Topic 6B: Alkanes Flashcards
what is an alkane
a saturated hydrocarbon with a general formula of CnH2n+2
what is a saturated hydrocarbon with examples
hydrocarbons which have no double bonds eg alkanes, cycloalkanes
what is a cycloalkane
a type of alkane in which the carbon atoms form a ring structure.
general formula CnH2n, yet it is saturated/has no double bond
what are some key features of alkanes
- consists solely of carbon and hydrogen atoms
- each carbon atoms form four single bonds
- they are saturated meaning they contain no double or triple bonds
what are some alkanes
methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12
How are alkane fuels are obtained
alkane fuels are obtained from the fractional distillation, cracking and reforming of crude oil
what is crude oil
a mixture of hydrocarbons extracted from underground oil reservoirs
- the main component alkanes
what is fractional distillation
- a technique used to separate hydrocarbon components of cruse oil based on their boiling point
describe and explain the process of fractional distillation
- crude oil is heated to above 350 degrees in a furnace making the crude oil vapours.
- these vapours enter a fractionating column that has a higher temperature at the bottom and lower temperatures near the top
- As the hot vapours rise through the column, they cool down. When the vapour temperature drops below the boiling point of a hydrocarbon in the mixture, it condenses from gas to liquid on the tray surface
- The condensed hydrocarbon liquids that accumulate on each tray are drawn off at specific intervals as fractions
describe the different properties of the fractional column as you move down the column
boiling point - BP increases down the column, as the alkane length increases
Viscosity - viscosity increases down the column
flammability - this decreases down the group as heavy fraction and long-chain hydrocarbons are the bottom are less flammable, making them harder to combust
what is cracking
a key process that involves breaking the carbon-carbon bonds in larger alkane chains to form smaller alkene and alkanes
a type of thermal decomposition reaction
eg
Decane ➔ octane + ethene
C10H22 ➔ C8H18 + C2H4
what are the two types of cracking
- thermal cracking
- catalytic cracking
describe and explain the process of cracking
thermal cracking
- thermal cracking operates at high temps (around 1000 degrees) and pressures ( about 70atm)
- produces a high yield of alkenes, used for creating numerous valuable products
describe and explain the process of catalytic cracking
- uses zeolite catalyst (hydrates aluminosilicate mineral) and requires a moderate temperature of 450 degrees and slightly above atmospheric pressure.
- primarily produces aromatic hydrocarbons and fuels for vehicles
explain two way catalyst enhance the efficiency of the cracking process
- they allow for lower temperatures and pressures, reducing energy consumption and production costs.
- they increase the rate of reaction, enabling faster production
what are the conditions of thermal cracking
High temperature (around 1,000°C) / High pressure (up to 70 atm)
what are the conditions of catalytic cracking
Moderate temperature (around 450°C) / Slightly above atmospheric pressure
what is reforming
a process used to convert less desirable straight-chin alkanes into more valuable branched, cyclic and aromatic hydrocarbons
what are the conditions of reforming
catalyst - platinum supported on aluminium oxide
Reaction conditions - High temperatures (450-520°C) and moderate pressures (10-40 atm).
what is an example of reforming using hexane
During reforming, alkanes undergo various transformations. For example:
Hexane (C6H14) can be reformed into cyclohexane (C6H12) and then benzene (C6H6):
C6H14 ➔ C6H12 + H2
C6H12 ➔ C6H6 + 3H2
what is combustion of alkanes
when alkanes are burnt in order to release a significant amount of energy
what are the two types of combustion
-complete combustion
- incomplete combustion
what is complete combustion
when there is an ample supply on oxygen so the alkane combnust complete and only forms carbon dioxide adn water as products
what is the complete combustion of methane
CH4(g) + 2O2(g) ➔ CO2(g) + 2H2O(g)
what is a condition of combustion of alkanes
- alkanes in their liquid state must be vaporised before combustion
what is the link between alkane size and combustion
- Smaller alkanes, due to their lower boiling points, vaporise and thus combust more readily
- Larger alkanes have more chemical bonds, hence when combusted, they release more energy per mole, making them better fuels
what is incomplete combustion
when the oxygen supply is limited, alkanes undergo incomplete combustion, leading to the formation of carbon monoxide and water vapour sometimes solid carbon (soot)
how is carbon monoxide formed and how is it bad for the environment
- produces during incomplete combustion
- binds with haemoglobin more effectively than oxygen.
- this prevents oxygen from transporting within the body
- High concentrations lead to asphyxiation, while lower levels may cause blurred vision, poor coordination, and headaches.
how is nitrogen oxide formed and how is it bad for the environment
- made when atmospheric nitrogen and oxygen react at high temperatures and pressures within an engine
N2(g) + O2(g) ➔ 2NO(g) - Nitrogen oxides contribute to the formation of photochemical smog when they react with unburnt hydrocarbons in the presence of sunlight
- Nitrogen oxides also contribute to the formation of acid rain. When nitrogen dioxide dissolves in water, it forms nitric acid (HNO3), which lowers the pH of rainwater:
2NO2(g) + H2O(l) + 1⁄2O2(g) ➔ 2HNO3(aq) - acid rain can cause damage to buildings through erosion and also damage to aquatic life and nature
how is sulfur dioxide formed and how is it bad for the environment
- Formed by oxidising sulfur impurities in some fossil fuels, as represented by the equation:
S(s) + O2(g) ➔ SO2(g) - Sulfur dioxide dissolves into clouds and oxidises into sulfuric acid, the main component of acid rain. This damages plants, animals and infrastructure.
what is a catalytic converters
- used to mitigate harmful pollutants
how does a catalytic converter work
- Installed on vehicle exhaust systems to remove pollutants.
- Contain a honeycomb structure coated with catalyst metals such as platinum, rhodium, and palladium.
- These metals catalyse the conversion of harmful compounds into less harmful substances as exhaust gases pass through.
- They oxidise carbon, carbon monoxide, and unburnt hydrocarbons into carbon dioxide and water, while reducing nitrogen oxides to nitrogen and oxygen gases.
eg
2CO(g) + 2NO(g) ➔ 2CO2(g) + N2(g)
what are biofuels
- Biofuels are renewable energy sources derived from organic matter over relatively short periods.
what are the different types of biofuels
- bioethanol - an alcohol produced by fermenting sugars from crops like maize or sugarcane
- biodiesel - refused from renewable fats and oils, such as vegetable oil
- biogas 0 generated through the decomposition of organic waste mass
why are biofuels considered carbon neutral
- when burnt, they release CO2 that was recently absorbed by plants during growth
- this creates a short-term carbon cycle
why arent biofuels actual carbon neutral
- CO2 is emitted during fuel refinement and transportation
- additional emissions come from fertiliser production and agricultural machinery used in crop cultivation
what are some advantages of biofuels
- renewable resource
- Reduced greenhouse gas emissions - The carbon dioxide released during biofuel combustion is offset by the carbon dioxide absorbed during photosynthesis.
- Sustainable resource - Biofuels can be produced from a wide range of plant materials and waste products.
-Enhanced energy security - Biofuels can be produced domestically, reducing dependence on foreign oil.
what are some disadvantages of biofuels
- Competition for land use
- Engine modifications - Some engines may require adjustments to efficiently use fuels with high ethanol content
- Reduced biodiversity - Large-scale monoculture plantations for biofuel crops can negatively impact biodiversity.
- Deforestation risk - Increasing demand for biofuels may lead to deforestation
what is free radical substitution
a photochemical reaction requiring UV light between alkanes and halogens to form halogenoalkanes.
- initiation, propagation, termination
why is UV light needed for free radical substitution
it is needed as alkanes are very unreactive so UV light is needed for he reaction to occur
what is heterolytic fission
The process of breaking a covalent within a molecule leading to the formation of
ions. Upon bond breaking, one atom receives the electron pair and becomes a negatively charged
ion. Other atom becomes a cation
Br-Br → Br+ + Br−
what is homolytic fission
the process of breaking a covalent bond within a molecule leading to the formation of radicals.
- when broken each atom receives one atom from the bonding pair and each atom becomes a radical
Br-Br → Br⋅ + Br⋅
what is a radical
A species with an unpaired electron. Represented in mechanisms by a single dot
what occurs in initiation
-UV light supplies the energy to break the bonds in a halogen eg Cl2
Cl2 → Cl⋅ + Cl⋅
what occurs in propagation
- the chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical
Cl⋅ + CH4 → ⋅CH3 + HCl - the methyl free radical reacts with a Cl2 molecules to produce the main product and another Cl free radical
⋅CH3 + Cl2 → CH3Cl + Cl⋅
what occurs in termination
- collision of wo free radicals does not generate further free radicals, so the chain is terminated
⋅CH3 + ⋅CH3 → CH3CH3
⋅CH3 + Cl⋅ → CH3Cl
Cl⋅ +Cl⋅ → Cl2
what are the limitations of free radical substitution
- it is difficult to control
