SUMMER 2026
PREDICTED PAPER
OCR
A Level Chemistry B (Salters)
H433/02 Scientific literacy in chemistry
Insert
Time allowed: 2 hours 15 minutes
INFORMATION
• This Insert contains the Advance Notice.
• This document consists of 5 pages.
for more: tyrionpapers.com
, 2
From Crude Oil to Plastic: The Chemistry of Hydrocarbons
Adapted from an article in Chemistry World, April 2018, by Dr Sarah Mitchell
A Liquid Archive of Ancient Life
Crude oil is one of the most chemically complex mixtures found in nature. Over millions of
years, the remains of marine organisms — algae, zooplankton and bacteria — accumulated
on the ocean floor and were buried beneath thick layers of sediment. Heat and pressure
transformed this organic matter through a process known as catagenesis into the dark, viscous
liquid we extract today. As a raw material, crude oil underpins much of modern industry —
from fuels that power our vehicles to the polymers found in clothing and packaging. Yet the
chemistry that transforms this viscous liquid into such varied and useful products is elegantly
systematic, relying on a relatively small number of reaction types applied with great precision
across refineries worldwide.
The economic importance of crude oil is difficult to overstate. In the United Kingdom alone,
the petrochemical industry directly employs over 100,000 people and contributes tens of
billions of pounds annually to the economy. The challenge for chemists and chemical
engineers is not simply to extract as much fuel as possible, but to tailor the range of products
obtained to precisely match market demand — a balance that requires sophisticated chemistry
at every stage.
Separating the Mixture: Fractional Distillation
Crude oil consists predominantly of alkanes — saturated hydrocarbons with the general
formula CnH2n+2. These range from methane (one carbon atom) to very long-chain
molecules with fifty or more carbon atoms. Their differing boiling points make fractional
distillation an effective and economically vital separation technique.
In an industrial fractionating column — a steel tower often exceeding 60 metres in height —
crude oil is heated to around 400 °C at the base. The resulting vapour mixture rises through
the column, which maintains a temperature gradient from hot at the bottom to cool at the top.
As each fraction cools to its boiling point, it condenses and is drawn off at a particular level.
The shorter the carbon chain, the weaker the induced dipole–dipole (London dispersion)
forces between molecules, and the lower the boiling point. Methane, ethane and propane are
gases at room temperature; petrol (gasoline, C5–C10) is a volatile liquid; kerosene and diesel
are less volatile; whilst the heaviest residues — bitumen and heavy fuel oil — remain liquid or
semi-solid at room temperature and are drawn from the very bottom of the column.
The magnitude of the London dispersion forces between alkane molecules depends critically
on the number of electrons in each molecule — which increases with chain length — and on
the surface area available for intermolecular contact. Longer chains can adopt a greater
variety of conformations and present more surface area for contact, further strengthening
these temporary dipole interactions. It is this principle that makes the boiling point of alkanes
such a reliable guide to their chain length, and fractional distillation such an efficient tool for
separating them.
Not all fractions are equally in demand. Petrol, kerosene and liquefied petroleum gas (LPG)
are highly sought after for use as fuels and chemical feedstocks, whilst heavier residues
accumulate in excess. This supply–demand imbalance creates a strong commercial incentive
to break down large alkane molecules into smaller, more useful ones through a process known
as cracking.
for more: tyrionpapers.com
PREDICTED PAPER
OCR
A Level Chemistry B (Salters)
H433/02 Scientific literacy in chemistry
Insert
Time allowed: 2 hours 15 minutes
INFORMATION
• This Insert contains the Advance Notice.
• This document consists of 5 pages.
for more: tyrionpapers.com
, 2
From Crude Oil to Plastic: The Chemistry of Hydrocarbons
Adapted from an article in Chemistry World, April 2018, by Dr Sarah Mitchell
A Liquid Archive of Ancient Life
Crude oil is one of the most chemically complex mixtures found in nature. Over millions of
years, the remains of marine organisms — algae, zooplankton and bacteria — accumulated
on the ocean floor and were buried beneath thick layers of sediment. Heat and pressure
transformed this organic matter through a process known as catagenesis into the dark, viscous
liquid we extract today. As a raw material, crude oil underpins much of modern industry —
from fuels that power our vehicles to the polymers found in clothing and packaging. Yet the
chemistry that transforms this viscous liquid into such varied and useful products is elegantly
systematic, relying on a relatively small number of reaction types applied with great precision
across refineries worldwide.
The economic importance of crude oil is difficult to overstate. In the United Kingdom alone,
the petrochemical industry directly employs over 100,000 people and contributes tens of
billions of pounds annually to the economy. The challenge for chemists and chemical
engineers is not simply to extract as much fuel as possible, but to tailor the range of products
obtained to precisely match market demand — a balance that requires sophisticated chemistry
at every stage.
Separating the Mixture: Fractional Distillation
Crude oil consists predominantly of alkanes — saturated hydrocarbons with the general
formula CnH2n+2. These range from methane (one carbon atom) to very long-chain
molecules with fifty or more carbon atoms. Their differing boiling points make fractional
distillation an effective and economically vital separation technique.
In an industrial fractionating column — a steel tower often exceeding 60 metres in height —
crude oil is heated to around 400 °C at the base. The resulting vapour mixture rises through
the column, which maintains a temperature gradient from hot at the bottom to cool at the top.
As each fraction cools to its boiling point, it condenses and is drawn off at a particular level.
The shorter the carbon chain, the weaker the induced dipole–dipole (London dispersion)
forces between molecules, and the lower the boiling point. Methane, ethane and propane are
gases at room temperature; petrol (gasoline, C5–C10) is a volatile liquid; kerosene and diesel
are less volatile; whilst the heaviest residues — bitumen and heavy fuel oil — remain liquid or
semi-solid at room temperature and are drawn from the very bottom of the column.
The magnitude of the London dispersion forces between alkane molecules depends critically
on the number of electrons in each molecule — which increases with chain length — and on
the surface area available for intermolecular contact. Longer chains can adopt a greater
variety of conformations and present more surface area for contact, further strengthening
these temporary dipole interactions. It is this principle that makes the boiling point of alkanes
such a reliable guide to their chain length, and fractional distillation such an efficient tool for
separating them.
Not all fractions are equally in demand. Petrol, kerosene and liquefied petroleum gas (LPG)
are highly sought after for use as fuels and chemical feedstocks, whilst heavier residues
accumulate in excess. This supply–demand imbalance creates a strong commercial incentive
to break down large alkane molecules into smaller, more useful ones through a process known
as cracking.
for more: tyrionpapers.com
