Basic Concepts: Compounds

Understanding compounds is fundamental in chemistry. Recognising their importance aids not only in examinations but also in practical applications across various industries and biological systems.

Definition of a Compound

• Compound : A chemical substance formed when two or more different elements are chemically bonded together in a fixed ratio.
• Unlike mixtures, compounds are formed through chemical bonding and exhibit new properties rather than a simple physical combination.

Introduction to Compounds

Compounds convert simple elements into entirely new substances with distinct characteristics. Much like a new dish created from raw ingredients, chemical bonds in compounds impart these unique properties.

Formation of Compounds

• Compounds are formed when elements combine through chemical reactions, resulting in substances with unique properties.
• Water (H₂O) and sodium chloride (NaCl) possess attributes distinct from those of sodium or chlorine on their own.

Types of Chemical Bonding

• Ionic Bonds

  • Definition: Involves the transfer of electrons forming positive and negative ions, e.g., $\text{Na}^+$ and $\text{Cl}^-$ in NaCl.
  • Properties: High melting points and conductivity when dissolved.

• Covalent Bonds

  • Definition: Involves the sharing of electron pairs, as seen in H₂O.
  • Properties: Low melting points and poor conductivity.

• Metallic Bonds

  • Definition: Characterised by electrons forming a 'sea of electrons' around metal ions.
  • Properties: Good conductivity and malleability.

• Lewis Structures
  • Water (H₂O): Example of a covalent bond.
  • NaCl: Example of an ionic bond.
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Understanding Stoichiometry

• Stoichiometry: The calculation of reactants and products in chemical reactions.

• Law of Conservation of Mass: Mass is neither created nor destroyed in chemical reactions.

• Example: Balancing $2H_2 + O_2 \rightarrow 2H_2O$, ensuring the correct ratio for the reaction.

Introduction to the Mole Concept

• Avogadro's Constant: $\approx 6.022 \times 10^{23}$, linking macro and micro scales.

• Formula for Mole Calculations:

  • $n = \frac{m}{MM}$
  • Components:
    • $n$: Moles
    • $m$: Mass
    • $MM$: Molar mass

Compound Properties

• Example: Sodium chloride exhibits stable properties, contrasting the reactivity of sodium and the toxicity of chlorine.

• Factors Affecting Properties:
  • Bond Types: Ionic, covalent, and metallic bonds influence stability and conductivity.

Real-World Examples of Compounds

• Water (H₂O): Acts as a universal solvent and is essential for biological processes.

• Sodium Chloride (NaCl): Used for flavouring, preservation, and de-icing.

• Carbon Dioxide (CO₂): Involved in photosynthesis and used in carbonated beverages.

• Activities:

  • Observing the effect of NaCl on freezing points.

Misconceptions in Chemistry

Key Takeaways

• Properties: Each compound possesses unique properties due to distinctive bonding.
• Chemical Formulas: They express the elements and their ratios.
• Considerations:
  • Exam preparation includes using flashcards and practical examples.

Exam Tips

• Practice stoichiometry and formula calculations.
• Use mnemonics for understanding bond types and compound characteristics.