Example questions
1.
Hydrogen peroxide is used in gels to whiten teeth. The ion –electron equation for the oxidation of
hydrogen peroxide is:
H 2 O 2 → O 2 + 2H + + 2e −
Using your knowledge of chemistry, comment on possible methods for measuring and comparing
the concentration of hydrogen peroxide present in two different gels.

As decomposition of hydrogen peroxide gives off oxygen, the volume of oxygen released could be
measured either in total or in a set time period. The more oxygen the higher the concentratio n.

As hydrogen ions are also released the pH could be measured, the lower the pH the higher the
concentration.

The concentration of hydrogen ions could be measured by titrating the gel against a known
concentration of alkali such as NaOH, with universal indicator.

Also a circuit could be set to measure the current as electrons a re given off during oxidation.
The higher the current the higher the concentration. Or the conductivity of a solution could be
measured, as ionic solutions conduct.
2.
A student makes the following statement:
‘Sugar can be used to produce alcohol, a carboxylic acid and the ester ethyl ethanoate’
Using your knowledge of chemistry, comment on the accuracy of the student’s statement.

Sugar in the form of glucose has the formula C 6 H 12 O 6 . In the presence of yeast, the glucose will
ferment to produce ethanol and carbon dioxide. The temperature that this should be done is
about 40 °C.

As the ethanol that is produced is a primary alcohol, in the presence of a suitable oxidising agent
such as acidified potassium dichromate or copper oxide, it can be oxidised to an aldehyde and
then a carboxylic acid. The carboxylic acid produced from etha nol would be ethanoic acid, also
known as vinegar.

If the ethanol made in fermentation is then reacted with the ethanoic acid the ester ethyl
ethanoate can be made in a condensation reaction. This would require concentrated sulphuric
acid as a catalyst, and the use of a water bath.

So the student is accurate, sugar can produce these substa nces with a little help from some
other reagents and catalysts.
3.
Aspirin is a widely used medicine. It is advised that it is stored in dry, cool conditions.
O
OH
C
O
C
CH3
O
Using your knowledge of chemistry, comment on the reasons why aspirin should be stored under these
conditions.

In the presence of water the ester could be hydrolysed at the ester link leaving an alcohol group
on the benzene ring and ethanoic acid.

At high temperatures the molecule could decompose.

As this is a reasonably small covalent molecule it will have a relatively low melting and boiling
point, and therefore should be kept at lower temperatures.

The presence of the OH in the carboxyl group, makes that part of the molecule highly polar.
Therefore aspirin is likely to be soluble in water which is also polar. Hydrogen bonding will occur
between the Oxygen of one molecule and the hydrogen of the other. So aspirin in likely to be
soluble in water.
4.
The Periodic Table groups together elements with sim ilar properties. In most Periodic Tables hydrogen
is placed at the top of Group 1, but in some it is placed at the top of Group 7.
Using your knowledge of chemistry, comment on the reasons for hydrogen being placed above
either Group 1 or Group 7.

Hydrogen atoms have one electron in total. Group one elements and hydrogen have one electron
in their outer shell. Therefore hydrogen and the group one elements will have similar properties.
They are in fact both highly reactive. Group one elements however are metals and hydrogen is a
non-metal.

Substances in group 1 lose 1 electron to become positively charged with a charge of 1+.

Group seven elements are non-metals like hydrogen. They have 7 electrons in their outer shell,
which is not like hydrogen. However they only need to gain one electron to have a full shell and
this is like hydrogen.
5.

Several of the halogens are diatomic gases just like hydrogen.

Atoms in group 7 gain one electron to have a charge of 1 -, hydrogen also does this.
Oxygen gas speeds up the rate at which food is spoiled. To improve the shelf -life of foods, food
manufacturers use several methods to remove oxygen from inside the food packaging. In one method,
an enzyme is added that catalyses a reaction between oxygen a nd the glucose which is often present in
foods.
glucose + oxygen + water → gluconic acid + hydrogen peroxide
Using your knowledge of chemistry, comment on why this method may not be suitable to improve
the shelf-life of all foods.

This method produces hydrogen peroxide. Hydrogen peroxide breaks down to give oxygen and
water.
H 2 O 2 →½ O 2 + H 2 O
So therefore oxygen could be produced again.

Also H 2 O 2 is a powerful oxidising agent, so in fact will spoil the food rapidly. Particularly if there
were any oils present in the food, they would oxidise and become rancid.

Hydrogen peroxide is also used as a cleaning and bleaching agent, it would therefore be
unsuitable and unpleasant to have near food.

Gluconic acid is likely to be a carboxylic acid. Many carboxylic acids have an unpleasant taste
and smell and this could affect the food.

The reaction removes glucose from the food, this could change the flavour of the food as the
sweet glucose is removed.
6.
Rasputin, often referred to as the ‘mad monk’, was a very powerful figure in the life of Alexandra, the
last Tsarina of Russia. His enemies decided to kill him using a cyanide compound. Cyanide compounds
are deadly poisons. Using a bottle of potassium cyanide, they attempted to poison a cake and some
wine. Rasputin ate the cake and drank the wine and yet was not harmed. Altho ugh his followers
claimed this was a sign of Rasputin’s supernatural powers, there are chemical explanations.
potassium cyanide + acid → potassium salt + hydrogen cyanide gas
(white powder)
(white powder)
Using your knowledge of chemistry and the follo wing information, comment on possible chemical
reasons as to why the cake and wine did not poison Rasputin.

The potassium cyanide would react with the acid in the cake and the wine to produce the
potassium salt, this would not have been harmful to him. T he hydrogen cyanide would have been
given off as a gas and therefore he would not have ingested it. The cooking of the cake would
speed up any reactions taking place due to the heat. So it is likely the cake only contained
potassium salt as all the hydrogen cyanide gas would have been released into the oven.
7.
Concentrated solutions of hydrogen peroxide are used in the propulsion systems of torpedoes.
Hydrogen peroxide decomposes naturally to form water and oxygen:
2H 2 O 2 (aq) → 2H 2 O(ℓ) + O 2 (g)
ΔH = −196∙4 kJ mol –1
Transition metal oxides act as catalysts in the decomposition of the hydrogen peroxide.
Unfortunately, there are hazards associated with the use of hydrogen peroxide as a fuel in torpedoes. It
is possible that a leak of hydrogen peroxide solution from a rusty torpedo may trigger an explosion.
Using your knowledge of chemistry, comment on why this could happen.
8.
An internet discussion board called ‘Bad Chemistry on TV’, has an entry referring to an episode of the
television drama CSI: Miami.
‘The episode of CSI: Miami last night had the deceased victim floating in a swimming pool
contaminated with sodium hydroxide. The concentration was high enough to eat through glass. When
the CSI guys realised it was an alkali, they knew they needed to neutralise it in order to retr ieve the
body. So they sent one of the team to the local grocery store for vinegar. They proceeded to pour the
vinegar from four litre jugs into the pool, dropping the pH from almost 13 to exactly 7∙0 – all within a
few seconds, and without any stirring!’
The volume of the swimming pool is 100,000 litres.
Vinegar is approximately a 1∙0 mol l –1 solution of ethanoic acid.
Using your knowledge of chemistry, comment on whether or not the events described in the
episode of CSI: Miami could take place.