Exam practice mark scheme C2: Discovering chemistry

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1 Exam practice mark scheme C: Discovering chemistry Foundation Tier (a)(i) Photo chlorine Any one correct for one mark Photo iodine Photo bromine Allow only one line from each photo and to each name Group 7 Allow halogens Fluorine / astatine Allow symbols (b)(i) Potassium + water potassium hydroxide + hydrogen Allow fully balanced symbol equation (solution is) alkaline Allow contains hydroxide ions C potassium ions surrounded by a sea of electrons Atoms have full outer shells So do not need to gain / lose / share electrons total 9 (a)(i) Nucleus Electron (b) C have the same number of protons Group Period (i) Magnesium oxide Mg(s) + O (g) MgO(s) Three correct formulae ( mark) Balancing correct formulae ( mark) State symbols ( mark) 4 + = 40 total 0 (a)(i) 9 C Exothermic (b)(i) (Conical) flask Reject other kinds of flask Effervescence Allow fizzing, bubbles etc. Chip gets smaller Allow chip disappears / dissolves Carbon dioxide / gas Escapes from flask (iv) Smaller chips have faster reaction Greater surface area Allows more frequent collisions total 0 Pearson Education 0. Edexcel GCSE Additional Science Teacher and Technician Planning Guide

2 Item Expected answers / 4(a) A charged atom / charged group of atoms / an atom that has gained or lost electron(s) Allow answer that only refers to a positive or negative ion e.g. atom that has lost an electron (b)(i) C BaSO 4 Barium sulfate is insoluble so does not get absorbed into bloodstream Allow does not get absorbed into body Hold sample in flame Allow any suitable method e.g. wire loop / damp splint Flame is lilac Allow purple Flame test: Advantages simple to perform / apparatus inexpensive Disadvantage red colour makes it difficult to distinguish from other similar elements / red colour may be obscured by more dominant colours Spectroscopy: Advantages definitive answer Disadvantages more difficult to carry out / more expensive apparatus - Might mention only one method and how this would work e.g. describes how a flame test would show caesium. Spelling may be poor and does not use specialist terms. -4 Mentions at least an advantage or disadvantage of both methods. Reasonable use of English and uses appropriate chemical terminology. 5- Evaluates the methods by comparing them and coming to a conclusion e.g. a flame test is better because it is simple and quick or spectroscopy is better because it gives a certain answer. Uses fluent communication skills and correct terminology. total Item Expected answers / 5(a)(i) Carbon Covalent (b)(i) Allow one mark for two correct O-H bonds Second mark for two lone pairs Four unbonded electrons do not need to be paired H O = 8 Give both marks for correct answer. Allow any number of significant figures. 00 = 89% 8 Sodium chloride: (soluble and) conducts so ionic High melting and boiling points so giant structure / lattice / crystal Hexane: (insoluble and) doesn t conduct so covalent Low melting and boiling point so molecular Silicon(IV) oxide: (insoluble and) doesn t conduct so covalent High melting and boiling point so giant - Identifies structure or bonding of at least compounds; or both bonding and structure for compound. Explanations may be limited. Language may be everyday. -4 Identifies and explains bonding and structure of or compounds; or identifies bonding and structure for all compounds with no explanation. Correct terminology used. 5- Identifies structure and bonding of all three compounds with well explained reasoning. Data in table used effectively. Uses detailed scientific terms. total Pearson Education 0. Edexcel GCSE Additional Science Teacher and Technician Planning Guide

3 Higher Tier (a) Fractional distillation ( mark) of liquid air ( mark) (b) D low melting point (i) Shared pair of electrons.00 C atoms combine with.7 O atoms 0. C atoms combine with 0. O atoms Formula = CO Can be all dots or all crosses 4 electrons must be shown to be shared between the two oxygen atoms 4 electrons shared ( from each atom) ( mark) Non-bonding electrons ( mark) Division of masses by relative atomic mass ( mark) Correct whole number ration and formula of oxide ( mark) Or by some other suitable method total 8 (a) Sodium atom loses an electron Chlorine atom gains the electron How the sodium atom becomes an ion ( mark) How the chlorine atom becomes an ion ( mark) (b)(i) B Na SO 4 Add dilute hydrochloric acid (to the sodium sulphate solution) Add barium chloride solution (to the sodium sulphate solution) White precipitate forms Accept correct formulae in place of chemical names Both colour and form are needed for the mark Strong electrostatic forces (i) ( )g NaCl 78g PbCl ( mark) g NaCl =.7 g PbCl ( mark) 58.5 Not all the sodium chloride solution reacts / insufficient lead nitrate solution added / some of the product lost when transferring between containers / lead chloride not completely insoluble Alternative calculation methods include: 0 NaCl ( mark) Mass PbCl = 78 =.7 g 58 ( mark) marks can be obtained from correct answer with no working Accept any one valid suggestion total 0 (a) Nucleus / protons and neutrons Accept nucleus of atom (b) particle charge proton negative neutron positive electron neutral Number of neutrons = mass number atomic number or 0 = Any two correct lines ( mark) Allow only line from each particle and to each charge Pearson Education 0. Edexcel GCSE Additional Science Teacher and Technician Planning Guide

4 (i).8 Do not accept a diagram Needs 0 electrons to balance the charge of 0 protons in the nucleus (e)(i) Different number of neutrons / one isotope has more neutrons than the other Allow,8 or -8 (0 90.9) ( 9.) Allow marks for correct answer of 0. with no working mark for (0 x 90.9) + ( x 9.) mark for division by 00 mark for correct answer total 0 Item Expected answers / 4(a)(i) Steeper line than graph A ( mark) Levels off at 95 cm earlier than A ( mark) All the magnesium has been used up Not the acid, as this was in excess Decreased temperature: decreases energy of the particles decreased frequency of collision decreased number of successful collisions Reverse argument in terms of increased temperature: increases energy of particles increased frequency of collision increased number of successful collisions each point mark, mark independently (b) : Polluting gases from the engine unburnt hydrocarbons, carbon monoxide, nitrogen oxides. Emitted gases from the converter carbon dioxide, nitrogen and water. Effect of the catalyst as well as the high surface area over which the catalyst is covered. The catalytic converter is near the engine so the temperature is high which speeds up the reactions. Reactions involved include: NO N + O CO + O CO - Knowledge of the polluting gases and gases coming out of the catalytic converter seen. Answer contains some credit but lacks any organisation or logical thought. Spelling punctuation and grammar used with limited accuracy. -4 Indication of a higher surface area on the catalyst seen as well as knowledge of the gases involved. Some technical terms used and shows some clarity and organisation. Spelling, punctuation and grams used with reasonable accuracy. 5- Knowledge of the gases involved as well as the operating conditions of the catalytic converter. Some chemical equations for the reactions involved. Good use of technical terms is made and answer shows clarity of thought. Spelling, punctuation and grammar used with considerable accuracy. total Pearson Education 0. Edexcel GCSE Additional Science Teacher and Technician Planning Guide

5 Item Expected answers / 5 (a) C their boiling points are usually high (b)(i) Reactions show that chlorine is more reactive than both bromine and iodine Further reaction needed between bromine water and sodium iodide solution / iodine solution and sodium bromide solution mark for each point Fe + Cl FeCl Correct formulae of reactants and products ( mark) Correct balancing ( mark) They all have full outer electron shells Allow: all the electron shells are full / they cannot lose or gain electrons : Trends within group correctly described softness, melting points, chemical behaviour towards water. Chemical behaviour with water all produce the hydroxide and hydrogen gas. Patterns indicate similarity and explanation of reactivity in terms of electron arrangements and ease of losing outermost electron. - Some trends identified correctly e.g. softness and melting point. Answer contains some credit but lacks any organisation or logical thought. Spelling punctuation and grammar used with limited accuracy. -4 Trends of physical properties identified as well as chemical behaviour towards water described. Some technical terms used and shows some clarity and organisation. Spelling, punctuation and grams used with reasonable accuracy. 5- Both trends and chemical behaviour towards water described. Explanation of behaviour in terms of electron arrangements and loss of outermost shell. Good use of technical terms is made and answer shows clarity of thought. Spelling, punctuation and grammar used with considerable accuracy. total Pearson Education 0. Edexcel GCSE Additional Science Teacher and Technician Planning Guide

(i) The atomic number of an atom is the number of... (1) (ii) The mass number of an atom is the number of (1)

This question is about atomic structure and elements. (a) Complete the sentences. (i) The atomic number of an atom is the number of... () The mass number of an atom is the number of... () (b) Explain why