Kirkcaldy High School - Chemistry Higher Assignment Pupil Guide. Antioxidants

Transcription

1 Kirkcaldy High School - Chemistry Higher Assignment Pupil Guide Name: Class: Antioxidants This assignment is worth 20% of the overall marks for the Higher course assessment. The course is graded A-D. In this assignment, you will have to investigate a relevant topic in chemistry and communicate your research in a report. The topic must have an application with an effect on the environment/society. Your report must be completed independently. The assignment has two stages research stage communication stage 1

2 Guidance on Producing your Report Your report should have: Appropriate structure with an informative title and headings where necessary Be clear and concise Aim The aim must describe clearly the question you are trying to answer. Introduction and Underlying Chemistry Here you must describe the uses of antioxidants. You should explain the underlying chemistry of antioxidants. You should include the definition of a fuel as well as formulae, chemical equations and calculations. The Chemistry must be at Higher level. Research Data/Information Here you must present two pieces of raw data/information taken unchanged from two sources that is relevant to your investigation. One of these pieces of data must be from the experiment you have carried out. You must also process these two pieces of data. This can include, for example, performing calculations, and plotting graphs from tables. One of your pieces of processed data must be a bar chart or line graph. It must be clear where the raw data that you processed came from. For example you could: Include raw data from an experiment/practical activity Include tables, graphs, diagrams, text taken from your sources Explain clearly where the data/information came from (reference your sources) On your bar chart/line graph. Check that you have included, as appropriate: Suitable scales Units Headings Labels You should also compare and analyse the data from your sources. For example, describe how they agree/disagree or describe other similarities/differences. You must comment on any trends and the value of the trend (i.e. twice as much, three times as much etc.). Risk Assessment You must include a risk assessment for the experiment you carried out. This cannot be simply safety glasses were worn and hair was tied back. You should comment on specific risks from the chemicals you used and how you reduced that risk. Conclusion You must clearly state the conclusion(s) of your investigation. Your conclusion(s) must answer every point in your aim and be supported by what the data from your sources. Evaluation You must evaluate your report, commenting on: robustness of findings validity of sources 2

3 reliability of data/information evaluation of (experimental) procedure References At the end of your report you must record the sources you have used, with enough detail to allow someone else to find them. If one of the sources is an experiment/practical activity, then you need to include the title and the aim. If you are including a reference, give the full web address including You must explain your choice of sources in terms of the following: Relevance: how useful they are for your topic? Reliability of sources: who wrote them? Who published them? Before submitting your report check you have completed the checklist at the back of this booklet to ensure you have included everything. 3

4 Antioxidants An antioxidant is a molecule that inhibits the oxidation of other molecules. It could be said that antioxidants sacrifice themselves in order to prevent other substances from being oxidised. Oxidation can happen due to the creation of free radicals atoms or molecules with unpaired electrons. Free radicals react according to three processes. These are: Initiation: Where free radicals are created from non-radical species Propagation: Where free radicals react and produce further free radicals (chain reaction) Termination: Where free radicals combine to make non-radical species For example, free radicals can be created in a sample of hydrogen and chlorine gasses using a flash light source. This explosive reaction occurs as follows: Initiation H2 2H Cl2 2Cl Propagation H + Cl2 HCl + Cl Cl + H2 HCl + H Termination 2H H2 2Cl Cl2 Overall H2+Cl2 HCl Free radicals can be created in human skin due to exposure to ultra-violet light (sunlight). Because of this it is often said that eating antioxidant-rich foods or by using antioxidant rich skin cream is good for you and can help to slow down the aging process. Common antioxidants are vitamin C, vitamin E and β-carotene. vitamin E β-carotene vitamin C In order to test the antioxidant levels in a number of substances, you will titrate solutions with iodine. Say for example, the antioxidant in the substance is vitamin C (ascorbic acid). It oxidises to dehydroascorbic acid as follows. 4

5 C6H8O(aq) C6H6O6(aq) This can be balanced to a full redox equation by adding 2 x H + and 2 x e - on the right hand side thus showing oxidation (loss of electrons): C6H8O(aq) C6H6O6(aq) + 2H + (aq) +2e - The corresponding reduction of iodine is: I2 2I - This can be balanced to a full redox equation by adding 2 x e- on the left hand side thus showing reduction (gain of electrons): I2(aq) + 2e - 2I - (aq) These two half equations for oxidation and reduction can then be combined and the electrons cancelled: C6H8O(aq) C6H6O6(aq) + 2H + (aq) + 2e - I2(aq) + 2e - 2I - (aq) C6H8O(aq) + I2(aq) C6H6O6(aq) + 2H + (aq) + 2I - (aq) Using starch as an indicator, the endpoint is reached when the solution turns blue/black i.e. the iodine is no longer reacting. Using this equation and an experimental result, the quantity of antioxidants in a sample could be calculated as follows. Say that a 100 cm 3 solution of 100 g of apple was made and 20 cm 3 of this solution reacted with 15 cm 3 of moll -1 iodine solution to reach the endpoint: C6H8O(aq) + I2(aq) C6H6O6(aq) + 2H + (aq) + 2I - (aq) N C (moll -1 ) V (l) niodine = nvit C = C x V = x = mol = mmol Therefore in 100 cm 3 of the solution there are x 5 mmol = mmol. So there are mmol of antioxidant per 100 g of apple. Your investigation Your aim is to answer the question Which foods contain the greatest quantities of antioxidants? 5

6 Practical This assignment has a practical associate with it. You should write this up separately to your final report but you must include raw and processed data from the practical in your final report. You should reference your experiment with the aim and title of the experiment. Apparatus Practical Which foods contain the greatest quantities of antioxidants? 1 burette and stand 100 ml or 200 ml standard flask 20 ml pipette 10 ml and 100 ml measuring cylinders 250 ml conical flasks Balance Starch indicator solution Iodine solution (0.005 moll -1 and moll -1 ) Fruit/veg./tea etc. Method 1. Cut a sample into small pieces and grind in a mortar and pestle or blend in a blender. 2. Add 10 cm 3 portions of distilled water several times while grinding the sample, each time decanting off the liquid extract into a 100 ml volumetric flask. 3. Strain the ground fruit/vegetable pulp through cheese cloth, rinsing the pulp with a few 10 cm 3 portions of water and collecting all filtrate and washings in the standard flask. 4. Make the extracted solution up to 100 cm 3 or 250 cm 3 with distilled water. 5. Pipette a 20 cm 3 aliquot of the sample solution into a 250 cm 3 conical flask and add about 150 cm 3 of distilled water and 1 cm 3 of starch indicator solution. 6. Titrate the sample with iodine solution. The endpoint of the titration is identified as the first permanent trace of a dark blue-black colour due to the starch-iodine complex. If the titration volume is too large (greater than 30 cm 3 or less than 4 cm 3 ) you may need to use an alternative concentration of iodine solution. 7. Repeat the titration with further aliquots of sample solution until you obtain concordant results (titres agreeing within 0.2 cm 3 ) with thanks! 6

7 Experiment Log Assessment Standard Evidence required Evidence produced 1.1 Planning an experiment or practical investigation Aim of experiment Dependent and independent variables Variable to be kept constant Measurements and observations are made Resources Method including safety 1.2 Following procedures safely Procedures have been followed safely and correctly 1.3 Making and recording observations and measurements accurately 1.4 Presenting results in an appropriate format 1.5 Drawing valid conclusions Observations and measurements taken are correct Results have been presented in an appropriate format What the experiment shows, with reference to the aim 1.6 Evaluating experimental procedures The suggestion given will improve the experiment 7

8 Aim 1 mark Research worth 6 marks total Category Selection of data / information Final Report Log Sub-category describe clearly what you are investigating. Show raw data from two, or more, sources. - only one of these can be an experiment. (this can be made in advance and used in the final write up) Make it clear where the data comes from Processing information Process the data from at least two sources this can include processing data you found into graphs or charts or diagrams (such as converting tables to line graph. table into pie chart or calculate averages from data you found or summarising text that you found Experiment Risk Assessment 1 mark Underlying chemistry 4 marks Analyse data/inform ation Conclusion 1 mark Evaluation 3 marks Presentation Presenting information Risk assess your experiment How the underlying chemistry relates to your topic. The information that you processed should be presented in two different formats: Summary Graph One must be a graph, table, chart or Table diagram Chart Diagram Check that you have included, as appropriate: Format 1 Format 2 Suitable scales Units Headings Labels This cannot be simply safety glasses were worn and hair was tied back. You should comment on specific risks from the chemicals you used and how you reduced that risk. Include the chemical principles involved. Have one or more of Formulae & or chemical equations Structures Calculations Data is analysed Interpreting data included in the report (which may/may not have been processed by the candidate) to identify relationships. This may include further calculations. Your conclusion(s) relates to aim Evaluate your report Appropriate presentation Conclusion is supported by what you have found out. Answer your original aim completely. Comment on: robustness of findings validity of sources reliability of data/information evaluation of (experimental) procedure 1 mark for each of: appropriate title and structure the references to at least two sources used in the report are given in sufficient detail to allow them to be retrieved by a third party to reference the experiment/practical activity, then the title and the aim should be recorded 8