Summary notes for AQA A-Level Chemistry Unit 3.1.3 - Bonding by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: States of Matter, Ionic Bonding, Covalent and Dative Covalent Bonds, Metallic Bonding, Shapes of Simple Molecules and Ions, Forces Between C...
Melting: change of state from solid to liquid. Temp. at which this occurs = m.p.
• Energy is taken in when a substance melts to overcome the forces or bonds. The stronger the forces
or bonds in a substance, the greater the energy required to melt the substance.
Freezing: change of state from liquid to solid. Temp. at which this occurs = m.p.
• Energy is released when a substance freezes as forces or bonds are formed. The stronger the forces
or bonds formed on freezing, the more energy is released.
Boiling: change of state from liquid to gas. Temp. at which this occurs = b.p.
• Energy is taken in when a substance boils. The stronger the bonds in the liquid substance, the
greater the energy required to boil the substance.
Condensing: change of state from gas to liquid. Temp. at which this occurs = b.p.
• Energy is released when a substance condenses. The stronger the bonds or forces formed on
condensing, the more energy is released.
Subliming: change of state from solid to gas on heating; change of state from gas to solid on cooling.
• Substances which sublime = solid iodine + solid carbon dioxide = A.K.A. dry ice.
Ionic Bonding
Lattice: a regular repeated 3-D arrangement of atoms, ions or molecules in a crystalline solid.
Ionic bonding: electrostatic attraction between opp. charged ions in a lattice. These ions are formed by
electrons being transferred from one atom to another.
Na + Cl (as dot and cross diagrams)
1 electron Formulae of Compound Ions:
transferred Ammonium: NH4+
Carbonate: CO32-
Hydroxide: OH-
Nitrate: NO3-
Sulfate: SO42-
Formula:
Giant Ionic Structure
1) Particles of the 3-D lattice are cations + anions.
2) Strong electrostatic attractions hold cations + anions together.
E.g. NaCl: The ions line up with every +ve ion surrounded by —ve ions and vice versa.
Hardness
Hard because the anions + cations are strongly attracted to each other +
dif cult to separate. However, when force is applied to an ionic solid then ions
of like charge may be forced closer to each other. The electrostatic repulsion
can be enough to split the solid, which is why ionic solids are also brittle.
Melting + Boiling Points
The giant ionic lattices are held together by a large no. of strong electrostatic attractions between
oppositely charged ions ∴ large amount of energy required to overcome these forces ∴ high b.p. + m.p.
• The smaller the ions + the higher the charge on the ions, the stronger the ionic bond.
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, Section 1 : Physical Chemistry
Electrical Conductivity
In general…
For anything to conduct electricity, they must satisfy the 2 criterion:
1. The substance must contain charged particles.
2. The charged particles must be free to move throughout the substance.
In application to ionic solids…
• In solid state, we satisfy the rst criterion (ions are charged particles) but they are xed in positions
in the lattice by strong ionic bond ∴ ionic compounds do not conduct in solid state.
• In molten state + in solution the ions are now free to move so it will now conduct.
Solubility
Ionic compounds are polar molecules, hence they tend to dissolve in polar solvents, such as water.
When moving water molecules hit the ionic lattice they can knock ions off + then water molecules
surround the ions, causing the substance to break up + dissolve.
• However, for ionic substances like aluminium oxide the electrostatic forces of attraction are so strong
that water cannot break up the lattice so the compound is insoluble in water.
Nature of Covalent and Dative Covalent Bonds
Covalent bonding: electrostatic attraction between the bonding pair of
electrons and the nuclei of the atoms involved in the bond.
• Atoms make 1 covalent bond for each electron that it needs to get a
full shell.
A normal covalent bond contains a shared pair of electrons w/ one electron coming from each atom.
But a coordinate (dative covalent) bond contains a shared pair of electrons w/ both electrons
supplied by one atom. Coordinate bonds form when a lone pair on one of the atoms is donated to
another atom (which doesn’t have any electrons available to share).
E.g. Ammonium
E.g. Hydroxium
Molecular (Simple Covalent) Structure
1) Particles of the 3-D lattice are covalent molecules.
2) The forces between the molecules are weak van der Waals forces.
E.g. Iodine (I2)
Lattice of iodine molecules
van der Waals forces (fairly weak)
Hardness
Relatively soft as they do not have strong intermolecular forces.
Melting + Boiling Points
Relatively low because the van der Waals forces between the molecules are weak ∴ little energy
required to overcome them.
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