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This thread is the older (archived) thread of my collection
of Chemistry questions, and is no longer being updated (ie. no new
questions will be added to this thread). Please view my other
thread for regular updates (ie. for new questions to be added
regularly).
http://sgforums.com/forums/2297/topics/392932
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Question Ambiguity in the 'O' or 'A' Level Exam
If you encounter ambiguous questions in the exams, write both
answers (yes it'll take up more time to do so, but at least your
marks are secure). Write a short note to explain to the
examiner/marker how the question could be interpreted in both
ways, then proceed to address the question in both ways.
Example, updated 20 Apr 09 :
davidche posted
: When
20 cm3 of a gaseous hydrocarbon were sparked with 150cm3 of oxygen
and the residual gases cooled to rtp, a contraction of 60cm3
occured. A further contraction of 80cm3 took place when the
residual gases were subjected to aq NaOH. All the vols were
measured at 20 degrees. Determine the formula of the
hydrocarbon.
Chemfreak022 posted : Vol of H2O produced = initial contraction = 60
cm3
Vol
of CO2 produced = 80 cm3 after NaOH
Therefore
vol of CO2 : H2O = 4:3
2CxHy + O2
---> 8CO2 + 6H2O
Therefore
hydrocarbon is C4H6
SBS261P posted :
CxHy + (x + y/4) O2 ---> x CO2 +
y/2 H2O.
Amount of CO2 absorbed by aq. NaOH
= 80cm^3.
Ratio of CxHy : CO2 = 20 : 80 =
1:4
=> x = 4.
There is a contraction of 60cm^3
=> final volume is 110cm^3, of which 80cm^3 is CO2. The other
30cm^3 is excess oxygen (since no water left)
=> 120cm^3 of oxygen
used.
Ratio of CxHy : O2 = 20 : 120 =
1:6
i.e. x + y/4 = 6
since x = 4
y/4 = 6 - 4 = 2
y = 8
Hence your hydrocarbon is
C4H8.
For the record, Chemfreak022 and SBS261P are both correct, depending on how
you choose to interpret this ambiguous question (contraction
of product gases or contraction of reactant gases?).
In the actual 'A' or 'O' level exam, should such critical
abiguity be found in a question, the intelligent candidate will
write out both alternative workings and answers, but with
qualification and explanation. "Dear Examiner/Marker Sir/Mdm, I
regret to inform that the question has failed to
unambiguously state whether the contraction of gases referred to
the product gases or the reactant gases. As such, I
am left with no choice but to provide alternative workings and
answers below. On the left below, I assume contraction of
product gases. On the right below, I assume contraction of reactant
gases. Thank you very much Sir/Mdm, for your kind understanding in
this critical matter."
Other examples of exam question ambiguity, and how the
intelligent candidate should respond :
Eg. "Dear Examiner Sir, the question did not specify
clearly the exact reaction conditions used. If the
temperature is below X deg C, then the products are such-and-such.
If the temperature is above X deg C, then the products are
such-and-such."
Eg. "Dear Examiner Sir, the question did not specify
clearly whether the KMnO4(aq) used on the alkene was
"alkaline, cold, dilute" or "acidified, hot, concentrated".
If "alkaline, cold, dilute" KMnO4 was used, then the resulting
product is...
If "acidified, hot, concentrated" KMnO4 was used, then the
resulting product is...
Eg. "Dear Examiner Sir, the question did not specify clearly
whether the body temperature had decreased due to ambient
environmental temperature, or due to physiological homeostatic
response. If the former, then the following explanation applies...
If the latter, then the following explanation applies..."
Eg. "Dear Examiner Sir, the question did not specify clearly
which categories of isomers are to be considered.
If structural isomers only, then the answers are...
If stereoisomers only, then the answers are...
If structural and stereoisomers, then the answers are...
You'll get your marks in this way (because after
showing the examiner that the question's ambiguity is at
fault, you've more importantly demonstrated to the examiner
you know all your facts/concepts that the question is asking for,
whichever interpretation turned out to be the one the question
setter or mark scheme had in mind), while candidates who only write
one interpretation have a 50% chance of getting zero marks.
-------------------------------------------
Cambridge Mark Schemes for Cambridge International Examinations
(CIE) and Oxford, Cambridge and Royal Society of Arts (OCR)
Examination Papers :
www.cie.org.uk
www.ocr.org.uk
-------------------------------------------
Balancing Redox Half-Equations ('A' levels)
Here is how you balance half-equations :
Hydrogen peroxide is reduced to water (chemical name : hydrogen
hydroxide aka dihydrogen monoxide aka hydrogen(I) oxide).
Hydrogen peroxide is oxidized to molecular oxygen gas.
(Acidified) Manganate(VII) ions are reduced to Manganese(II)
cations.
Nitrogen dioxide is oxidized to Nitrate(V) anions.
[reduction]
H2O2 --> H2O
To balance oxygen,
H2O2 --> 2H2O
To balance hydrogen,
2H+ + H2O2 --> 2H2O
To balance charges,
2H+ + 2e- + H2O2 --> 2H2O
[oxidation]
H2O2 --> O2
To balance hydrogen,
H2O2 --> O2 + 2H+
To balance charges,
H2O2 --> O2 + 2H+ + 2e-
[reduction]
MnO4 - --> Mn2+
To balance oxygen,
MnO4 - --> Mn2+ + 4H2O
To balance hydrogen,
8H+ + MnO4 - --> Mn2+ + 4H2O
To balance charges,
8H+ + MnO4 - + 5e- --> Mn2+ + 4H2O
[oxidation]
NO2 --> NO3 -
To balance oxygen,
H2O + NO2 --> NO3 -
To balance hydrogen,
H2O + NO2 --> NO3 - + 2H+
To balance charges,
H2O + NO2 --> NO3 - + 2H+ + e-
To obtain overall balanced redox equations, we ensure no. of
electrons lost = no. of electrons gained (by multiplying one or
both half-equations to obtain the lowest common multiple for
coefficient of electrons), then add up the half equations
(left hand side + left hand side, right hand side + right hand
side), and finally cancel away any common species on both sides,
invariably including all electrons.
For the reaction of using acidified KMnO4(aq) on H2O2, we
have
[reduction]
8H+ + MnO4 - + 5e- --> Mn2+ + 4H2O
16H+ + 2MnO4 - + 10e- --> 2Mn2+ + 8H2O
[oxidation]
H2O2 --> O2 + 2H+ + 2e-
5H2O2 --> 5O2 + 10H+ + 10e-
[Balanced Redox]
(16H+ + 2MnO4 - + 10e-) + (5H2O2) --> (2Mn2+ + 8H2O) +
(5O2 + 10H+ + 10e-)
6H+ + 2MnO4 - + 5H2O2 --> 2Mn2+ + 8H2O + 5O2
For the reaction of bubbling NO2 into H2O2, we have
[reduction]
2H+ + 2e- + H2O2 --> 2H2O
[oxidation]
H2O + NO2 --> NO3 - + 2H+ + e-
2H2O + 2NO2 --> 2NO3 - + 4H+ + 2e-
[Balanced Redox]
(2H2O + 2NO2) + (2H+ + 2e- + H2O2) --> (2NO3 - + 4H+ + 2e-) +
(2H2O)
2NO2 + H2O2 --> 2NO3 - + 2H+
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Did you know?
Two common errors in Chemistry are :
"The 3 isotopes of hydrogen are 1) hydrogen 2) deuterium 3)
tritium", and
"Isotopes are atoms with the same no. of protons but different
no. of neutrons."
Actually, the 3 isotopes of hydrogen are in fact protium,
deuterium and tritium; atoms of which should all be written as 'H'
(but with different mass numbers indicated), but the common (though
technically erroneous) practice of writing 'D' and 'T' for
atoms/ions of deuterium and tritium isotopes, is acceptable by
IUPAC.
In addition, the proper definition of isotopes (which means "types"
of any given element) is "Isotopes of elements have atoms and ions
with the same number of protons, but different number of neutrons",
or alternatively, "Different isotopes of the same element,
have atoms/ions that have the same number of protons but different
number of neutrons."
But the (technically erroneous) common definition of isotopes which
presume they are "atoms" is commonplace and acceptable at 'O' and
'A' levels.
To reiterate :
Isotopes are NOT atoms. They are "types" of an element
(and elements are defined by their unique number of
protons).
The correct idea of isotopes is :
"Different isotopes of the same element, have atoms/ions
that have the same number of protons but different number of
neutrons."
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Balancing Chemical Equations (both 'O' and 'A'
levels)
I find the following equation to be good practice for my
students (both 'O' levels and 'A' levels). Note that this is a
simple, non-ionic, non-redox equation.
At 'O' levels, an equation you should be familiar with, in
regard to how nitrogen dioxide contributes to acid rain, is
4NO2(g) + O2(g) + 2H2O(l) --> 4HNO3(aq)
Similarly, for sulfur dioxide contributing to acid rain :
2SO2(g) + O2(g) + 2H2O(l) --> 2H2SO4(aq)
But for the purpose of the exercise of balancing chemical
equations, let's say you're given the following equation to
balance :
