Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
THE CHEMISTRY HANDBOOK A2-LEVEL NAME:________________________ TEACHER:__________________ WHERE WILL IT TAKE YOU? ______________________________ CONTENTS PAGE Introduction The Chemistry A-Level Commitment 3 4 Assessment Information & Record of Achievement Internal Assessment Record of achievement - End of unit tests & Internal mock exams Record of Achievement - Homework External Assessment information 6 7 8 9 General Information Mathematical Requirements Strategies for reaching and exceeding your target grade Homework 11 12 13 Contact with your teacher 13 Tools Reading Lists Television / DVD / iplayer Phone Apps Internet Definitions Graphs Table of common ions and molecules Properties of common acids/alkalis/bases Equations Balanced equations Practical Techniques Practical Observations Practical Accuracy (experimental error) Significant Figures Yields Hazards /precautions GCE Chemistry Data Sheet The Periodic Table of the Elements 15 15 15 16 16 17 18 19 20 20 22 23 24 26 27 28 30 31 2 Dr. mravetz Introduction Welcome back to the Helsby High School Chemistry A-level course. This handbook is aimed at providing you with a reference source for some common queries on some common subjects. Keep it handy for the duration of your course and refer to it often. To compliment this hand book there are also a series of ‘How to…’ booklets available on Moodle. If you are stuck on anything then refer to these booklets, and if you have any improvements that you can suggest email me at [email protected]. The specification that you will be following is the AQA GCE (AS & A Level) Chemistry specification. You will be given the specifications for the units of the course by your teachers, keep them in your file so you can refer to them in lessons and at home. The specifications tell you what the exam board expects you to know and do. You can also find the specification on the AQA website. You have studied Chem 1 and Chem 2 for AS-level and you will study Chem 4 and Chem 5 for Alevel. You will also be completing a practical element of the course which is contained within Chem 6 (A2). Any theory that you have learnt in Chem 1, 2, 4 & 5 may be needed/used/tested within this practical unit. Come to the edge. We might fall. Come to the edge. It’s too high! Come to the edge! And they came, And he pushed, And they flew…. Christopher Logue, ‘Come to the Edge’, 1969 3 Dr. mravetz The Chemistry A-level commitment Chemistry A-level is a highly sought after commodity by individuals, universities and employers. In addition to teaching you a particular branch of science it helps you develop your transferable skills such as problem solving, analysis, evaluation and communication skills. With Chemistry you can enter medicine, law, banking, office based work, engineering, architecture and of course work in the Chemical industry in a variety of careers. But as with anything worth having in life you need to work at it. Chemistry A-level requires a commitment - if you commit you will succeed. The commitment you make when choosing the Alevel Chemistry course is as follows; To commit to 9 hours per fortnight of independent study most of which will be directed by your teacher in the form of homework. To commit to using all the resources at your disposal while completing all your homework to the best of your ability. To commit to handing your homework in before or on the deadline given by your teacher. To come to lessons fully equipped including pens, pencils, rulers, erasers, calculator, text book, file, paper, planner, lab coat and a periodic table. To keep an organised and complete file of notes, work completed & homework that is brought to every lesson. To attend every lesson and arrive on time and ready to work. To revise fully and effectively for every internal unit test & mock exam as well as for all external exams. I have read and understood the commitment necessary to succeed at A-level Chemistry. Signed: __________________________ date:___________________ 4 Dr. mravetz ASSESSMENT INFORMATION & RECORD OF ACHIEVEMENT 5 Dr. mravetz Internal Assessment Between September and May you will be assessed within each module (Chem 4 & 5) through a series of standard homework tasks, end of unit tests and mock exams. The result of these assessments will contribute to your attainment level which will be reported to you at regular intervals but particularly at consultation evenings and in written reports. Your reported attainment level is aimed at giving you an indication of what level you are working at and whether you are on or above target. It will also be used to inform the grades reported on your UCAS application form. The assessment tasks within the academic year are; Chem 4: o o o 2 x mock exams 5 x end of unit tests 5 x standard homework tasks o o o 2 x mock exams 4 x end of unit tests 4 x standard homework tasks o 2 x standard ISA homework tasks Chem 5: ISA: You will also be given normal homework tasks which are aimed at helping you learn. Each assessment task will be given a different weighting when applied to the calculation of your overall attainment level for each module and your overall attainment. 6 Dr. mravetz Record of achievement - End of unit tests & Internal mock exams My Target Level is: _________ It is very important that you take all tests and exams seriously and revise well. These tests will closely match what you will need to do in the external A-level exams. From the results you will be able to identify areas you need to work on – this could be knowledge based or exam technique. Record the details of all your internal assessments in the table below. Date Subject of end of unit test or internal exam Mark & grade CHEM 4 Carbonyls Aromatics & Acylation Amines, Amino Acids & Polymers Rates & Equilibrium pH CHEM 5 Periodicity Redox Thermodynamics Transition Metals MOCKS Mock 1 Mock 2 7 Dr. mravetz Possible improvements Record of Achievement - Homework My Target Level is:________ It is very important that you put the maximum amount of effort and thought into your homework. It is part of the learning process. Use the table below to record details of your standard homework tasks and attainment. Date Task Mark & grade CHEM 4 Carbonyls Organic Synthesis Structure Determination Rates & Equilibrium pH CHEM 5 Redox Thermodynamics Transition Metals – Complexes & Aqueous Chemistry Transition Metals – Redox Reactions & Catalysts 8 Dr. mravetz Possible improvements External Assessment information You have completed the Chem 1 and Chem 2 papers, if you think a resit would help your overall attainment then discuss the appropriate time to take the resit with your teacher. Chem 4 and Chem 5 papers will be taken in the June of Year 13. For Unit 6 the school follows the CHM6T route. The PSAs will be completed during normal practical lessons (the best six, each worth 2 raw marks, will be submitted to the exam board). There will be two opportunities in Year 13 to complete the ISA (only one will be submitted to the exam board). The ISAs consist of a 2 hour practical exam followed within a week by a 1 hour written exam and are usual held in the Jan/Feb & April/May of each year. Record the date of your external exams in the table below. ISA 1 ISA 2 Chem 4 Chem 5 9 Dr. mravetz GENERAL INFORMATION 10 Dr. mravetz Mathematical Requirements 11 Dr. mravetz Strategies for reaching and exceeding your target grade The following are a list of strategies proven to help students reach and exceed their target grade: 1. Review work done in class by reading around the subject. Look up web sites on the internet and read for understanding. Read (for understanding) your notes and text book. If it doesn’t make sense keep researching and working through the problem until it does. 2. Complete all homework set to a high standard. a. To do this you must first understand the area of chemistry it relates to (by looking at notes and reading around the subject), b. Secondly try the homework questions using the text book and notes to help you, c. Thirdly check your answers, d. Finally review your answers prior to handing it in. In this whole process make sure you focus and concentrate on the work completely (no music, TV or chatting), correct any mistakes. Check : units; calculations; rearranging of equations; inputs into calculators; use of words e.g. is it molecules or ions?; definitions; state symbols; chemical equations; charges; is it logical?; do you contradict yourself?; does it actually answer the question?; does it cover all the mark points?; have you written down all relevant working out?; Have you made all numerical substitution into equations clear?; Have you written down all relevant information or have you summarised it too much?; Have you used full sentences? 3. If you have finished work set then devise your own study e.g. practise exam questions which can be found in the text book, on Moodle and on the AQA website – www.aqa.org.uk/qualifications/a-level/chemistry-key-materials 4. Memorise important information such as definitions, equations, sequences, examples. 5. Organise your file. 6. Organise your life by organising your work time and play time so your play time does not affect your work time. 7. Organise your study time so you make the most of it. 8. Learn definitions off by heart. 9. Don’t study in the break out space – you are wasting your time! Study in the library or the other quiet study areas for Year 12 & 13. 10. Be strict with your work ethic at school. You should be working between 9am and 3.05pm. The only time you should be taking a break from work in school is between 11am to 11.15am and 1.20pm to 2pm. This means you need to use your study periods productively. 11. Draw up mind maps & spider diagrams for each topic. Make brief notes using your text book and internet searches. 12 Dr. mravetz 12. Keep reviewing and revising past work – you will need it. 13. Read for understanding again! Make connections. If anything doesn’t make sense don’t leave it until tomorrow, try to understand it today by concentrating on trying to understand it. Read for understanding again! 14. You must spend at least 9 hours a fortnight outside lessons on Chemistry homework & review. 15. Review marked homework. Analyse to understand why you got things right and why you missed marks. Don’t make the same mistake twice! Homework You are expected to complete homework set by the deadline given. Follow the advice given in step 2 in the section ‘Strategies for reaching and exceeding your target grade’ as well as the advice below. Do Start the homework the day you are given it Using all the resources available, including text books and notes, complete and review at least twice more before handing it in Leave time to be able to work on any problems you encounter Arrange to see your teacher at least 2 days before the homework is due if you are having problems Attempt all homework questions Do Not Go to your teacher the day the homework is due and tell them you couldn’t do it Hand in incomplete HW Fail to hand in the HW on time Contact with your teacher You have 9 lessons of Chemistry a fortnight in which you can ask anything. If you are struggling outside of class, although it is always better to try and solve a problem yourself (even if it takes you hours – you will learn from the experience) then ask because if you are having problems then you can bet other people are having problems and your teacher will either go over the relevant area again in class or arrange a separate tutorial at lunch time. BUT: before asking your teacher will expect you to have had a go at the problem and be able to tell them what you have done to try and solve the problem yourself. 13 Dr. mravetz TOOLS 14 Dr. mravetz Reading List Core text books (provided by the school on loan) 1. AQA Chemistry AS, Nelson Thornes (for Year 12) 2. AQA Chemsitry A2, Nelson Thornes (for Year 13) Support material (Available for purchase at various outlets such as Amazon) 3. Student support material for AQA AS Chemistry Unit 1: Foundation Chemstry 4. Student support material for AQA AS Chemistry Unit 2: Chemistry in Action 5. Student support material for AQA AS Chemistry Unit 4: Kinetics, Equilibria and Organic Chemistry (Year 13) 6. Student support material for AQA AS Chemistry Unit 5: Energetics, Redox and Inorganic Chemstry (Year 13) 7. There are many revision books available, choose one that is appropriate to your learning style. Ensure it is for the AQA syllabus being studied. Other useful books (available to use in the Chemistry Dept.) 8. Calculations in AS/A Level Chemistry, Jim Clark, Longman 9. Advanced Level AS Chemistry for AQA, J. Atkinson & C. Hibbert, Heinemann 10. Advanced Level A2 Chemistry for AQA, J. Atkinson & C. Hibbert, Heinemann Other – for reading around the subject 11. Bill Bryson: A short history of everything 12. Periodic Tales, Hugh Aldersey-Williams 13. The Elements – A very Short Introduction, Philip Ball 14. Molecules – A very Short Introduction, Philip Ball 15. Molecules at an Exhibition, John Emsley 16. The Periodic Kingdom, Peter Atkins Television / DVD / iplayer Watching scientific television programs related to Chemistry is sometimes a more enjoyable way of broadening your Chemistry knowledge. Watch out for science related programs or try to get hold of the following TV & radio programs. Possible TV programs to watch out for: Bang goes the theory Brainiac (a bit of fun!) Past TV programs which are now on DVD, BBC website, iplayer or even Youtube: Chemistry: A Volatile History with Prof. Jim Al-Kahlili Earth: The power of the Planet with Dr. Iain Stewart Radio programs: Radio 4 – Material world APPs for phones Look out for any mobile phone apps. There are many periodic table and Chemistry quiz question apps available for free. 15 Dr. mravetz Internet 1. Moodle You can access Moodle from any computer within school or outside of school. If you go to the school website there is a link to the Moodle log in page. Your login details are the same as your normal login details for accessing the school system. On Moodle there are various resources that will be helpful to you over the course of the year such as the Standard homework you will be set & past papers. 2. www.aqa.org.uk/qualifications/a-level/chemistry-key-materials The AQA board website is where you will find the course specifications, examiners reports, past paper questions & answers. 3. The following are a list of websites that contain information to help you with homework, review work or revision. I have not vetted all of these web sites so when using them read for understanding – if they don’t make sense then check the chemistry. www.s-cool.co.uk/a-level/chemistry www.chemguide.co.uk alevelchem.com www.docbrown.info www.chembook.co.uk 4. Kerboodle It is hoped that the department will be offering access to the Kerboodle learning package. You will be told your password and login details which you should keep safe. Centre ID:_____________________ Username:_____________________ Password:_____________________ Definitions It is essential that you learn all the definitions you come across (and there really is NO substitute to just memorizing them). The text books you are provided with have a glossary of definitions at the back of the book. Use the table below to record any that you want to have quick reference to. It is very important in Chemistry to be precise in your definitions. Unit Word Definition 16 Dr. mravetz Graphs Graphs are easy to get full marks on if you follow these pointers. The plotted points must take up at least half a page. Make sure the plotted points are accurate – you will get marked on accuracy. Complete the axis / points / lines of best fit in sharp pencil Make sure the axis are drawn on the major lines of the graph paper, not between or coming off the lines Label all axis and give the graph an overall title Put units onto the axis. Check the units e.g. should it be J (Joules) or kJ (kilojoules) Use sensible and equally spaced numerical graduations on the axis Know how to draw (and practise) lines of best fit If asked to extrapolate a graph continue the line of best fit to the point specified Make the lines of best fit a singularly drawn line not sketched or thick Should the line of best fit go through the origin? If so draw it through the origin. (you will have to use your scientific knowledge to determine this and will be able to do this by gaining experience through the course). 17 Dr. mravetz Table of common ions and molecules You will need to know these common ions so you must memorise them. You will use them throughout your Chemistry A-level. There are gaps in the table so you can add to the ions as you learn more. Positive ions H+ Li+ K+ Mg2+ Ca2+ Zn2+ Cu2+ Fe2+ Co2+ Al3+ Fe3+ Cr3+ NH4+ Pb2+ Negative ions FClBrIO2S2OHNO3NO2HCO3CO32SO42CrO42Cr2O72MnO4C2O42- Hydrogen ion Lithium ion Potassium ion Magnesium ion Calcium ion Zinc ion Copper (II) ion Iron (II) ion Cobalt ion Aluminium ion Iron (III) ion Chromium ion Ammonium ion Lead ion Fluoride ion Chloride ion Bromide ion Iodide ion Oxide ion Sulfide ion Hydroxide ion Nitrate (V) ion Nitrate (III) ion Hydrogencarbonate ion Carbonate ion Sulphate ion Chromate (VI) ion Dichromate (VI) ion Manganate (VII) ion Ethandioate ion You will need to know these common molecules so you must memorise them. You will use them throughout your Chemistry A-level. Formula H2 F2 Cl2 Br2 I2 O2 N2 CH4 NH3 SO2 NO NO2 CO CO2 Name Hydrogen Fluorine Chlorine Bromine Iodine Oxygen Nitrogen Methane Ammonia Sulphur dioxide Nitrogen monoxide Nitrogen dioxide Carbon monoxide Carbon dioxide 18 Dr. mravetz Properties of common acids/alkalis/bases Acids Bases Slippery to touch pH > 7 turns red litmus blue turns phenolphthalein pink Turn methyl orange to yellow When soluble in water contain OH- ions Sour pH < 7 turns blue litmus paper red corrosive Turn methyl orange to red Contain H+ ions when in solution Alkali: Has two possible definitions 1) A basic (as in it is a base) salt of an alkali metal (Group 1) or alkaline earth metal (Group 2). 2) A base that dissolves in water General chemical reactions of acids Acid + Metal → Salt + Hydrogen e.g. 2HCl + Mg → MgCl2 + H2 Acid + Metal oxide → Salt + Water e.g. 2HNO3 + MgO → Mg(NO3)2 + H2O Acid + Metal hydroxide → Salt + Water e.g. H2SO4 + Mg(OH)2 → MgSO4 + 2H2O Acid + Carbonates → Salt + Water + Carbon Dioxide e.g. 2HCl + MgCO3 → MgCl2 + H2O + CO2 Acid + Bicarbonates → Salt + Water + Carbon Dioxide e.g. HNO3 + Mg(HCO3)2 → Mg(NO3)2 + H2O + CO2 Some common acids – with their common states 1) Hydrochloric acid – HCl gas dissolved in water to make a solution 2) Sulphuric acid – solution 3) Nitric acid – solution 4) Citric acid – solid 5) Ethanoic acid – liquid (in solution is vinegar) 6) Tartaric acid – solid 7) Phosphoric acid - solution Some common bases - For the following then sodium can be substituted for other group 1 or group 2 metals 8) Sodium hydroxide – solid, dissolves to form a solution 9) Sodium Carbonate – solid 10) Sodium hydrogencarbonate – solid 11) Sodium Oxide - solid 19 Dr. mravetz Equations Equations represent what happens when chemical reactions take place. There are different types of equation and you will need to be able to use the correct equation in the correct context. Word equations Describes how a reaction progresses. e.g. Magnesium + oxygen → Magnesium oxide Symbol equations - Formula equations These are equations using the formulae of substances that show how many atoms of each element are involved and where they go in the products. e.g. Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g) Symbol equations - Ionic equations These equations only consider the ions taking part in the reaction e.g. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) if the equation is split into just the reacting ions H+ + Cl- + Na+ + OH- → Na+ + Cl- + H 2O we can cancel the ions that don’t change and appear on each side of the equation so overall the ionic equation is H+ + OH- → H 2O Symbol equations - Ion-electron equations / Half equations These show the ionisation of the species. Li+(g) e.g. Li(g) → + e- When asked for an equation in a question a balanced symbol equation, not a word equation, is always expected. Balanced equations It is essential that you are able to balance full equations, ionic equations and half equations. Remember these rules when balancing equations. The rules apply to any equation, even the most complicated looking one. 1. Write down the formulae for the reactants and products. The formula is fixed and can not be changed. C3H8 + O2 → CO2 + H2O 2. The only way you can balance an equation is to place numbers in front of the formulae (you can not change the formula by changing the subscript numbers). You must place 20 Dr. mravetz numbers in front of the formulae until there are the same number of atoms of an element on the reactant side of the equation as there are on the product side of the equation. NO! C3H8 + O3 → C3O2 + H 8O YES! C3H8 + 5O2 → 3CO2 + 4H2O There are now 3 carbons, 8 hydrogens & 10 oxygens on each side. 3. You must also ensure that the charges balance. H+ + OH- → H 2O The positive and negative charges make an overall zero charge on the reactant side while the products have no charge so the charges are balanced. 2Na+ + SO42- → Na2SO4 By placing a 2 in front of the sodium ion not only have we balanced the sodium but also the charges – there are two positive and two negative charges on the reactant side resulting in an overall zero charge and zero charge on the products side. 4. If required you then put state symbols in the balanced equation to show the state of the reactants and products. Remember to think carefully; is the water produced a gas or a liquid in your reaction?; Precipitates are solids; Is a gas given off?: Acids tend to be dissolved in water therefore they are aqueous. (s) = solid (l) = liquid (g) = gas (aq) = aqueous (dissolved in water) MgCO3(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) + CO2(g) 5. NOW CHECK – do all the elements balance, do all the charges balance, are the state symbols correct. 21 Dr. mravetz Practical techniques Throughout the course you will be shown and have the opportunity to practise practical techniques. You are required to know how to carry out these techniques and why you use them. You may be asked about them in the ISA exam. Use the following Youtube clips to help you better understand some of the processes you will learn – especially before an ISA exam. The information presented below is not exhaustive and may be added to. 1. Go to www.youtube.com 2. In the search box put in the following key words/titles to bring up the appropriate clip a) RSCteacherfellows then Recrystallisation Hot filtration Melting point determination Running an Infrared Spectrum Distillation Vacuum Filtration Thin Layer Chromatography Heating under reflux Weighing compounds using a balance b) Preparing a standard solution c) Pipetting Technique d) Titration technique using a burette e) Organic Chem: How to Flute filter paper f) Recrystallization of an impure compound (note: they refer to a conical flask as an ergimyer flask) g) Reflux reactions h) Distillation i) Extraction j) Calorimetry, heat of neutralization.avi You may be asked as part of your exam (particularly the ISA exam) the reasons for carrying out a particular practical technique. The following information will help but you will have to add to this knowledge throughout the course. Technique Why it may be used Filtration To remove an insoluble impurity or to collect a recrystallised product or to collect a precipitate or to separate a solid product from a solution. Hot filtration To remove an insoluble impurity from a sparingly soluble solute or from a solute (in a solution) requiring recrystallization. It is necessary to keep the solution hot to stop the product crystallising out Fluted filter paper To increase surface area to allow a faster filtration Recrystallisation To purify the product / To remove a soluble impurity Distillation To remove / purify / separate a liquid Fractional distillation To separate liquids with different boiling points Reflux Gives all reactants chance to react. Speeds up reactions without the reactants and products escaping 22 Dr. mravetz Practical Observations It is important when making practical observations, or answering exam questions which ask for observations that you are descriptive and precise. The following are key descriptive words and phrases that you should use where appropriate. a) Where there is the word ‘colour’ then insert the appropriate colour e.g. Blue solution b) Where there is the word ‘specific’ then insert the appropriate word e.g. carbon dioxide gas produced White precipitate produced ’colour’ precipitate produced Clear, colourless solution Clear, colourless liquid ‘Colour’ solution Gas produced Bubbles of gas produced Colourless gas produced ‘Specific’ gas produced Liquid condenses No visible change White solid produced ‘Colour’ solid produced Increase / decrease in temperature observed Fizzing / popping sound heard Sharp / pungent smell detected Vapour given off Just saying Lime water goes cloudy is no longer enough detail, instead you would say a white precipitate was produced. 23 Dr. mravetz Practical Accuracy In an experiment there is an experimental uncertainty (often called 'experimental error'). Experimental uncertainty arises because of: Limits in the precision of the measuring apparatus. Imperfections in the experimental procedure itself. Judgements made by the operator. Operator error To improve accuracy by you, the operator: always record results to the precision allowed by the apparatus. For example, if the balance reads to 0.01g, write 6.78g as 6.78g - make sure you don’t round it up to 6.8g or 7g. always record results the moment you take them e.g. write down the mass used when next to the balance, not after you have returned to your desk. When doing calculations do not round up numbers too early – if you do you may loose accuracy (see also section on significant figures). Apparatus Errors You must be able to calculate the size of errors in practical work. Percentage error = Margin of error Quantity measured x 100 Mass 1. Consider weighing 1g of solid. If you use a two decimal place balance, the mass recorded will be to the nearest 0.01g. In this example, the % error will be: 0.01 (margin of error) x 100 = 1% 1 (quantity measured) 2. Consider weighing the same 1g of solid on a three decimal place balance. The mass recorded will be to the nearest 0.001g, and so the % error will be: 0.001 x 100 = 0.1% 1 There is much less error involved in this procedure. 3. Consider weighing 10g of solid on the two decimal place balance. In this case the % error will be: 0.01 x 100 = 0.1% 10 This error is less than weighing 1g on this balance Choose the right balance for the amount of material to be weighed. 24 Dr. mravetz Volume 4. Consider measuring 25cm3 in a 25cm3 measuring cylinder. The measurement will normally be to the nearest 0.5cm3 (the accuracy will be quoted on the apparatus itself), so the % error will be: 0.5 x 100 = 2.0% 25 5. Sometimes on equipment the accuracy is quoted as a ± a value e.g. on a biuret the accuracy is given as ±0.05cm3 so if 25cm3 of solution was dispensed then the % error will be: 0.05 x 100 = 0.2% 25 However if 2cm3 of solution was dispensed then the % error will be: 0.05 x 100 = 2.5% 2 If choosing equipment to measure a volume choose the correct equipment based on the amount to be measured and the possible error. Other equipment The errors for other equipment can be calculated in a similar way. Total apparatus error To work out the total error attributed to the apparatus for a particular experiment, then add all the separate equipment errors together. For example, imagine a pupil doing an experiment where she measured out 1.245 g of a base, made it up to 250 cm3 of solution in a volumetric flask, pipetted 25 cm3 of that solution into a conical flask, and then found that it reacted with 23.30 cm 3 of acid in a titration using a burette. Balance ( 0.001 g) 100 x (0.001/1.245) = 0.08% Pipette ( 0.1 cm3) 100 x (0.1/25) = 0.40% 100 x (0.1/250) = 0.04% 3 Volumetric flask ( 0.1 cm ) Burette 3 ( 0.15 cm ) 100 x (0.15/23.30) = 0.64% Total apparatus error = 1.16% This means that the result of the experiment should be within 1.16% of the correct value. When you design experiments, you should aim to ensure that the total apparatus error is minimised by working on a suitable scale and with suitable apparatus. A very small titre for example (e.g. 5 cm3) leads to a very large apparatus error for the burette (3%). 25 Dr. mravetz Significant figures Counting the significant figures in a number Read the number from left to right and count all the digits starting with the first digit that is not zero. The examples below all have four significant figures: 0.09047 1.794 2.560 (because this number has a zero that is to the right of a decimal place and is shown it is counted as a significant figure) 0.007503 2.085 x 105 (using scientific notation like this shows without ambiguity the number of significant figures – if it was shown as 2085000 we would not know if this was 4,5,6 or 7 s.f. Accuracy and significant figures: 1) Always work out formula masses to one decimal place if data is taken off of the periodic table. If data is taken from a question then use the appropriate level of significant figures for the data given. 2) Mole calculations should always be to three significant figures. 3) If a problem asks for an answer to a certain number of significant figures or to a certain number of decimal places then you must do as the question asks. E.g. a) If the answer is 5.67g but the question requires an answer to 1 decimal place then the answer must be rounded up to 5.7g b) If the answer is 52.63cm3 but the question requires an answer to 3 significant figures then the answer must be rounded down to 52.6cm3. 4) General addition and subtraction: The value with the fewest decimal places (0.74 – 2 decimal places) determines the number of significant figures in the answer. The answer can contain no more decimal places than the least accurate measurement. 519.8754 - 0.74 + 5.337 = 524.4724 = 524.47 (2 decimal places) Answer must be rounded to 524.47 5) General multiply and divide: The quantity with fewest significant figures determines the number of significant figures in the answer. e.g. If an object has a mass of 29.1143g (6 significant figures) and a volume of 25.0cm3 (3 significant figures) then its density is given by 29.1143 = 1.164572g/cm3 = 1.16g/cm3 25.0 Answer in 3 significant figures NOTE: Round off numbers at the END of calculations so errors are not carried through. . 26 Dr. mravetz Yields 1) Atom economy tells us in theory how many atoms must be wasted in a reaction or how many atoms enter the desired product: % atom economy = mass of desired product x 100% Total mass of reactants The mass is calculated from the balanced equation. 2) The yield tells us about the practical efficiency of the process: Yield of a chemical reaction = The number of actual moles of a specified product x 100% Theoretical maximum number of moles of the product Or = actual yield in grams x 100% Theoretical yield in grams 27 Dr. mravetz Hazards / precautions Hazards New international hazard symbols are coming into use. You must know these symbols. SYMBOL (red diamond) HAZARD Caution – used for less serious health hazards like skin irritation. Flammable Dangerous to the environment Explosive Oxidising Longer term health hazards such as carcinogenicity Corrosive 28 Dr. mravetz Gas under pressure Toxic Precautions In ISA exams in particular you may be asked about how to minimise risk to do with hazards. When answering these sorts of questions you must be specific to the question. E.g if the question asks ‘How can contact with the skin be minimised?’ then it is no use answering ‘by washing hands’ because this is not a precaution – this is something you do after contact has been made. The following table gives some precautions to be taken against hazards but it is by no means exhaustive and you must continue to add to it as you gain experience in the laboratory. Hazard Chemical substance Possible harm Contact with the person causes irritation, burns, poisoning, death (dependent on hazard symbols) Breaking glass e.g. breakage, putting bungs in test tubes, putting pipette holders on pipettes Cuts Bunsen burner, heating mantle, hot equipment Burns / fires Bags / stools Trip hazard – someone falls over Possible contact with substance causing irritation, burns, poisoning (dependent on hazard symbols) Chemical spills 29 Dr. mravetz Precaution to reduce risk Use gloves Place lids on containers Mop up spillages Wear safety glasses Wear a mask Wear safety glasses Use appropriate apparatus to hold equipment e.g. test tube rack Hold glassware close to where the bung/holder is being inserted Tie hair back Leave hot equipment to cool Do not have flammable substances nearby Bags and stools under the table out of the way Keep tops on bottles when not in use Mop up spills straight away 31 Dr. mravetz