Hydrocarbons: Alkanes and Alkenes: Chemistry 0620 (Cambridge O Level / IGCSE)

A hydrocarbon belongs to the alkene homologous series. One molecule of this hydrocarbon contains 9 carbon atoms.

Using the general formula for alkenes, $C_nH_{2n}$, how many hydrogen atoms are in one molecule of this hydrocarbon?

A student collects four samples, labelled Fraction P, Q, R and S, from different heights of a laboratory fractionating column used to separate a sample of petroleum. The table shows data recorded for each fraction, listed in a random order.

(a) State which fraction, P or Q, is collected closer to the top of the fractionating column. Justify your answer using one piece of data from the table. [2]

(b) Arrange fractions P, Q, R and S in order of increasing average chain length. [1]

(c) Explain, in terms of the size of the hydrocarbon molecules, why fraction R flows much more slowly (has a much higher viscosity) than fraction Q. [2]

(d) Petroleum is a mixture of many different hydrocarbons rather than a single pure compound. Explain why fractional distillation is still able to separate this mixture into fractions such as P, Q, R and S. [1]

A student mixes gaseous pentane, $C_5H_{12}$, with chlorine gas in a clear glass flask and exposes the flask to strong sunlight. Over several minutes, the pale green colour of the chlorine fades, and a colourless, acidic gas escapes from the flask.

The student writes three conclusions in their notebook.

1. "This must be an addition reaction, because a chlorine atom has added onto the pentane molecule."
2. "Sunlight only speeds the reaction up. The same reaction would still happen at roughly the same rate if the flask were instead left in a dark cupboard at the same temperature."
3. "Since monosubstitution occurs and one mole of pentane reacts with one mole of chlorine, the organic product must have the molecular formula $C_5H_{11}Cl$."

(a) State, with a reason, whether conclusion 1 is correct. [2]

(b) State, with a reason, whether conclusion 2 is correct. [2]

(c) State whether conclusion 3 is correct. [1]

(d) State what provides the energy needed to break the Cl–Cl bond and start this reaction, and name the type of bond that is broken. [2]

A refinery has a surplus of a long-chain hydrocarbon fraction with molecular formula $C_{14}H_{30}$, but there is much greater demand for shorter-chain alkanes (used as fuels) and for alkenes (used as a chemical feedstock). The $C_{14}H_{30}$ fraction is heated strongly with a catalyst to crack it.

(a) State two reasons why the refinery cracks the $C_{14}H_{30}$ fraction rather than selling it as it is. [2]

(b) In one cracking reaction, a single molecule of $C_{14}H_{30}$ splits into one molecule of a saturated product, $C_9H_{20}$, and one molecule of another product, X.

(i) Using conservation of atoms, deduce the molecular formula of product X. [2]

(ii) State whether X belongs to the alkane or the alkene homologous series. Justify your answer using its general formula. [2]

(c) The $C_9H_{20}$ product and product X are collected separately in two unlabelled gas jars. Describe a simple chemical test, including the observation for each jar, that would let a student identify which jar contains product X. [2]

But-2-ene, $CH_3CH=CHCH_3$, is an unsaturated hydrocarbon. It undergoes three separate addition reactions: with bromine, with hydrogen (nickel catalyst), and with steam (acid catalyst). Each reaction produces a single organic product.

Which statement about the three products is correct?

An airline's fuel supplier buys petroleum fractions from a nearby oil refinery. One of these fractions is loaded onto tanker trucks and delivered to the airport, where it is used as the fuel burned in aircraft engines.

Which fraction obtained from the fractional distillation of petroleum is normally used as fuel for aircraft?

A school science technician finds three unlabelled gas cylinders left over from a delivery. A supplier's note confirms that each cylinder contains a pure hydrocarbon, and gives its molecular formula:

• Gas L: $\text{C}_4\text{H}_{10}$
• Gas M: $\text{C}_4\text{H}_8$
• Gas N: $\text{C}_7\text{H}_{16}$

(a) State the general formula of the alkane homologous series and the general formula of the alkene homologous series, using $n$ for the number of carbon atoms in one molecule. [2]

(b) Using the general formulae from part (a), state whether each of Gas L, Gas M and Gas N belongs to the alkane homologous series or the alkene homologous series. [3]

(c) State what is meant by the term "homologous series". [1]

(d) Ethane, $\text{C}_2\text{H}_6$, is the simplest member of the alkane homologous series after methane. Describe, in words, the displayed formula of ethane, stating the number of carbon atoms, hydrogen atoms, carbon–carbon bonds and carbon–hydrogen bonds it contains, and show that this is consistent with the general formula for $n=2$. [2]

A technician tests two alkane fuels, propane, $\text{C}_3\text{H}_8$, and ethane, $\text{C}_2\text{H}_6$, by burning a sample of each separately in a plentiful (excess) supply of oxygen, so that each fuel undergoes complete combustion.

(a) Write a balanced symbol equation, including state symbols, for the complete combustion of propane in a plentiful supply of oxygen. [3]

(b) Write a balanced symbol equation, including state symbols, for the complete combustion of ethane in a plentiful supply of oxygen. [3]

(c) Alkanes form a homologous series with general formula $\text{C}_n\text{H}_{2n+2}$. Using this general formula, write a general balanced symbol equation, in terms of $n$, for the complete combustion of any alkane in a plentiful supply of oxygen. [2]

Propene, $\text{CH}_2\text{=CHCH}_3$ (molecular formula $\text{C}_3\text{H}_6$), is reacted with hydrogen gas in the presence of a nickel catalyst. This addition reaction forms a single saturated organic product.

What is the molecular formula of this organic product?

A chemical plant cracks decane, $\text{C}_{10}\text{H}_{22}$, by heating it strongly in the presence of a catalyst. In one type of cracking reaction taking place in the plant, a single molecule of decane splits into one molecule of an alkene with the same number of carbon atoms as decane, plus one molecule of hydrogen gas, $\text{H}_2$.

(a) Using conservation of atoms, deduce the molecular formula of the alkene product. [2]

(b) Write a balanced symbol equation for this cracking reaction, including state symbols. (All species are gaseous under the high-temperature conditions used for cracking, even though decane itself is a liquid at room temperature.) [2]

(c) State the two conditions needed to bring about this cracking reaction. [2]

(d) Give one use for the hydrogen gas produced by cracking reactions like this one. [1]