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Unit DF "Developing Fuels" - part of module 2850

PLEASE REMEMBER, THESE ARE NOT 'STAND ALONE' NOTES, and were designed for my classes for use alongside the Salters resources - Chemical Ideas, Chemical Storylines, Practical Activities-Investigations and the AS-A2 Revision guides all published by Heinemann Secondary Series, to reduce the reading workload and offer a study strategy. From your teacher (not me!), its handy to have the answers to the Chemical Ideas, Storylines Assignments and Activities Questions side by side with the texts and these strategy pages. You haven't time to redo the Q's but a quick read of the Q's and connecting with the official answers is valuable revision - there is too much hit and miss revision from doing past papers in my opinion.

Storylines DF1 "PETROL IS POPULAR"

• petrol is a high density energy source e.g. in kJ/kg, much more efficient in terms of 'delivery' rate compared charging batteries for an electric road vehicle
• crude oil is a very important chemical feedstock i.e. an important organic synthesis starting raw material as well as valuable finite resource!
• but burning fossil fuels has it price e.g. greenhouse gas and polluting emissions, so there is a need to burn petrol etc. as efficiently and 'cleanly' as possible, and its a finite resource

• meaning of the words: exothermic, endothermic, and enthalpy change H
• need to appreciate enthalpy diagrams like Figs 1/2 (and later including activation energy 'humps') GCSE notes help beginners
• interpretation of equations in mole ratios, relate to the H given for the equation e.g. to scale or relate to reacting masses
• even for the written exam you should be able to describe a simple calorimeter experiment and process data to deduce a delta H for a reaction and interpret delta H patterns (see CI p58 and Activities DF 1.2-1.3)
• you should know the basic design and operation of the bomb calorimeter and appreciate how much more accurate it is by reducing heat losses to almost zero AND the fact it can be accurately calibrated by burning a known substance of known H^Ø_comb
• must be able to use and clearly define standard enthalpies of (1) reaction  H^Ø_react,298K, (2) combustion H^Ø_comb,298K and (3) formation H^Ø_form,298K AND be able to state the standard conditions AND the need for them
• you must be fluent in solving problems using the Hess's Law thermo chemical cycle (the 'triangle') system - practice!
  • even if its on the same ones you did for homework!
  • view the 'triangle' from any angle to see x + y = z or change arrow direction
  • watch the direction of arrows, changing direction changes the delta H sign
  • if you can do CI 4.1 Q's, you can do em' in the exam!, particularly Q10-13
• a few odd scribbles to help and extra delta H questions scroll down to the DF section of My AS homepage

Chemical Ideas 1.3 "Using equations to work out reacting masses"

• read equations as a molar ratio
• convert mole ratio to reacting mass ratio
• quite a lot of help on GCSE calculations page

• know the basic experiment,

• its error sources,

• how to process the data to get the delta H in kJmol^-1,

• be able to write and balance combustion equations for alkanes and alcohols (and don't forget the O in the alcohol!)

Storylines DF2 "GETTING ENERGY FROM FUELS"

• enthalpy of combustion H^Ø_comb,298K, -ve very exothermic, varies from fuel to fuel, reaction with oxygen to release energy
• Fig 3 - bigger molecule = bigger H^Ø_comb, but note for oxygenated molecules of the same C number, H^Ø_comb is less, since already partially oxidised (compare methane and methanol)
• important ideas:
  1. The net energy change H^Ø_comb is the difference between the energy absorbed (endothermic) in bond breaking and the energy released on bond formation (exothermic).
  2. The use of equations set out in structural formula style p21 which help in numerical problem solving in CI 4.2 below (again GCSE notes can help too)
• assignments 1 and 2 are useful exam type Q's e.g. equation balancing, using energy density concept to compare different fuels and relating this to molecular structure

Chemical Ideas 4.2 "Where does the energy come from?

• very important key phrase: bond enthalpy (bond energy) - its definition and use of the values to get delta H for a reaction (My GCSE notes may help beginners)
• appreciate why average bond enthalpies are used ...
  • each particular bond e.g. C-C or C-H can be slightly different in a different molecule due to subtle changes in the 'electronic environment'.
  • by using a typical average value it enables the best estimation to be made.
  • also bond enthalpies themselves, have to be calculated through enthalpy cycles from known  H values, this calculation can't take into account each individual C-C or C-H bonds but just the average of the 'sum' of each type of bond.
• the net energy change H^Ø_comb is the difference between the energy absorbed (endothermic) in bond breaking and the energy released on bond formation (exothermic)
• set out the problems carefully e.g. a column for the endothermic bond breakings and another for the exothermic bond makings, tot up both columns separately and work out the difference for delta H (reaction)
• other important ideas: e.g. (1) the shorter the bond, the stronger the bond (usually as atoms get smaller), (2) bond strength increases single < double < triple (e.g. N and C based molecules)
• Note there are two reasons why enthalpy changes calculated from bond energies are different and less accurate than those calculated using accurately measured enthalpy values e.g. from a bomb calorimeter.
  • Bond energy values are still usually based on 1 atm (101kPa) and 298K BUT only for species in the gaseous state. So if the equation involves H₂O_(l) or C₈H_18(l) etc. the enthalpies of vaporisation have not been taken into account.
  • Quoted values are 'average' values of the electronic situation. This means individual bond energies can be slightly different e.g. the C-H value in the chain -CH₂- is not quite exactly the same as the C-H in the end -CH₃. Also going from CH₄ => CH₃ =>CH₂ => CH => C, i.e. for the stepwise removal of H's from a molecule like methane, each C-H bond is significantly different and what is 'quoted' is 1/4 of the energy to go from CH_4(g) ==> C_(g) + 4H_(g).

Activity DF2.1 "Using spreadsheets to calculate enthalpy changes of combustion"

• of little use for written exam? but make sure you are competent at balancing combustion equations and can do the bond energy calculations to get a delta H as outlined on CI pages 65-66

Storylines DF3 "FOCUS ON PETROL"

• crude oil is a mixture of hydrocarbons (molecules made of a chemical combination of carbon and hydrogen atoms) which are separated by fractional distillation (works because the molecules have different boiling/condensation points)
• many of these oil hydrocarbons are alkanes, and are sorted into fractions in the distillation i.e. narrow boiling ranges of limited carbon number (e.g. petrol is C₅ to C₇)
• have an idea from Table 2 about how the fractions are used BUT two supply/demand problems ... (solutions in CS DF4)
  • 'straight run' gasoline from primary distillation doesn't make good petrol, most needs further treatment to fulfill commercial petrol demands
  • the surplus of high boiling hydrocarbons which need to be cracked to make more volatile hydrocarbons suitable for petrol etc. (AND NOTE in the process making valuable alkenes - important secondary chemical feedstock for a huge number of other chemicals including plastics)
• the 'refinery's job' is to convert the crude oil fractions into useful products - wide range of hydrocarbons - be able to recognise the aliphatic alkanes (linear or branched and name them), and cycloalkanes (and name them), arenes (aromatic hydrocarbons) - look for the circle in the hexagonal ring!), and the cracking products alkenes. These notes have links to naming quizzes.
• vacuum distillation allows low temperature distillation to minimise thermal decomposition products
• volatility - winter and summer petrol - need for a more volatile fuel mixture in lower winter temperatures
• assignments 3 and 4 useful data interpretation questions
• the knocking problem - p26 -auto-ignition at the wrong time - extra explosion on compression before the desired spark induced one - causes engine damage (green box notes)
• the octane rating is a measure of the fuels auto-ignition capacity - the higher the octane rating the less likely auto-ignition on compression will occur - the 'standard' is in the green box p26 (a good one to revise a more awkward alkane to name!)
• alkane hydrocarbons have a higher octane rating when they are shorter (smaller C number) or more branched for the same carbon number (both these factors makes them more volatile).
  • [and aromatic hydrocarbons (arenes like benzene) and oxygenated molecules like ethers and alcohols also have higher octane ratings than linear alkanes - see also CS DF4 and 7]

CI 12.1 "Alkanes"

(and bits of 12.2/12.3 to complete DF 'molecular picture' of hydrocarbons)

• generic name alkanes, [compare and recognise structure compared to other aliphatic hydrocarbons - alkenes (p 274/282-283) and the aromatic hydrocarbons - arenes (p274/288-291)
• be able to do covalent o and x electron diagrams for the smaller alkanes
• huge variety of organic compounds possible - due to catenation, can form chains, rings, double/triple bonds, lots of other groups of compounds when combined with O, N etc. - i.e. formation of functional groups e.g. C=C, C-O-C or C-OH etc. which give organic molecules a more reactive and distinctive chemistry. Summary of AS-A2 functional groups
• alkanes are relatively unreactive - strong C-C or C-H bonds and described as saturated - combined with the maximum number of H atoms i.e. other atoms can't add to the alkane molecule (alkenes are prime examples of unsaturated molecules - atoms like bromine easily add across the double bond - e.g. bromine water test: orange ==> colourless)
• alkanes form a homologous series - a series of compounds linked by a general formula (in this case C_nH_2n+2 where n = 1,2, 3 etc.), each differing by a -CH₂- unit, have very similar chemical properties, but physically, although showing similarities, they show gradual changes as the C number changes e.g. increase in melting/boiling point or density.
• other general formulae to watch out for: cycloalkanes (p277-278) and alkenes with one double C=C bond (p282) are isomerically related by C_nH_2n 
• must know how to deduce the empirical formula (simplest whole number atomic ratio as found from experiment) and molecular formula (summary of the actual number of atoms of each element in the molecule) from combustion data p275-276: (1) convert mass of H₂O and CO₂ to mass of H and C, (2) empirical formula calc. (3) need molecular mass to finally deduce molecular formula
• representing the structure of alkanes (plenty of examples on 276-279  ): full and shortened structural formula and skeletal formula, and 3D diagrams (p278-279), you must be clear on all formats (and bond angles - easy for alkanes, all 109^o but watch out for 120^o around the C=C in alkenes. My notes on alkane naming and structure and molecule shapes.
• be able to work out all the structural isomers (molecules with the same molecular formula but different arrangements of the atoms), particularly of the lower alkanes (and alcohols too) - draw and analyse carefully, and remember its the bond connections that count - not how ziggy-zaggy you draw the structure or its style!!!!!
• nomenclature of alkanes: naming based on - methane, ethane, etc. methyl, ethyl, etc., longest C chain, lowest number for substituent side chains, di, tri, etc. if more than one of same substituent in the main chain, study/practice of examples and the web notes of examples and quizzes m/c and type in name (often avoided at some cost in marks!).
• physical properties of alkanes:C₁-C₄ colourless gases, C₅-C₁₈ colourless liquids, >C₁₈ white waxy solids, all mix well with each other but not soluble in highly polar solvents like water or methanol
• chemical reactions of alkanes: (1) combustion: products and equations, (2) cracking, isomerisation and reforming - products and equations (CI p280, CS p27-29)

Activity DF3.3 "Auto-ignition in a test-tube":  just the basic Q and rule on octane number versus isomer structure - more branched, more volatile, higher octane rating

CS DF4 "MAKING PETROL - GETTING THE RIGHT OCTANE RATING"

• need to avoid lead additives (good at raising the octane number of a petrol), which give brain toxic emissions* and lead poisons catalytic converters by coating the active sites (*if visiting the west country, look up the history of cider drinking where they put lead rods in to sweeten it due to vinegar formation - lead acetate was called 'sugar of lead' - and find out the result!)
• the shorter or more branched the alkane the higher its octane rating - also makes them more volatile so handy for 'cold-starting' BUT can't be too volatile so you need to get the right blend. Aromatic hydrocarbons (arenes) also have a higher octane rating than the equivalent linear or cycloalkane (see p29 Fig 16). So how do we get these molecules for a better petrol blend? ...
• to get the right blend of the right molecular ingredients several processes of are used
  • isomerisation - carbon number stays constant but produces linear ==> branched (catalysed by Pt/Al₂O₃), and the products can be separated by zeolite molecular sieves, filters out the branched, allows linear through which are recycled over the catalyst again (no waste!)
  • reforming at ~500^oC - also catalysed by Pt/Al₂O₃, means of producing various cyclic hydrocarbons - converts linear alkanes ==> cycloalkanes ==> arenes (see Fig 16 AND note the increasing octane number)
  • cracking - the catalytic thermal decomposition of hydrocarbons - lots of products all with a same/smaller C number - linear alkanes ==> smaller linear alkanes,  more branched alkanes, cycloalkanes, alkenes etc. Zeolite beds catalyse the cracking but surface gets coated in carbon which has to be burned off at times with hot air to regenerate the catalytic performance!
• assignments 6 and 7 are exam like questions involving rates of reaction and molecular structure ideas
• adding oxygenates is an important method of raising the octane number of petrol - ethers and alcohols (must be able to recognise them via their functional group structure: C-O-C and C-O-H, and be able to name the alcohols too) See also green box on p26 and p31
• Ethers and alcohols have much higher octane ratings than the 'straight run' alkanes and generally produce less pollution, but they add to the cost of the petrol and there are health and safety doubts about their use e.g. MTBE is water soluble and spillages can cause water pollution
• another of the problems in producing a good petrol blend is one of solubility - hydrocarbons readily mix with each other to give a homogeneous mixture (see green box to learn initial ideas on entropy)
• BUT some oxygenated molecules like methanol don't readily mix with petrol hydrocarbons - if one component produces strong intermolecular forces, it will tend to stick to itself.
• If H stands for hydrocarbon and A stands for oxygenated molecule like the alcohols, there are three possible interactions in terms of inter-molecular forces* : (1) H....H, weakest, non-polar bonds, (2) H...A, stronger, highly polar alcohol will induce some polarisation in H, (3) A....A, strongest, highly polar O-H bond, produces greatest intermolecular force due to hydrogen bonding*) and so hydrocarbons get 'squeezed out' to form their own layer!
• (* do NOT confuse intermolecular forces with covalent bonds, hydrogen bonding is the strongest intermolecular attractive force and is unfortunately very miss-named for rookie students!)

• be clear on definition of structural isomerism and its 3 'varieties' and examples you have likely to have encountered so far
  • chain - e.g. alkanes from C₄ onwards, ( and aromatics like the dimethylbenzenes p290, these come from reforming)
  • positional - e.g. alcohols from C₃ onwards
  • functional group - e.g. alcohols and ethers, alkenes and cycloalkanes

• structure of alcohols, functional group hydroxyl C-O-H, watch the bond lines in the skeletal formula - don't muddle the dash in -OH with carbon chain /\/ etc., polyhydric just means more than one -OH group to form a diol or triol, watch out for need of polar diagrams in answers relating to physical properties (see below)
• must be able to recognise and draw ethers too p307 (but not name them), they are slightly polar but closer physically to alkanes rather than to alcohols, useful but flammable solvents, slightly soluble in water but readily mix with alkanes
• alcohol nomenclature: name rules based on the alkane system but now has a suffix of ..ol (and after ethanol, preceded by a position number e.g. ...-2-ol etc.), again study and practice - Alcohol/Ether naming and structure notes (2 pages), tests on naming m/c and type in name (they do include ethers but don't worry about it, the tests serve other courses)
• physical properties: polar molecule diagrams, polar bond originates from the difference in electronegativity of O and H, hydrogen bonding (strongest intermolecular force BUT NOT a chemical ionic/covalent bond!), you must be able to argue
  • (1) why lower alcohols dissolve in water (longer chains become more like alkanes, so increasingly less soluble)
  • (2) why alkanes will not dissolve in water and not very soluble in alcohols
  • (3) why the boiling points of alcohols are much higher than similar length alkanes
  • and the arguments are based on A...A, A...B and B...B intermolecular interaction
• chemical properties: later in the course except to balance combustion equations for alcohols (and don't forget the O in the alcohol!)

Chemical Ideas 4.3 "Entropy and the direction of chemical change"

• events that happen, are the most probable! always obvious?!
• concept: the entropy of a system is a measure of the number of ways a system can be arranged
• the more ways an event can, the more probable is that event and the higher the entropy of the system
  • or if there are more ways to arrange a 'system' than another, the more likely that system arrangement will be formed.
  • Don't just talk about order and disorder - not enough for full marks (if any?) on an entropy Q.
• the entropy of a system will always try to increase, determining the direction of change e.g.
  • gases will always diffuse into each other
  • liquids will mix (unless intermolecular forces prevent this - see above in alcohols)
  • production of a gas in reactions is quite a driving force for 'unfavourable' reactions e.g. thermal decompositions like limestone ==> lime OR cracking of alkanes are both very endothermic, but the increase in gas molecules drives the reaction in the decomposition direction*
• entropy content increases** gas > liquid >> solid 
• entropy content increases in a series of increasingly longer molecules e.g. from lower to higher alkanes
• ** often described as increasing chaos - but always express your answers in terms of ... 'more probable ways of arranging the outcome'

• backs up Storylines DF4 on basic octane number rules as regards chain length and branching so worth re-reading

and DF6 "TACKLING THE EMISSIONS PROBLEM"

• you need to able to describe concisely the origin and effects of various exhaust emissions (and even evaporating petrol from the tank is also a pollutant)
• reducing emissions and pollution effects is done by technologically by
  1. car design (e.g. engine - compression ratio, fuel/air ratio e.g. 'lean burn' engines, exhaust catalyst etc.)
  2. the composition of the fuel
  3. changing peoples habits e.g. car sharing, public transport etc. can all play their part. 
     • This area is often tested by the harder Q's e.g. 10 blank lines for 6 marks!
     • In exams make SPECIFIC POINTS, not just vague answers
       • its no good just saying 'improve car design'!!!!!
• carbon dioxide is not a harmful emission in terms of 'health and safety' BUT it is the major contributor to 'greenhouse' gas emissions and may lead to global warming and its possible effects
• unburned hydrocarbons C_xH_y from inefficient combustion can be carcinogenic and are also involved in the chemistry of photochemical smogs.

• The CO is from the inefficient combustion of the hydrocarbon fuel,

  • C_xH_y + (^x/₂ + ^y/₄)O₂ ==> xCO + ^y/₂H₂O 

• nitrogen monoxide NO is formed at the high temperatures in the engine  
  • N_2(g) + O_2(g) ==> 2NO_(g) and rapidly gets oxidised to nitrogen dioxide NO₂ in air 
    • 2NO_(g) + O_2(g) ==> 2NO_2(g) 
  • Nitrogen dioxide is a lung irritant, acidic gas => rain, and is involved in the complex chemistry of 'photochemical smog'.
• carbon monoxide CO is formed by the inefficient (incomplete) combustion of the fuel and is a highly toxic gas
• both unburned hydrocarbons C_xH_y, carbon monoxide and nitrogen monoxide emissions can be significantly reduced by use of a catalytic converter - see the three example reactions given on p37, platinum and rhodium are typical transition metal catalysts used. However catalysts are easily poisoned by e.g. lead compounds (so need lead-free fuel) and most only work efficiently when the temperature is high enough, so some extra pollution when the car is started. Catalytic converters are examples of heterogeneous catalysts - reactants (gas) are in a different phase (~ state) to the catalyst (solid).

  • 2NO_(g) + 2CO_(g) ==> N_2(g) + 2CO_2(g)

  • These transition metal catalysts can also oxidise unburned hydrocarbons from inefficient combustion.

    • C_xH_y + (x+^y/₄)O₂ ==> xCO₂ + ^y/₂H₂O 

• the oxides of sulphur SO₂ and SO₃ are formed from combustion of any sulphur or sulphur compounds in the fuel. Sulphur dioxide is a lung irritant, and both oxides contribute to acid rain (affecting building corrosion, aquatic eco-systems etc.)
• at the oil refinery most of the sulphur is removed before the fuels are burned (see Activity DF5)
• the chemistry of photochemical smog is complex but when sunlight (acts as 'catalyst') shines on primary pollutants like NO_x and C_xH_y mixed with O₂ and H₂O the secondary pollutant ozone O₃ is formed. Ozone is a very reactive irritant toxic gas, weakens immune system, attacks lung tissue. Further reactions of hydrocarbons produce more irritating and eye-watering compounds (Fig 24 summary gives you the 'big picture').
• much of this 'photochemical' chemistry involves free radicals which are highly reactive molecular fragments because they have a lone unpaired electron.
• all of these products contribute to a very unhealthy atmosphere, and particularly vulnerable people like very young children and those with respiratory problems such as asthmatics. Not surprisingly the peaks and troughs in pollution levels are related to 'rush hours' and sunlight levels.
• Exhaust emissions do not significantly affect the ozone layer in the upper atmosphere, its depletion is primarily due to molecules such as CFC's used in aerosol cans and refrigerant liquids (dealt with in Unit A "The Atmosphere".
• don't get bogged down with the three way catalyst, you would be given information on it, should be no problem if you know the basic ideas outlined above.
• assignments 8, 9, 10 and 11a-e are exam like questions, 8 may come along in the way of a more open-ended free response Q and you must be able to punch through good scientific points AND NOT VAGUE WAFFLE, 9 is good example of a more quantitative Q.
  • This sort of Q is always the most difficult to get high marks on!

• you must be able to interchange mass <=> moles <=> volume using the formula mass and molar volume and readily apply these relationships to combustion equations.

• reacting volume ratio questions are less tricky, the mole ratio of gases in the equation gives the basic reacting volume ratio in any units (as long as you use the same units!), this stems from Avogadro's Law. My GCSE calculation notes help at bit

Chemical Ideas 10.4 "What is a catalyst?"

• be able to clearly define what we mean by a catalyst
• the catalyst affects the rate and is chemically involved in the process, but it does NOT make more product from what you start with AND is chemically unchanged in the end
• need to be able to sketch the process of heterogeneous catalysis (e.g. Fig 18) and be able to quote some examples
• know the difference between a homogeneous and heterogeneous catalysts
• know about catalytic poisoning, its causes, effects e.g. absorbed molecules block the catalytic site at the 'atomic/molecular level'.

Activity DF5 "What happens to the sulphur"

• the most important points are (1) good idea to remove the sulphur after fractional distillation, (2) the sulphur/sulphur compounds are converted into hydrogen sulphide H₂S which is toxic!, (3) the H₂S is oxidised to form sulphur which is sold to make sulphuric acid - so you recover some of the cost of making fuels less polluting on combustion

DF8 "HYDROGEN - THE FUEL OF THE FUTURE"

• arenes (aromatic hydrocarbons), volatile butane are added to petrol because of their high octane ratings BUT aromatic hydrocarbons are potentially carcinogenic (benzene should not be used outside a fume cupboard!) and dissolved butane gas readily vapourises
• so there is a need to find alternatives like 'oxygenates', organic molecules containing oxygen atoms, namely alcohols and ethers.
• the idea is to reduce (1) volatility (reduce evaporative emissions), (2) harmful aromatic molecules in the blend, (3) polluting emissions BY adding oxygenates to improve cleaner and more efficient burning
• methanol CH₃OH burns cleanly with a high octane number BUT there are various problems:
  • it does not readily mix with petrol (see near end of CS DF4 above) and forms two layers of methanol/petrol
  • it is hygroscopic (absorbs water) and this causes increased corrosion problems and the formation of two layers alcohol-water/petrol
  • it is a brain toxin and causes blindness (its the deliberate 'methylated' poison added to the 'spirits') so long term exposure via  breathing in the vapour is not good for you!
  • it is 40% less energy dense than 'ordinary' petrol, so you need larger and heavier fuel tanks
• ethanol CH₃H₂OH ('alcohol') is similar and has some advantages over methanol ...
  • readily made from ethene (from cracking) + water
  • ethanol can be made from distilling fermented sugar cane solution (a renewable resource, unlike oil, but needs cheap labor, has high energy distillation costs, and subject to the influence of the world price for oil)
  • but note that Brazil uses 'gasohol' a mixture of gasoline and ethanol
• hydrogen is an alternative to fossil fuels, if it might be made economically from water by electrolysis and safely stored and distributed.
  • primary energy sources can be solar light cells, wind turbines, hydroelectric, solar heat panels
  • a.c. from generators is converted to d.c. and water electrolysed
  • advantages: hydrogen can be compressed and stored and distributed by pipeline; it can be burned in an internal combustion engine; there are no pollution emissions; it would help conserve oil stocks - which are a valuable chemical feedstock other than for fuels; it can be used in small scaled fuel cells to re-generate electricity e.g. in an electric car
  • disadvantages: a large compressed volume of hydrogen is needed compared to a tank of petrol even though energy density in kJ/kg is much higher than petrol
  • assignment 12 is a typical 'applied' calculation Q and so useful revision

ALL my KS3 SCIENCE Revision Quizzes (~US K12 grades 6,7,8)

GCSE-IGCSE-KS4 Science-CHEMISTRY notes & quizzes (~US K12 grades 9-10)

Advanced Level CHEMISTRY GCE AS A2 IB notes and quizzes (~US K12 grades 11-12)

All my GCSE-IGCSE Science-CHEMISTRY etc. syllabus help links

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