Understanding Alcohol Combustion and Enthalpy

Expert reviewed • 23 November 2024 • 6 minute read

Introduction

Alcohols are important organic compounds that undergo combustion reactions, releasing significant amounts of energy. Understanding these reactions is crucial for both theoretical chemistry and practical applications.

Complete Combustion of Alcohols

When alcohols undergo complete combustion with oxygen, they produce carbon dioxide and water. For example, ethanol combustion follows this equation:

C2H5OH(l)+3O2(g)→2CO2(g)+3H2O(l)\text{C}_2\text{H}_5\text{OH}_{(l)} + 3\text{O}_{2(g)} \rightarrow 2\text{CO}_{2(g)} + 3\text{H}_2\text{O}_{(l)}

This reaction is exothermic, meaning it releases energy. The energy release occurs because:

  • The energy released during bond formation in products (C=O and H-O bonds)
  • Exceeds the energy required to break bonds in reactants (C-C, C-H, and C-O bonds)

Enthalpy Trends in Alcohol Combustion

Molar Enthalpy

The molar enthalpy of combustion (ΔH) becomes more negative as the alcohol chain length increases. For each additional carbon atom:

  • Two C-H bonds and one C-C bond are broken
  • Two C=O bonds form in CO₂
  • Two O-H bonds form in H₂O

Mass-Based Enthalpy

The enthalpy of combustion per gram also increases with molecular size because:

  • Each additional carbon adds a consistent mass (CH₂ unit)
  • The O-H group's mass becomes proportionally smaller
  • More energy is released per unit mass

Isomer Effects

Position and chain isomers (like 1-propanol and 2-propanol) have identical heats of combustion because:

  • They contain the same number and types of bonds
  • They form the same products upon combustion
  • The total energy change is independent of molecular structure

Experimental Determination of Combustion Enthalpy

Equipment Setup

  • Spirit burner containing alcohol
  • 500 mL beaker with known water volume
  • Thermometer
  • Electronic balance

Procedure

  • Record initial mass of spirit burner with alcohol
  • Measure initial water temperature
  • Ignite alcohol and heat water
  • Record maximum water temperature
  • Measure final mass of spirit burner

Calculations

The heat absorbed by water: q=mcΔTq = mc\Delta T where:

  • m = mass of water (kg)
  • c = specific heat capacity of water (4.18 × 10³ J kg⁻¹ K⁻¹)
  • ΔT = temperature change (K)

The enthalpy of combustion: ΔH=−qn\Delta H = -\frac{q}{n} where n = moles of alcohol consumed

Experimental Considerations

Validity

The experiment is valid for comparing different alcohols but may not provide absolute values due to:

  • Potential incomplete combustion
  • Heat loss to surroundings

Improving Accuracy

  • Minimize heat loss:
    • Reduce flame-to-beaker distance
    • Control air currents
    • Use insulation
  • Ensure complete combustion:
    • Maintain proper oxygen supply
    • Use appropriate apparatus

Reliability

Results are reliable when:

  • Procedures are standardized
  • Conditions are controlled
  • Multiple trials are performed

Up Next

Enthalpy of Combustion: Calculations and Key Concepts

Return to Methods of Alcohol Production in Organic Chemistry