Oxford Cambridge and RSA
2024 OCR A Level Chemistry B (Salters) H433/02 Scientific literacy in
chemistry question paper and mark scheme combined
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[Contents]:
Advance Notice Article
Question Paper
Mark Scheme
#By: Panoptic
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Tuesday 18 June 2024 – Morning
A Level Chemistry B (Salters)
H433/02 Scientific literacy in chemistry
Advance Notice Article
Time allowed: 2 hours 15 minutes
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INFORMATION
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• This document has 4 pages.
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Reactive Oxygen Species
Adapted from Oxygen – the molecule that made the world by Nick Lane, Oxford 2002, pages 115–119.
Radiation is present all around us. It can interact directly with all kinds of molecules, but in our bodies it is
most likely to interact with water, knocking out electrons and forming oxygen. The three intermediates
formed by irradiating water, the hydroxyl radicals, hydrogen peroxide and superoxide radicals are called
reactive oxygen species and react in very different ways. However, because all three are linked and can
be formed from each other, they might be considered equally dangerous. Indeed the three work
together as part of an insidious catalyst system. We will consider each in turn,in the order they are
produced by the radiation on route from water to oxygen.
H2 O OH H2 O 2 O2– O2
water hydroxyl H2 O hydrogen superoxide oxygen
radical peroxide radical
Hydroxyl radicals (OH) are the first to be formed. These are extremely reactive fragments or random
muggers. They can react with all biological molecules at speeds approaching their rate of diffusion. This
means that they react with the first molecules in their path, and it is virtually impossible to stop them
from doing so. When a hydroxyl radical reacts with a protein, lipid (fat) or DNA, it snatches a proton and
an electron to itself and sinks back into the sublime chemical stability of water. But of course, the act of
snatching an electron leaves the reactant short of an electron. So, another radical is formed, this time
part of the protein, lipid or DNA. This is a fundamental feature of all free radical reactions – one radical
creates another, and if this radical is also reactive, then a chain reaction will ensue. Thus, the essential
feature of a radical is an unpaired electron, while the essential feature of free-radical chemistry is the
chain reaction.
We are all familiar with radical chain reactions when they happen in fatty foods such as butter: they are
responsible for rancidity. The fats in the butter oxidise and taste disgusting. The same type of reaction
also takes place in cell membranes, which are made mostly of lipids. The process is then called lipid
peroxidation. Radical damage is less obvious when it affects proteins or DNA, but radical damage to DNA
is one of the main causes of genetic mutation and accounts for the high rates of cancer suffered by
radiation victims.
A dramatic non-biological example of the power of radical chain reactions is the hole in the ozone layer.
The devastation that can be caused by chlorofluorocarbons (CFCs) such as freon is a result ofthe formation
of radicals in the upper atmosphere. CFCs are quite robust molecules that can survivebuffeting by the
weather in the lower atmosphere. However, they are shredded by ultraviolet rays
in the upper atmosphere and disintegrate to release chlorine atoms. Being one electron short of a full
pack, chlorine atoms are dangerously reactive radicals. They can steal electrons from almost anything,
setting in motion a chain reaction. According to the US Environmental Protection Agency,a single gram
of freon will often destroy as much as 70 kg of ozone. The radical chain reaction endswhen two radicals
react with each other, and their unpaired electrons conjoin in blissful chemical union.
If radiation strips a second electron from water, the next fleeting intermediate is hydrogen peroxide
(H2O2) – whose bleaching properties give its name to the peroxide blonde. Bleaching is caused bythe
oxidation of organic pigments as hydrogen peroxide strips electrons from them. The oxidising
properties of hydrogen peroxide can kill bacteria. Most industrial uses of hydrogen peroxide also drawon
its power as an oxidising agent.
Despite its widespread use as an oxidising agent, hydrogen peroxide is unusual in that it lies
chemically half-way between oxygen and water. This gives the molecule something of a split
© OCR 2024 H433/02 Jun24
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personality. It can go either way in its reactions (losing or gaining electrons) depending on the
© OCR 2024 H433/02 Jun24
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chemical company it keeps. It can even go both ways at once, when reacting with another hydrogen peroxide
molecule. In this case, one of the molecules gains two electrons to become water, while theother loses two
electrons to become oxygen.
A far more dangerous and significant reaction, however, takes place in the presence of iron, which
can pass on electrons one at a time to hydrogen peroxide to generate hydroxyl radicals. If
dissolved iron is present, hydrogen peroxide is a real hazard. Organisms go to great lengths to avoid
contamination with dissolved iron. The reaction between hydrogen peroxide and iron is called the Fenton
reaction, after the Cambridge chemist Henry Fenton who first discovered it in 1894.
He later showed that the reaction could damage almost any organic molecule. Thus, the main reasonthat
hydrogen peroxide is toxic is that it produces hydroxyl radicals in the presence of dissolved iron. Ironically,
the greatest danger lies in its slow reactivity in the absence of iron. It has time to diffuse throughout the
cell. Hydrogen peroxide may diffuse into the cell nucleus, for example, and there
mix with the DNA before it encounters iron, which transforms it into a brutish hydroxyl radical. The
insidious infiltration of hydrogen peroxide means that it is more dangerous than the hydroxyl radicals
produced outside the nucleus. Some proteins, such as haemoglobin, also contain iron. If they happento
run into hydrogen peroxide they can be mutilated on the spot.
What, then, of the third of our intermediates, the superoxide radical, O – ? 2Like hydrogen peroxide, the
superoxide radical is not terribly reactive. However, it too has an affinity for iron, dissolving it from
proteins and storage depots. To understand why this is harmful, we need to think again about the
Fenton reaction. The Fenton reaction is dangerous because it produces hydroxyl radicals but it grinds to
a halt when all the accessible iron is used up. Any chemical that regenerates dissolved iron
is capable of re-starting the reaction. Because the superoxide ion is one electron away from molecular
oxygen, it is more likely to lose that electron to form oxygen than it is to gain three electrons to form
water. Only a few molecules are able to accept a single electron, however. One of the best places forthe
superoxide to jettison its spare electron is iron. This converts iron back into the form where it can
participate in the Fenton reaction:
2 2
In summary, then, the three intermediates between water and oxygen operate as an insidious catalyticsystem
that damages biological molecules in the presence of iron.
END OF ADVANCE NOTICE ARTICLE
© OCR 2024 H433/02 Jun24
