Aim, Underlying chemistry and Experimental approach. Candidate A. mark awarded Comments. Max Mark. Section. 1 Aim

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1 Aim, Underlying chemistry and Experimental approach Candidate A 1 Aim An aim that describes clearly the purpose of the 2 Underlying chemistry An account of chemistry relevant to the aim of the Max Mark mark awarded 1 0 The aim does not clearly state the purpose of the 3 1 The candidate has demonstrated a limited understanding of chemistry relevant to the aim of the Correct chemistry relevant to the aim includes: molecular formula of vitamin C mention of atmospheric oxidation of vitamin C attempt to define what is meant by the term antioxidant However, there are errors in chemistry which are key to the investigation: (a) A brief summary of the approach used to collect incorrect ion-electron equation for oxidation of vitamin C incorrect structural formula for dehydroascorbic acid incomplete word/formula equation for titration reaction statement that vitamin C contains antioxidants statement that free radical scavengers steal electrons from cellular structures 1 0 The candidate has not demonstrated the ability to summarise the method used, as a diagram on its own is insufficient. The candidate has identified a specific safety measure associated with working with dilute iodine solutions in this SQA 1 of 10

2 Candidate B 1 Aim An aim that describes clearly the purpose of the 2 Underlying chemistry An account of chemistry relevant to the aim of the (a) A brief summary of the approach used to collect Max Mark mark awarded 1 1 The aim clearly describes the purpose of the 3 3 A good understanding of the underlying chemistry, at a depth appropriate to Higher level, is given. Correct chemistry relevant to the aim includes: use of sulfur dioxide and sulphites to prevent bacterial or fungal growth use to prevent oxidation (by inhibiting enzymes) the oxidation of ethanol, in wines, to form ethanal and ethanoic acid definition of oxidation balanced equation for the redox reaction occurring in the titration description of colour change observed when iodine reacts with starch stoichiometry of titration reaction 1 1 By describing the use of sodium hydroxide and sulphuric acid and the iodine titration, the candidate has provided sufficient detail to allow the nature of this multistep experiment to be visualised by a marker. Precautions necessitated by the use of sulfuric acid and sodium hydroxide solutions in this experiment are described. SQA 2 of 10

3 Candidate C 1 Aim An aim that describes clearly the purpose of the 2 Underlying chemistry An account of chemistry relevant to the aim of the Max Mark mark awarded 1 1 The aim clearly describes the purpose of the 3 2 The candidate has demonstrated a reasonable understanding of chemistry relevant to the aim of the Correct chemistry relevant to the aim includes: structural formulae for three alcohols description of viscosity names for different types of intermolecular forces mention of the fact an uneven distribution of electrons within an individual atom can result in the creation of a temporary dipole correct electronegativity difference for O-H However, the candidate has made some errors in statements: (a) A brief summary of the approach used to collect incorrect term - London dispersal forces incorrect label temporary dipole in diagram of London dispersion forces statement implying that permanent dipolepermanent dipole forces exist whenever there is a difference in electronegativities between atoms incorrect statement that hydrogen bonding only occurs with the molecules N-H, O-H and F-H 1 1 The candidate has provided sufficient detail to allow the nature of this experiment to be visualised by a marker. By describing the need to keep the alcohols used in this experiment away from flame, the candidate has identified a safety measure specific for this The correct procedure for filling a burette is not a safety measure specific to this SQA 3 of 10

4 Candidate D 1 Aim An aim that describes clearly the purpose of the 2 Underlying chemistry An account of chemistry relevant to the aim of the Max Mark mark awarded 1 1 The aim clearly describes the purpose of the 3 0 The candidate has demonstrated no understanding of the chemistry relevant to their aim at a depth appropriate to Higher level. The stated aim of the assignment is to determine the % sucrose (q/v) in a variety of alcopops with the method used being the measurement of solution density. Although the candidate has written many pages of underlying chemistry, little is relevant to this aim. There is no chemical information regarding sucrose, nor any explanation of how density could be expected to relate to the concentration of a solute. Chemistry that is not relevant to the aim includes: uses of alcohols not related to alcopops classification of alcohols oxidation of alcohols formation of esters fats and oils structure of glycerol (a) A brief summary of the approach used to collect 1 0 The candidate has demonstrated the ability to summarise the method. However, the safety considerations given are general safety measures. No additional safety measures specific to the experiment are given. SQA 4 of 10

5 Candidate E 1 Aim An aim that describes clearly the purpose of the 2 Underlying chemistry An account of chemistry relevant to the aim of the Max Mark mark awarded 1 1 The aim clearly describes the purpose of the 3 3 A good understanding of the underlying chemistry, at a depth appropriate to Higher level, is given. Correct chemistry relevant to the aim includes: vinegar contains ethanoic acid, a carboxylic acid general formula of carboxylic acids full structural formula for ethanoic acid neutralisation reactions of acids balanced equation for the titration reaction Although the experimental procedure does not involve the oxidation of ethanol, the ethanoic acid present in vinegar can be produced by oxidation, so also relevant are: (a) A brief summary of the approach used to collect the description of the production of ethanoic acid by oxidation of ethanol the definition of oxidation 1 0 The candidate has demonstrated the ability to summarise the method, however no comment regarding safety has been made. SQA 5 of 10

6 Data collection and Graphical presentation Candidate A Max mark Mark awarded (b) Sufficient raw data from the candidate s (c) Data presented in correctly produced table(s) and correct units shown for calculated values. (d) Values calculated correctly using a chemical 4 Graphical presentation (a) An appropriate format from the options of scatter graph, line graph or bar graph. (b) The axis/axes of the graph has/have suitable scale(s). (c) The axes of the graph have suitable labels and units. (d) Accurately plotted data points and, where appropriate, a line of best fit. 1 1 Three different juices is a suitable range of values and the candidate has repeated the titrations. 1 0 In the results tables, the candidate has not listed initial and final burette readings clearly, and the text iodine level is ambiguous. In addition, the units have been omitted from the iodine used row in the Apple (juice 2) table. For these reasons, the mark cannot be awarded. The final values calculated from experimental data do have correct units ( mg per 25 ml and mg per 100 ml ). 1 1 Chemical calculations involving the relationship between moles, concentration and volume, and the relationship between moles and gram formula mass have been correctly carried out. In all cases, the candidate has discarded the rough titre, and has calculated average titres for results concordant to 0 2 cm As the graph is showing the vitamin C content for different types of juice, a bar graph is an appropriate format. 1 1 A suitable linear scale for the y-axis has been chosen allowing the plotting of the bars to be readily checked. 1 0 The y-axis label incorrectly states 'average volume of vitamin C when the values plotted are average concentrations of vitamin C. 1 0 The bar for grapefruit has been drawn to show a value of 45 8 when the true value is SQA 6 of 10

7 Candidate B Max mark Mark awarded (b) Sufficient raw data from the candidate s (c) Data presented in correctly produced table(s) and correct units shown for calculated values. (d) Values calculated correctly using a chemical 4 Graphical presentation (a) An appropriate format from the options of scatter graph, line graph or bar graph. (b) The axis/axes of the graph has/have suitable scale(s). (c) The axes of the graph have suitable labels and units. (d) Accurately plotted data points and, where appropriate, a line of best fit. 1 1 Three different vinegars is an acceptable range of values and the candidate has repeated the titrations. 1 0 The candidate has tabulated the experimental results in tables with appropriate headings and units. However, the concentration values calculated from experimental data do not have correct units. 1 1 Chemical calculations involving the relationship between moles, concentration and volume, and the relationship between moles and gram formula mass have been correctly carried out. In all cases, the candidate calculated average titres for results concordant to 0 2 cm As the graph is showing the ethanoic acid content for different types of vinegar, a bar graph is an appropriate format. 1 0 The scale for the y-axis is not linear as the major gridline value of 0 6 has been missed. 1 0 The y-axis has no label or units and the x-axis does not have a category label. 1 0 The bar for Sarsons Classic Malt pickling shows a value of 1 07 when the true value is 1 08 (1 0765). The bar for Tesco Malt shows a value of 0 86 when the true value is 0 87 (0 8665). SQA 7 of 10

8 Candidate C Max mark Mark awarded (b) Sufficient raw data from the candidate s (c) Data presented in correctly produced table(s) and correct units shown for calculated values. (d) Values calculated correctly using a chemical 4 Graphical presentation (a) An appropriate format from the options of scatter graph, line graph or bar graph. (b) The axis/axes of the graph has/have suitable scale(s). (c) The axes of the graph have suitable labels and units. (d) Accurately plotted data points and, where appropriate, a line of best fit. 1 1 Five different temperatures evenly spaced from 30 o C to 70 o C is an acceptable range of values to use. The measurement of rate has been repeated at each temperature. 1 1 The experimental data and the values calculated from the measured values, namely the average reaction times and relative rates, are tabulated with acceptable column headings and units. 1 1 The chemical relationship between relative rate and reaction time has been correctly applied, and both the average time and relative rate values have been calculated correctly. 1 1 As the graph has been plotted to establish the relationship between two numerical variables, relative rate and temperature, a scatter graph is an appropriate format. 1 1 The graph has been drawn to a suitable size that allows the accuracy of plotting of the data points to be checked. Both axes have acceptable linear scales. 1 1 Both axes have appropriate labels and units. 1 1 The values have all been plotted accurately and an appropriate curved best-fit line has been drawn that illustrates the trend observed in the results. SQA 8 of 10

9 Candidate D Max mark Mark awarded (b) Sufficient raw data from the candidate s (c) Data presented in correctly produced table(s) and correct units shown for calculated values. (d) Values calculated correctly using a chemical 4 Graphical presentation (a) An appropriate format from the options of scatter graph, line graph or bar graph. (b) The axis/axes of the graph has/have suitable scale(s). (c) The axes of the graph have suitable labels and units. (d) Accurately plotted data points and, where appropriate, a line of best fit. 1 1 Four temperatures were used over the range 0 o C to 90 o C. This is an acceptable range of values. The time taken for a ball bearing to fall has been measured five times at each temperature. 1 0 The second table, showing the calculated rate at which the ball bearings travel, does not have a unit for temperature in the first column heading. 1 0 No values have been calculated using a chemical The calculation of the speed at which an object travels is not a chemical calculation. 1 1 As the graph has been plotted to establish the relationship between two numerical variables, speed and temperature, a scatter graph is an appropriate format. 1 1 The graph has been drawn to a suitable size that allows the accuracy of plotting of the data points to be checked. Both axes have acceptable linear scales. 1 1 Both axes have appropriate labels and units. 1 0 The value for 0 o C has been plotted at 1 25 cm s -1 when the actual value is 1 53 cm s -1. The value for 23 o C has been plotted at 4 05 cm s -1 when the actual value is 4 57 cm s -1. An appropriate best-fit straight line has been drawn that illustrates the trend observed in the results. SQA 9 of 10

10 Candidate E Max mark Mark awarded (b) Sufficient raw data from the candidate s (c) Data presented in correctly produced table(s) and correct units shown for calculated values. (d) Values calculated correctly using a chemical 4 Graphical presentation (a) An appropriate format from the options of scatter graph, line graph or bar graph. (b) The axis/axes of the graph has/have suitable scale(s). (c) The axes of the graph have suitable labels and units. (d) Accurately plotted data points and, where appropriate, a line of best fit. 1 1 The candidate has collected sufficient raw data to verify Hess s law and has repeated each reaction. 1 1 The experimental data and calculated values have been presented in correctly produced tables with column headings and units. Appropriate units are stated for the final values calculated from 1 1 The candidate has carried out chemical calculations using the relationship E h = cmδt and the relationship between mass and the gram formula mass. All arithmetic is correct, and averages have been correctly calculated. 1 1 The graph produced is based on the candidate s A bar graph is used which is an appropriate format for this data. 1 1 The graph has been drawn to a suitable size that allows the accuracy of plotting of the bars to be checked. The y-axis is inverted, however as the scale is linear, and the values clearly show the direction in which the values are changing, this is an acceptable scale. 1 1 The axes are labelled appropriately, with the stacked bars for route 2 identified by additional labels. 1 1 The heights of the bars are plotted correctly, within a half-box tolerance. SQA 10 of 10