In-Depth Notes on Chemical Calculations, Reactions, and Equilibrium Principles

Calculations from Equations

Understanding Chemical Equations

  • Chemical Reaction: Carbon reacts with oxygen to form carbon dioxide.
  • Molecular Representation:
  • Molecules: 1 Carbon (C) + 1 Molecule of Oxygen (O₂) → 1 Molecule of Carbon Dioxide (CO₂)
  • Shorthand Equation: C (s) + O₂ (g) → CO₂ (g)
Proportions and Masses
  • Entities Involved:
  • 1 Carbon Atom (Relative Atomic Mass = 12)
  • 1 Oxygen Molecule (Relative Mass = 32)
  • 1 CO₂ Molecule (Relative Mass = 44)
  • Mass Relationships:
  • Mass ratios in chemical reactions are consistent.
  • Proportions from the equation:
    • 12g of C reacts with 32g of O to yield 44g of CO₂.
    • Example variations:
    • 6g of C + 16g of O → 22g of CO₂

Law of Conservation of Mass

  • Concept: The total mass remains unchanged during a chemical reaction.
  • Example Calculation:
  • Starting mass: 12g (C) + 32g (O) = 44g
  • Ending mass: 44g (CO₂)
  • As a result, no atoms disappear; they merely rearrange.
  • When mass seems to change (e.g., burning coal), it’s due to gas escapes but in a closed system, mass is conserved.

Steps for Mass Calculations

  1. Write the balanced equation: Example for hydrogen and oxygen = 2H₂ + O₂ → 2H₂O.
  2. Relative Atomic Masses: H = 1, O = 16.
  3. Identify Reacting Amounts: 2H₂ (4g), O₂ (32g), 2H₂O (36g).
  4. Start with known mass: 1g of H requires 8g of O to form 9g of H₂O.
Example Calculations
  • Example 1: Hydrogen Combustion:
  • Masses needed to react: 1g H → 8g O → 9g H₂O.
  • Example 2: Iron and Sulphur Reaction:
  • 7g Fe + S → FeS yields 11g FeS and uses 4g S.

Practical Application of Gases in Reactions

Gas Volume Relationships
  • Particle Volume Principle: Equal volumes of gases at the same temperature and pressure contain equal numbers of particles.
  • Avogadro’s Law: This sets the foundation for gas calculations.
  • The molecular weight of a gas indicates that:
  • 44g CO₂, 2g H₂, and 32g O₂ occupy the same volume at rtp (Room Temperature and Pressure = 20°C, 1 atm).
Volume Calculations Steps
  1. Identify Balanced Equation: Example: S + O₂ → SO₂.
  2. Relative Atomic Masses: S = 32.
  3. Calculate Reacting Volumes: Treat gases similarly with the volume of gas representing their molecular weight.
  • 8g of S yields 6 dm³ SO₂ at rtp.
Electrolysis Calculations
  • Principles of Electrolysis
  • Copper(II) Chloride yields Copper and Chlorine.
  • Half Equations for each reaction.
  • For each unit of one element produced, the equivalent of another can be calculated:
  • E.g. for lead electrolysis, 2.07g of lead gives 1.6g of bromine.
Rates of Reaction
  • Definition: Rate measures the change over time - essential for industrial applications.
  • Measurement Techniques: Utilize hydrogen production in reactions (as in Mg + HCl) over time.
  • Plotting: Graphs can illustrate reaction rates and indicate completion (flatlines).
Factors Affecting Reaction Rates
  1. Concentration: Higher concentrations can increase reaction speed.
  2. Temperature: Elevating temperature usually accelerates reactions.
  3. Pressure: For gaseous reactions, increasing pressure can shift equilibrium and improve yields.
  4. Catalysts: These speed up reactions without being consumed.

Energy Changes in Reactions

Types of Reactions
  • Exothermic: Release energy (e.g., combustion).
  • Endothermic: Absorb energy (e.g., ammonium chloride reaction).
  • Highlight bond breaking (endothermic) vs. bond forming (exothermic).
Bond Energies & Calculations
  • Bond Energy: Energy required to break or form bonds expressed in kJ. Used to determine heat exchanges during reactions.

Reversible Reactions and Equilibrium

  • Example: Copper(II) Sulphate transition between hydrated and anhydrous forms.
  • Dynamic Equilibrium: Forward and backward reactions occur at the same rate, maintaining constant concentrations.

Conclusion

  • Understanding these principles of chemistry will assist in calculations of mass, gas volumes, and the dynamics of various reactions, ensuring a well-rounded preparation for exams in this subject matter.