When it comes to A2/A-level Chemistry, do you know the process of fusion and vaporisation?

• When heating, a solid or supplying energy, it vibrates to a more fixed position therefore increasing the distance between particles so it expands.
• Supplying more energy can turn a solid into a liquid – this is required to weaken the forces that act between the particles.
• Energy known as the enthalpy change of melting.
• When a solid is melting, the temperature does not change as the heat is absorbed as the forces are weakened.

• Enthalpy is the heat energy change measured under constant pressure.

• Temperature depends on the average kinetic energy which is also related to their speed.

• Particles move further apart after being supplied energy therefore liquids also expand when heated.
• Energy is required to break all the intermolecular forces between the particles.
• Energy known as enthalpy change of vaporisation.
• No temperature change during the process of boiling.
• Gases expand a great deal when heated because they also gain kinetic energy and move further apart.

Can you distinguish crystals, diamond and graphite in A2/A-level Chemistry?

• Have a regular arrangement and are held together by forces of attraction.
• The strength of forces of attraction has an effect on the physical properties of crystals.
• The stronger the force, the higher the melting point/temperature and a greater the enthalpy.

• Molecular crystals – consist of molecules held in a regular structure by intermolecular forces and molecules held together by covalent bonds however are much weaker so have a low melting temperature and low enthalpy.

• Crystals are soft and break easily.
• They have low melting point temperature.
• They do not conduct electricity because there are no charged particles.

• It is a polymorph or allotrope of carbon.
• It consists of pure carbon with covalent bonding between every carbon atom.
• It has a giant structure – bond spreads throughout the structure.
• Each carbon forms four single covalent bonds with other carbon atoms.
• These four electrons repel from each other – electron pair repulsion theory.
• The bond angles is 109.5°.

• It is a very hard material.
• It has a very high melting temperature (over 3,700k).
• It does not conduct electricity because there are no delocalised electrons to carry the charge.

• It consists of pure carbon atoms which are bonded and arranged differently.
• It has strong covalent and weak van der Waals forces.
• It bonds at angle 120°.
• It leaves each carbon atom with a spare electron in a p-orbital that is not part of the three single covalent bonds.
• Electrons are able to move anywhere within the layer (delocalised).
• Weak intermolecular forces allows layers to slide across one another.

• It is a soft material.
• It has a very high melting temperature – breaks down before it melts due to strong network of covalent bonds.
• It conducts electricity.

• Ionic compound conduct electricity only in liquid state.
• Current is carried by the movement of ions towards the electrodes of opposite charge.
• Covalent bonded substances do not conduct electricity as they have no charged particles which are able to carry a current.
• They are mostly insoluble in water and can react to form ions, e.g. Ethanoic acid

• Simple molecules have low melting and boiling points.
• Giant structured molecules have high melting and boiling points.

• Strength of force determines melting and boiling points.

References:

1. https://www.tutormyself.com/edexcel-igcse-2017chem-150/

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