Organic Chemistry On​ Octane rating, knocking, and fuel quality

 

WAEC Chemistry Practice Quiz: Octane Rating, Knocking & Fuel Quality

Test your knowledge with these 10 WAEC-standard multiple-choice questions. Click "Show Answer" at the bottom of each question to view the correct option and detailed solution.

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1. Premature ignition of the fuel-air mixture in the cylinder of an internal combustion engine is known as ________.

A. Crackling B. Knocking C. Reforming D. Pyrolysis

Correct Answer: B

Explanation: Knocking (or detonation) is the sharp metallic noise resulting from the premature, uneven combustion of the fuel-air mixture in an engine cylinder before the spark plug fires. This reduces engine efficiency and can cause damage.

2. Which of the following hydrocarbons is assigned an octane rating of 0?

A. n-heptane B. Isooctane C. Benzene D. n-octane

Correct Answer: A

Explanation: Straight-chain alkanes cause severe knocking. Straight-chain heptane (n-heptane) is assigned an arbitrary performance value of 0 on the octane rating scale, while highly branched isooctane is assigned 100.

3. IUPAC name of the reference hydrocarbon assigned an octane rating of 100 is ________.

A. 2,2,3-trimethylbutane B. n-octane C. 2,2,4-trimethylpentane D. 2,4,4-trimethylpentane

Correct Answer: C

Explanation: "Isooctane" is the common name for 2,2,4-trimethylpentane. Because it resists knocking exceptionally well under high compression, it defines the 100-mark on the standard octane rating scale.

4. A fuel mixture consisting of 85% 2,2,4-trimethylpentane and 15% n-heptane has an octane rating of ________.

A. 15 B. 70 C. 85 D. 100

Correct Answer: C

Explanation: The octane number is calculated as the percentage by volume of isooctane (2,2,4-trimethylpentane) in a reference mixture with n-heptane. Therefore, 85% isooctane gives an octane rating of 85.

5. Which structural characteristic generally yields a higher octane rating in petrol?

A. Long, unbranched carbon chains B. Highly branched carbon chains and aromatic rings C. High concentration of low molecular weight alkynes D. Complete absence of volatile cyclic compounds

Correct Answer: B

Explanation: Fuel quality increases with branching, cyclization, and aromaticity. Highly branched alkanes and aromatic hydrocarbons (like benzene or toluene) burn more smoothly and possess high anti-knock properties.

6. In oil refineries, straight-chain alkanes are converted to branched-chain isomers to improve fuel quality. This commercial process is called ________.

A. Cracking B. Isomerization C. Polymerization D. Fractional distillation

Correct Answer: B

Explanation: Isomerization rearranges straight-chain molecules into branched-chain isomers (e.g., converting normal pentane into isopentane). This directly raises the octane number of the petrol pool without changing molecular weight.

7. Tetraethyllead(IV), $\text{Pb(C}_2\text{H}_5)_4$, was formerly added to petrol as an anti-knock agent. Why has its use been banned globally?

A. It dramatically reduces the octane number of fuel. B. It corrodes steel engines faster than standard fuel. C. Lead emissions pollute the environment and act as a cumulative nervous system toxin. D. It causes petrol to evaporate instantly at room temperature.

Correct Answer: C

Explanation: Although tetraethyllead (TEL) is an excellent anti-knock additive, it releases highly poisonous lead compounds into the atmosphere via exhaust fumes and permanently destroys catalytic converters in vehicles.

8. Which of the following compounds is a common, environmentally safer additive used to improve the octane rating of unleaded petrol today?

A. Methanoic acid B. Ethanol (or MTBE) C. Tetrachloromethane D. Chlorofluorocarbons

Correct Answer: B

Explanation: Oxygenates like ethanol and Methyl Tert-Butyl Ether (MTBE) are blended into modern unleaded gasoline to enrich oxygen levels, promote complete combustion, and boost the overall octane rating safely.

9. Petroleum fraction used as petrol (gasoline) generally consists of hydrocarbons containing how many carbon atoms per molecule?

A. $\text{C}_1$ to $\text{C}_4$ B. $\text{C}_5$ to $\text{C}_{12}$ C. $\text{C}_{12}$ to $\text{C}_{18}$ D. Above $\text{C}_{20}$

Correct Answer: B

Explanation: Petrol (gasoline) is a volatile liquid fraction obtained from fractional distillation of crude oil, containing a mixture of liquid hydrocarbons spanning from pentane ($\text{C}_5\text{H}_{12}$) up to dodecane ($\text{C}_{12}\text{H}_{26}$).

10. High-quality fuel prevents engine knocking because it ________.

A. burns very slowly and leaves a lot of soot residues B. ignites instantly upon compression before any spark is introduced C. burns smoothly and evenly when triggered by the spark plug D. contains large quantities of water vapor to cool the engine

Correct Answer: C

Explanation: High-quality gasoline features a high octane number, meaning it withstands compression pressures well and only ignites via the spark plug flame front, delivering a smooth stroke instead of erratic shocks.