AQA A-level CHEMISTRY 7405-1 Paper 1 Inorganic and Physical

AQA A-level CHEMISTRY 7405/1 Paper 1 Inorganic and Physical Chemistry Mark scheme June 2021 Version: 1.0 Final  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 in mark schemes. In the event that a student provides, for example, both KCN and cyanide ion, it would be usual 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. Organic structures Where students are asked to draw organic structures, unless a specific type is required in the 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, but need not show correct bond angles.  Skeletal formulae must show carbon atoms by an angle or suitable intersection in the skeleton 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 the isomeric 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 clearly shown. 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, if students 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, given that 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 be allowed, 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 tolerance is the half-way position between the vertical bond and the relevant atoms in the attached group. 3 2 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 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 of the structures. CH2 = CH2 for ethene, H2C=CH2 CH3CHOHCH3 for propan-2-ol, CH3CH(OH)CH3 AQA A-level CHEMISTRY 7405/1 Paper 1 Inorganic and Physical Chemistry Mark scheme June 2021 Version: 1.0 Final  In most cases, the use of ‘sticks’ to represent C ─ H bonds in a structure should not be penalised. 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. Organic names As a general principle, non-IUPAC names or incorrect spelling or incomplete names should 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 MARK SCHEME – A-LEVEL CHEMISTRY – 7405/1 – JUNE 2020 10 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 towards the 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 or the 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 bond breaks, both the bond and the H must be drawn to gain credit. 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 content statement 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, then go to the next one and decide if it meets this level, and so on, until you have a match between the level descriptor and the answer. 3 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 well as 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 communication statement: • If the answer completely matches the communication descriptor, award the higher mark within the level. • If the answer does not completely match the communication descriptor, award the lower mark within the level. The exemplar materials used during standardisation will help you to determine the appropriate level. There will be an exemplar in the standardising materials which will correspond with each level of the mark scheme and for each mark within each level. This answer will have been awarded a mark by the Lead Examiner. You can compare the student’s answer with the exemplar to determine if it is the same standard, better or worse than the example. You can then use this to allocate a mark for the answer based on the Lead Examiner’s mark on the exemplar. You may well need to read back through the answer as you apply the mark scheme to clarify points and assure yourself that the level and the mark are appropriate. Indicative content in the mark scheme is provided as a guide for examiners. It is not intended to be exhaustive and you must credit other chemically valid points. Students may not have to cover all of the points mentioned in the indicative content to reach the highest level of the mark scheme. The mark scheme will state how much chemical content is required for the highest level. An answer which contains nothing of relevance to the question must be awarded no marks. For other extended response answers: Where a mark scheme includes linkage words (such as ‘therefore’, ‘so’, ‘because’ etc), these are optional. However, a student’s marks for the question may be limited if they do not demonstrate the ability to construct and develop a sustained line of reasoning which is coherent, relevant, substantiated and logically structured. In particular answers in the form of bullet pointed lists may not be awarded full marks if there is no indication of logical flow between each point or if points are in an illogical order. The mark schemes for some questions state that the maximum mark available for an extended response answer is limited if the answer is not coherent, relevant, substantiated and logically structured. During the standardisation process, the Lead Examiner will provide marked exemplar material to demonstrate answers which have not met these criteria. You should use these exemplars as a comparison when marking student answers