Alkanes Flashcards
What are alkanes?
A homologous series of saturated hydrocarbons
What are hydrocarbons?
Compounds which only contain hydrogen and carbon
What is a saturated hydrocarbon?
A hydrocarbon which only contains single bonds, no double bonds
How is methane produced?
Anaerobic respiration in cows and sheep, compost heaps, rice fields and landfill waste tips
Why are alkanes non-polar?
Because there’s a small difference in electronegativity between the carbon and the hydrogen
What intermolecular forces exist between alkane molecules?
Van der Waals forces
How does the Mr of compounds affect the strength of the intermolecular forces?
Greater Mr = stronger forces
Why do branches chained isomers have a lower melting and boiling point?
Smaller surface area in contact with each other and do not fit together as closely
What substances can you dissolve alkanes in?
Non-polar liquids like hexane and cyclopetane
How can you separate mixtures of alkanes?
Fractional distillation
What is crude oil?
A mixture containing mainly alkane hydrocarbons that can be separated by fractional distillation
How is crude oil made?
Remains of animals and plants get covered in mud at the bottom of the ocean and the high pressure and temp change the mud so the animal and plant remains turn into crude oil
What is fractional distillation?
The continual evaporation and condensation of a mixture causing it to separate into its components due to a difference in their boiling points
What is a fraction?
Group of compounds with similar boiling points and are removed at the same level of a fractionating column
Explain how fractional distillation works
Crude oil heated in a furnace until part of it turns into a vapour. Vapour rises up a fractionating column where it’s cooler as you go up and it condenses at its boiling point and it’s piped off
Where are short chain molecules collected in a fractionating column?
At the top bc its cooler there and they have a lower boiling point
What is cracking?
The process where long chain hydrocarbon molecules are broken into shorter chain molecules which are high in demand bc they’re more useful
Describe the conditions of thermal cracking
High temp of 1000-1200K and high pressure of 70atm for 1s
What is produced when long chain alkanes are cracked thermally?
Short chain alkanes and alkenes
Why are the conditions in thermal cracking applied for a short amount of time?
Because the high temp and pressure could decompose the alkanes into carbon and hydrogen
Describe the conditions needed in catalytic cracking
Temp of 800-1000K, pressure of 1-2atm and a zeolite catalyst
What is a zeolite catalyst?
Acidic mineral with a honeycomb structure made from aluminium oxide and silicon dioxide
Why is zeolite’s honeycomb structure good?
Large surface area which increases the rate of reaction
Why are high temperatures needed in cracking?
To break the strong carbon-carbon bonds in the alkanes which require a lot of energy to overcome
What do alkanes that burn in oxygen produce?
Carbon dioxide and water
What do alkanes that burn in a limited supply of oxygen produce?
Water and carbon monoxide
Why is carbon monoxide dangerous?
Reacts with haemoglobin in the blood and prevents them from carrying oxygen to all of the cells in the body
What does catalytic cracking produce?
motor fuels and aromatic hydrocarbons
What pollutants do internal combustion engines form?
nitrogen oxides, carbon monoxide, carbon particles, sulfur dioxide and unburnt hydrocarbons
Why are nitrogen oxides formed in an internal combustion engine?
Bc of the high tempeerature and pressure
Why is sulfur dioxide formed in an internal combustion engine?
Because there may be sulfur in the impurities of the crude oil
What is the greenhouse effect?
The effect of trapping the energy from the sun
What is global warming?
The increasing average temperature of the atmosphere at the surface of the Earth
Name three greenhouse gases
Water vapour, methane and carbon dioxide
What is the origin of water vapour?
Occurs naturally in the atmosphere
What is the origin of carbon dioxide?
Produced in respiration and combustion
What is the origin of methane?
Product of digestion from cows and sheep
What causes global warming?
Changing the balance of the concentration of greenhouse gases bc they trap more heat from the sun so the Earth’s temperature increases
Why is rain water a weak acid?
Bc of naturally occuring Co2 in the atmosphere
What causes acid rain?
Sulfur dioxide from the impurities of fossil fuels dissolving in rain water then being oxidised to form sulfuric acid
Why is acid rain harmful?
Bc it destroys trees and vegetation, corrodes buildings, kills fish and also can fall far from the source
What causes photochemical smog?
Gases like nitrogen oxides, sulfur dioxide and unburnt carbon fuels react with sunlight and carbon atoms from incomplete combustion
How do catalytic converters reduce greenhouse gas emissions?
Converting harmful gases into less harmful gases using a catalyst
What catalysts are used in a catalytic converter?
Rhodium and platinum
Why is a thin layer of metal catalyst used in a catalytic converter?
Large surface area increases the rate of conversion and also small amount of expensive metal is used
How is the amount of sulfur dioxide released into the atmosphere reduced?
Flue gas desulfurisation
Explain the process of flue gas desulfurisation
Gases passed through a slurry of calcium oxide which neutralises the acid bc it’s a base and forms calcium sulphite
What is done with the products of the flue gas desulfurisation?
Calcium sulphite is oxidised to calcium sulfate to be used as a construction material
