Real-world Examples of Spontaneous Reactions

Background Context and Historical Significance:

Spontaneous reactions are reactions that occur naturally under given conditions without any external intervention. Their inherent tendency to occur is based on a combination of entropy changes and enthalpy changes, as summarized by the Gibbs free energy equation. Historically, spontaneous reactions have been central to many discoveries and technologies, from the rusting of iron to the generation of electricity in batteries.

Detailed Content:

  1. Rusting of Iron:
    • Iron reacts with oxygen in the presence of moisture to form iron(III) oxide (rust).
    • 4 Fe + 3 O₂ → 2 Fe₂O₃
    • This process is exothermic, releasing energy, and the formation of solid rust represents an increase in entropy due to increased dispersion of particles.
  2. Decomposition of Hydrogen Peroxide:
    • In the presence of a catalyst (like manganese dioxide), hydrogen peroxide will spontaneously decompose to water and oxygen.
    • 2 H₂O₂ → 2 H₂O + O₂
  3. Respiration in Cells:
    • Glucose is oxidized in cells to produce carbon dioxide, water, and energy.
    • C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O
    • This reaction releases energy, which is harnessed in the form of ATP, the cell’s energy currency.
  4. Natural Dissolution of Salt in Water:
    • When you add table salt (sodium chloride) to water, it spontaneously dissolves.
    • NaCl → Na⁺ + Cl⁻
  5. Formation of Ammonium Chloride:
    • When ammonia and hydrogen chloride gases are combined, they spontaneously form solid ammonium chloride.
    • NH₃(g) + HCl(g) → NH₄Cl(s)
  6. Spontaneous Combustion:
    • Some materials, like oily rags, can undergo oxidation at a slow rate. If heat accumulates faster than it can dissipate, the temperature rises, and the material can ignite spontaneously.

Patterns and Trends:

  • Entropy and Enthalpy: For a spontaneous process, the Gibbs free energy change (ΔG) must be negative. ΔG is determined by both the change in enthalpy (ΔH) and the change in entropy (ΔS). Either a large negative ΔH or a large positive ΔS can drive a reaction to be spontaneous.
  • Temperature Dependency: Some reactions are spontaneous at certain temperatures but not at others. This is because the temperature influences the Gibbs free energy change.

Influential Figures or Works:

  • Josiah Willard Gibbs: Developed the Gibbs free energy concept, a foundational principle to understand and predict spontaneous reactions.

Relevance in the Broader Framework:

  • Industrial and Daily Life: Many industrial processes and everyday phenomena, like the rusting of vehicles or the souring of milk, can be traced back to spontaneous reactions.
  • Environmental Implications: Spontaneous reactions like combustion contribute to environmental issues, e.g., CO₂ emissions from burning fossil fuels.


Spontaneous reactions, while they might seem “simple” because they “just happen,” have deep thermodynamic underpinnings. They play a significant role in both nature and technology, driving processes that shape our world and our daily experiences.