8303235-67-Why-Students-Lose-Marks-1-Redox

8303235-67-Why-Students-Lose-Marks-1-Redox - Chem Factsheet

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Why Students Lose Marks: AS Redox Questions 1 This Factsheet analyses students’ real answers to exam questions on reduction and oxidation. By the end of this Factsheet, you should be more confident about: What the examiners want The kinds of things you are likely to be asked Common mistakes and misunderstandings As you read the students' answers to the questions and the comments, try to work out what the student should have done - using the hints and comments if necessary - before you read the markscheme. Number 67 C hem F actsheet www.curriculum-press.co.uk What do you have to know? In this type of question, the examiner is trying to assess whether you can: recognise examples of redox reactions, oxidising and reducing agents calculate oxidation numbers usingthe standard rules write half-equations for standard redox conversions write half-equations for new reactions, given appropriate information combine half equations (a) Oxidation used to be defined as ‘combination with oxygen’. Explain why, even though the definition has now been broadened, it is still generally true to say that combination with oxygen is oxidation. ................................................................................................................................................................................................................................. ............................................................................................................................................................................................................................ [2] (b) When oxygen reacts with fluorine, converting it into oxygen difluoride, OF 2 , does the fluorine become oxidised? Explain your answer. ................................................................................................................................................................................................................................. ............................................................................................................................................................................................................................ [2] No, fluorine always has an oxidation state of -1 in its compounds Because oxygen is an effective oxidising agent - it takes electrons away from other elements easily Although this answer is not on the markscheme, 1 mark awarded for the idea that "oxygen takes electrons away from other elements easily", which demonstrates an understanding of oxidation ! " Although the rule quoted for fluorine is true, a rule is not an explanation - the student should have referred to fluorine being the most electronegative element - or to it being more electronegative than oxygen Hints and Comments Although it's vital to know the rules for assigning oxidation numbers, they are not explanations - you need to understand why fluorine always has an oxidation number of -1 in compounds, oxygen always -2 except with fluorine or in peroxides/superoxides etc. Unless a previous part of the question has asked for a definition of oxidation - or told you it! - there are likely to be marks available for showing that you understand oxidation is loss of electrons or increase in oxidation number. Markscheme (a) Oxygen is a highly electronegative element (1) so the other element generally loses electrons and is oxidised (1) (b) Fluorine is not oxidised (1) because it is more electronegative than oxygen/ is the most electronegative element (1) TiO 2
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8303235-67-Why-Students-Lose-Marks-1-Redox - Chem Factsheet

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