3.5.4Physical Chemistry
Combustion of Alcohols — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 85 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6Complete Combustion
Complete Combustion
Reactants and products
- Reactants and products With plenty of oxygen, an alcohol burns to give carbon dioxide and water only.
Ethanol
- Ethanol C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l), where one oxygen atom comes from the alcohol itself.
Methanol
- Methanol 2CH₃OH(l) + 3O₂(g) → 2CO₂(g) + 4H₂O(l), or CH₃OH + 1½O₂ → CO₂ + 2H₂O.
General equation
- General equation CₙH₂ₙ₊₁OH + 1.5n O₂ → nCO₂ + (n+1)H₂O for a saturated alcohol with one –OH.
Section 2 of 6Measuring the Enthalpy of Combustion
1
Weigh a spirit burner containing the alcohol, and measure a known mass of water into a metal calorimeter.
2
Record the starting temperature of the water, then light the burner and stir the water as it is heated.
3
After the temperature has risen by a suitable amount, extinguish the flame and record the highest temperature.
4
Reweigh the burner so the mass of alcohol burned is found by difference.
5
Calculate q = mcΔT using the mass of water, then divide by the moles of alcohol burned to find ΔH.
Section 3 of 6Sources of Error
Heat loss
- Heat loss Much of the heat escapes to the surroundings, so the measured temperature rise is too small.
Calorimeter
- Calorimeter Some heat warms the metal calorimeter rather than the water, which is ignored in the calculation.
Incomplete combustion
- Incomplete combustion A sooty flame means carbon monoxide or carbon formed, so less energy was released than for complete combustion.
Reducing errors
- Reducing errors A lid, a draught shield and a short flame-to-calorimeter distance reduce losses but cannot remove them.
Section 4 of 6Ethanol as a Fuel
Ethanol as a Fuel
Advantages
- Advantages It burns cleanly and can be made by fermentation from renewable crops, reducing the use of crude oil.
Lower energy density
- Lower energy density Ethanol releases less energy per kilogram than petrol, so vehicles need larger tanks or refuel more often.
Blending
- Blending Mixing ethanol with petrol raises the energy density of the blend and makes the flame easier to see.
Land use
- Land use Large areas of farmland used for fuel crops cannot grow food, which raises social and economic concerns.
Section 5 of 6Worked Example
Question
- 1.00 g of ethanol (Mr = 46.1) was burned and heated 150 g of water from 20.0 °C to 44.0 °C.
- Calculate the enthalpy of combustion of ethanol from these results.
- (c = 4.18 J g⁻¹ K⁻¹)
- Temperature change ΔT = 44.0 − 20.0 = 24.0 K.
- q = m c ΔT = 150 × 4.18 × 24.0 = 15 048 J = 15.05 kJ.What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
- Moles of ethanol = 1.00 ÷ 46.1 = 0.02169 mol.
- ΔH = −q ÷ n = −15.05 ÷ 0.02169 = −693.7 kJ mol⁻¹,
- which rounds to −694 kJ mol⁻¹, with a negative sign because the reaction is exothermic.What the symbols mean: ΔH = enthalpy change (kJ mol−1) · q = heat energy measured · n = amount of the limiting reactant (mol); negative sign = exothermic
- (J, divide by 1000 for kJ)
- which rounds to −694 kJ mol⁻¹, with a negative sign because the reaction is exothermic.
Answer ΔH is about −694 kJ mol⁻¹, much less exothermic than the data-book value because of heat losses.
Section 6 of 6Exam Checklist & Tips
3.5.4 – Key Points for the Exam
- Complete combustion Burning in plentiful oxygen to give carbon dioxide and water only.
- Enthalpy of combustion The enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
- Energy density The energy released per unit mass of fuel, for example in kJ per kg.
- Calorimeter A container in which the heat transferred to water by a reaction is measured.
Common Mistakes
- Forgetting to count the oxygen atom in the alcohol when balancing the oxygen on the left-hand side.
- Using the mass of the alcohol rather than the mass of the water in q = mcΔT.
- Leaving ΔH positive for a combustion, which is always exothermic.
- Quoting the answer in joules when the question asks for kJ mol⁻¹.
Last updated:
\n