Alkanes Flashcards
What is the general formula for alkanes?
CnH2n+2
What is a hydrocarbon?
A compound made up of carbon and hydrogen only.
Why are alkanes described as being saturated?
The carbon atoms in alkanes are joined together by single covalent bonds.
Describe and explain the arrangement of bonds around each carbon atom in an alkane.
Each carbon atom in an alkane forms four single bonds, either with hydrogen atoms (C-H) or other carbon atoms (C-C). These bonds are arranged in a tetrahedral geometry around the central carbon atom, with bond angles of ≈109.5°.
The tetrahedral arrangement of the bonds is due to the equal repulsion between the bonding pairs of electrons.
State and explain the effect of increasing chain length on the boiling point of alkanes.
As the chain length of alkanes increases, boiling point increases.
This is because the surface contact between molecules increases which leads to stronger induced dipole-dipole forces between molecules which require greater energy to overcome.
State and explain the effect of branching on the boiling point of alkanes.
Branched alkanes generally have lower boiling points compared to straight-chain isomers.
This is because branched alkanes cannot pack as closely together which means there are fewer surface points of contact. This leads to weaker induced dipole-dipole forces which require less energy to overcome.
What is the general formula of cycloalkanes?
CnH2n
What is crude oil?
Crude oil is a mixture of hydrocarbons, primarily alkanes.
What is the name of the process we use to separate the components of crude oil?
Fractional distillation
What physical property of hydrocarbons allows them to be separated by fractional distillation?
The different boiling points of hydrocarbons.
Outline the process of fractional distillation.
- Crude oil is heated to vaporise it into a gas and then passed into the fractionating column.
- The fractionating column is cooler at the top compared to the bottom. As the gas rises up the column, it gradually cools down.
- When the individual hydrocarbon components in the gas mixture reach a height where the temperature is lower than their respective boiling points, they condense into liquids.
- These liquids are collected in trays placed at specific intervals along the column and are drawn off as separate fractions.
What are the names and uses of crude oil fractions?
- Petrol is used as a fuel for vehicles.
- Naptha is used as a petrochemical feedstock.
- Kerosene is used as jet fuel.
- Diesel is used as a fuel for vehicles.
- Mineral oil is used as a lubricant.
- Bitumen is used in road surfacing.
What is meant by the term fraction, in the context of fractional distillation?
A fraction is a mixture of hydrocarbons with similar boiling points
How do the boiling points of hydrocarbons change as you move down the fractionating tower?
The boiling points of hydrocarbons increase as you move down the fractionating tower.
What happens to the temperature as you move higher up the fractionating tower?
The temperature decreases as you move higher up the fractionating column.
Give 3 physical properties of short-chain hydrocarbons.
- Low boiling points (they are typically gases or volatile liquids).
- High flammability.
- Low viscosity.
Give 3 physical properties of long-chain hydrocarbons.
- High boiling points.
- Low flammability.
- High viscosity (they are often in the form of thick residues like tar or bitumen).
Are hydrocarbons with long or short chains collected at the top of the fractionating column?
shorter chains
Do hydrocarbons with long or short chains have a lower boiling point?
shorter chains have lower boiling points.
Why do short-chain hydrocarbons make good fuels?
They are very flammable and volatile, so are easy to ignite and combust for energy.
Why are long-chain hydrocarbons bad fuels?
They are not very flammable or volatile. This means they are difficult to ignite and combust for energy.
What is cracking?
Cracking is the process by which longer chain hydrocarbons are split into shorter, more useful hydrocarbons.
Why are the products of cracking useful?
- They can be used as chemical feedstocks for the petrochemical industry.
- They make more efficient fuels (than longer-chain hydrocarbons).
What type of reaction is cracking an example of?
Thermal decomposition
Name the 2 types of cracking.
Thermal cracking and catalytic cracking.
Compare the conditions used in thermal cracking and catalytic cracking.
-Thermal cracking uses very high temperatures (≈1,000°C) and high pressures (about 70 atm).
-Catalytic cracking uses lower temperature (≈450°C), lower pressure, and a zeolite catalyst.
Compare the main types of hydrocarbon formed in thermal cracking and catalytic cracking
-Thermal cracking mainly produces alkenes.
-Catalytic cracking mainly branched alkanes and aromatic hydrocarbons.
Outline the process of thermal cracking.
- Long chain alkanes are heated to very high temperatures (around 1,000°C) under high pressure (about 70 atm).
- At these extreme conditions, the long chain alkanes break into smaller fragments.
- The fragments recombine to form a mixture of shorter chain alkanes and alkenes.
Why is cracking an economically important process in the petrochemical industry?
-converts less valuable, longer chain hydrocarbons into more valuable, shorter chain hydrocarbons.
- These products, such as alkenes and aromatic compounds, are essential chemical feedstocks for the petrochemical industry and are in high demand for manufacturing plastics, chemicals, and fuels.
Outline the process of catalytic cracking.
- Long chain alkanes are heated until they vaporise into a gas.
- The gaseous alkanes are passed over a zeolite catalyst (hydrated aluminosilicate) at around 450°C.
- This breaks the long chain alkanes into a shorter chain alkane and an alkene.
What are the products of cracking?
an alkene and an alkane.
e.g decane can be cracked to give ethene and octane
Why are alkanes used as fuels?
they release a large amount of energy when they undergo combustion.
What is combustion?
the reaction between a fuel and oxygen, resulting in the release of energy as heat.
What is the difference between complete and incomplete combustion?
-In complete combustion, there is a plentiful supply of oxygen so the fuel reacts completely to form carbon dioxide and water vapour.
-In incomplete combustion, there is an insufficient supply of oxygen so the fuel doesn’t react completely, producing carbon monoxide and water vapour
What is the word equation for the complete combustion of a hydrocarbon?
Hydrocarbon + oxygen ➔ carbon dioxide + water
What products can be formed during incomplete combustion of an alkane?
Carbon monoxide, water vapour, carbon and unburnt hydrocarbons can be formed.
Why is carbon monoxide harmful?
poisonous and can prevent oxygen from binding to haemoglobin in the blood.
What are the consequences of unburnt hydrocarbons being released into the atmosphere?
Some unburnt hydrocarbons are carcinogenic and can contribute to the formation of photochemical smog.
How are oxides of nitrogen produced during combustion?
Oxides of nitrogen are produced when nitrogen and oxygen in the air combine at the high temperatures and pressures present in internal combustion engines.
What are the environmental consequences of oxides of nitrogen being released into the atmosphere?
nitrogen oxides can react to form nitric acid which contributes to acid rain. They are also a cause of photochemical smog.
What is a catalytic converter?
A catalytic converter is a device fitted to a vehicle’s exhaust system which uses transition metal catalysts (such as platinum, palladium or rhodium) to convert polluting gases into less harmful products.
Explain the origin of sulfur dioxide pollution.
Sulfur dioxide pollution originates from sulfur-containing impurities in fuels like coal and crude oil. When burned, these impurities are oxidised to form sulfur dioxide.
What is the environmental consequence of sulfur dioxide being released into the atmosphere?
In the atmosphere, sulfur dioxide reacts with oxygen and water to form sulfuric acid, contributing to acid rain.
Explain, with the use of equations, why sulfur dioxide can be removed from flue gases using calcium oxide or calcium carbonate.
- CaO(s) + H2O(l) + SO2(g) ➔ CaSO3(s) + H2O(l)
- CaCO3(s) + 2H2O(l) + SO2(g) ➔ CaSO3(s) + 2H2O(l) + CO2(g)
Sulfur dioxide can be removed from flue gases by reacting it with an alkali. A slurry of calcium oxide or calcium carbonate is sprayed onto the flue gases, neutralising the SO2 and producing a solid waste product, calcium sulfite
Explain the environmental consequence of carbon dioxide being released into the atmosphere.
Carbon dioxide is a greenhouse gas which absorbs infrared radiation (heat) in the atmosphere and emits it back towards Earth in a process known as the greenhouse effect. This causes the Earth’s temperature to rise, which is the process we call global warming.
What is the word equation for the reaction that takes place within a catalytic converter between carbon monoxide and nitrogen monoxide?
Carbon monoxide + nitrogen monoxide ➔ nitrogen + carbon dioxide
Explain the low reactivity of alkanes.
-The strong C-C and C-H covalent bonds require a lot of energy to break.
-The non-polar nature of C-C bonds make it difficult for alkanes to attract nucleophiles or electrophiles.
What is a free radical?
A free radical is a species with an unpaired electron (represented by a dot).
Describe the steps in a free radical reaction.
- Initiation (formation of free radicals).
- Propagation (repeated reactions that regenerate more free radicals).
- Termination (when two free radicals combine to form a stable product).
What is the general equation for the reaction between an alkane and a halogen?
Alkane + halogen ➔ halogenoalkane + hydrogen halide
State and explain the conditions are required for alkane molecules to undergo reactions with halogens?
Alkanes react with halogens when exposed to ultraviolet (UV) light.
The energy from the UV light breaks the halogen-halogen bond to initiate the reaction.
What type of reaction occurs when alkanes react with halogens?
Free radical substitution
What is the equation for the initiation step in the reaction between methane and chlorine?
Cl2 ➔ Cl* + Cl*
What type of bond fission (homolytic or heterolytic) occurs during the initiation step of free radical substitution reactions?
Homolytic fission
How do the relative amounts of chlorine and methane present determine the identity and ratio of products produced in the reaction between methane and chlorine?
-If methane is in excess, more chloromethane (CH3Cl) will be produced.
-If chlorine is in excess, further substitutions of H with Cl may take place which will form dichloromethane (CH2Cl2), trichloromethane (CHCl3), and tetrachloromethane (CCl4).
Give three equations for possible termination steps in the reaction between methane and chlorine.
- Cl* + Cl* ➔ Cl2
- CH3* + CH3* ➔ C2H6
- Cl* + CH3* ➔ CH3Cl
Give two equations for possible propagation steps in the reaction between methane and chlorine.
- Cl* + CH4 ➔ HCl + CH3*
- CH3* + Cl2 ➔ CH3Cl + Cl*
