November 2020: Paper 1, Periodic table, elements and physical chemistry

Answer

D

Mark scheme

Notes from the mark scheme and report

• rinsing the pipette with water dilutes the NaOH

Skills: PAG2 Acid–base titration

Answer

B

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Notes from the mark scheme and report

• 0.5 mol Ca loses 1 mol of electrons

Skills: M0.2 Use ratios, fractions and percentages

Answer

B

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Answer

C

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Notes from the mark scheme and report

• 2H3PO4 : 3Ca(OH)2

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Answer

A

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Answer

A

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Answer

D

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Answer

D

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Notes from the mark scheme and report

• Ea = –gradient × R, then convert J to kJ

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; M3.3 Determine the slope and intercept of a linear graph

Answer

B

Mark scheme

Notes from the mark scheme and report

• p(NH3) = 25 – 18.75 – 2.50

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Answer

C

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Notes from the mark scheme and report

• overall order deduced from the units of k

Skills: M0.0 Recognise and make use of appropriate units in calculations

Answer

D

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Notes from the mark scheme and report

• excess H+ in total volume 30 cm3

Skills: M0.2 Use ratios, fractions and percentages; M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Answer

C

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Notes from the mark scheme and report

• 3I– + 2MnO4– + H2O → 3IO– + 2MnO2 + 2OH–

Skills: M0.2 Use ratios, fractions and percentages

Answer

B

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Answer

D

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Notes from the mark scheme and report

• Sr2+ has 8 s electrons
• Ba forms Ba(OH)2

Answer

C

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Notes from the mark scheme and report

• mean taking into account the relative abundances of the isotopes
• DO NOT ALLOW 'average mass of the isotopes'

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Notes from the mark scheme and report

• Mg2+ with no (or 8) outer electrons and two Br– each with dot-and-cross octet
• correct charges
• 1 mark if only one Br– drawn

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Notes from the mark scheme and report

• 3 ions per formula unit
• 3 sig figs required (1.71 × 10^22)
• common error 5.69 × 10^21 (no × 3) scores 2

Skills: 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, with the levels and indicative points

Notes from the mark scheme and report

• Mg giant metallic with mobile delocalised electrons
• Br2 simple molecular with London forces between molecules (DO NOT ALLOW between atoms)
• MgBr2 giant ionic – ions fixed in solid, mobile when liquid (DO NOT ALLOW electrons)
• metallic and ionic bonds stronger than London forces
• IGNORE 'carry' charge

Mark scheme

Examiner's comment

Many candidates got the second ionisation energy equation. Very few candidates got the correct state symbol on the lower line for Br2, with solid being the common response.

Notes from the mark scheme and report

• Mg2+(g) + 2Br(g) + 2e– and Mg(s) + Br2(l)
• state symbols required – bromine is a liquid

Mark scheme

Examiner's comment

OCR support Further support can be found in: The AS delivery guide ‘Theme: Identifying Unknowns’ https://www.ocr.org.uk/Images/208563-identifying-unknowns.pdf ‘Transition elements’: https://www.ocr.org.uk/qualifications/as-a-level-gce- chemistry-a-h032-h432-from-2015/delivery-guide/module-cam05-module-5- physical-chemistry-and-transition-elements/delivery-guide-cadg016- transition-elements The colours of the transition metal ions and complexes can be reviewed at: https://www.ocr.org.uk/Images/598991-colours-of-inorganic-ions-and- complexes-poster-.pdf Themes in candidate responses Calculation based questions Questions 16(d)(ii) and 16(d)(iii) This Born Haber cycle combined the traditional cycle with the enthalpy solution/hydration cycle.

Notes from the mark scheme and report

• –346.5 (–693 ÷ 2)
• 2 × atomisation and 2 × electron affinity of Br
• one sign or factor-of-2 error scores 1 (e.g. –693, +346.5, –509, –290.5)
• two errors score 0

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

Notes from the mark scheme and report

• Mg2+(g) + 2Br–(g) → MgBr2(s) with state symbols
• –2433
• ALLOW ECF from (d)(ii)

Skills: M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

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Notes from the mark scheme and report

• high pressure AND low temperature
• fewer gaseous moles on right
• forward reaction exothermic
• marks independent

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Notes from the mark scheme and report

• lower temperature/energy demand
• less fossil fuel burnt/less CO2
• ALLOW reactions with greater atom economy/less waste

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Notes from the mark scheme and report

• ∆S = –222 J K–1 mol–1 (2 × H2)
• convert to kJ or ∆H to J
• ∆G = +25.55 kJ mol–1 with units
• not feasible because ∆G > 0

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

Mark scheme

Examiner's comment

This was an unfamiliar expression linking ∆G and Kp. Many candidates correctly calculated this number. Some candidates calculated ln Kp as 10.0 but then put a – in front for the e calculation. The question required the answer to 3 significant figures. Higher-attaining candidates were able to work out the units as any pressure unit -2 .

Notes from the mark scheme and report

• ln Kp = –∆G/RT = 10.01
• Kp = 2.22 × 10^4 (3 sig figs)
• units atm–2 (or Pa–2 etc.)
• common errors 22400 (R = 8.31) and 4.50 × 10^–5 (sign) score 1

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.4 Use calculators to find and use power, exponential and logarithmic functions; M1.1 Use an appropriate number of significant figures; M2.2 Change the subject of an equation; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Notes from the mark scheme and report

• Mg + 2CH3COOH → (CH3COO)2Mg + H2
• Mg 0 to +2 oxidised
• H +1 to 0 reduced
• 1 mark for correct oxidation numbers wrongly linked to redox

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Notes from the mark scheme and report

• HCOOH is the stronger acid so donates: HCOO– + CH3COOH2+
• both charges required
• only ECF is pairs labelled for proton transfer the wrong way round

Mark scheme

Notes from the mark scheme and report

• mark is for working

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.2 Change the subject of an equation; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

This question required the candidate to calculate the original concentration of ethanoate ions in the buffer. Higher-attaining students gained full credit. Most students calculated the concentration in the buffer solution but did not factor for the original solution. Lower-attaining candidates often scored the first two marking points but did not use the buffer equation.

Notes from the mark scheme and report

• n(CH3COOH) = 0.0816 in 1 dm3
• × 1000/600 = 2.4 × 10^–2
• common errors 8.7 × 10^–3 (600/1000 inverted), 3.6 × 10^–6 (ratio inverted), 3.6 × 10^–2 (no volumes)
• ALLOW Henderson–Hasselbalch

Skills: M0.2 Use ratios, fractions and percentages; M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Most candidates drew a circuit containing a voltmeter and the silver half cell but very few candidates included the H+ in the MnO4-/Mn2+ cell.

Notes from the mark scheme and report

• voltmeter and labelled salt bridge
• Ag in 1 mol dm–3 Ag+
• Pt in H+/MnO4–/Mn2+ (1 mol dm–3 or equimolar)
• IGNORE pressure
• 1 mark for both half-cells if concentrations omitted

Skills: PAG8 Electrochemical cells

Mark scheme

Examiner's comment

Higher-attaining candidates described two oxidations starting from HCHO to end up with CO2. Many candidates used the data correctly but stopped at the first oxidation to form HCOOH. Lower-attaining candidates did not state that the direction of reaction of redox equilibria is dependent on the relative negativity/positivity of the standard electrode potentials. Some candidates are still using higher/lower to compare the E cell values, and should be encouraged to instead use the phrasing ‘more positive’ or ‘more negative’. Many candidates wrote a correct first equation, although some did not cancel down the H+ and/or water. Group 17

Notes from the mark scheme and report

• E of MnO4–/Mn2+ more positive than systems 1 and 2
• more negative system shifts left
• HCHO oxidised to HCOOH then to CO2 (the gas)
• two overall equations
• DO NOT ALLOW uncancelled species
• IGNORE higher/lower

Mark scheme

Notes from the mark scheme and report

• 2H+ + ½O2 + 2e– → H2O
• E = 1.34 + (–0.11) = +1.23 V

Skills: M2.2 Change the subject of an equation

Mark scheme

Examiner's comment

Most candidates used incorrect ideas about reaction going to completion or the methanol not being limiting.

Notes from the mark scheme and report

• IGNORE 'methanol does not run out/not limiting'

Skills: PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Very few candidates were given full marks. Higher-attaining students calculated one half life in range but very few could come up with a second half life as the graph did not allow another successive half life to be obtained. Higher-attaining candidates chose alternative half lives from the data given.

Notes from the mark scheme and report

• two half-lives shown on graph (102–110 min) AND statement that half-life is constant
• values on the graph.
• Further guidance on rates of reaction can be found at: https://www.ocr.org.uk/Images/371956-experiments-on-rates-of-reaction.doc

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question required the candidate to draw a line of best fit and then draw a tangent at t=0. Many candidates did not draw a line of best fit, and many did not get a tangent in the acceptable range. Very few candidates processed the gradient by using the correct subtraction on the y axis (scale was from 1 to 5) or by using the 10-3 on the axis label. Electrode potentials

Notes from the mark scheme and report

• tangent at t = 0 (gradient 2.9–4.0 × 10^–5) OR k = ln 2/t½ then rate = k × 5.0 × 10^–3
• note 10^–3 on y-axis
• ALLOW ECF from t½

Skills: M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; 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; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question asked the candidate to calculate Kc. Higher-attaining students tended to gain full marks. Some candidates made full use of tables (e.g. RICE: Reaction, Initial concentration, Change in concentration, Equilibrium concentration) which allowed for credit to be given through error carried forward. Some candidates did not use 0.03 as the change, and lower-attaining candidates did not use water in the Kc expression. Candidates should remember to provide written indications of what it is they are working out – presenting the calculations without any annotations can make it harder for error carried forward marks to be given if there is an error in their calculation.

Notes from the mark scheme and report

• initial moles 0.3 and 0.2
• 0.17 mol reacted so 0.13, 0.03, 0.17, 0.17 at equilibrium
• Kc = 7.4
• V cancels but expression must be correct

Skills: 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

This question required the candidate to explain the reactivity of the halogens given experimental observations. Higher-attaining candidates were able to explain the observations with ionic equations and explain the reactivity in terms of gaining electrons. Some candidates did not associate the colour with the halogen and linked it with the halide ion, but then did explain the trend in reactivity due to the ability to gain electrons. Lower-attaining candidates explained the reaction in terms of displacement (which was ignored) and they did not proceed with ionic equations or describe the ability to gain electrons. Key teaching and learning points – comments on improving performance Candidates could improve their performance in this examination by: • Practising the skills required to complete multiple choice questions. • Showing clearly structured working in their calculation questions so that methods are understood and followed. • Practising linking experimental observations with theoretical understanding to improve the quality of descriptions and explanations. • Making sure that they use the appropriate terminology. • Developing their ability to write ionic equations. • Improving their ability to process experimental results from graphical data. Guidance on using this paper as a mock This paper should be used in conjunction with H432/02 and H432/03. Candidates should sit the paper under examination conditions in the allotted time. The marked paper should be reviewed, in conjunction with the mark scheme, by the student to identify the terminology and structure required within the questions to achieve full credit. Internal moderation will add validity to the credit given. If any of your students’ results are not as expected, you may wish to consider one of our review of results services. For full information about the options available visit the OCR website. If university places are at stake you may wish to consider priority service 2 reviews of marking which have an earlier deadline to ensure your reviews are processed in time for university applications. Our priority is supporting you and your students this autumn and to support you as you prepare for summer 2021 exams. We’ll update our website information regularly with resources, guidance and key information. • Teachers • Students • Exams officers • Assessment specialists We are sending a weekly roundup to tell you about important updates. You can also sign up for your subject specific updates. If you haven’t already, sign up here. Attend one of our popular CPD courses to hear directly from a senior assessor or drop in to a Q&A session. All our courses for the academic year 2020-2021 are being delivered live via an online platform, so you can attend from any location. Please find details for all our courses on the relevant subject page on our website or visit OCR professional development. ExamBuilder is the question builder platform for a range of our GCSE, A Level, Cambridge Nationals, Cambridge Technicals and Functional Skills qualifications. See the full list of available qualifications in the sign up form. ExamBuilder is free for all OCR centres with an Interchange account and gives you unlimited users per centre. We need an Interchange username to validate the identity of your centre's first user account for ExamBuilder. If you do not have an Interchange account please contact your centre administrator (usually the Exams Officer) to request a username, or nominate an existing Interchange user in your department. Review of results

Notes from the mark scheme and report

• orange = bromine, no reaction with Cl–
• violet = iodine formed
• Br2 + 2I– → 2Br– + I2
• reactivity/oxidising power decreases down group: larger radius/more shielding, less nuclear attraction for gained electron
• DO NOT ALLOW losing electrons/ionisation energy
• IGNORE explanations in terms of displacement
• rather than electron affinity. The colour of the organic layer was also associated with the halide ion rather than the halogen.
• Further guidance can be found in the AS Level delivery guide ‘Theme: Patterns’ (Group 2 and Group 17): https://www.ocr.org.uk/Images/231740- patterns.pdf Links to the legacy coursework tasks and PAG practice question sets can be found on OCR Interchange.

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Notes from the mark scheme and report

• benefit AND risk needed for the one mark: kills bacteria
• toxic/forms chlorinated hydrocarbons
• IGNORE harmful/dangerous
• IGNORE 'chlorine is carcinogenic'

Mark scheme

Notes from the mark scheme and report

• n = 0.209/29.0 (given molar volume, not 24.0)
• Mr = 174.8
• BrF5
• formula dependent on Mr

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme, with the levels and indicative points

Notes from the mark scheme and report

• B = FeSO4, D = Fe(OH)2, G = BaSO4
• C = CrCl3, E = Cr(OH)3, F = [Cr(NH3)6]3+, H = AgCl
• ionic equations for D–H
• Level 3 needs at least six identified and equations mostly correct

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Notes from the mark scheme and report

• Ni : S : N = 1 : 4 : 8 so x = 4
• N count 2 + x + y = 8 so y = 2
• ALLOW ECF for y from incorrect x

Skills: M0.2 Use ratios, fractions and percentages; M2.4 Solve algebraic equations

Mark scheme

Notes from the mark scheme and report

• +2
• + sign required

Mark scheme

Examiner's comment

This question asked the candidate to determine the amount of sodium sulfite in food. Many candidates gained full marks. Most candidates calculated the number of moles of MnO4- and SO32-. Some candidates calculated the mass in 525 g of meat, although some used the wrong Mr, e.g. 80, for the sulfite ion. The lower ability candidates did not process the scaling to 1 Kg. Rates of reaction

Notes from the mark scheme and report

• n(MnO4–) × 2.5 for SO3 2–
• × 10 for 250 cm3
• × 126.1 = 0.397 g in 525 g
• scale to 1 kg = 757 mg which is below 850 mg so complies
• ALLOW ECF throughout

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