structure and properties of materials (chap 5) -olevel pure chem

what is an element

a pure substance that cannot be broken down into two or more simpler substances by chemical methods

what is a compound

a pure substance containing 2 or more elements that are chemically combined in a fixed ratio

what are the differences between compounds and mixtures

compounds has a fixed ratio while a mixture has no fixed ratio

compounds cannot be separated into its constituent elements easily but mixtures can

compounds have different properties from its constituent elements but mixtures usually have similar properties to its constituent elements

mixtures boil over a range of temperatures while compounds have a fixed melting and boiling point

what are the structural properties of ionic compounds

• ionic compounds have giant ionic crystal lattice structures

• it consists of large numbers of alternating positive and negative ions held together by electrostatic forces of attraction

what are the physical properties of ionic compounds

• high melting and boiling points

  • because the mutual electrostatic attraction between oppositely charged forces are strong, a large amount of energy is needed to overcome these forces

• hard but brittle

  • they are hard as the strong attractive forces cause ions to resist motion

  • if enough force is applied, ions move away from their lattice positions and ions of the same charge approach each other. repulsive forces between ions of the same charge become larger than the attractive forces and lattice structure shatters, hence ionic compounds are hard but brittle

• soluble in water and insoluble in organic solvents

  • dissolved in water - aqueous state

• can conduct electricity in the aqueous state

  • when in aqueous state, the ions are mobile and can conduct electricity

what are the structural properties of covalent substances

• atoms in the molecule are held by strong covalent bonds, but the molecules are held together by weak intermolecular forces of attraction

what are the physical properties of covalent substances

• low melting and boiling points

  • the weak intermolecular forces of attraction can be easily overcome with a small amount of energy

  • exists mostly as gases and liquids at room temperature

  • as the molecule gets larger, the intermolecular forces of attraction are stronger, so the melting and boiling points are higher

  • eg: iodine is a solid at room temp, bromine is a liquid at room temp (iodine higher molar mass)

• most are insoluble in water and soluble in organic solvents

• cannot conduct electricity

  • have no mobile charge carriers to conduct electricity as they typically exist as neutral molecules

  • some can disassociate in water to dissolve into its individual ions which are mobile and can conduct electricity as a solution (eg: HCL hydrogen chloride)

what is an allotrope

different forms of the same element with different structural arrangements of atoms

what are examples of giant covalent structures

1. diamond (carbon allotrope)

2. graphite (carbon allotrope)

3. silicon dioxide

what is the structural of diamond

• each carbon atom forms strong covalent bonds with four other carbon atoms

• this gives diamond a three dimensional network structure in a tetrahedral arrangement

what is the structural properties of graphite

• each carbon atom forms covalent bonds with three other carbon atoms

• gives a layered structure consisting of hexagonal rings of carbon atoms

• strong covalent bonds are found within each layer, but weak intermolecular forces of attraction exist between the layers

what are the physical properties of diamond

• hard

  • made up of only strong covalent bonds

  • large amount of energy is needed to break the structure apart

• very high melting and boiling points

  • due to the large number of bonds in the network

  • very large amount of energy is required to break the strong covalent bonds

• insoluble in water and in organic solvents

• do not conduct electricity

  • has no mobile electrons

what is the physical properties of graphite

• soft and slippery

  • only a small amount of energy needed to overcome the weak intermolecular forces of attraction between the layers of carbon atoms

• high melting and boiling points

  • due to the large number of bonds in the network

  • a large amount of energy is required to break the strong covalent bonds between the atoms

• insoluble in water and organic solvents

• conducts electricity

  • each carbon atom is bonded to three other carbon atoms

  • this leaves one unbonded electron per carbon atom, which is free and mobile to conduct electricity

why are carbon nanotubes such a good conductor of electricity

the delocalised mobile electrons are trapped within the cylinder. this means that electricity can be conducted up and down the nanotube

why is fullerene (the soccer ball shaped carbon allotrope) a good carrier of substances

they have a large surface area which would make them good catalysts

what is the metallic bond

the mutual electrostatic attraction between positively charged metal ions and the “sea of delocalised electrons”

what is an alloy

is a mixture of a metal with one or more other elements

what are the structural properties of metals and alloys

metals:

• have a regular structure

• they are malleable and ductile

• sea of delocalised electrons will hold the metal ions together until a much greater force is applied

alloys:

• atoms are of different sizes, so it has a irregular arrangement

• a larger force is needed to make the layers slide over each other

• thus, alloys tend to be less malleable and ductile than the pure metals they are made from

• alloys are harder and stronger than pure metals

what are the physical properties of metals and alloys

• high melting and boiling points

  • atoms are held together by strong electrostatic forces of attraction between positively charged metal ions and sea of delocalised electrons

  • alloys melt over a range of temperatures and metals have fixed mp and bp

• good electrical conductors

  • the sea of delocalised electrons found in metals and alloys make them good electrical conductors