Reactivity 1.3. Energy from fuels

When 11 g of an organic compound is burnt in an excess of oxygen, 22 g of carbon dioxide and 9.0 g of water are produced as the only products of combustion. What is the  empirical formula of the organic compound? 

A. CH₂O. 

B. CH₂. 

C. C₄H₇O. 

D. C₂H₄O. 

Answer: D. C₂H₄O. 

Step 1: Determine moles of carbon and hydrogen 

a. Carbon from CO₂: 

`""n_(CO_2)` = `frac{22}{44}` = 0.5 mol 

Each CO₂ contains 1 carbon atom → `n_C` = 0.5 mol 

b. Hydrogen from H₂O: 

`n_(H_2O)` = `frac{9.0}(18)` = 0.5 mol 

Each H₂O has 2 hydrogens → `"n_H` = 2 × 0.5 = 1.0 mol 

Step 2: Determine mass of carbon and hydrogen 

`"m_C` = 0.5 × 12 = 6.0 g 

`"m_H` = 1.0 × 1 = 1.0 g 

Step 3: Determine mass of oxygen in the compound 

`"m_O` = 11.0 − (6.0 + 1.0) = 4.0 g 

`"n_O` = `frac{4.0}{16}` = 0.25 mol 

Step 4: Find simplest ratio 

⇒ Empirical formula: C₂H₄O. 

The hydrocarbon C₄H₈ was burnt in air. Incomplete combustion occurred. Which equation, A, B, C or D, correctly represents an incomplete combustion reaction?

A. C₄H₈ + 4O → 4CO + 4H₂. 

B. C₄H₈ + 4O₂ → 4CO + 4H₂O. 

C. C₄H₈ + 6O₂ → 4CO₂ + 4H₂O. 

D. C₄H₈ + 8O → 4CO₂ + 4H₂. 

Answer: B. C₄H₈ + 4O₂ → 4CO + 4H₂O. 

Recall possible products: 

   • Complete combustion: produces CO₂ and H₂O

   • Incomplete combustion: produces CO (and/or C) and H₂O 

A. Incorrect: C₄H₈ + 4O → 4CO + 4H₂

   • “O” is not diatomic oxygen (should be O₂) 

   • Produces H₂ gas, not H₂O → not a combustion reaction. 

B. Correct: C₄H₈ + 4O₂ → 4CO + 4H₂O

   • Oxygen is O₂ (correct species) 

   • Produces CO (incomplete combustion) and H₂O 

   • Balanced: 

      o C: 4 on both sides 

      o H: 8 on both sides 

      o O: 8 on both sides 

This represents incomplete combustion perfectly. 

C. Incorrect: C₄H₈ + 6O₂ → 4CO₂ + 4H₂O

This is complete combustion, since CO₂ forms (not CO). 

D. Incorrect: C₄H₈ + 8O → 4CO₂ + 4H₂

   • Uses O (not O₂) 

   • Produces H₂ gas, not water → not combustion. 

Which of the following compounds is most likely to have some incomplete combustion when burnt in oxygen? 

A. C₂H₅OH. 

B. CH₃OH. 

C. C₆H₅CH₃. 

D. C₃H₈. 

Answer: C. C₆H₅CH₃. 

Incomplete combustion occurs when: Oxygen supply is limited. The compound is large, non-polar, or aromatic, so it’s harder to oxidize completely. 

Hence, larger or aromatic hydrocarbons tend to burn less completely.

A. Incorrect: C₂H₅OH 

   • Small alcohol, already partially oxidized (contains oxygen). 

   • Burns cleanly and completely → efficient combustion. 

→ Not likely incomplete combustion.

B. Correct: CH₃OH 

   • Even smaller alcohol, very easily oxidized. 

   • Burns completely → clean flame. 

→ Not likely incomplete combustion. 

C. Incorrect: C₆H₅CH₃ 

   • Aromatic hydrocarbon, large molecule. 

   • High carbon-to-hydrogen ratio, requires a lot of oxygen. 

   • Often produces sooty flame (carbon particles) due to incomplete combustion. → Most likely to undergo incomplete combustion. 

D. Incorrect: C₃H₈ 

   • Small hydrocarbon, gaseous fuel, burns quite completely under normal  conditions. 

→ Not the most likely. 

Ethanol is an example of a biofuel used in place of gasoline. 

a. State two advantages and two disadvantages associated with the use of ethanol as a biofuel.

The table shows some information about some alcohols and the amount of heat energy released when one mole of each alcohol is completely burnt in oxygen. 

b. Write a chemical equation for the complete combustion of butan-1-ol.

c. Sandro suggests that filling a car’s fuel tank with butan-1-ol instead of methanol  would mean there is more than 3 times the chemical energy in the tank. Explain why his  statement is incorrect. 

d. Ethanol and gasoline have different physical and chemical properties. When 1.00 dm³ of petrol is burnt completely in air, 38,000 kJ of energy is released. Using the data from  the table, calculate the amount of energy released when 1.00 dm³ of ethanol is burnt  completely in air given that the density of ethanol is 0.780 g cm⁻³. 

a. Advantages and disadvantages of ethanol as a biofuel 

   • Advantages: 

     o Renewable resource – ethanol is made from crops (e.g. sugarcane, corn),  so it can be replenished. 

     o Carbon-neutral (partially) – the CO₂ released during combustion is  roughly balanced by that absorbed during plant growth. 

   • Disadvantages: 

     o Lower energy density – ethanol releases less energy per litre than gasoline,  so vehicles need more fuel for the same distance. 

     o Competition with food production – using farmland to grow biofuel crops can reduce food supply or increase food prices. 

b. Equation for complete combustion of butan-1-ol 

C₄H₉OH + 6O₂ → 4CO₂ + 5H₂O 

c. Why Sandro’s statement is incorrect

Sandro said: filling a car with butan-1-ol instead of methanol gives more than 3× the  energy. 

This is incorrect because: 

   • Butan-1-ol releases 3.69× more energy per mole (2676 ÷ 726).

   • But per volume of fuel, fewer moles of butan-1-ol fit in the tank (it’s denser and  has a larger molar mass). 

   • Therefore, energy per litre is only about 1.6× higher – not more than 3×.

⇒ Fewer moles fit in the same volume, giving less than 3× the energy.

d. Energy released by 1.00 dm³ ethanol 

Given: 

   • Density = 0.780 g cm^-3 = 780 g dm^-3

   • Molar mass of ethanol = 46.1 g mol^-1

   • Energy = 1367 kJ mol^-1

n = `frac{780}{46.1}`= 16.9 mol 

E = 16.9 × 1367 = 2.31 × 10⁴ kJ. 

⇒ Energy released ≈ 2.3 × 10⁴ kJ = 23 MJ per 1 dm³ of ethanol 

This question is about two fossil fuels, natural gas and coal. Select the row which shows  the correct answers. 

Answer: C. 

CO₂ produced per gram: 

   • Coal = almost all carbon → produces more CO₂ per gram. 

   • Natural gas = partly hydrogen → produces less CO₂ per gram. 

⇒ Coal produces the most CO₂ per gram. 

Energy released per gram: 

   • Natural gas combustion releases more energy per gram because hydrogen oxidation gives high energy and low molar mass. 

   • Coal releases less energy per gram. 

⇒ Natural gas releases the most energy per gram.

Intercontinental jet airlines use kerosene as fuel. The formula of kerosene may be taken as C₁₄H₃₀.

a. State the homologous series kerosene belongs to. 

The flight path from Singapore to London is approximately 10,700 km. A typical intercontinental jet airliner burns 10.8 kg of kerosene for each kilometre covered.

b. i. Calculate the mass, in tonnes, of C₁₄H₃₀ burnt on a flight from Singapore to London. [1 tonne = 1000 kg] 

ii. Calculate the mass of carbon dioxide, CO₂, produced during this flight.

a. Alkanes (Kerosene is a mixture of saturated hydrocarbons; C₁₄H₃₀ fits CₙH₂ₙ₊₂.)

b. 

i.Fuel used = 10,700 km × 10.8 kg km⁻¹ = 115,560 kg = 115.6 tonnes (≈ 1.16 × 102t) 

ii. Combustion: 

C₁₄H₃₀ + 21.5O₂ → 14CO₂ + 15H₂O.

Per mole fuel (198 g), 14 mol CO₂ form — mass ratio: 

`frac{m_(CO_2)}{m_"fuel"}=frac{14*44}{198}`= 3.11

→ `"m_(CO_2)`= 3.11 × 115,560 kg ≈ 3.60 × 105 kg 

⇒ 360 tonnes of CO₂ (3 s.f.) 

Which of the following is not a greenhouse gas? 

A. CH₄. 

B. N₂. 

C. CO₂. 

D. H₂O. 

Answer: B. N₂. 

A. Incorrect: Methane is a major greenhouse gas, trapping heat very effectively.

B. Correct: Nitrogen is not a greenhouse gas – it is diatomic and does not absorb infrared  radiation because it has no dipole moment. 

C. Incorrect: CO₂ strongly absorbs infrared radiation → main contributor to the  greenhouse effect. 

D. Incorrect: Water vapor is the most abundant greenhouse gas in the atmosphere. 

Which of the following is correct? 

Answer: D 

A. Incorrect: CO₂ absorbs infrared (IR) radiation, not UV. 

B. Incorrect: CO₂ does not cause acid rain and is a reactant, not product, in photosynthesis. 

C. Incorrect: CO₂ is used (reactant) in photosynthesis, not produced.

D. Correct: CO₂ absorbs infrared, causes greenhouse effect, and is produced in  fermentation (e.g. yeast making CO₂ + ethanol).

Which statement about diesel and biodiesel is correct? 

A. Diesel consists of long-chain hydrocarbons; biodiesel contains benzene and its derivatives. 

B. Diesel has greater viscosity and hence flows more slowly along fuel lines than biodiesel. 

C. Diesel emits soot (small carbon particles) when burning is incomplete; biodiesel does not produce any harmful emissions when burnt.

D. Diesel is produced by distilling crude oil; biodiesel is produced by transesterification  of oils in plants.

Answer: D. Diesel is produced by distilling crude oil; biodiesel is produced by transesterification of oils in plants. 

A. Incorrect: Biodiesel is not aromatic; it’s made of esters of fatty acids, not benzene compounds. 

B. Incorrect: Actually, biodiesel is more viscous because of its polar ester groups; it flows more slowly than diesel. 

C. Incorrect: Biodiesel emits less soot, but can still produce CO₂, NOₓ, etc. So not completely harmless. 

D. Correct: This is the only fully correct statement. 

Which of these is not a disadvantage of using a hydrogen–oxygen fuel cell?

A. Very low efficiency. 

B. Fuel storage can be problematic. 

C. Needs constant supply of fuel. 

D. Expensive metals are needed on the electrodes.

Answer: A. Very low efficiency. 

A. Correct: Hydrogen–oxygen fuel cells are actually highly efficient (~60–70%) compared to combustion engines (~25–30%). 

B. Incorrect: Hydrogen is difficult to store due to low density; needs high pressure or low temperature. 

C. Incorrect: Fuel cells are not batteries; they require continuous flow of H₂ and O₂ to keep generating electricity. 

D. Incorrect: Catalysts like platinum are used – very costly.