geometry
Electrons in outer shell of central atom
Electrons added from other atoms (and any charge on an ion)
No. of pairs of electrons
No. of bonding pairs
Diagram of molecule (including
bond angles)
Description of shape
BF3 3 3 3 3 Trigonal planar
CCl4 4 4 4 4 Tetrahedral
NH3 5 3 4 3 Trigonal pyramidal
H2O 6 2 4 2 Bent (or V-shaped)
Shapes of molecules
SF6 6 6 6 6 Octahedral
CO2 4 4 2 2 Linear
C2H6 4 4 4 4 Tetrahedral
C2H4 4 4 3 3 Trigonal planar
ClF4- 7 5 6 4 Square
planar
Shapes of molecules
AS Chemistry
Metallic bonding
Lesson ObjectivesCandidates should be able to describe metallic bonding in terms of a lattice of positive ions surrounded by mobile electrons.
Starter Activity
This is sometimes described as "an array of positive ions in a sea of electrons".
Metallic Bonding
Close packed structures?
Dense metals
Group 1 metals
Malleability
Metal grains
AS Chemistry
Intermolecular forces
Lesson ObjectivesCandidates should be able to describe intermolecular forces (van der Waals’ forces), based on permanent and induced dipoles, as in CHCl3(l), Br2(l) and the liquid noble gases.
Starter Activity
Particles in solids, liquids and gasesIn a liquid or a solid there must be forces between the molecules causing them to be attracted to one another, otherwise they would move apart from each other and become a gas.
Intermolecular attractions are attractions between one molecule and a neighbouring molecule.
Intermolecular forces
The lozenge-shaped diagram represents a small symmetrical molecule - H2, perhaps, or Br2. The even shading shows that on average there is no electrical distortion (i.e. the molecule is non-polar).
How do intermolecular (or van der Waals) forces arise?
Temporary or instantaneous dipoles
Temporary or instantaneous dipolesElectrons are mobile. The constant "sloshing around" of the electrons in the molecule causes rapidly fluctuating dipoles even in the most symmetrical molecule.
It even happens in monatomic molecules - molecules of noble gases, like helium, which consist of a single atom.
Temporary dipole - induced dipole interaction
How temporary dipoles give rise to intermolecular attractions
This diagram shows how a whole lattice of molecules could be held together in a solid using van der Waals’ forces.
An instant later, of course, you would have to draw a quite different arrangement of the distribution of the electrons as they shifted around - but always in synchronisation.
van der Waals’ forces
van der Waals lattice
helium -269°Cneon -246°Cargon -186°Ckrypton -152°Cxenon -108°Cradon -62°C
How molecular size affects the strength of the dispersion forces
The boiling points of the noble gases are:
How molecular size affects the strength of the dispersion forcesThere is a gradual increase in the very low boiling temperatures of the noble gases with increasing atomic size.
As the size of the atoms increases the number of electrons increases and the magnitude of the van der Waals forces increases.
How molecular shape affects the strength of the temporary dipole interactions
Butane has a higher boiling point because the intermolecular forces are greater. The molecules are longer and can lie closer together than the shorter, fatter 2-methylpropane molecules.
Permanent dipoles
Permanent dipoles
AS Chemistry
Hydrogen bonding
Lesson ObjectivesCandidates should be able to describe hydrogen bonding, using ammonia and water as simple examples of molecules containing N-H and O-H groups.
Starter Activity
The increase in boiling point happens because the molecules are getting larger with more electrons, and so van der Waals forces become greater.
Boiling points of the Group 4 hydrides
Boiling points of the hydrides in Groups 5, 6 and 7.
The origin of hydrogen bonding
Hydrogen bonding is a particularly strong intermolecular force that involves three features:
a large dipole between an H atom and the highly electronegative atoms N, O or F;
the small H atom which can get very close to other atoms;
a lone pair of electrons on another N, O or F, with which the positively charge H atom can line up.
The origin of hydrogen bonding
Drawing hydrogen bonds
1 mark for indicating bond polarity
1 mark for showing lone pair
1 mark for showing H-bond
Hydrogen bonding accounts for many of the other unusual properties of water including:
its high specific heat capacity
its very high surface tension
its high viscosity and
the low density of ice compared to water
Hydrogen bonding in water
Which type of intermolecular force?
AS Chemistry
A summary Bonding, structure
and properties
Lesson ObjectivesCandidates should be able to:describe, interpret and/or predict the effect of
different types of bonding on the physical properties of substances.
describe, in simple terms, the lattice structure of a crystalline solid which is ionic, simple molecular, giant molecular, hydrogen-bonded and metallic.
suggest from quoted physical data the type of structure and bonding present in a substance.
Starter Activity
The properties of substances are decided by their bonding and structure.
Bonding means the way the particles are held together: ionic, covalent, metallic or weak intermolecular bonds.
Structure means the way the particles are arranged relative to one another. You have already met the major types of structure at IGCSE.
Bonding, structure and properties
Bonding, structure and properties
Bond Average bond enthalpy/kJmol-1
Bond length/nm
C – C +347 0.154C = C +612 0.134C ≡ C +838 0.120C – H +413 0.108O – H +464 0.096C – O +358 0.143C = O +805 0.116
Bond energies
GIANT LATTICE COVALENT MOLECULARIonic Covalent network Metallic Simple molecular Macromolecular
What substances have this type of structure?
Compounds of metals with non-metals.
Some elements in Group 4 and some of their compounds.
Metals Some non-metal elements and usually some non-metal/non-metal compounds.
Polymers
Examples NaCl SiO2 Cu H2O Poly(ethene)What type of particle does it contain?
ions atoms positive ions and
delocalised electrons
molecules molecules
How are the particles bonded together?
Strong ionic bonds; attraction between oppositely charged ions
Strong covalent bonds; attraction of atoms’ nuclei for shared electrons
Strong metallic bonds; attraction of atoms’ nuclei for delocalised electrons
Weak intermolecular bonds between molecules; strong covalent bonds between atoms within each molecule.
What are the typical properties?M. pt and b.pt.
high very high generally high lowmoderate (often
decompose on heating)
Hardness hard but brittle
very hard (if 3D)
hard but malleable
soft variable
Electrical conductivity
conduct when (l) or (aq)
do not normally conduct
conduct when (s) or (l)
do not conduct do not normally conduct
Solubility in wateroften soluble insoluble insoluble (but
some react)
usually insoluble (but
some H-bond)
sometimes soluble
Solubility in non-polar solvents (e.g. hexane)
insoluble insoluble insoluble usually soluble sometimes soluble
Structure table
AS Chemistry
The modern use of materials
Lesson ObjectivesCandidates should be able to:Explain the strength, high melting point and
insulating properties of ceramics in terms of their giant molecular structure.
Relate the uses of ceramics to their properties.
Describe and interpret the uses of the metals aluminium and copper (and their alloys) in terms of their physical properties.
Understand that materials are a finite resource and the importance of recycling processes.
Starter Activity
Five most common metals
Aluminium
Copper
Zinc
Steel
Brass
Low density, corrosion resistance and strength make it ideal for construction of aircraft, lightweight vehicles, and ladders.
Malleability, low density, corrosion resistance and good thermal conduction make it a good material for food packaging.
Good electrical conduction, corrosion resistance and low density leads to its use for overhead power cables hung from pylons (low density gives it an advantage over copper).
Uses of aluminium
Uses of copperCopper is an excellent conductor of electricity and heat.
Copper is soft and malleable.
Copper is very unreactive and therefore corrosion resistant.
Copper Alloy
Other metal it contains
Main properties
Uses
BrassZinc
Fairly soft and
malleableScrews and
hingesBronze
Tin StrongPropellors
and bearings
Alloys of copper
A mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties.
Ceramics
Ceramic: Any of various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing a nonmetallic mineral.
Ceramicsfurnace: an enclosed chamber in which heat is produced to heat buildings, destroy refuse, smelt or refine ores, etc.
heat shields glass and crockery
furnace linings
brake pads electrical insulators
Raw materials extracted (removed by chemical means) from the Earth cannot last forever. Although some materials are more (present in great quantity) abundant than others, they are all finite (have a limit) resources. Increasing demand for raw materials (items used to produce something else), coupled with ever growing problems of waste disposal, have led to considerable interest in recycling (processing for reuse) waste.
Recycling has a number of possible advantages (beneficial factors):
It leads to reduced demand for new raw materials;
It leads to a reduction in environmental damage (harm to the surroundings);
It reduces the demand for landfill sites (a place for burying waste) to dump waste;
It reduces the cost of waste disposal;
It may reduce energy costs.
Recycling
AS Chemistry
The kinetic-molecular model of
liquids
Lesson ObjectivesCandidates should be able to describe using a kinetic-molecular model, the liquid state; melting; vaporisation and vapour pressure.
Starter Activity
Solid Liquid GasArrangement of particles
very orderly short-range order, longer range disorder
almost complete disorder
Movement of particles
vibrate about fixed positions
some movement from place to place
continuous, rapid, random movement
Proximity of particles
close (~10-10m) close (~10-10m) far apart (~10-
8m)Compressibility of substance
very low very low high
Conduction of heat
poor except metals and graphite
metals very good; others poor
very poor
The Kinetic-Molecular Model of Liquids
The Kinetic-Molecular Model of LiquidsLiquids do not have a fixed __________ because the particles can move about. However, they remain very __________ together. This shows that the inter-particle forces have not been __________ broken. If sufficient __________ is supplied, the particles overcome the inter-particle forces almost completely and __________ from the liquid. This is called __________ or boiling. The energy required to boil a liquid is always __________ than that required to melt the same substance and is a better __________ of the strength of inter-particle forces.
Vapour pressureVapour pressure is the pressure of a vapour over a liquid at equilibrium.
Vapour pressure
Even at low temperature there are particles with high energy.
Vapour pressure
At equilibrium, the rate at which molecules leave the liquid equals the rate at which molecules join the liquid.
Measuring vapour pressure
AS Chemistry
More on ideal gases
Lesson ObjectivesCandidates should be able to explain qualitatively in terms of intermolecular forces and molecular size the limitations of ideality at very high pressures and very low temperatures.
Starter Activity
More on ideal gases
More on ideal gasesY is hydrogen. It is closest to ideal under all conditions. Hydrogen has the weakest intermolecular forces and is the smallest molecule.
Z is ammonia. It is the least ideal at lower pressures. Ammonia molecules can hydrogen bond.
X is nitrogen. Deviates greatly from ideality at high pressures where its larger molecular volume becomes important.
