Doc Brown's Chemistry Revision

Extra Notes on chemical bonding for advanced A level chemistry students

A comparison of types of crystal structure and their properties

(a few extra notes and links to other detailed notes)

CHEMICAL BONDING INDEX

You need to be able to ...

Be able to relate the melting point and conductivity of materials to the type of structure and the bonding present, explain the energy changes associated with changes of state and draw diagrams to represent these structures involving specified numbers of particles. There are two tables at the end summarising various structures and their properties.

Abbreviations used: mpt. melting point, bpt. boiling point

1. Ionic lattice (giant ionic lattice) e.g.

The structure and properties of sodium chloride and other ionic compounds are discussed in detail on my IONIC BONDING page.

Extra A level notes on sodium chloride

Sodium chloride: mpt. 801^oC/1074K, enthalpy of fusion 29 kJ mol^-1, bpt. 1467^oC/1740K, enthalpy of vaporisation 171 kJ mol^-1

The relative high melting and boiling points and large enthalpies for the state changes are due to very strong bonding between the ions in the crystal lattice.

You need to be able to write the electron configuration of ions in terms of s, p and d orbital notation.

e.g. sodium ion Na⁺ is 1s²2s²p⁶, [Ne], and the chloride ion Cl^- is 1s²2s²p⁶3s²3p⁶, [Ar]

More on electron configuration of ions and oxidation states

2. Covalent molecular crystals e.g.

Iodine and water

The electronic structure and properties of simple molecular elements/compounds including water are discussed on my COVALENT BONDING page.

Iodine and water (apart from hydrogen bonding) have the typical properties described and explained on the above page.

Extra A level notes on molecular lattices

 Iodine

The molecular lattice of iodine consists of a neat arrangement of molecules in the crystal lattice which is held together by the weak intermolecular forces. Being a non-polar molecule, the weak intermolecular bonding is due to instantaneous dipole - induced dipole interactions (the weakest of the Van der Waals forces).

The iodine atoms in the I₂ molecule are joined by a single covalent bond - one electron short of a noble gas configuration.

Iodine: mpt. 114^oC, enthalpy of fusion 15.7 kJ mol^-1, bpt. 184^oC, enthalpy of vaporisation 44 kJ mol^-1

Iodine tends to sublime at temperatures approaching 114^oC and only melts under pressure, low melting/boiling/sublimation points and low state change enthalpies due to weak intermolecular bonding.

Water

Ice is a molecular lattice where the small covalent water molecules are held together by hydrogen bonds.

This is discussed in detail on my hydrogen bonding page, including the anomalous density behaviour.

Water: mpt. 0^oC/273 K, enthalpy of fusion 6.02 kJ mol^-1, bpt. 100^oC/373K, enthalpy of vaporisation 41.1 kJ mol^-1,

Low melting/boiling points and low state change enthalpies due to weak intermolecular bonding.

Links to pages on intermolecular bonding forces for relatively low melting/boiling 'simple' covalent molecular elements and compounds

Introduction to intermolecular forces - intermolecular bonding

Detailed comparative discussion of boiling points of 8 organic molecules

Boiling point plots for six organic homologous series and explaining the trends and differences

Other case studies of boiling points related to intermolecular forces for a variety of compounds

Evidence and theory for hydrogen bonding in simple covalent hydrides

3. Giant covalent lattice (a type of macromolecular molecule lattice) e.g.

The structure and properties of diamond and graphite are fully described on my giant covalent structures page

Extra A level notes on giant covalent structures like diamond and graphite

Carbon (diamond)

The structure and properties

Diamond

Carbon (diamond): mpt ~3900^oC, bpt. ?, enthalpy of vaporisation ? ~715 kJ mol^-1 ?, but its big!

You can only melt or boil diamond under great pressure, otherwise it sublimes.

Very high melting point and high enthalpy of vaporisation/sublimation due extremely strong 3D covalent bond network.

Carbon (graphite)

graphite

Carbon (graphite): mpt. ~3900^oC, bpt. ?, enthalpy of sublimation 715 kJ mol^-1

You can only melt or boil graphite under great pressure, otherwise it sublimes.

Very high melting point and high enthalpy of vaporisation/sublimation due extremely strong 2D covalent bond network.

4. Giant metallic lattice e.g.

magnesium

The structure and properties of metal structures are described on my metallic bonding page

Extra notes on metallic structures like magnesium

magnesium metal lattice

Magnesium metal: mpt. 650^oC enthalpy of fusion 8.95 kJ mol^-1, bpt. 1107^oC, enthalpy of vaporisation 132 kJ mol^-1

Relatively high melting/boiling point and enthalpy of vaporisation due to strong metallic bonding.

More on the enthalpy changes in physical changes of state Changes of physical state i.e. gas <==> liquid <==> solid are also accompanied by energy changes. To melt a solid, or boil/evaporate a liquid, heat energy must be absorbed or taken in from the surroundings, so these are endothermic energy changes. The system is heated to effect these changes. To condense a gas, or freeze a solid, heat energy must be removed or given out to the surroundings, so these are exothermic energy changes. The system is cooled to effect these changes. Generally speaking, the greater the forces between the particles, the greater the energy needed to effect the state change AND the higher the melting point and boiling point. A comparison of energy needed to melt or boil different types of substance The heat energy change involved in a state change can be expressed in kJ/mol of substance for a fair comparison. In the table below ΔHmelt is the energy needed to melt 1 mole of the substance (formula mass in g). ΔHvap is the energy needed to vaporise by evaporation or boiling 1 mole of the substance (formula mass in g). For simple small covalent molecules, the energy absorbed by the material is relatively small to melt or vaporise the substance and the bigger the molecule the greater the inter–molecular forces. These forces are weak compared to the chemical bonds holding atoms together in a molecule itself. Relatively low energies are needed to melt or vapourise them. These substances have relatively low melting points and boiling points. Covalent Bonding – small simple molecules For strongly bonded 3D networks e.g. (i) an ionically bonded lattice of ions (ionic bonding), (ii) a covalently bonded lattice of atoms (covalent bonding – giant covalent structures), (iii) and a metal lattice of ions and free outer electrons (metallic bonding), the structures are much stronger in a continuous way because of the continuous chemical bonding throughout the structure. Consequently, much greater energies are required to melt or vaporise the material. This is why they have so much higher melting points and boiling points.
Substance	formula	Type of bonding, structure and attractive forces operating	Melting point K (Kelvin) = oC + 273	Energy needed to melt substance	Boiling point K (Kelvin) = oC + 273	Energy needed to boil substance
methane	CH4	small covalent molecule – very weak intermolecular forces	91K/–182oC	0.94kJ/mol	112K/–161oC	8.2kJ/mol
ethanol  ('alcohol')	C2H5OH	larger covalent molecule than methane, greater, but still weak intermolecular forces	156K/–117oC	4.6kJ/mol	352K/79oC	43.5kJ/mol
sodium chloride	Na+Cl–	ionic lattice, very strong 3D ionic bonding due to attraction between (+) and (–) ions	1074K/801oC	29kJ/mol	1740K/1467oC	171kJ/mol
iron	Fe	strong 3D bonding by attraction of metal ions (+) with free delocalised outer electrons (–)	1808K/1535oC	15.4kJ/mol	3023K/2750oC	351kJ/mol
silicon dioxide (silica)	SiO2	giant covalent structure, strong continuous 3D bond network of O-Si-O bonds	1883K/1610oC	46.4kJ/mol	2503K/2230oC	439kJ/mol

 

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COMPARISON OF VARIOUS STRUCTURES and their PROPERTIES

	Giant lattice			covalent molecular
	Ionic	Covalent network	Metallic	simple molecular	macromolecular
Examples	compounds of metals combined with non-metals eg NaCl, K2O	Group 4 eg carbon C (diamond & graphite), silica SiO2, some thermosetting polymers	metals like magnesium Mg and iron Fe	some non-metal elements and non-metal + non-metal compounds eg O2, H2O	polymers eg poly(ethene), PVC, proteins, DNA
Type of particles and bonding	strong ionic bonds, strong electrostatic attraction between oppositely charged ions	2D or 3D network of strong covalent bonds between atoms (mutual attraction of two +ve nuclei to shared -ve electrons	strong metallic bond, lattice of positive metal ions strongly attracted to a 'sea' of surrounding delocalised negative electrons	small molecules with strong intramolecular covalent bonds eg O=O, O-H within the molecule, BUT weak inter-molecular bonds of attraction between the molecules	strong intramolecular covalent bonds but weak intermolecular bonds - but stronger than in small simple molecules
Melting point and boiling point	high, strong electrostatic attraction between ions	very high, strong covalent bond network	generally high (except mercury and some group 1 metals)	low due to weak intermolecular forces	moderately high and tend to decompose on strong heating - smell!
Electrical conductivity	only conduct (electrolytes) when molten or dissolved in water - ions then are mobile	rarely conduct, graphite and graphenes are important exception	conduct when solid or liquid conduct via delocalised electrons	do not conduct, no ions or mobile electrons	do not normally conduct, but there some 'smart' plastic materials that can
Solubility in water	many soluble - ions can be hydrated (solvated by polar water molecules)	insoluble	insoluble but some react eg group 1 alkali metals, but this is a dissolving-reaction	most are insoluble, but, if it is a polar molecule that can hydrogen bond with water, dissolving can occur eg ammonia or ethanol	usually insoluble. but you can make special water soluble polymers with polar -OH groups on the polymer chain
Solubility in non-polar solvents	insoluble	insoluble	insoluble	often soluble	tend to be insoluble
Hardness - physical strength	hard but brittle	3D structure very hard, 2D graphite soft and slippery!	usually hard but malleable except mercury and some group 1 metals	usually relatively soft crystals	quite variable, but usually flexible, can be quite hard in high % of crystallinity
Structure type	ionic	giant covalent	metallic	small molecules	macromolecules

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