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State what eutrophication is,
what species is/are measured to monitor its likely occurrence, and
how, where and when such measurements are made. If you have some idea of how
phosphate or nitrate is measured, give some information.
The paragraphs in italics are
probably not necessary to get full marks, but if you have time put one or
both in.
(p. 286-8, 280)
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Eutrophication
is the process in which a water body becomes enriched with nutrients such
as phosphate and nitrate to such an extent that it becomes susceptible to
an algal bloom, which is an excessive growth of algae which damages the
aquatic ecosystem.
Waterways are monitored for eutrophication by regularly
measuring the concentration of phosphate (most importantly) and sometimes
nitrate as well. Such monitoring is performed at several locations across
the water body and at regular time intervals (every few weeks).
Monitoring is intensified at times when an algal bloom is most likely
low water levels, low flows and high temperatures.
At the low concentrations involved (<1 ppm) phosphate is measured by a
colorimetric method based on the development of an intense blue colour,
molybdenum blue, when molybdate and a reducing agent are added to
water containing phosphate.
An algal bloom is likely if the phosphate concentration is greater than
0.05 ppm in still water (lake or dam) or greater than 0.1 ppm in running
water (creek or river).
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Question 26 (8 marks)
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Include sufficient detail about
the two processes to convince the examiner that you know what is involved in
each process. For the petroleum chart this means mentioning catalytic
cracking (and probably zeolites), that the ethylene is just a byproduct of
that process, and that fractional distillation is needed to obtain the
ethanol. From cane sugar you need to mention fermentation, the use of
yeast, the temperature (body temperature) and the need to exclude oxygen.
Equations are not asked for and so are not necessary.
You could use steam cracking (thermal cracking) to form ethylene as a main
product from petroleum instead of collecting it as a byproduct of catalytic
cracking.
(p. 5-7, 33-5)
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(a)
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Explain why making ethanol from
petroleum is completely unsustainable and how getting it from sugar cane is
partly sustainable. Do not claim that getting it from sugar cane is
completely sustainable. Note it is a comparison that is required: hence the
'more' in the answer opposite.
(p.34-5)
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(b)
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Producing
ethanol from petroleum is not environmentally sustainable because it uses
a non-renewable starting material, crude oil, and it uses a lot of energy
in the various processes show, energy that usually comes from fossil fuels
which are also non-renewable resources.
Producing ethanol from sugar cane is to more environmentally sustainable
in that the starting material is a renewable resource (new crops can be
grown). However it is not completely sustainable in that large quantities
of energy, usually from non-renewable fossil fuels, are needed to produce
the fertiliser essential for crop growth, to cultivate, harvest and
process the cane, to maintain the fermentation tank at the required
temperature and to distil the ethanol from the 15% solution.
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Question 27 (3 marks)
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You need to understand how these
two different types of radiation are used medically. Fortunately the
information about emissions is given.
(p. 86-7, 76-8)
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(a)
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For
both diagnosis and therapy the patient ingests (or is injected with) the
radioactive material which accumulates in the tissue or organ to be
examined. For diagnosis radiation from this isotope is detected from
outside the body so needs to have high penetrating power to be able to
pass through body tissue. Gamma rays not beta rays can be detected outside
the body. Hence both isotopes can be used for diagnosis.
For radiation therapy we want the radiation to damage the cancerous tissue
in which the isotope has lodged and not damage the surrounding good
tissue, so we need radiation that does not penetrate very far. Beta rays
with their low penetrating power are good for this so we use iodine-131
for radiation therapy.
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(b)
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Question 28 (3 marks)
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A straight forward calculation;
a trap is to use number of moles instead of molarity for calculating pH.
Remember the rule on significant figures: the number of decimal places in
the pH should equal the number of significant figures in the hydrogen ion
concentration. Since the input data have three and four significant figures,
the pH should have three decimal places. Two would be acceptable but 1.4
should not get full marks.
(p. 134-5)
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Moles of HCl provided = 0.075 X 0.120 = 0.00900
mol
Moles of NaOH provided = 0.025 X .200 =
0.00500 mol
These react in the ratio of 1:1 so after reaction
moles
of HCl left = 0.00400 mol
This is now in 100 mL
Concentration of HCl (and so of H+) = 0.00400/0.100
= 0.0400 mol/L
pH = log10 0.0400 = 1.398
(1.40 is acceptable)
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Question 29 (5 marks)
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Draw a simple diagram
showing the four regions. You have to show two pollutants from different
layers would probably be preferable but not essential.
CFCs are the major ones for the stratosphere. Although use of these
compounds has been phased out, there are still significant concentrations of
them in the stratosphere (and will be for another 20 to 30 years). You could
use oxides of nitrogen from high flying aircraft as an alternative, but only
if you can't remember the CFCs.
Alternative pollutants for the troposphere are
CO and hydrocarbons from the incomplete combustion of fossil fuels; oxides of nitrogen (NO and NO2) from power stations and motor
cars (burning fossil fuels); and ozone formed from hydrocarbons and oxides of nitrogen in photochemical
smog. But note that you cannot call ozone a pollutant in the stratosphere
it has no detrimental effects there, only good ones.
(p.
236-40, 250-8)
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Question 30 (6 marks)
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(a) and (b) are straight forward
volumetric calculations. Give your full working: that way if you make a
numerical error you will get some marks. The equation is not really
necessary just state the 1:1 ratio.
(p. 159-62)
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(a)
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Moles
of HCl used = 0.0250 X 0.1034 = 0.002585 mol
HCl and NaOH react in a 1:1 ratio, so
moles of NaOH in the titration = 0.002585
This was in 25.75 mL
Molarity of NaOH solution = 0.002585/0.02575 =
0.1004 mol/L
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Note that the answer is wanted
in milligrams.
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(b)
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(i) |
Average volume of
NaOH used = 16.55 mL
Moles of NaOH in this = 0.01655 X 0.1004 =0.001662 mol
Because aspirin and NaOH react in a 1:1 ratio, this is also the number of
moles of aspirin in each tablet.
Molar mass of aspirin = 9 X 12.0 + 8 X 1.0 +4 X 16.0
= 180.2 g/mol
Mass of aspirin in each sample = 0.001662 X 180.2
= 0.2994 g = 299.4 mg
(The data have four significant figures but in the answer three would
probably be acceptable 299 mg.)
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Bit in italics is probably not
necessary.
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(ii) |
To make the aspirin
dissolve in the solvent. Aspirin is only slightly polar so does not
dissolve easily in highly polar water. By making the solvent less polar by
using ethanol, which though polar is less polar than water, the solubility
of aspirin increases. We need the aspirin to be in solution: otherwise it
will react too slowly for an accurate titration to be done.
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Question 31 ( 5 marks)
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Plot the points and identify them with a circle,
square, triangle or cross. Whether to draw a straight line or a curve
through the points is debatable but under the time constraints of an exam a
straight line is the best option.
An error of ± 2 seems reasonable in view of the fact that the straight line
is not an ideal fit.
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(a)
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The answer you get will depend
on whether you drew a curve or straight line, but regardless your answer
should be within 2 degrees of 120.
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(b)
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120°C
(± 2°C) |
This is a trap for the unwary.
The strongest intermolecular forces in these compounds are hydrogen bonds
but they are approximately the same strength in all of the compounds: it is
why they all have boiling points much higher than the corresponding alkanes.
It is dispersion forces, which increase in strength as the number of carbon
atoms increases, that explain the increase in boiling point of these
compounds with number of carbon atoms.
(p.172-3, exercise 33)
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(c)
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Dispersion
forces
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Question 32
(3 marks)
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The main difficulty with this
calculation is getting masses and concentrations for solution and fish
sorted out.
(p. 225-30)
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From
the graph concentration of Hg in the analysed sample = 0.17
mg/kg (± 0.01)
This was in 25 mL (= 0.025L)
Mass of Hg in this sample = 0.17 X 0.025 = 0.00425
mg
This is the mass of Hg in 18.6 g (= 0.0186 kg) of fish
Concentration of Hg in the fish = 0.00425/0.0186
= 0.23 mg/kg
This is below 0.5 mg/kg so the consumer can eat the fish..
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Question 33 (6 marks)
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Name the cell and give two
environmental risks associated with it (one may have to be a bit trivial),
list the types of chemists that need to collaborate (environmental,
analytical and electrochemical and possibly polymer), and state why
collaboration is necessary (no one chemist can be an expert in all fields).
It is hard to see that being worth six marks or requiring 20 lines to
answer. You may need to pad out the answer to make it look like it is worth
six marks! Hence the inclusion of some description of the cell.
The obvious choices of cell
are the lead-acid cell and the nickel-cadmium cell; the environmental threats
from dry cells or alkaline cells are quite trivial and would be hard to pad
out.
(p. 58-60, 197-9)
310313
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The
electrochemical cell to be discussed is the lead-acid cell (motor car
battery). This cell consists of plates of lead (one set coated with
lead(IV) oxide, the other with lead sulfate) dipping into a moderately
concentrated solution of sulfuric acid all enclosed in a sturdy
acid-resistant plastic case. There is a huge human demand for such
batteries, primarily in the ever-growing number of motor cars in use and
also for storage of electricity in diverse situations.
To assess and minimise the environmental
impact of the lead-acid cell collaboration between environmental,
analytical and electrochemical chemists is necessary environmental
chemists to identify the possible environmental impacts of the product
(the wide dispersion of lead, a poisonous heavy metal, after disposal of
the batteries, and the localised spillage of the sulfuric acid from the
batteries in use or at disposal), analytical chemists to monitor lead in
the environment (particularly near disposal sites) and electrochemists to
improve the design of the cell in order to minimise leakage of dangerous
chemicals into the environment, to increase its lifetime and to maximise
recycling of components or materials (so that fewer batteries are sent to
waste dumps). Collaboration with polymer chemists is also needed to
improve the design and durability of the plastics used in the cases and in
the spacers used to keep the lead plates apart inside the cell and to stop
distortion of the plates which is the main cause of failure of such
batteries.
Collaboration is necessary because modern
chemistry is so complex that no one person can be an expert in all aspects
of the subject. The ways that trace amounts of numerous chemicals cause
damage to the environment and spread through it require good understanding
of the chemistry (and biology) of the environment. Because very low
concentrations of some chemicals, for example lead, can damage the
environment, very sophisticated analytical instruments and techniques such
as atomic absorption spectroscopy are needed to measure them and this
requires skilled analysts. The detailed design and functioning of
electrochemical cells is another specialised area requiring it own
experts.
Alternative answer
The electrochemical cell to be discussed is the nickel-cadmium cell;
it consists of a cadmium electrode and a nickel one with a coating of
nickeloxyhydroxide NiO(OH) on it. This is an easily rechargeable cell for
which there has been a widespread demand for several decades for cameras
and portable electronic devices.
To assess and minimise the environmental
impact of the nickel-cadmium cell collaboration between environmental,
analytical and electrochemical chemists is necessary environmental
chemists to identify the possible environmental impacts of the product
(the wide dispersion of cadmium, a poisonous heavy metal, after disposal
of the batteries, and the localised leakage of corrosive sodium hydroxides
from the cells in use or at disposal), analytical chemists to monitor
cadmium in the environment (particularly near disposal sites) and
electrochemists to improve the design of the cell in order to minimise
leakage of dangerous chemicals into the environment and to increase its
lifetime (so that fewer batteries end up in waste dumps); the factors that
control how many recharge cycles a cell can be put through before it fails
and how to minimise the cell's detrimental memory effect are quite complex
and may require input from metallurgical chemists as well as
electrochemists.
Collaboration is necessary because modern
chemistry is so complex that no one person can be an expert in all aspects
of the subject. The ways that trace amounts of numerous chemicals cause
damage to the environment and spread through it require good understanding
of the chemistry (and biology) of the environment. Because very low
concentrations of some chemicals, for example cadmium, can damage the
environment, very sophisticated analytical instruments and techniques such
as atomic absorption spectroscopy are needed to measure them and this
requires skilled analysts. The detailed design and functioning of
electrochemical cells is another specialised area requiring it own
experts.
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H2CO3(aq) DH
is negative