June 2019: AS Paper 2, Depth in chemistry

Mark scheme

Examiner's comment

Most candidates were given at least one mark but lost the second mark due to omitting the word “atom”, or “mean” or “one-twelfth”

Notes from the mark scheme and report

• weighted mean mass of an ATOM (DO NOT ALLOW mean mass of an element)
• compared with 1/12th mass of carbon-12

Mark scheme

Examiner's comment

Algebra was used very well here and allowed most candidates to obtain at least one mark, with the majority obtaining 2 marks

Notes from the mark scheme and report

• third isotope is 10.2%
• DO NOT ALLOW Sr-86 with no working

Skills: M0.2 Use ratios, fractions and percentages; M1.2 Find arithmetic means; M2.2 Change the subject of an equation; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

In general, the diagrams were poorly drawn or inadequately labelled. Many candidates lost marks as they did not specify the movement of electrons as the reason why metals conduct electricity. Too many candidates wrote “delocalised electrons carry the charge” or negated the metallic bonding description with “intermolecular forces” in their responses. This candidate scored 3 marks for the description but lost 2 marks as the electrons were not throughout the structure, merely drawn around the ions, and the positive ions were not labelled. This candidate scored 5 marks for the excellent description with a well drawn and labelled diagram, clearly showing the electrons throughout the structure in-between the ions.

Notes from the mark scheme and report

• diagram needs at least two rows of labelled + ions with labelled electrons between
• strong attraction between cations and delocalised electrons
• delocalised electrons MOVE for conductivity
• DO NOT ALLOW intermolecular forces

Mark scheme

Examiner's comment

Nearly half of the candidates did not answer this question correctly, mainly because of incorrect balancing or the formation of strontium oxide instead of strontium hydroxide.

Notes from the mark scheme and report

• Sr + 2H2O → Sr(OH)2 + H2
• IGNORE state symbols

Mark scheme

Examiner's comment

Most candidates were able to identify at least one difference, although a significant number of responses stated the opposite trend

Notes from the mark scheme and report

• less vigorous fizzing, dissolves more slowly, lower pH/less alkaline, precipitate forms
• IGNORE less heat, less reactive, less gas (not observations)

Mark scheme

Examiner's comment

Most students managed to gain some marks on this question. The most common error was rounding to 6, something they have been taught to do for water of crystallisation. This caused them to lose a mark as the question asked for two significant figures. Many rounded too early so a variety of responses were seen.

Notes from the mark scheme and report

• use final constant mass 17.58 g
• answer must be to 2 SF (5.8)
• 5.83 scores 2
• ALLOW ECF from n(SrCl2) and n(H2O)

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; PAG1 Moles determination

Mark scheme

Examiner's comment

The majority of candidates answered this correctly, the main incorrect answer was “to achieve constant mass”.

Notes from the mark scheme and report

• to make sure ALL the water has been removed
• IGNORE just 'to weigh to constant mass'

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Most candidates identified either using a larger mass or a more accurate balance, not many stated both. The most common incorrect answers involved heating for longer or taking less measurements.

Notes from the mark scheme and report

• balance reading to 3 or more decimal places
• use a larger mass
• IGNORE lid on crucible, repeating/averaging, weighing straight after heating

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG1 Moles determination

Mark scheme, with the levels and indicative points

Examiner's comment

Most candidates focused on removing the water of crystallisation, often going to great depths of explanation, with apparatus diagrams, of how to remove it. Some candidates then went on to explain how to make a standard solution and could be given marks. Calculations for the mass required were often correct or could be given some marks. This in a response of a common approach to this question, it was given 4 marks. Despite calculating the mass properly, the candidate then put all their efforts into describing removing the water of crystallisation and left out the valuable fine detail required for a Level 3 response.

Notes from the mark scheme and report

• mass 25.63 g of Mg(NO3)2•6H2O (14.83 g if water of crystallisation omitted)
• dissolve in distilled water, transfer with washings to 250 cm3 volumetric flask, make up to mark with bottom of meniscus on mark, invert to mix
• IGNORE removing water of crystallisation

Skills: M0.0 Recognise and make use of appropriate units in calculations; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

This question was answered well, with the majority of the candidates obtaining 3 marks.

Notes from the mark scheme and report

• 2:1 ratio HNO3:Mg(NO3)2
• 3 SF required (38.5)
• ALLOW ECF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.2 Change the subject of an equation

Mark scheme

Examiner's comment

Less than half the candidates answered this question correctly. This may be because they are not used to assigning oxidation numbers within formulae that contain brackets.

Notes from the mark scheme and report

• O oxidised –2 to 0, N reduced +5 to +4
• MAX 1 if no '+' sign
• 1 mark if all numbers correct but oxidised/reduced reversed

Mark scheme

Examiner's comment

Those that had learnt this important mechanism scored all 3 marks with very precisely drawn arrows and partial charges. Although many candidates wrote out the correct organic product, many wrote NaI instead of I- which lost them the final mark. This candidate scored all 3 marks. The lone pair of electrons on the oxygen and partial charges on the C-I bond are clearly marked on the diagram and the arrows are precisely drawn. Connectivity is good on the organic product and I- identified as the additional product.

Notes from the mark scheme and report

• curly arrow from lone pair or – charge on O of OH– to C of C–I
• dipole Cδ+–Iδ– AND arrow from C–I bond to I
• product AND I– (NaI does not show I–)
• no double-headed or half arrows

Mark scheme

Examiner's comment

The question asked for the measurement AND observation, many students did not answer both parts and therefore did not gain any marks. “How fast” and “how long” were not given as they did not detail the measurement.

Notes from the mark scheme and report

• TIME for PRECIPITATE to appear - both required

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Answers were too vague to be given in most cases. Candidates referred to bonds being broken or overcome, but did not specify C-I bonds breaking, or discussed iodine and bromine in terms of reactivities.

Notes from the mark scheme and report

• C–I weaker/lower bond enthalpy than C–Br (comparison required)
• carbon–halogen bond breaks

Mark scheme

Examiner's comment

This was generally well answered. The most common incorrect answer was atomic mass.

Notes from the mark scheme and report

• molecular MASS/Mr
• IGNORE 'molecular ion' alone

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

Mark scheme

Examiner's comment

Most students gained one mark on this question as they omitted the + sign or wrote the molecular formula instead of the structural formula.

Notes from the mark scheme and report

• Y CH3CH2CH2CH2CH2+ and Z CH3CH2CH2+
• one mark only if + charge missing or molecular formulae C5H11+ and C3H7+ given

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

Mark scheme

Examiner's comment

This was well answered by most candidates

Notes from the mark scheme and report

• CH3C(CH3)ICH2CH3
• any unambiguous formula

Mark scheme

Examiner's comment

Most candidates answered the similarity part correctly, many wrote vague answers to the difference and were not specific. A common error focused on the peak at m/z = 71, very few recognised that the Z peak at 43 would not be present for 2-iodo-2-methylbutane

Notes from the mark scheme and report

• similarity: both have peak at m/z 198 (or 71 or 29)
• difference: 2-iodo-2-methylbutane has no peak at 43
• IGNORE 'different ion peaks' with no m/z

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

Mark scheme

Examiner's comment

This was a well answered question, with the majority of candidates obtaining all 3 marks

Notes from the mark scheme and report

• HCl is limiting
• 120 cm3 (no ÷2) or 240 cm3 (×2) score 2
• ALLOW ECF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Most candidates obtained this mark, some lost the mark because they did not use a linear scale or provide units.

Notes from the mark scheme and report

• linear x scale AND label Time / s AND points plotted ±1 small square
• ALLOW up to 3 plotting errors

Skills: M3.2 Plot two variables from experimental or other data; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Nearly all the candidates obtained this mark

Notes from the mark scheme and report

• point at 80 s circled

Skills: M3.2 Plot two variables from experimental or other data; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Nearly all the candidates obtained this mark

Notes from the mark scheme and report

• smooth curve through all points EXCEPT the 80 s point

Skills: M3.2 Plot two variables from experimental or other data; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Many students did not sketch this curve or sketched a curved that was less steep and did not finish at 58cm3.

Notes from the mark scheme and report

• steeper initial slope AND levels off earlier
• same final volume (58 cm3) because same moles of HCl

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

Mark scheme

Examiner's comment

Many responses detailed why the graph was steep at the beginning, rather than answering the question. Those that did explain the decrease often omitted the words concentration and frequency so the majority did not gain 2 marks. A large number of candidates discussed particles “losing energy” and “less successful collisions” so were not given any marks.

Notes from the mark scheme and report

• concentration of acid decreases (IGNORE amount decreases)
• fewer collisions per second/less frequent - 'fewer collisions' alone insufficient

Mark scheme

Examiner's comment

Most candidates scored the first marking point but many did not achieve the second marking point as their explanations were too vague.

Notes from the mark scheme and report

• lowers activation energy by alternative route
• greater proportion of molecules have energy ≥ Ea
• IGNORE more successful collisions

Mark scheme

Examiner's comment

Almost half of the candidates answered this question correctly, the remainder did not realise that the question gave them the answer to the state that the copper sulphate solution was in. Many answers stated that it could be solid or aqueous, so difficult to classify.

Notes from the mark scheme and report

• reactants are in different physical states - CuSO4(aq) same phase as acid but different from zinc

Mark scheme

Examiner's comment

Most candidates answered this question well and achieved full marks. The most common errors were to put the chlorine on the wrong carbon, or to put both bromines on the same carbon.

Notes from the mark scheme and report

• hexane
• 2-chlorohexane as major product (Markownikoff)
• 1,2-dibromohexane
• ALLOW part molecular formulae but not full

Mark scheme

Examiner's comment

Most candidates correctly stated nickel, although it was spelled incorrectly a lot of the time, which was ignored. “Acid” was the most common incorrect answer

Notes from the mark scheme and report

• Ni
• ALLOW Pt, Pd or Rh

Mark scheme

Examiner's comment

Many candidates wrote the colour change the wrong way around, or thought that a gas would be evolved, or wrote “clear” instead of “colourless”. A large proportion merely stated what type of reaction it was, rather than what they would observe.

Notes from the mark scheme and report

• (orange to) colourless/decolourised
• IGNORE 'colour change'

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme, with the levels and indicative points

Examiner's comment

This question discriminated well and most were able to attempt to calculate a mass and explain the purification steps with some fine detail. It was evident that most candidates were aware of the apparatus required but the logic in the order was sometimes out of sequence. Drying agents were mentioned, some of the examples used were incorrect and errors were made with the boiling points if candidates mentioned distillation. This in a response of an excellent approach to this question, it was given 6 marks. The correct mass has been calculated and all of the fine detail, except distilling at 63°C

Notes from the mark scheme and report

• 0.0500 mol hex-1-ene needs 0.0800 mol hexan-1-ol = 8.16 g
• partly correct: 5.1 g (% yield omitted), 3.19 g (% yield inverted)
• separating funnel (upper organic layer), dry with anhydrous MgSO4/CaCl2, distil collecting fraction at 63 °C
• incorrect purification method not creditworthy

Skills: M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Most candidates obtained the first mark, but very few stated that hex-2-ene forms which is why the yield is less. Many candidates stated this was due to secondary alcohols being more stable than primary ones and a significant number thought the yield would be the same as they had the same Mr.

Notes from the mark scheme and report

• yield of hex-1-ene is less because a mixture with hex-2-ene forms

Mark scheme

Examiner's comment

Over half of the candidates answered this question incorrectly. Many drew the wrong repeat units or did not show two repeat units. The structural diagrams were very hard to decipher and not well drawn. Although the connectivity is not exact, this was allowed. Many students drew full displayed structures which were very difficult to decipher as they often encroached over the writing in the question.

Notes from the mark scheme and report

• two repeat units with C4H9 side chains and side links (end bonds)
• IGNORE brackets and n

Mark scheme

Examiner's comment

Most students did not gain many marks on this question, with a significant number stating that the plastics could be used to feed livestock or as fertilisers. Those that mentioned combustion, merely stated plastics could be used as a fuel. Some candidates made references to making them biodegradable or recycling, but these answers were not given.

Notes from the mark scheme and report

• combustion for energy production
• organic feedstock for plastics/other organic chemicals