Key concepts in chemistry Revision Notes

C60 (Buckminsterfullerene)

C60 is a spherical fullerene with 60 carbon atoms covalently bonded together in pentagonal and hexagonal rings, forming a 'buckyball' structure. Each carbon atom forms three covalent bonds. While the covalent bonds within the molecule are strong, C60 molecules are held together by weak intermolecular forces. This results in C60 being a soft solid with a low melting point. Its electrons are localised, making it a poor electrical conductor.

Graphene

Graphene is a single, two-dimensional layer of carbon atoms arranged in a hexagonal lattice. Each carbon atom is covalently bonded to three others, creating strong covalent bonds across the entire sheet. The presence of delocalised electrons in graphene allows it to be an excellent conductor of electricity and heat. Its flat, single-atom-thick structure gives it exceptional strength and lightness. Graphene exhibits high electrical and thermal conductivity due to its delocalisation.

Shared Characteristics and Differences

Both C60 and graphene are allotropes of carbon, meaning they consist solely of carbon atoms but in different structural arrangements. Both feature carbon atoms forming three covalent bonds. Key differences arise from their structures: C60 is a discrete molecular structure, while graphene is an extended two-dimensional network. This leads to C60 having localised electrons and being a poor conductor, while graphene has delocalised electrons and is an excellent conductor. Graphene's 2D structure also contributes to its extraordinary strength compared to the more brittle C60.

Purpose of the Experiment

The primary goal is to find the simplest whole-number ratio of atoms of each element in a compound. This ratio is known as the empirical formula. For example, in magnesium oxide, we aim to find the ratio of magnesium atoms to oxygen atoms.

Experimental Procedure for Magnesium Oxide

1. Weigh a clean crucible and lid. 2. Add a known mass of magnesium ribbon to the crucible and reweigh to find the mass of magnesium. 3. Heat the crucible strongly, occasionally lifting the lid to allow oxygen to enter, but preventing loss of magnesium oxide 'smoke'. 4. Continue heating until all magnesium has reacted, indicated by the absence of bright burning. 5. Allow the crucible and its contents to cool completely. 6. Weigh the crucible, lid, and magnesium oxide product. 7. Calculate the mass of oxygen that reacted by subtracting the initial mass of magnesium from the final mass of magnesium oxide.

Calculating the Empirical Formula

After determining the masses of magnesium and oxygen that reacted: 1. Convert the mass of each element to moles using their atomic masses (Ar: Mg = 24.3, O = 16.0). Moles = Mass / Ar. 2. Divide the number of moles of each element by the smallest number of moles calculated. 3. If the ratios are not whole numbers, multiply all ratios by a small integer to obtain the simplest whole-number ratio. This gives the empirical formula.

Sources of Error and Improvements

Potential errors include incomplete reaction of magnesium, loss of magnesium oxide product as 'smoke' (fine powder), and reaction with nitrogen from the air to form magnesium nitride. To improve accuracy: ensure strong heating until no unreacted magnesium remains; lift the lid only briefly to allow oxygen in; if magnesium nitride forms, add a few drops of water to convert it to magnesium hydroxide and ammonia, then re-heat until the water evaporates to reform magnesium oxide.

Metallic Structure and Bonding

Metals consist of a giant lattice of positive metal ions surrounded by a 'sea' of delocalised electrons. This unique bonding, often called metallic bonding, is key to many metallic properties.

Malleability

Malleability means metals can be hammered or pressed into shapes without shattering. When force is applied, layers of metal ions slide over each other without breaking the metallic bonds. The delocalised electrons maintain electrostatic attraction, holding the structure together even after deformation.

Electrical Conductivity

Metals are good electrical conductors because of their delocalised electrons. These electrons are not bound to any specific atom and can move freely throughout the metallic structure. When an electric field is applied, these mobile electrons can carry a charge, resulting in an electric current.

Other Common Properties

In addition to malleability and electrical conductivity, metals typically have high melting and boiling points, are good thermal conductors, and are typically dense. They are also usually shiny when polished.

Understanding Model Limitations

All scientific models are simplifications of reality, designed to help us understand complex phenomena. While useful, it is vital to acknowledge their limitations to avoid misconceptions. Different models highlight different aspects and omit others.

Specific Model Limitations

Dot and cross diagrams show electron transfer or sharing but ignore 3D shape and actual particle size. Ball and stick models depict 3D shape and bond angles but don't show electron clouds, relative sizes accurately, or dynamic movement. Two-dimensional representations are limited by their inability to show bond angles and molecular geometry accurately. Three-dimensional models are better for geometry but still simplify electron distribution and dynamic processes inherent in molecules.