2.5Chemical Bonding, Application Of Chemical Reactions and Organic Chemistry
Crude Oil, Fuels and Organic Chemistry — WJEC GCSE Chemistry Revision Notes & Topical Questions
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Complete PDF with figures and original page layoutPage 1 WJEC GCSE Chemistry Your notes 2.5 Crude Oil, Fuels & Organic Chemistry Contents Crude Oil Fractions in Crude Oil Impact of the Oil Industry Combustion of Hydrocarbons Energy Density of a…
Section 1 of 38Crude oil
- Nature
- a finite, thick, sticky black liquid found in porous rock under the ground and sea, and it is a complex mixture of hydrocarbons
- Hydrocarbon = a compound containing hydrogen and carbon atoms only, and the word "only" must be stated in a definition
- Structure
- a carbon backbone as a chain or ring with hydrogen atoms attached
- Formation
- over millions of years from the effects of high pressure and temperature on buried remains of plants and animals, mainly plankton in mud
- Finite resource
- it is used up much faster than it forms
- Uses
- a source of hydrocarbons for fuels such as petrol and diesel, and of feedstock for the petrochemical industry
Section 2 of 38Fractional distillation of crude oil
- Fractions
- the hydrocarbons are separated into groups of similar chain length and similar boiling points
- Content
- most fractions contain mainly alkanes, which have only single bonds
- Column
- the fractionating column is very hot at the bottom and cooler at the top
- Crude oil is heated and vaporises.
- The vapours enter the column, which has a temperature gradient.
- Vapours of hydrocarbons with high boiling points condense at the bottom.
- Vapours with lower boiling points rise and condense at the top.
- Rule
- the smaller the hydrocarbon, the lower its boiling point, so small molecules condense at the top and large molecules at the bottom
Section 3 of 38Fractions and their uses
- Importance
- the petrochemical industry supplies transport fuels, polymers, lubricants, solvents, detergents and adhesives
- Reason for variety
- carbon forms multiple strong covalent bonds with itself, giving many organic compounds
| Fraction | Use |
|---|---|
| Petroleum gases | Fuel for domestic heating and cooking |
| Gasoline (petrol) | Fuel for cars |
| Naphtha | Making chemicals |
| Kerosene | Jet fuel |
| Diesel | Fuel for cars and some trains |
| Lubricating oil | Reduces friction in machinery |
| Fuel oil | Fuel for ships and power stations |
| Bitumen | Surfacing roads and roofs |
Section 4 of 38Trends in properties of fractions
- Boiling point
- it increases with molecule size
- Intermolecular forces get stronger and need more energy to overcome.
- Viscosity
- it increases with chain length
- Thick long-chain hydrocarbons would clog a car engine.
- Viscous long-chain alkanes make good lubricants because they burn less easily and reduce friction.
- Flammability
- smaller molecules ignite more easily, so they make good fuels that release a lot of energy
- Colour
- small fractions at the top are colourless, middle fractions are yellow to brown, and the bottom fraction is black
- Cleanliness of burn
- short hydrocarbons burn with a cleaner, bluer flame, and larger ones burn with a yellow, smoky flame because of insufficient oxygen
| Going down the column | Trend |
|---|---|
| Boiling point | Increases |
| Flammability | Decreases |
| Viscosity | Increases |
| Colour | Gets darker |
Section 5 of 38Environmental impact of the oil industry
- Carbon dioxide
- burning oil releases this greenhouse gas, contributing to global warming and climate change
- Sulfur dioxide
- sulfur impurities oxidise on burning and cause acid rain
- Reach
- the pollutants affect everywhere, not just the countries using oil
- Spillages
- oil spills kill animals and destroy habitats locally
Section 6 of 38Political, social and economic importance
- Political
- growing countries such as China use more oil each year, which is not sustainable, and conflict in producing countries can cut supplies
- Economic
- oil companies control the price, giving them global influence and making oil hard for poorer countries to afford
- Prices
- rising demand for a non-renewable resource raises fuel costs, affecting heating, travel and food prices
- Other uses
- plastics and medicines are made from crude oil components, so a choice may be needed between burning reserves and using them for other purposes
Section 7 of 38Combustion of hydrocarbons
- Combustion
- fuels react with oxygen to release energy, so the reaction is exothermic
- Complete combustion
- with plenty of oxygen, carbon forms carbon dioxide and hydrogen forms water
- Examples
- methane, ethane and ethanol each give carbon dioxide + water
- Methane equation
- CH4 + 2O2 → CO2 + 2H2O
- Balance the carbon atoms first.
- Balance the hydrogen atoms next.
- Balance the oxygen atoms last.
- Propane
- C3H8 + 5O2 → 3CO2 + 4H2O, with 3 CO2 for carbon, 4 H2O for hydrogen, and 10 oxygen atoms needing 5 O2
Section 8 of 38Energy released by fuels: calorimetry
- Method
- a fuel burned in a spirit burner heats water in a copper can, and a lid reduces heat loss
- Measure a fixed volume of water into a copper can.
- Weigh the spirit burner containing the fuel.
- Measure the initial water temperature.
- Burn the fuel and stir the water.
- Extinguish the flame when the temperature has risen by about 20 °C.
- Record the final temperature and re-weigh the burner.
- Specific heat capacity, c
- the energy needed to raise 1 g of a substance by 1 °C, which is 4.18 J/g/°C for water
- Equation
- Q = m × c × ΔT, where Q is heat energy change in J, m the mass of water in g and ΔT the temperature change in °C
- Example
- 200 g of water rising by 30 °C gives Q = 200 × 4.18 × 30 = 25 080 J, with the negative sign for the fuel showing it is exothermic
- Exam note
- the equation is provided but must be used to calculate energy released
Section 9 of 38Practical: energy released by alcohols
- Aim
- to find which alcohol releases the most energy per gram, as alcohol + oxygen → carbon dioxide + water
- Apparatus
- clamp stand, boss and clamp, 250 cm3 conical flask, 100 cm3 measuring cylinder, thermometer, balance (±0.01 g),
- and spirit burners of methanol, ethanol, propanol and butanol
- clamp stand, boss and clamp, 250 cm3 conical flask, 100 cm3 measuring cylinder, thermometer, balance (±0.01 g),
- Measure 100 cm3 of water into the conical flask.
- Clamp the flask at a height that fits the spirit burner underneath.
- Record the water temperature.
- Record the mass of the spirit burner with its lid.
- Place the lit burner under the flask.
- Heat until the temperature rises by about 40 °C and record it.
- Extinguish the flame and record the burner mass.
- Repeat for each other alcohol.
- Safety and accuracy
- The thermometer must not touch the bottom of the flask.
- Alcohols are highly flammable, so work in a ventilated lab away from other flames.
Section 10 of 38Practical: calculations and conclusion
- Results table
- initial and final burner mass, mass change, initial and final temperature, temperature change, and energy released per gram
- Equation
- energy per gram = mass of water × temperature change × 4.2 ÷ mass of alcohol burnt
- Evaluation
- calculate each temperature rise and the mass of alcohol burnt, then the energy per gram
- Example
- butanol gives the most energy at about 11 357 J per gram
- Conclusion
- the larger the alcohol molecule, the more energy released per gram
- Order of size
- methanol < ethanol < propanol < butanol
Section 11 of 38Hydrogen as a fuel
- Uses
- rocket engines and fuel cells that power some cars and buses
- Reaction
- 2H2 + O2 → 2H2O, an exothermic reaction
- Advantages
- It is made from water, so it is renewable.
- The only product is water, so it does not add to global warming or acid rain.
- Disadvantages
- It needs large amounts of electricity to make by electrolysis of water.
- It is difficult and dangerous to store, needing bulky pressurised containers.
- It forms explosive mixtures with air.
- Cost
- hydrogen remains expensive compared with fossil fuels
Section 12 of 38The fire triangle
- Three factors
- a fire needs fuel, heat and oxygen
- Principle
- removing any one side stops the fire
- Removing fuel
- the main focus of fire prevention
- Clothing and materials are treated to be fire-resistant.
- Buildings have fire-proof compartments in walls, ceilings and internal doors.
- Forests and moors have fire breaks where all fuel is cleared.
- Removing heat
- spraying large amounts of water
- It is unsuitable for electrical fires because of the risk of shock.
- It is unsuitable for oil fires because the oil breaks into droplets with greater surface area and burns more.
- Removing oxygen
- carbon dioxide extinguishers, foam extinguishers and fire blankets cover the fire
- The fire goes out once it uses up the available oxygen.
- Doors are closed to hold a fire in a compartment with limited oxygen, and opening a door gives it fresh oxygen.
Section 13 of 38Supply and demand of fractions
- Supply
- how much of a fraction refining the crude oil produces
- Demand
- how much customers want to buy
- Imbalance
- demand for short-chain fractions such as petrol, kerosene and diesel exceeds supply,
- and demand for long-chain fractions such as fuel oil is less than supply
- demand for short-chain fractions such as petrol, kerosene and diesel exceeds supply,
Section 14 of 38Cracking
- Cracking = an industrial process that breaks low-demand long-chain hydrocarbons into more useful small-chain hydrocarbons
- Purpose
- to convert excess unwanted fractions into the more useful ones that are in demand
- Scope
- any long-chain hydrocarbon can be cracked into smaller chain hydrocarbons
Section 15 of 38Cracking process
- Example
- kerosene and diesel are cracked to give petrol, alkenes and hydrogen
- Heat the hydrocarbon to about 600–700 °C to vaporise it.
- Pass the vapour over a hot powdered catalyst of alumina or silica.
- Covalent bonds break as the molecules touch the catalyst surface, which is thermal decomposition.
- The molecules break in a random way, giving a mixture of shorter alkanes and alkenes.
- Alkanes
- saturated molecules with carbon-carbon single bonds only
- Alkenes
- unsaturated molecules containing carbon=carbon double bonds
- Higher temperature and pressure
- they give more hydrogen and a greater proportion of alkenes
- Example
- decane is cracked to give octane for petrol and ethene for polymerisation and ethanol synthesis
Section 16 of 38Why cracked products are useful
- Shorter alkanes
- they have lower boiling points and are more flammable, which makes them better fuels
- Shorter alkenes
- the reactive C=C double bond makes alkenes, such as ethene, a starting point for polymers and plastics
Section 17 of 38Equations for cracking
- Conservation of mass
- the reactant must have the same numbers of carbon and hydrogen atoms as all the products combined
- Reactant
- it must be an alkane with general formula CnH2n+2
- Products
- a mixture of alkanes (CnH2n+2) and alkenes (CnH2n)
- Hexane example
- C6H14 → C4H10 + C2H4, with butane as a fuel and ethene for polythene
- Checking an equation
- Carbons6 on the left and 4 + 2 = 6 on the right.
- Hydrogens14 on the left and 10 + 4 = 14 on the right.
- Butane and hexane fit CnH2n+2 and ethene fits CnH2n.
- Eicosane example
- C20H42 → C18H38 + C2H4, and the C18H38 is an alkane because it fits CnH2n+2
Section 18 of 38Alkanes and their names
- General formula
- CnH2n+2
- Saturated
- carbon always has four bonds, and all carbon-carbon bonds are single
- Properties
- they are colourless with gradually changing physical properties, generally unreactive, but they burn and can be cracked
- Naming
- the stem shows the number of carbons and -ane shows single bonds only
| Carbons | Stem | Name | Formula |
|---|---|---|---|
| 1 | meth | methane | CH4 |
| 2 | eth | ethane | C2H6 |
| 3 | prop | propane | C3H8 |
| 4 | but | butane | C4H10 |
| 5 | pent | pentane | C5H12 |
| 6 | hex | hexane | C6H14 |
Section 19 of 38Alkenes and their names
- General formula
- CnH2n
- Unsaturated
- each has at least one carbon-carbon double bond, C=C, which is the functional group
- Minimum size
- an alkene needs at least two carbons
- Properties
- they are colourless and reactive, they burn, undergo addition reactions and form polymers
- Naming
- the stem shows the carbons and -ene shows a double bond, with a number for the double bond position where needed
| Name | Formula |
|---|---|
| ethene | C2H4 |
| propene | C3H6 |
| but-1-ene | C4H8 |
| pent-1-ene | C5H10 |
- Exam requirements
- Name methane to pentane and ethene and propene.
- Write the molecular formula for any alkane or alkene.
- Identify unfamiliar formulae as an alkane or an alkene.
Section 20 of 38Naming branched alkanes (Higher tier)
- Side chains
- they are limited to methyl (–CH3) and ethyl (–C2H5) groups
- Find the longest carbon chain and the type of bonds, which gives the stem and -ane.
- Identify the alkyl side-chain.
- Number the chain so the side-chain gets the lowest possible number.
- Examples
- 3-methylpentane has a 5-carbon chain with a methyl on carbon 3, and 3-ethylpentane has an ethyl on carbon 3
- Short chains
- 2-methylpropane and 2-methylbutane can be named without the 2, because the methyl can only be in that position
- Common error
- naming 2-methylpropane as 2-ethylpropane, because propane is not the longest chain
Section 21 of 38Naming branched alkenes (Higher tier)
- Find the longest carbon chain that includes the C=C double bond.
- Find where the double bond is located.
- Identify the alkyl side-chain and the carbon it is on.
- Example
- 2-methylbut-1-ene has a 4-carbon chain, a double bond on carbon 1 and a methyl on carbon 2
- Drawing
- structures can be drawn "backwards", with carbon 1 on the right
- Structure from name
- 3-methylpent-2-ene has 5 carbons, a double bond from carbon 2 to 3 and a methyl on carbon 3
Section 22 of 38Isomerism and alkane isomers (Higher tier)
- Isomers = chemicals with the same molecular formula but different structural formulae
- No isomers
- methane, ethane and propane have no isomers because carbon always has four bonds and hydrogen one
- Misconception
- two drawings of propane are the same molecule because the longest chain is 3 carbons in both
- C4H10
- two isomers
- Butane, the straight chain.
- 2-methylpropane, formed by moving a CH3 from the end to the second carbon.
- C5H12
- three isomers
- Pentane, the straight chain.
- 2-methylbutane, a methyl moved to the second carbon.
- 2,2-dimethylpropane, two methyls on the central carbon.
- Requirement
- draw structural formulae of all isomers of C4H10 and C5H12
Section 23 of 38Alkene isomers (Higher tier)
- No isomers
- ethene and propene have none
- C4H8
- three isomers
- But-1-ene, with the double bond between carbons 1 and 2.
- But-2-ene, with the double bond between carbons 2 and 3.
- 2-methylpropene, with a methyl moved to the second carbon of a three-carbon chain.
- Numbering
- number the longest chain from the left
- Syllabus note
- skeletal formulae are not needed
Section 24 of 38Addition reactions of alkenes
- Reactivity
- the C=C functional group makes all alkenes react similarly, mainly by addition
- Addition
- atoms of a small molecule add across the C=C, which opens to a single bond, forming a saturated compound
- Hydrogenation
- an alkene reacts with hydrogen to give an alkane
- CH2CH2 + H2 → CH3CH3.
- It is used to make margarine, where polyunsaturated vegetable oils are partially hydrogenated to increase Mr and make solid fats.
- Bromine addition
- CH2CH2 + Br2 → CH2BrCH2Br
- The bromoalkane need not be named.
Section 25 of 38Bromine water test
- Purpose
- it distinguishes alkenes (unsaturated) from alkanes (saturated)
- Reagent
- bromine water, an orange solution, is safer and easier than bromine
- Alkane
- the solution stays orange because there is no C=C for bromine to react with
- Alkene
- bromine adds across the C=C, forming a colourless bromoalkane, so the solution decolourises
Section 26 of 38Addition polymerisation
- Process
- small reactive monomers, each with a C=C, join to form a polymer
- Mechanism
- one bond in each C=C breaks, so the molecule joins to an adjacent one, leaving a polymer with only single bonds
- Polymers
- polythene, poly(propene), poly(vinylchloride) and poly
- (tetrafluoroethene)
- polythene, poly(propene), poly(vinylchloride) and poly
- Naming
- put the monomer name in brackets with poly- before it, except poly(ethene) which is polythene
- Draw the monomer with a lower case n in front.
- Draw the repeating unit with a C–C single bond.
- Put brackets round the unit.
- Add continuation bonds on each carbon through the brackets.
- Add n after the brackets.
- Requirement
- draw the monomers ethene, propene, vinylchloride and tetrafluoroethene and write equations using n
Section 27 of 38Properties and uses of plastics
- General properties
- flexible, strong, good thermal and electrical insulators, corrosion-resistant, low density and do not rot
- Choosing a plastic
- properties decide the use, such as PVC insulating electrical wiring to prevent shocks and overheating
- Polythene shopping bags
- they are flexible, low density, waterproof, non-toxic and unreactive
| Plastic | Use |
|---|---|
| Poly(propene) | Ropes, crates |
| Poly(vinylchloride) | Drain pipes, window frames, wiring insulation |
| Poly(tetrafluoroethene) | Non-stick pans |
Section 28 of 38Disposal of plastics
- Problem
- many polymers are unreactive and non-biodegradable
- Landfill
- It takes up valuable land.
- Decomposers cannot break plastics down, so they last hundreds of years and fill sites quickly.
- Incineration
- Plastics release a lot of heat, but burning produces carbon dioxide that contributes to climate change.
- Some release toxic fumes, such as hydrogen chloride from PVC.
- Recycling
- It reduces waste to landfill or incineration.
- It conserves crude oil and fossil fuels, because recycling needs less energy than manufacture and avoids raw materials.
Section 29 of 38Making ethanol by fermentation
- Uses of ethanol
- alcoholic drinks such as wine and beer, car fuel and solvent
- Dissolve sugar or starch in water and add yeast.
- Ferment at 15–35 °C without oxygen for a few days.
- Yeast enzymes break down sugar to carbon dioxide and ethanol.
- Equation
- C6H12O6 → 2CO2 + 2C2H5OH, by anaerobic respiration of yeast
- Temperature
- too low makes the reaction too slow, and too high denatures the enzymes
- Batch process
- the yeast dies at about 15% alcohol, so the vessel is emptied and the process restarted
Section 30 of 38Hydration of ethene
- Reaction
- an alkene adds steam, so water adds across the double bond to make an alcohol
- Conditions
- about 330 °C, 60–70 atm and a concentrated phosphoric acid catalyst
- Industrial value
- it is a more efficient way to make ethanol for industrial uses
Section 31 of 38Test for alcohols
- Reagent
- acidified potassium dichromate(VI), meaning dichromate in dilute acid such as sulfuric acid
- Observation
- the orange solution turns green when an alcohol is present
Section 32 of 38Alcoholic drinks: health, economic and social impacts
- Context
- ethanol has been used in drinks for thousands of years, and consumption is now an issue because alcohol is readily available
- Health
- binge drinking and long-term misuse cause damage
- High blood pressure, heart disease, stroke, liver disease and digestive problems.
- Cancers of the breast, mouth, throat, oesophagus, voice box, liver, colon and rectum.
- A weakened immune system, learning and memory problems including dementia, mental health concerns and dependence.
- Economic
- Alcohol duties raise about £13.1 billion in 2023–2024.
- Exports add £6 billion, and the industry directly employs 650,000 people and supports over 1 million more jobs.
- Misuse costs over £21 billion a year in healthcare, policing and lost productivity.
- Social
- About 20% of reports of violence happen near pubs or clubs, suggesting alcohol is a factor.
- Alcohol-related crime costs £8–13 billion a year.
Section 33 of 38Ethanol as solvent and bioethanol
- Solvent
- a substance that dissolves solutes to form a solution, and ethanol dissolves substances including some insoluble in water
- Methanol
- it is a solvent in the pharmaceutical industry and an antifreeze, and it is poisonous if swallowed
- Bioethanol
- a biofuel made by fermenting glucose from plants
- Plants absorb CO2 in photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2.
- The glucose is fermented to ethanol, and the name only shows how it was made.
- Carbon neutral
- CO2 absorbed by the plants equals CO2 released on burning, and production is renewable
Section 34 of 38Advantages and disadvantages of bioethanol
| Advantages | Disadvantages |
|---|---|
| Carbon neutral | Lower specific energy than fossil fuels |
| Renewable and sustainable if crops are replanted | Developed countries lack the land, which is needed for food |
| Reduces greenhouse emissions and pollution | Engines and machinery must be converted |
| Could earn money for countries with space to grow crops | Crops depend on climate |
- Reality
- emissions from harvesting, transporting and processing mean more CO2 is emitted than absorbed overall, so it contributes to global warming
Section 35 of 38Names and formulae of alcohols
- Functional group
- the hydroxyl group, –OH, is responsible for the characteristic reactions, and is not the hydroxide ion OH-
- Series
- alcohols are a homologous series with formula CnH2n+1OH, differing by one CH2
- Naming
- the same stems as alkanes, with the final -e replaced by -ol
| Name | Formula |
|---|---|
| Methanol | CH3OH |
| Ethanol | C2H5OH |
| Propanol | C3H7OH |
| Butanol | C4H9OH |
Section 36 of 38Positional and chain isomers of alcohols (Higher tier)
- Positional isomers
- the –OH is on different carbons
- Butan-1-ol has –OH on carbon 1, and butan-2-ol has it on carbon 2.
- Chain isomers
- the longest chain differs because of branching
- Butan-1-ol and 2-methylpropan-1-ol both have formula C4H9OH.
- In 2-methylpropan-1-ol the longest chain has 3 carbons, with a methyl on carbon 2 and –OH on carbon 1.
- Naming rule
- use the lowest possible numbers, so butan-4-ol is butan-1-ol and butan-3-ol is butan-2-ol
Section 37 of 38Microbial oxidation of alcohols
- Microbial oxidation
- microbes such as Acetobacter oxidise ethanol to ethanoic acid
- Equation
- C2H5OH + 2[O] → CH3COOH + H2O, where [O] represents oxidation and water forms because hydrogen is removed
- Use
- it is how vinegar is made from ethanol
- Spoilage
- beer and wine go off if left exposed to air for days
| Alcohol | Oxidation product |
|---|---|
| Ethanol | Ethanoic acid |
| Propanol | Propanoic acid |
| Butanol | Butanoic acid |
- Higher tier
- the carboxylic acid has a higher proportion of oxygen and a lower proportion of hydrogen than the alcohol
Section 38 of 38Infrared spectroscopy
- Method
- infrared light is shone through a substance, and different bonds absorb different wavelengths, so functional group bonds can be identified
| Bond | Wavenumber / cm-1 |
|---|---|
| O–H (alcohols) | 3230–3550 |
| C–H | 2850–3300 |
| O–H (carboxylic acids) | 2500–3000 |
| C=O | 1680–1750 |
| C=C (alkenes) | 1620–1680 |
| C–C | 750–1110 |
- Propanoic acid
- its spectrum shows key peaks for O–H and C=O
- Fingerprint region
- the complex part of the spectrum is unique to each molecule and identifies a particular compound
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