3.2 Marking procedure for calculations
Full marks should be awarded for a correct numerical answer, without any working shown, unless the question states
... [Show More] ‘Show your working’ or ‘justify your answer’. In thiscase, the mark scheme will clearly indicate what is required to gain full credit.
If an answer to a calculation is incorrect and working is shown, process mark(s) can usually be gained by correct substitution / working and this is shown in the ‘Comments’column or by each stage of a longer calculation.
3.3 Errors carried forward, consequential marking and arithmetic errors
Allowances for errors carried forward are most likely to be restricted to calculation questions and should be shown by the abbreviation ECF or consequential in the markingscheme.
An arithmetic error should be penalised for one mark only unless otherwise amplified in the marking scheme. Arithmetic errors may arise from a slip in a calculation or from anincorrect transfer of a numerical value from data given in a question.
3.4 Equations
In questions requiring students to write equations, state symbols are generally ignored unless otherwise stated in the ‘Comments’ column.
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Examiners should also credit correct equations using multiples and fractions unlessotherwise stated in the ‘Comments’ column.
3.5 Oxidation states
In general, the sign for an oxidation state will be assumed to be positive unless specifically shown to be negative.
3.6 Interpretation of ‘it’
Answers using the word ‘it’ should be given credit only if it is clear that the ‘it’ refers to thecorrect subject.
3.7 Phonetic spelling
The phonetic spelling of correct scientific terminology should be credited unless there is a possible confusion with another technical term or if the question requires correct IUPAC nomenclature.
3.8 Brackets
(…..) are used to indicate information which is not essential for the mark to be awardedbut is included to help the examiner identify the sense of the answer required.
3.9 Ignore / Insufficient / Do not allow
Ignore or insufficient is used when the information given is irrelevant to the question or notenough to gain the marking point. Any further correct amplification could gain the markingpoint.
Do not allow means that this is a wrong answer which, even if the correct answer is given, will still mean that the mark is not awarded.
3.10 Marking crossed out work
Crossed out work that has not been replaced should be marked as if it were not crossedout, if possible. Where crossed out work has been replaced, the replacement work and not the crossed out work should be marked.
3.11 Reagents
The command word ‘Identify’, allows the student to choose to use either the name or theformula of a reagent in their answer. In some circumstances, the list principle may apply when both the name and the formula are used. Specific details will be given in mark schemes.
The guiding principle is that a reagent is a chemical which can be taken out of a bottle orcontainer. Failure to identify complete reagents will be penalised, but follow-on marks (eg for a subsequent equation or observation) can be scored from an incorrect attempt (possibly an incomplete reagent) at the correct reagent. Specific details will be given in mark schemes.
For example, no credit would be given for:
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• the cyanide ion or CN– when the reagent should be potassium cyanide or KCN;
• the hydroxide ion or OH– when the reagent should be sodium hydroxide or NaOH;
• the Ag(NH ) + ion when the reagent should be Tollens’ reagent (or ammoniacal silver nitrate). In this example, no credit is given for the ion, but credit could be given for a correct observation following on from the use of the ion. Specific details will be given inmark schemes.
In the event that a student provides, for example, both KCN and cyanide ion, it would beusual to ignore the reference to the cyanide ion (because this is not contradictory) and credit the KCN. Specific details will be given in mark schemes.
3.12 Organic structures
Where students are asked to draw organic structures, unless a specific type is required inthe question and stated in the mark scheme, these may be given as displayed, structural or skeletal formulas or a combination of all three as long as the result is unambiguous.
In general
• Displayed formulae must show all of the bonds and all of the atoms in the molecule, butneed not show correct bond angles.
• Skeletal formulae must show carbon atoms by an angle or suitable intersection in theskeleton chain. Functional groups must be shown and it is essential that all atoms other than C atoms are shown in these (except H atoms in the functional groups of aldehydes, secondary amines and N-substituted amides which do not need to be shown).
• Structures must not be ambiguous, e.g. 1-bromopropane should be shown as CH3CH2CH2Br and not as the molecular formula C3H7Br which could also represent theisomeric 2- bromopropane.
• Bonds should be drawn correctly between the relevant atoms. This principle applies in all cases where the attached functional group contains a carbon atom, eg nitrile, carboxylic acid, aldehyde and acid chloride. The carbon-carbon bond should be clearlyshown. Wrongly bonded atoms will be penalised on every occasion. (see the examples below)
• The same principle should also be applied to the structure of alcohols. For example, ifstudents show the alcohol functional group as C ─ HO, they should be penalised on every occasion.
• Latitude should be given to the representation of C ─ C bonds in alkyl groups, giventhat CH3─ is considered to be interchangeable with H3C─ even though the latter would be preferred.
• Similar latitude should be given to the representation of amines where NH2─ C will beallowed, although H2N─ C would be preferred.
• Poor presentation of vertical C ─ CH3 bonds or vertical C ─ NH2 bonds should not be penalised. For other functional groups, such as ─ OH and ─ CN, the limit of toleranceis the half-way position between the vertical bond and the relevant atoms in the attached group.
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By way of illustration, the following would apply.
CH3 C C
CH3
C
CH3CH2
OH C C
OH
allowed allowed not allowed not allowed not allowed
NH2 C C
NH2
NH2
NO2
allowed allowed allowed
allowed not allowed
CN C C
CN
COOH C C
COOH
C
COOH
not allowed not allowed not allowed not allowed not allowed
CHO C C
CHO
C
CHO
COCl C C
COCl
not allowed not allowed not allowed not allowed not allowed
• Representation of CH2 by C-H2 will be penalised
• Some examples are given here of structures for specific compounds that should not
gain credit (but, exceptions may be made in the context of balancing equations)
CH3COH for ethanal
CH3CH2HO for ethanol
OHCH2CH3 for ethanol
C2H6O for ethanol
CH2CH2 for ethene
CH2.CH2 for ethene
CH2:CH2 for ethene
• Each of the following should gain credit as alternatives to correct representations ofthe structures.
CH2 = CH2 for ethene, H2C=CH2 CH3CHOHCH3 for propan-2-ol, CH3CH(OH)CH3
• In most cases, the use of ‘sticks’ to represent C ─ H bonds in a structure should not
bepenalised. The exceptions to this when “sticks” will be penalised include
• structures in mechanisms where the C ─ H bond is essential (eg elimination reactions in halogenoalkanes and alcohols)
• when a displayed formula is required
• when a skeletal structure is required or has been drawn by the candidate.
3.13 Organic names
As a general principle, non-IUPAC names or incorrect spelling or incomplete namesshould
not gain credit. Some illustrations are given here.
Unnecessary but not wrong numbers will not be penalised such as the number ‘2’ in 2-methylpropane or the number ‘1’ in 2-chlorobutan-1-oic acid.
but-2-ol should be butan-2-ol
2-hydroxybutane should be butan-2-ol
butane-2-ol should be butan-2-ol
2-butanol should be butan-2-ol
ethan-1,2-diol should be ethane-1,2-diol
2- methpropan-2-ol should be 2-methylpropan-2-ol 2-methylbutan-3-ol should be 3-methylbutan-2-ol 3- methylpentan should be 3-methylpentane
3- mythylpentane should be 3-methylpentane
3-methypentane should be 3-methylpentane
propanitrile should be propanenitrile
aminethane should be ethylamine (although aminoethane can gain credit)2- methyl-3-bromobutane should be 2-bromo-3-methylbutane
3-bromo-2-methylbutane should be 2-bromo-3- methylbutane 3-methyl-2-bromobutane should be 2-bromo-3- methylbutane 2-methylbut-3-ene should be 3-methylbut-1-ene difluorodichloromethane should be dichlorodifluoromethane
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3.14 Organic reaction mechanisms
Curly arrows should originate either from a lone pair of electrons or from a bond.
The following representations should not gain credit and will be penalised each time
within a clip.
H3C Br
: _
H C . Br H3C. .Br
.. _
OH OH
For example, the following would score zero marks
H
H3C C Br
HO H
When the curly arrow is showing the formation of a bond to an atom, the arrow can go directly to the relevant atom, alongside the relevant atom or more than half-way towardsthe relevant atom.
In free-radical substitution:
• the absence of a radical dot should be penalised once only within a clip.
• the use of half-headed arrows is not required, but the use of double-headed arrows orthe incorrect use of half-headed arrows in free-radical mechanisms should be penalised once only within a clip.
The correct use of skeletal formulae in mechanisms is acceptable, but where a C-H bondbreaks, both the bond and the H must be drawn to gain credit.
3.15 Extended responses
For questions marked using a ‘Levels of Response’ mark scheme:
Level of response mark schemes are broken down into three levels, each of which has a descriptor. Each descriptor contains two statements. The first statement is the Chemistry contentstatement and the second statement is the communication statement.
Determining a level
Start at the lowest level of the mark scheme and use it as a ladder to see whether the answer meets the Chemistry content descriptor for that level. The descriptor for the level indicates the qualities that might be seen in the student’s answer for that level. If it meets the lowest level, thengo to the next one and decide if it meets this level, and so on, until you have a match between thelevel descriptor and the answer.
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When assigning a level you should look at the overall quality of the answer and not look to pick holes in small and specific parts of the answer where the student has not performed quite as wellas the rest. If the answer covers different aspects of different levels of the mark scheme you should use a best fit approach for defining the level.
Once the level has been decided, the mark within the level is determined by the communicationstatement:
• If the answer completely matches the communication descriptor, award the higher mark withinthe level.
• If the answer does not completely match the communication descriptor, award the lower markwithin the level. [Show Less]