. Fuels (Cambridge (CIE) O Level Chemistry): Theory Questions

Petrol is a mixture of hydrocarbons and additives. The combustion of petrol in car engines is a major source of air pollution. This is reduced by catalytic converters.

Petrol is obtained from the gasoline fraction, boiling point range 40 °C to 100 °C, from the distillation of petroleum.

Explain the term fraction.

Octane, C₈H₁₈, is a constituent of petrol.

i) Write the equation for the complete combustion of octane.

C₈H₁₈ + .......... O₂ → .......... + ..........

[2]

ii) Some of the petrol burned in a car engine undergoes incomplete combustion.

Name the toxic gas produced by the incomplete combustion of petrol.

[1]

iii) Dibromoethane is a compound that was once added to petrol.

What does each of the following tell you about the structure of dibromoethane?

dibromo ..........................................................................................................

eth ..........................................................................................................

ane ..........................................................................................................

[2]

iv) What additional information is needed to draw the structural formula of dibromoethane?

[1]

Alkanes have the general formula C_nH_2n+2.

A sample of an alkane contains 0.026 moles of alkane molecules and 0.104 moles of carbon atoms.

What is the value of n? Show how you arrived at your answer.

Oxides of nitrogen are among the pollutants emitted by vehicle exhausts.

Explain how oxides of nitrogen form in a car engine, and describe how a catalytic converter removes them.

Petroleum contains hydrocarbons which are separated by fractional distillation.

i) Complete the following definition of a hydrocarbon:

A hydrocarbon is a compound which ............................................

[1]

ii) Explain what is meant by the term fractional distillation.

[1]

Some of the fractions obtained from petroleum are given below. State a use for each fraction.

bitumen ....................................................................

lubricating fraction ..............................................

paraffin fraction ........................................................

gasoline fraction ......................................................

Petroleum is separated into useful fractions at an oil refinery.

Describe how petroleum is separated into fractions.

The fractions collected near the top of the column have lower boiling points than those collected near the bottom.

Explain why.

Name the fraction that is used as a fuel in ships and in home heating systems.

Bitumen and refinery gas are both obtained from petroleum.

State which of the two is the more viscous.

Explain your answer in terms of the molecules each contains.

Explain why the refinery gas fraction is supplied to customers in sealed metal cylinders rather than in open containers.

Two fractions are collected at almost the same height in a fractionating column.

Suggest what this tells you about the hydrocarbon molecules in the two fractions.

An oil refinery finds that it produces more fuel oil than its customers want to buy, and less gasoline than they ask for.

Explain why the amounts of each fraction that a refinery obtains do not match the amounts its customers want.

Charcoal burns well and is made by heating wood in the absence of air.

Charcoal is not a fossil fuel.

Name the three fossil fuels.

Petrol is obtained from petroleum by fractional distillation.

Explain why petroleum, rather than petrol, is named as a fossil fuel.

Methane and ethanol can both be burned as fuels.

Explain why methane is classed as a hydrocarbon but ethanol is not.

The table shows the boiling ranges of four fractions obtained from petroleum.

Room temperature is 25 °C.

Identify the fraction that is a gas at room temperature.

Explain your answer using the data in the table.

State which fraction contains the molecules with the longest carbon chains.

Explain how you can tell this from the table.

State whether fraction M is collected above or below fraction J in the column.

Fraction J enters the column as a vapour but is collected as a liquid.

Explain why.

The table shows the number of carbon atoms per molecule and the viscosity of four fractions obtained from petroleum.

Describe the relationship between the number of carbon atoms per molecule and the viscosity.

Explain, in terms of the molecules, why the viscosity changes in this way.

A fifth fraction, E, contains molecules with 26 to 35 carbon atoms.

Predict the viscosity of fraction E.

Justify your prediction using the data in the table.

The fraction with the highest viscosity is also the least volatile.

Explain why viscosity and volatility change in opposite directions as the fractions are collected further up the fractionating column.

A fraction collected from a fractionating column boils between 150 °C and 250 °C.

A pure sample of dodecane boils at 216 °C.

Explain why the fraction has a range of boiling points but dodecane does not.

The boiling ranges of two fractions collected next to each other in the column overlap slightly.

Explain what this tells you about the hydrocarbons in the two fractions.

Explain why the hydrocarbons in petroleum can be separated from one another by fractional distillation, rather than by a chemical reaction.

A chemist is given a sample of one fraction from a fractionating column.

The only information given about the sample is that its molecules contain between 20 and 30 carbon atoms.

The kerosene fraction contains molecules with 11 to 16 carbon atoms.

Predict whether the sample is collected above or below the kerosene fraction in the column.

Justify your prediction.

Predict how each of the following properties of the sample compares with the same property of kerosene.

i) viscosity

[1]

ii) volatility

[1]

Suggest one use for the sample.

Explain why two of its properties make it suitable for that use.

Petroleum is separated into fractions in a fractionating column. The column is heated at its base.

The table shows the temperature inside the column at each of three outlets, X, Y and Z.

The second table shows the boiling range of each of three fractions obtained from the column.

Deduce which outlet, X, Y or Z, each of fractions J, K and L is collected from.

Explain how the boiling range of a fraction and the temperature inside the column together determine the height at which that fraction is collected.

The heater at the base of the column develops a fault. The temperature inside the column at outlet Z falls to 200 °C. The temperatures at outlets X and Y are unchanged.

Explain the effect this has on fraction K and on fraction L.