Describe the combustion of ethanol

Organic Chemistry – Alcohols (Cambridge IGCSE 11.6)

1. Ethanol – definition and classification

• Systematic name: ethanol (also called ethyl alcohol).
• Molecular formula: C₂H₅OH or CH₃CH₂OH.
• Classification: primary alcohol – the –OH group is attached to a carbon that is bonded to only one other carbon (CH₃‑CH₂‑OH).
• Functional group: hydroxyl (‑OH) attached to a saturated carbon atom.

2. Manufacture of ethanol

Two routes are required by the syllabus. Both overall equations are shown, followed by a brief rationale and a comparison of advantages and disadvantages.

Advantages / disadvantages

3. Physical properties relevant to its use

4. Uses of ethanol (linked to properties)

• Fuel – high vapour pressure and low boiling point give a readily ignitable vapour; energy density makes it suitable for spirit burners and bio‑ethanol blends (E85).
• Solvent – complete miscibility with water and many organic compounds allows it to dissolve a wide range of substances.
• Antiseptic / disinfectant – 70 % (v/v) solutions denature proteins; volatility ensures rapid drying.
• Beverage alcohol – pleasant taste and low toxicity at moderate concentrations; produced by fermentation.
• Chemical feed‑stock – can be dehydrated to ethene or esterified to ethyl acetate; the hydroxyl group is a versatile functional handle.

5. Combustion of ethanol – overall reaction

Complete oxidation occurs when ethanol vapour mixes with excess oxygen and is ignited.

Balanced equation (complete combustion)

$$\displaystyle \color{blue}{C_2H_5OH} + 3\,\color{red}{O_2} \;\longrightarrow\; 2\,\color{green}{CO_2} + 3\,\color{purple}{H_2O}$$

Stoichiometric oxygen requirement: 3 mol O₂ per 1 mol ethanol (useful for AO2 calculations).

6. Reaction conditions

• Ignition source – flame, spark or hot surface.
• Excess atmospheric oxygen (≥ 3 mol O₂ mol⁻¹ ethanol).
• Usually at atmospheric pressure; temperature rises sharply because the reaction is highly exothermic.
• Vapour phase of ethanol is the reactive species – the liquid must evaporate first.

7. Simplified radical mechanism (illustrative only)

1. Heat breaks the C–O bond → ethoxy radical CH₃CH₂O· + H·.
2. Radicals react with O₂ → peroxy radical CH₃CH₂OO·.
3. Fragmentation gives CO, CO₂ and additional radicals.
4. Hydrogen radicals combine with O₂ or OH· → H₂O.

Only the overall stoichiometry is required for IGCSE; the radical steps explain the rapid, exothermic nature of the reaction.

8. Energy change

Standard enthalpy of combustion

$$\Delta H^\circ_{\text{comb}} = -1367\ \text{kJ mol}^{-1}$$

• Negative sign = exothermic (heat released to the surroundings).
• Value is used in quantitative AO2 questions (e.g., energy released from a given mass).

9. Products and their significance

10. Practical applications of the combustion reaction

• Laboratory spirit burners and Bunsen‑type burners.
• Bio‑ethanol fuel for spark‑ignition engines (e.g., E85 blends).
• Demonstrations of energy release, enthalpy calculations, and fire‑safety principles.

11. Safety considerations

• Ethanol vapour is highly flammable – keep away from open flames, sparks and hot surfaces.
• Store in a tightly sealed container, labelled “Flammable liquid – Class 3”.
• Use heat‑resistant tongs, gloves and safety goggles when handling burning ethanol.
• Ventilate the area to avoid accumulation of CO₂ and to prevent asphyxiation.
• First‑aid: If skin is exposed, rinse with plenty of water; if in eyes, use an eye‑wash station for at least 15 min; in case of fire, use a CO₂ or dry‑powder extinguisher.

12. Sample calculation – energy released from 10 g of ethanol

Given: molar mass of ethanol = 46.07 g mol⁻¹.

Number of moles:

$$n = \frac{10\ \text{g}}{46.07\ \text{g mol}^{-1}} = 0.217\ \text{mol}$$

Energy released (using the magnitude of ΔH°):

$$q = n \times |\Delta H^\circ_{\text{comb}}| = 0.217\ \text{mol} \times 1367\ \text{kJ mol}^{-1} \approx 2.97 \times 10^{2}\ \text{kJ}$$

Thus, burning 10 g of ethanol liberates roughly 297 kJ** of heat.

13. Link to Assessment Objectives (AO)

• AO1 – Knowledge
  • Definition, classification and formula of ethanol (Section 1).
  • Manufacture routes and balanced equations (Section 2).
  • Physical properties (Section 3) and uses (Section 4).
  • Balanced combustion equation, ΔH° value and stoichiometric O₂ requirement (Section 5‑8).
• AO2 – Application
  • Calculate moles of ethanol or oxygen required (Section 5, bullet on O₂).
  • Energy‑release calculation (Section 12).
  • Use the combustion equation to predict products in a given scenario.
• AO3 – Analysis
  • Explain why ethanol is a good fuel (volatility, energy density – Section 4).
  • Discuss advantages/disadvantages of the two manufacture methods (Section 2‑table).
  • Evaluate environmental impact of the combustion products (CO₂ – Section 9) and safety aspects (Section 11).