The halogens Module 3

Answer

C

Mark scheme

Examiner's comment

The correct answer was C. Candidates generally understood the intermolecular nature of this trend was due to London forces. The most common incorrect answer was D. It is important to make sure that candidates can appreciate that London forces can be more significant/stronger, as the molecule increases in size, compared to other intermolecular forces.

Mark scheme

Examiner's comment

This question required the candidates to know the observations of the reactions of group 7 and to recognise a change in colour to show the evidence of a chemical reaction. This question proved difficult for candidates with few being given all three marks. Colours were not well-known with I2 in aqueous solution often described as purple. Other incorrect answers included no colours stated, effervescence and descriptions of the formation of the halide precipitates. Several candidates gave an equation instead of an observation. The PAGs – section 1.2.1 (d) mentions accurate use of terminology to describe change instead of just final colours. A good number of candidates were able to provide the correct ionic equation, although several incorrectly included Na in their equations or wrote half equations. Occasionally candidates wrote more than one incorrect equation, often forming Cl2.

Notes from the mark scheme and report

• NaI: orange to brown (DO NOT ALLOW purple)
• NaCl: stays orange/no colour change (IGNORE 'no reaction'
• DO NOT ALLOW 'turns orange')
• Br2 + 2I– → 2Br– + I2
• IGNORE half-equations
• We have produced a teacher and delivery guide to assist with learning about the reaction of group 7 elements and their compounds: Teach Cambridge (ocr.org.uk) Links to the legacy coursework tasks and PAG practice question sets can be found on Teach Cambridge.
• Exam hints for students can be found on the OCR website.

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

This was generally a well answered question from most candidates. The definition requires the use of ‘element’ rather than the more general term ‘species’. Candidates often went on to identify N as the element. Less successful candidates identified H or O from the redox change. A misconception for a few was that an ‘atom’ was disproportionated without realising this would be impossible. Oxidation numbers were mostly correct and correct oxidation and reduction identified, either by naming compounds in the answer space or labelling them in the equation. Candidates would be advised to make sure that there is a clear link between the oxidation number and the element that it refers to.

Notes from the mark scheme and report

• oxidation and reduction of the same ELEMENT (IGNORE species)
• N +3 to +5 (in HNO3) AND +3 to +2 (in NO)
• oxidised to HNO3 and reduced to NO
• + signs required
• DO NOT ALLOW N3+, N5+, N2+

Mark scheme

Examiner's comment

Over half of candidates scored full marks on this question, clearly stating the definition of disproportionation. Where a response did not state ‘element’ in the definition, the candidate usually went on to gain the first mark by clearly stating that bromine is oxidised and reduced. Oxidation numbers were usually assigned correctly, but marks were missed by not linking changes to chemicals, or by not stating which change was oxidation and/or reduction. These marks could sometimes be given from working around the equation, but candidates should be encouraged to include these links in their written response.

Notes from the mark scheme and report

• 'atom' insufficient for element
• oxidation numbers need sign and number and must be linked to NaBrO and NaBr
• 1 of 2 redox marks if oxidation/reduction assigned the wrong way round
• IGNORE electron loss/gain

Mark scheme

Examiner's comment

This long response question was approached well and there were some excellent and concise answers. However, some candidates tried to link reactivity to electronegativity or confused the question with boiling point trends and London forces. Candidates are well-versed with tackling this type of question and most were able to write sensible explanations. However, marks could not be given for insufficiently specific responses, for example, repeating the idea of attracting an electron for the third marking point. Responses could be improved if candidates read through what they write to see if it makes cohesive sense. There was a fairly even spread of marks obtained, with 2 marks given most frequently.

Notes from the mark scheme and report

• comparison required throughout
• IGNORE 'nuclear charge' for nuclear attraction and 'effective nuclear charge'
• IGNORE fewer orbitals/sub-shells
• IGNORE electronegativity

Mark scheme

Examiner's comment

This question was generally well answered, although a few candidates put an advantage down instead such as ‘kills harmful bacteria’.

Notes from the mark scheme and report

• IGNORE 'harmful/dangerous'
• IGNORE chlorine is carcinogenic
• IGNORE forms HCl

Mark scheme

Examiner's comment

This question was a very good discriminator. Most candidates knew that silver nitrate should be added to the solutions and knew the colour of precipitates produced. Some candidates did not score the second mark by only giving colours and not stating they were solids. Many candidates were also able to give a correct ionic equation, with only the highest scoring candidates explicitly stating it was the AgX precipitates which had different solubilities in ammonia. A small number of candidates mistook this for a question on halogen displacement, scoring no marks. naming the solution and its result. Ionic equations are given (although only one is needed) and relative solubilities of the precipitates in ammonia are stated. This response was given the full 4 marks.

Notes from the mark scheme and report

• AgNO3
• any two precipitate colours
• ionic equation
• AgCl dissolves in dilute NH3, AgBr in concentrated NH3, AgI insoluble (must be clear the precipitate dissolves)
• HCl/H2SO4 or BaCl2 is a CON (IGNORE HNO3)

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Candidates produced a range of answers, and the question discriminated extremely well. The higher attaining candidates often gave 6 or 7 correct formulae. The flowchart contained many clues which most candidates used. Most were able to provide at least 3-4 of the required formulae and there was no consistently wrong answer. Most candidates gave H+ and Cl– as two of the three ions, and AgCl for the white precipitate with Ag+(aq). Both S and S8 were allowed for sulfur but S2 was not allowed. As SO42– ions are present, Ag2SO4 was also allowed for the precipitate with Ag+(aq). Common errors included acid formulae such as H2SO4 in place of water, an incorrect formula for BaSO4, giving BaSO3, writing SO4– for the sulfate ion, and OH– as one of the three ‘acidic’ ions. Lower attaining candidates added formulae without using information in the question. Examples include full formulae such as HCl in the box labelled ‘ions’ and CuCl42– for the yellow solid, despite Cu being absent in the flowchart.

Notes from the mark scheme and report

• all responses must be formulae: AgCl (ALLOW Ag2SO4), BaSO4, three ions from H+, Cl–, SO42– (ALLOW ClO–, HSO4–), S (ALLOW S8
• DO NOT ALLOW S2 or 'sulfur'), H2O

Skills: PAG4 Qualitative analysis of ions

Answer

C

Mark scheme

Examiner's comment

The correct answer was C. This question was answered well, alongside Q2. Candidates should aim to separate explanations regarding chemical and physical properties. B was a common wrong answer and a few candidates suggested that the covalent bonds need to be broken, selecting A.

Answer

D

Mark scheme

Examiner's comment

The correct answer was D. This proved a more challenging question. Successful candidates often presented oxidation numbers above the equations to identify the element that was simultaneously oxidised and reduced. Most candidates recognised that A and B could be ruled out, with C being the most common error.

Answer

B

Mark scheme

Examiner's comment

Overall, candidates had little difficulty with this question and most selected B. Options C and D were the main distractors, with few choosing option A. The question did not discriminate well across the ability range, suggesting that many candidates guessed.

Answer

A

Mark scheme

Examiner's comment

About half of the candidates chose A correctly. Most candidates wrote oxidation numbers below the chlorine in the equations, which is good practice, with C proving to be the main distractor. Note also the point made in Question 6 about underlining the word ‘not’.

Answer

D

Mark scheme

Examiner's comment

The correct answer was D. Most candidates were able to select this response, but the common error was the selection of A. It is important that candidates can distinguish the difference between oxidation states and charge on the ions. Oxidation state is the measure of the number of electrons that an atom uses to bond with atoms of another element.

Mark scheme

Examiner's comment

Candidates performed well on this question with most knowing that Cl2 kills the bacteria. Some stated that chlorine removed bacteria or purified the water.

Notes from the mark scheme and report

• kills bacteria/microorganisms
• IGNORE 'removing' bacteria

Mark scheme

Examiner's comment

Most students scored the equation mark, although some presented unbalanced equations. Nearly all candidates used the ionic equation. Explanations were well argued with most candidates using the order of reactivity. A few considered the oxidising power of the halogen. Candidates are advised to ensure that both comparisons are clearly made, and it is obvious which of the two halogens the response is referring to. It is also important that candidates can distinguish between a halogen and a halide. Some candidates explained in terms of electronegativity and displacement.

Notes from the mark scheme and report

• ionic equation required (DO NOT ALLOW full equation)
• compare reactivity/oxidising strength of Cl2, Br2 and I2
• care with -ide/-ine
• We have produced a teacher and delivery guide to assist with learning about the reaction of group 7 elements and their compounds: Teach Cambridge (ocr.org.uk)

Mark scheme

Examiner's comment

Generally, this question was well answered with a clear understanding of how and why ions are formed. However, approximately a quarter of students only gained 1 mark as they either didn’t explain electrons being lost by Mg and gained by Cl or gave no justification. A common slip was stating Cl has one electron in its outer shell. Some described bonding between Mg and Cl, which wasn’t what the question asked, but this didn’t prevent them from scoring both marks.

Notes from the mark scheme and report

• Mg loses 2 electrons AND Cl gains 1
• to obtain a full outer shell/noble gas configuration

Mark scheme

Examiner's comment

Candidates were required to write a straightforward ionic equation that they would have encountered many times during the A Level Chemistry course. It was surprising that only just over half the candidates produced an equation that could be given. Common errors included the following. • Omission of state symbols or incorrect state symbols, especially (aq) in AgCl(s). • Inclusion of nitrate ions or use of AgNO3 instead of Ag+. • An equation using Cl2 and forming AgCl2. Some candidates used the ideal gas equation to determine the moles of hydrogen, choosing suitable values for temperature and pressure. This approach was allowed, although the exercise would have wasted candidate time compared to the much simpler division by 24 for using RTP, which is stated in the question.

Notes from the mark scheme and report

• ALL 3 state symbols required

Answer

B

Mark scheme

Examiner's comment

Almost all candidates selected the correct response of B. The main distractor proved to be D, the other type of dipole–dipole interaction.

Answer

D

Mark scheme

Examiner's comment

Most candidates answered this correctly with D. Errors came from candidates not realising HCl provided Cl- ions that would react with AgNO3 and therefore they did not recognise the formation of the white precipitate.

Mark scheme

Examiner's comment

Another tricky question with less than half gaining credit. Many were able to recognise the addition of silver nitrate as the test for halide ions but did not realise that it could be used quantitatively. Many didn’t read the question carefully and assumed Na2SO4 was still present, giving a mixture of two precipitates. Some, despite recognising the formation of AgCl, could not then see how to calculate the mass of NaCl i.e. “you won’t have formation of BaSO4”. Some suggested that barium nitrate would also form a precipitate, perhaps confused by the (s) state symbol in the question.

Notes from the mark scheme and report

• AgCl precipitate would form
• mass of NaCl calculated from mass/moles of AgCl
• IGNORE incorrect formula of silver chloride

Skills: PAG4 Qualitative analysis of ions

Mark scheme, with the levels and indicative points

Examiner's comment

Very few candidates managed to access the higher level on this question. They found the calculation very challenging. In addition to a typical mole calculation, they needed to use the tonnes conversion (given on the data sheet), look at the ratio of I2 to KI and use a mass concentration. Many used the mass concentration as a molar concentration. Even if a balanced equation was given, it didn’t guarantee the correct ratio in calculations and a common error was to find moles of I2 using the relative atomic mass of iodine (126.9). As with previous calculations on this paper, it was often challenging to follow the logic in the steps of the calculation. It is important to encourage candidates to label each step of their calculation, showing them as independent steps and only using = when terms are equal. Explanations of relative reactivities of chlorine and iodine often lacked detail or had extra irrelevant information, such as reference to electronegativity, ionisation energy, bond enthalpy or intermolecular forces. Some candidates gave responses for the general trend in reactivity in the group rather than comparing Cl and I. Confusion around nuclear charge and nuclear attraction was also often seen. A key skill for longer answer questions is to plan out answers to avoid contradictions. Candidates should also be encouraged to re-read their answers to check that what they have written makes sense. This exemplar demonstrates a L3 6 mark response. A comprehensive explanation of relative reactivities of chlorine and iodine is given. The steps in the calculation are logical with each step labelled, making it easy to follow. A balanced equation is also given.

Notes from the mark scheme and report

• comparison required throughout
• IGNORE 'nuclear charge' for nuclear attraction
• 2:1 ratio I–:I2
• n(I2) = 1 × 10^6/253.8
• acceptable errors: 126.9 used for I2 (1.74 × 10^7) or missing × 2 (4.36 × 10^6), not both for L3

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

Mark scheme

Examiner's comment

Most were able to explain the term disproportionation. Some missed the mark by not stating an element or chlorine. A very common error was giving final oxidation numbers of Cl as +2 and/or -2, rather than per atom. The link between oxidation number and species was not always clearly indicated or changes not specified as oxidation/reduction (or given as the wrong way round). It is vital to set out answer s clearly showing oxidation numbers, species and stating if oxidised or reduced. It is not enough to write on the equation given in the question as it often challenging to read these numbers, or they contradict the main answer. Some attempted to show that Ca has been disproportionated.

Notes from the mark scheme and report

• oxidation AND reduction of the same element
• 0 to +1 in Ca(OCl)2 and 0 to –1 in CaCl2
• IGNORE 'species'/'reactant' for element
• 1 mark for three correct oxidation numbers without oxidation/reduction stated

Mark scheme

Examiner's comment

The majority of candidates scored both marks. Some scored 1 mark only mainly for correct colours of the precipitates but with incorrect formulae. Common errors included AgBr2 and AgI2, giving the formula of the haloalkane or formulae involving a halogen and a nitrate ion.

Notes from the mark scheme and report

• AgBr cream, AgI yellow: formula AND colour for each mark
• 1 mark if both formulae OR both colours correct

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Although most candidates did choose F, a substantial number chose Br (most common), K and H, seemingly confused between gaining and losing electrons in forming ions.

Notes from the mark scheme and report

• F

Answer

D

Mark scheme

Examiner's comment

This question proved to be difficult, with only the most able candidates selecting the correct answer of D. A was often given as an incorrect answer as candidates recognised that AgCl would be the only halide precipitate to show a change with dilute ammonia but did not realise that as it would redissolve, it would be the only one not in the filtrate.

Notes from the mark scheme and report

• AgCl dissolves in dilute NH3
• AgBr and AgI do not

Skills: PAG4 Qualitative analysis of ions

Answer

D

Mark scheme

Examiner's comment

This question was direct recall of specification content and most candidates selected D as the correct answer.

Mark scheme

Examiner's comment

Candidates answered this question well and most were given the mark. Where candidates didn’t receive credit, it was mainly because they used the term ‘same atom’ instead of ‘same element’. Some less successful responses responded with completely incorrect chemistry and had clearly not learnt this specification content.

Notes from the mark scheme and report

• 'atom' is insufficient for element
• IGNORE 'species'

Mark scheme

Examiner's comment

Candidates found the equation hard, despite this reaction being specification content and the inclusion in the earlier part of the stem of ‘NaClO’ as one product. The correct response required candidates to realise that NaCl would be a product and to balance the resulting equation. Some did not add the balancing ‘2’ before NaOH, and many selected HCl as the second product, a compound that would react further with NaOH to produce NaCl. The explanation worked the same whether NaCl or HCl had been identified as the second product. There were some excellent responses, providing the correct oxidation number changes, linking these to the species involved and identifying the changes as either oxidation or reduction. Two explanation marks were available with marks not being given for omission of one of the three features described above. This exemplar has been included to emphasise the points made above. It was only possible to award this response 1/3 marks. The equation shows the common error of the second chlorine-containing product being HCl and not NaCl: 0 marks The candidate has identified the oxidation number changes and has linked these to the correct species. The last statement in brackets is correct but the candidate has not communicated which oxidation number change is oxidation and which is reduction: 1/2 marks

Notes from the mark scheme and report

• DO NOT ALLOW Cl2 + NaOH → NaClO + HCl
• signs required on oxidation numbers
• 1 of 2 redox marks if changes correct but oxidation/reduction assigned the wrong way round
• IGNORE electron loss/gain

Mark scheme

Examiner's comment

Candidates generally answered the first part of this question well. Most candidates were able to identify silver nitrate (or a halogen displacement method), to describe the expected observations, supported with mainly correct ionic equations. Candidates found it much harder to identify B and C as NaBr and CaCl2. They could do this in various ways by matching possible formula with the provided molar mass ranges. The mark scheme did allow marks to be given when candidates described the identification process, although this was often very muddled, so, only the most able few candidates fully identified the unknown B and C.

Notes from the mark scheme and report

• AgNO3 then any two precipitate colours
• ionic equation for at least one halide
• B = NaBr and C = CaCl2 from molar mass range
• HCl CONs AgNO3 (IGNORE HNO3)
• IGNORE solubility in NH3
• ALLOW halogen displacement alternative

Skills: M0.2 Use ratios, fractions and percentages; PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Candidates were required to write a balanced equation from provided reactants and products and to use oxidation numbers to illustrate disproportionation. In the equation, the reactants and products were sometimes unbalanced, or incorrectly balanced. A common error was to balance the equation after first adding O2 as an extra reactant. Illustrating disproportionation proved to be easier with the oxidation number changes from the initial +1 being required. Otherwise, more successful responses sometimes missed out on marks if they omitted detail. For example, the oxidation number changes were stated but the candidate omitted to state which change was oxidation and which was reduction. The best responses identified Ca(ClO3)2 as the oxidation product and CaCl2 as the reduction product. One unexpected error on many scripts was for calcium to be identified as the element undergoing disproportionation, with oxidation number changes from +6 to +14 and +2. It was difficult to see why this happened, with Ca forming +2 compounds, but the error was common.

Notes from the mark scheme and report

• 3Ca(ClO)2 → 2CaCl2 + Ca(ClO3)2
• Cl reduced +1 to –1 and Cl oxidised +1 to +5
• 1 of 2 redox marks if changes correct but oxidation/reduction mis-assigned or not stated
• signs required
• IGNORE ionic charges such as Cl5+

Mark scheme

Examiner's comment

Almost all candidates realised that Cl– ions would react with the added AgNO3 at time = t1.

Notes from the mark scheme and report

• Cl– reacts with Ag+ to form AgCl (precipitate)
• IGNORE chlorine for chloride

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

Answer

D

Mark scheme

Mark scheme

Notes from the mark scheme and report

• Mn oxidised from +6 to +7 AND reduced from +6 to +4
• signs required
• 1 mark if numbers correct but not linked to oxidation/reduction

Mark scheme

Notes from the mark scheme and report

• toxic or forms chlorinated hydrocarbons
• IGNORE 'harmful'/'dangerous'
• IGNORE chlorine is carcinogenic

Mark scheme

Notes from the mark scheme and report

• mix each halogen with a different halide
• Cl2 + bromide orange, Br2 + iodide violet, I2 no change
• ionic equation
• IGNORE addition of halogen to same halide

Skills: PAG4 Qualitative analysis of ions

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, with the levels and indicative points

Examiner's comment

1(a) A considerable number of candidates did not score the mark here despite knowing that it was an oxidation reaction. They either omitted to give the number of electrons lost or gave their justification using oxidation numbers. This question highlights the importance of reading the question carefully and answering the question asked, not one that may have been seen previously. 1(b)(i) Most candidates were able to make some progress with this calculation. Common errors included omitting to multiply by two to find the number of moles of HCl or not multiplying their volume in dm3 by 1000 to give an answer in cm3. 1(b)(ii) More candidates struggled with this part of calculation. A significant number of candidates used the number of moles rounded to 3 SF but then gave a final answer to 4 SF, giving an answer of 78.48 cm3. This was given credit as correct calculation had been carried out for one mark. A couple of candidates correctly used the ideal gas equation and were given credit for this, but made it more complicated than necessary for one mark. 1(b)(iii) Very few candidates managed to gain marks here. Most did not recognise that HCl was the limiting reagent in the reaction and suggested that a greater volume of H2 would be produced, as same mass of Mg would give a greater number of moles. 1(c)* Very few candidates managed to score full marks for this question. Even the highest-attaining candidates struggled with writing balanced chemical equations. The most successful candidates used ionic equations with state symbols in their responses. A large proportion of candidates gave unnecessary details such as testing for CO2 using limewater or the colours of other silver halide precipitates. The best responses broken down their response to cover each test in turn, giving clear and concise details for each.

Notes from the mark scheme and report

• acid then fizzing for carbonate
• Ba2+(aq) white precipitate for sulfate
• AgNO3(aq) cream precipitate for bromide
• sequence carbonate then sulfate then halide
• equations ionic or full, IGNORE state symbols

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

3(a) More able candidates scored well in this question, setting out their answers in a logical order. They often first discussed Ca and its bonding and structure, linking this to the physical properties and then doing the same for Br. A number of candidates discussed the chemical properties of Ca and Br, such as their ability to bond with other elements, ionisation energy and their reactivity based on their position in the periodic table. Some candidates gave a good description of metallic bonding but then went on to discuss melting point in terms of intermolecular forces. 3(b)(i) Most candidates were able to give the correct diagrams for ionic bonding, although care needs to be taken that diagrams are well drawn with both charges given. Some gave diagrams for covalent bonding. 3(b)(ii) It was important to answer the question asked. A number of responses lost marks for describing the general trend down group 2 without making reference at all to calcium and barium. Most candidates managed to score at least one mark here but a considerable proportion missed the second marking point explaining that nuclear attraction was less in Ba. 3(c)(i) This was dependent on candidates’ ability to give correct formula of aluminium hydroxide, Al(OH)3. Writing correct formulae is an important skill in chemistry, which should be practised regularly in different topics to help candidates master the skill. 3(c)(ii) Most candidates were able to recognise that water had the higher boiling point due to hydrogen bonding. However, it was evident in a number of responses that they were unclear that this is an example of an intermolecular force and doesn’t just refer to the covalent bond formed between O and H. Most avoided discussing the intermolecular forces present in H2Se even although both London forces and permanent dipole-dipole interactions were accepted as they may not have come across this specific example and had not been given electronegativity data. 3(d)(i) Very few candidates scored this mark. Although a number of candidates did give sodium bromate as the answer (with the omission of the oxidation state), many other answers were seen suggesting candidates are not aware of naming conventions for inorganic compounds. 3(d)(ii) This is the first time in a reformed chemistry AS paper that the question space has been left unstructured for oxidation number changes. The highest-attaining candidates set out their responses clearly, dealing with changes for oxidation and reduction separately, and giving the correct oxidation numbers. Some struggled to obtain an oxidation state of Br in NaBrO3 as +5, suggesting +1 instead.

Notes from the mark scheme and report

• same element both oxidised (0 to +5) and reduced (0 to –1)
• 1 mark if all three oxidation numbers correct but oxidation/reduction mis-assigned

Answer

D

Mark scheme

Answer

C

Mark scheme

Examiner's comment

This question proved to be difficult, with only the most able candidates selecting the correct answer of C. Many candidates confused the trends in oxidising power of the halogens with trends in the reducing power of the halide ions. Down the group, the oxidising power of the halogens decreases but the reducing power of the halide ions increases. Option A is incorrect because iodine is a weaker oxidising agent than bromine, option B (the most common incorrect answer) is incorrect because chlorine is not a reducing agent (it’s an oxidising agent) and option D is incorrect because the chloride ion is not an oxidising agent (it’s a reducing agent).

Answer

D

Mark scheme

Examiner's comment

Despite most scripts being covered with annotations of oxidation numbers, only about half of all candidates obtained the correct answer of D.. Option B was the most common incorrect response, followed by option C. The annotations on these scripts often showed incorrect assignments of oxidation numbers.

Mark scheme

Examiner's comment

This question discriminated well. Candidates were expected to link increasing boiling point down the group with the increasing number of electrons and stronger induced dipole–dipole forces or London forces between molecules that must be broken. To be given all 3 marks, candidates needed to show that intermolecular forces (or forces between molecules) are broken during boiling. This mark proved to be the most difficult. The response shows a typical response that did not include intermolecular forces. Lower attaining candidates often let themselves down by being unable to construct a well-reasoned, relevant response. Many discussed nuclear charge, shielding, atomic radius and the ease of losing or gaining an electron to produce a valid explanation for ionisation energy or electron gain trend but not for different boiling points. The contrast in the clarity of low- and high-attaining candidate responses was particularly pronounced for this question.

Notes from the mark scheme and report

• IGNORE van der Waals/vdW and abbreviations (LDF, IDID)
• IGNORE 'more shells'/shielding
• breaking covalent bonds is a CON for the last mark
• needs idea of forces between molecules

Mark scheme

Examiner's comment

This question was best discriminator of the paper and rewarded the well-prepared candidates who were competent in writing equations. Most candidates were given the 2 marks for the bromide test with silver nitrate and the related equation (usually shown ionically). Many found the test for the ammonium ion more challenging to describe. The alkaline nature of ammonia was well-known and the indicator colour change to blue was often seen. Many candidates omitted the NaOH reagent and tested the compound with indicator, thinking that the ammonium ion itself is alkaline. Few candidates were able to write the equation for the ammonium test. Lower attaining students often outlined electrolysis as a test and many candidates wrote about the carbonate and sulfate tests prior to the halide test.

Notes from the mark scheme and report

• NaOH (NOT ammonia) and heat, damp indicator paper turns blue
• AgNO3 gives cream precipitate
• IGNORE nitric acid
• ionic or full equations allowed
• ALLOW displacement by Cl2 giving orange solution (DO NOT ALLOW precipitate)

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Most candidates realised that as the group is descended, each molecule has more electrons, resulting in stronger London forces (or greater induced dipole-dipole interactions), which require greater energy to overcome. Common errors included describing the intermolecular forces as van der Waals’ forces or permanent dipoles; stating that more energy is needed to overcome bonds (there are covalent bonds in these molecules) having previously used the term forces or interactions; and relating boiling point to reactivity based upon ease of electron loss due to differences in nuclear attraction.

Notes from the mark scheme and report

• London forces increase down the group
• more electrons
• more energy to break the intermolecular forces
• DO NOT ALLOW covalent bonds break
• DO NOT ALLOW hydrogen bonds or permanent dipole interactions
• IGNORE van der Waals'

Mark scheme

Examiner's comment

Most candidates recognised that silver nitrate and chloride ions react together to form a white precipitate, but many did not make the link between this observation and whether any silver nitrate was left unreacted. Many candidates did not give a correct equation, with missing or incorrect state symbols being common. This question discriminated extremely well.

Notes from the mark scheme and report

• state symbols required in equation
• observation must be linked to conclusion (white precipitate = not all reacted)

Skills: PAG4 Qualitative analysis of ions

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

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 chose the correct response of chlorine, although hydrogen was a common incorrect response, presumably by linking to the acidic properties of H+ ions. Other candidates focused on ‘reacts with water’ and chose sodium (which does form a solution with water, but on that is alkaline rather than acidic).

Notes from the mark scheme and report

• chlorine OR fluorine

Answer

D

Mark scheme

Answer

D

Mark scheme

Answer

D

Mark scheme

Mark scheme

Notes from the mark scheme and report

• same element oxidised and reduced
• Cl +5 to +7 and +5 to –1
• signs required
• IGNORE ionic charges such as Cl5+
• 1 redox mark if numbers correct but oxidation/reduction wrongly assigned

Mark scheme

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols
• IGNORE explanation in terms of electronegativity

Mark scheme

Notes from the mark scheme and report

• benefit AND risk both needed for the mark
• IGNORE 'harmful'/'dangerous'
• IGNORE 'chlorine is carcinogenic'

Mark scheme

Notes from the mark scheme and report

• ALLOW Ag+
• IGNORE acidified/HNO3

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Notes from the mark scheme and report

• all three colours (white, cream, yellow) required for the mark

Skills: PAG4 Qualitative analysis of ions