June 2018: AS Paper 2, Depth in chemistry

Mark scheme

Examiner's comment

Many candidates gave a good description of standard conditions or stated 1 mol dm-3, but that did not answer the question so no marks could be credited.

Notes from the mark scheme and report

• solution of known concentration

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Many candidates stated a Brønsted–Lowry definition or gave pH values. Of the candidates that did mention OH- ions, most did not state ‘releases’ OH- ions in solution, although they were credited with the mark.

Notes from the mark scheme and report

• releases OH– ions in aqueous solution
• IGNORE mention of pH

Mark scheme

Examiner's comment

Most candidates were able to accurately record the burette readings and made the correct subtractions. Despite the examination question requesting the mean titre to be recorded to the accuracy of the burette, many candidates did not do this. A common error was taking a mean of all three readings instead of only the concordant results; this led the candidates to give an answer of 26.58 which lost them 2 marks.

Notes from the mark scheme and report

• readings to 2 d.p. ending in 0 or 5
• mean from concordant titres only (26.45)
• DO NOT ALLOW 26.5
• ALLOW ECF from incorrect burette readings

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

A good attempt by many candidates but some did not know how to calculate this or did not multiply by 2.

Notes from the mark scheme and report

• two readings so 2 × 0.05
• ALLOW ECF from (c)(i) or use of any titre

Skills: M0.2 Use ratios, fractions and percentages; M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Although there were some excellent descriptions of the correct processes, such as inverting the apparatus to ensure mixing and then making the solution up to the mark, many candidates could not name a volumetric flask.

Notes from the mark scheme and report

• volumetric flask instead of beaker
• IGNORE graduated flask

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

This was a good discriminator. Many candidates were credited the first two marks, and higher ability candidates went on to obtain 3 or 4 marks. Common errors were not rounding to a whole number at the end and not scaling up by 250/27.30. Many candidates did not scale up at all, obtaining an answer of 1081, or merely multiplied by 250/25 and obtained a molar mass of 108.

Notes from the mark scheme and report

• acid A is in the burette so scale by 250.0/27.30
• 1:2 ratio A:NaOH
• common ECF answers 108, 1081, 59, 114, 99 (3 marks)

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates that obtained a sensible value for the previous question managed to draw a creditable structure. Allowing error carried forward meant that feasibly derived structures could be credited a mark.

Notes from the mark scheme and report

• structure must match Mr from (d)(i) within 10
• ALLOW ECF only if 2 × COOH possible
• ALLOW incorrect connectivity e.g. –HO

Mark scheme

Examiner's comment

The majority of candidates obtained full marks on this question. The most common errors were incorrect charges or covalent structures.

Notes from the mark scheme and report

• S with 8 electrons (6 dots + 2 crosses or vice versa)
• correct charges
• Na with 0 or 8 electrons
• brackets not required

Mark scheme

Examiner's comment

The majority of candidates obtained 2 or 3 marks on this question. Many candidates seemed unaware that sodium was a metal.

Notes from the mark scheme and report

• marked by column
• Na2S giant, poor solid, good liquid
• Na giant, good, good
• S simple, poor, poor

Mark scheme

Examiner's comment

Most candidates answered this correctly. The most common error seen was 4p6 instead of 4p4

Notes from the mark scheme and report

• ALLOW 3d10 after 4s2 or 4p4
• DO NOT ALLOW [Ar] shorthand

Mark scheme

Examiner's comment

The majority of candidates obtained 1 or 2 marks on this question. The most common errors seen were identifying the gas as H2S or incorrect balancing.

Notes from the mark scheme and report

• H2Se
• DO NOT ALLOW H2S for gas B but ALLOW ECF for equation
• IGNORE state symbols

Mark scheme

Examiner's comment

Around 50% of candidates answered this correctly. The most common error was not balancing the equation, and many did not know what an ionic equation was.

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols

Mark scheme

Examiner's comment

This question was answered well by most candidates, although some omitted the word ‘nucleus’ when explaining attracting electrons, or answered in terms of losing electrons, and did not have the final marking point credited. This candidate was credited all three marks. Some candidates managed to be credited for all 3 marks in one short sentence that covered all 3 marking points.

Notes from the mark scheme and report

• larger atomic radius
• more shielding
• weaker nuclear attraction on electron gained
• DO NOT ALLOW mention of losing electrons
• IGNORE ionisation energy
• IGNORE just 'greater attraction'

Mark scheme

Examiner's comment

Most candidates were aware of disproportionation but lost marks by not stating the species or whether the process was oxidation or reduction. Here the candidate has lost a mark for not stating the initial oxidation number of elemental iodine as 0.

Notes from the mark scheme and report

• same element oxidised and reduced
• 0 to +1 in HIO and 0 to –1 in HI
• 1 mark only for three correct oxidation numbers without the words oxidation/reduction

Mark scheme

Examiner's comment

The majority of candidates stated that chlorine is toxic or forms carcinogens, although some stated that chlorine is a carcinogen which was not credited.

Notes from the mark scheme and report

• toxic OR forms chlorinated hydrocarbons/carcinogens
• IGNORE 'chlorine causes cancer', harmful/dangerous

Mark scheme

Examiner's comment

Most candidates made a good attempt at this question. The most common mistake was to use the wrong sign when incorporating the enthalpy change into the calculation, or not to incorporate it at all.

Notes from the mark scheme and report

• 431.5
• –431.5 scores 1 (wrong sign)
• 247.5 scores 1 (wrong expression)
• must divide by 2

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

A good attempt by many candidates but some lost marks by having the wrong state of bromine, even though the question stated it was a liquid changing to a gas. Many added water or oxygen, some confused the equation with bond enthalpy and answers such as Br2 (l)  2Br(g) were commonly seen.

Notes from the mark scheme and report

• Br2(l) → Br2(g)
• state symbols essential

Mark scheme

Examiner's comment

The majority of candidates answered this question incorrectly. Only 10% of candidates mentioned intermolecular/London forces. Most stated ‘exothermic’ or described breaking covalent bonds.

Notes from the mark scheme and report

• endothermic AND energy needed to overcome London forces/intermolecular forces
• DO NOT ALLOW van der Waals' forces

Mark scheme

Examiner's comment

Most candidates were able to gain the first mark, but many lost the second mark by putting a double headed arrow or -∆H.

Notes from the mark scheme and report

• products below reactants with ∆H arrow downwards
• Ea to top of curve
• DO NOT ALLOW –∆H or double-headed ∆H arrow
• 1 mark for a correctly labelled endothermic diagram

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

Most candidates were able to convert from tonnes to moles and then went on to complete the majority of the calculation steps. Many omitted to divide by 4 and were credited 3 marks. Some candidates lost marks by not stating the answer to standard form or quoted their answer to more than 3 significant figures. A number of candidates attempted to use Q = mc∆T and did not get very far in the calculation.

Notes from the mark scheme and report

• tonnes to g
• ÷4 for per mole NH3
• 3 sig figs AND standard form (6.79 × 10^7)
• common errors 2.72 × 10^8 (no ÷4), 1.09 × 10^9 (×4), 6.79 × 10^1 (no tonnes to g)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.1 Recognise and use expressions in decimal and ordinary form; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures

Mark scheme

Examiner's comment

Generally, this question was well answered with only a small proportion of candidates adding the values together instead of multiplying.

Notes from the mark scheme and report

• square brackets required
• IGNORE state symbols

Mark scheme

Examiner's comment

Most candidates answered this question very well, with the most common mark being 4/5. Many candidates put a lot of effort into explaining, in depth, Le Chatelier’s principle, which was not required. The first three marking points were credited to most candidates. Responses were confident in their descriptions of equilibrium shifts and many candidates then went on to qualify their answers with operational factor considerations and/or rate. The explanation for pressure was described less commonly than temperature and many candidates did not appreciated that increased rate would lead to a decreased equilibrium yield. This candidate scored all five marks for this well-reasoned approach to the question.

Notes from the mark scheme and report

• low temperature (exothermic) and low pressure (fewer gaseous moles on left)
• compromise must relate temperature/pressure to rate
• IGNORE catalyst and stated values

Mark scheme, with the levels and indicative points

Examiner's comment

Candidates were not prepared to answer this type of question and the diagrams were hard to give credit to. Many had significant safety implications such as open beakers of butan-1-ol being heated by a Bunsen burner. Most mis-read the question and just outlined the method for purification and struggled to recall the practical details. Very few candidates mentioned the use of anhydrous salts, referring instead to ‘boiling off’ the water. This candidate was credited 4 marks for this level 2 answer. Although they have drawn distillation apparatus instead of reflux, they have considered the boiling point of the product, detailed using a separating funnel, a drying agent and that the lower organic layer would be drawn off first.

Notes from the mark scheme and report

• reflux with water in at bottom of condenser and open system
• separating funnel – organic layer is the lower (denser) layer
• dry with anhydrous salt
• redistil collecting at 102 °C

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

This question was well answered, but a significant number of candidates incorrectly used the Mr of butan-1-ol when calculating the mass of 1-bromobutane.

Notes from the mark scheme and report

• 3 sig figs required (12.6)
• common error 33.4 (dividing by yield)
• DO NOT ALLOW 6.82 (Mr of butan-1-ol)

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Most candidates knew they had to draw a tangent to the curve but calculating the gradient led to careless errors. Many drew tiny triangles and mis-read readings of coordinates. It is far easier, and more accurate, to use large triangles using the axes for coordinates. Lower ability candidates just interpolated the graph at 30s and were not credited marks for this.

Notes from the mark scheme and report

• tangent drawn at t = 30 min (±10)
• DO NOT ALLOW interpolation (direct reading)
• ALLOW ECF from incorrect tangent
• IGNORE units and sign

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; M3.5 Draw and use the slope of a tangent to a curve as a measure of rate of change

Mark scheme

Examiner's comment

Many candidates knew the answer to this question but forgot that water must be in the gaseous state. There were numerous responses stating nickel as the catalyst, but most knew that an acid catalyst was required.

Notes from the mark scheme and report

• steam AND acid/H+ catalyst both required

Mark scheme

Examiner's comment

This question was generally well answered, although some candidates made careless mistakes such as not writing -di or writing 1,2-dibromo-1-dichloroethane

Notes from the mark scheme and report

• 1,2-dibromo-1,1-dichloroethane

Mark scheme

Examiner's comment

Many candidates gained all three marks on this question and the diagrams were clear and easy to read. Lower ability candidates had incorrect dipoles or curly arrows that could not be traced back to the correct origin. Candidates should be encouraged to consider what the arrows mean rather than memorising mechanisms with no understanding.

Notes from the mark scheme and report

• arrow from C=C to Br
• dipole on Br–Br and arrow from bond to Brδ–
• arrow from Br– lone pair/charge to C+
• DO NOT ALLOW partial charge on C=C or δ+ on carbocation
• DO NOT ALLOW half-headed arrows

Mark scheme

Examiner's comment

Most candidates correctly drew the repeat unit and were credited with one mark, but many placed the n position in the wrong place on the left-hand side of the equation or forgot to write it in at all.

Notes from the mark scheme and report

• displayed repeat unit with side bonds and brackets
• n on both sides (subscript on RHS)
• DO NOT ALLOW ECF from incorrect repeat unit

Mark scheme

Examiner's comment

With all the media interest in plastic pollution this question was answered well, although many gave the answer ‘quick and efficient’ as an advantage which was not credited. Candidates should beware of vague statements such as these. Many wrote ‘harmful’ instead of toxic, or ‘bad for the environment’ instead of being specific about the environmental issue.

Notes from the mark scheme and report

• advantage: energy production
• disadvantage: HCl/acid rain OR CO2/greenhouse gases OR CO
• ALLOW less landfill

Mark scheme, with the levels and indicative points

Examiner's comment

This question was a good discriminator. Most candidates were able to deduce the empirical formula and the C=O peak on the IR spectrum. However, many mistook the C-H peaks around 3000 cm-1 for an alcohol O-H peak or assumed from the empirical formula that it was an alcohol so made the spectra ‘fit’ their hypothesis. At AS, the exposure of candidates to IR and MS spectra is not as comprehensive as in the second year of A Level and this was evident. There was very little annotation of the spectra and candidates should be encouraged to do this as it is helpful to them in their deductions (and to the examiners for crediting marks). Analysis of the IR spectrum was much more detailed than the mass spectrum. Most candidates just referred to the molecular ion peak and made no attempt, or an incorrect attempt, at discerning the peak at 41. Those that did quickly realised what the structure was and gained 5 or 6 marks. Some candidates, despite ascertaining that a trans stereoisomer should be drawn, drew the cis version instead. This candidate has very logically worked through all the information provided and has come up with the correct structure from the deductions. Like most candidates, the crucial information about F being a trans isomer was not picked up so they drew the double bond in the wrong place. Copyright acknowledgements Q7, graph 1 © SDBSWeb : https://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi (National Institute of Advanced Industrial Science and Technology, accessed 31 January 2017). Q7, graph 2 © SDBSWeb : https://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi (National Institute of Advanced Industrial Science and Technology, accessed 31 January 2017).

Notes from the mark scheme and report

• empirical formula C4H6O
• M+ at m/z 70 so molecular formula C4H6O
• IR C=O 1630–1820 cm–1
• m/z 41 = C3H5+ (loss of CHO)
• trans-but-2-enal (cis accepted for Level 3)

Skills: M0.2 Use ratios, fractions and percentages; M3.1 Translate information between graphical, numerical and algebraic forms