Essential Chemistry Concepts: Structure, Bonding & Reactions

Sodium Chloride: Structure and Ionic Bonding

Sodium chloride is an ionic compound. It contains positive sodium ions (Na⁺) and negative chloride ions (Cl⁻). These ions are held together in a giant ionic lattice by strong electrostatic forces. These forces act in all directions, contributing to its characteristic high melting point.

Potassium vs. Lithium: Reactivity Differences

Both potassium and lithium are in Group 1 of the periodic table and possess one electron in their outer shell. However, potassium's outer electron is significantly further from the nucleus than lithium's. This increased distance results in less attraction between the nucleus and the outer electron, making it more easily lost. Consequently, potassium is more reactive than lithium.

Graphite vs. Diamond: Electrical Conductivity

Both graphite and diamond are allotropes of carbon, forming giant covalent structures. The key difference lies in their electron arrangement:

• In graphite, each carbon atom forms three covalent bonds, leaving one electron per atom delocalised and free to move. These delocalised electrons can carry charge, allowing graphite to conduct electricity.
• In diamond, each carbon atom forms four strong covalent bonds, with no free electrons available. Therefore, diamond does not conduct electricity.

Electrolysis of Molten Lead Bromide

When molten lead bromide (PbBr₂) undergoes electrolysis, the following reactions occur at the electrodes:

• At the Cathode (−): Positive lead ions (Pb²⁺) migrate to the cathode, where they gain electrons to form lead metal (Pb). This process is called reduction.
• At the Anode (+): Negative bromide ions (Br⁻) move to the anode, where they lose electrons to form bromine gas (Br₂). This process is called oxidation.

The products are lead and bromine. A useful mnemonic is OIL RIG: Oxidation Is Loss, Reduction Is Gain. The entire process requires electricity to split the molten compound.

Separating Soluble Salts: Filtration & Crystallisation

To separate a soluble salt from a solution using filtration and crystallisation, follow these steps:

1. Use filtration to remove any undissolved or excess solid impurities from the solution.
2. Gently heat the filtrate (the liquid that passed through the filter) to evaporate some of the water, creating a more concentrated solution.
3. Allow the remaining concentrated solution to cool slowly. As it cools, the solubility of the salt decreases, and pure crystals will form.
4. Carefully dry the formed crystals, typically by pressing them between sheets of filter paper.

Exothermic Reactions: Definition and Examples

An exothermic reaction is a chemical process that releases energy to its surroundings, typically in the form of heat. This energy release causes a noticeable rise in the temperature of the surroundings.

Examples: Common examples include combustion (e.g., burning fuel) and neutralisation reactions (e.g., acid + alkali).

Metal Reactivity vs. Carbon in Extraction

The reactivity series dictates how metals can be extracted, particularly in relation to carbon:

• A metal positioned above carbon in the reactivity series is more reactive than carbon and cannot be extracted by reduction using carbon.
• A metal positioned below carbon in the reactivity series is less reactive than carbon and can be extracted by reduction using carbon. This is because carbon is able to displace less reactive metals from their oxides.

Simple Molecular vs. Giant Covalent Structures

Here's a comparison of simple molecular and giant covalent substances:

• Simple Molecular Substances:
  • Composed of discrete molecules.
  • Held together by weak intermolecular forces.
  • Result in relatively low melting and boiling points.
  • Generally do not conduct electricity (e.g., water, CO₂).
• Giant Covalent Structures:
  • Consist of a vast network of atoms held by strong covalent bonds.
  • Require significant energy to break the strong covalent bonds throughout the structure.
  • Possess very high melting and boiling points.
  • Most do not conduct electricity (e.g., diamond), but some, like graphite, do due to delocalised electrons.