3.3.2 Alkanes Flashcards
Coal is high in _____/______, and large amounts of _____/_____ are emitted from _____/_______.
Coal is high in sulfur content, and large amounts of sulfur oxides are emitted from power stations
State the conditions for catalytic cracking.
450°C
Slight pressure
Zeolite catalysts
What is petroleum?
A mixture consisting of mainly alkane hydrocarbons
What is a petroleum fraction?
A mixture of hydrocarbons with a similar chain length and similar boiling point
Describe the steps of the fractional distillation of crude oil (7)
- Crude oil is vaporised
- The tower is cooler at the top (negative temperature gradient)
- Vaporised crude oil enters the bottom and rises up the tower
- Fractions have different boiling points
- Each fraction condenses at the level where the temperature is lower than its boiling point
- Larger molecules (high bpt) condense lower in the tower
- Small molecules (low bpt) rise to the top of the tower
What are the different fractions of crude oil and their boiling points?
GOOGLE DOC
What is vacuum distillation and it’s purpose?
This is will heavy residues from the fractionating column are distilled again under a vacuum, which will make them more volatile.
It allows the heavier fractions to be separated without high temperatures, which could break them down.
How do we carry out fractional distillation in a lab?
Fractional distillation separates liquids with different boiling points
1) Heat the flask with bunsen burner
2) Vapours from the components are produced
3) Vapour of the substance with the lower boiling point reaches the top of the fractionating column first
4) Vapours with higher boiling points will condense back into the flask
5) The most volatile vapour passes into the condenser
6) The condenser cools the vapours and condenses them to a liquid
What is cracking?
conversion of large hydrocarbons to smaller, more useful hydrocarbon molecules by breaking C-C bonds
Why does cracking require high temperatures?
It’s a chemical process involving the splitting of strong covalent bonds, which requires lots of energy
What are the 3 economic reasons for cracking?
➤ Fractions of shorter C chains are more in demand
➤ To make use of excess larger hydrocarbons and to supply demand for shorter ones
➤ Products of crackign are more valiable than the starting materials
Describe Catalytic Cracking (3)
Catalytic Cracking
➤ slight/moderate presure, high temperature, zeolite catalyst
➤ produces branched, cyclic alkanes
➤ used mainly to produce motor fuels and aromatic hydrocarbons
What are the two types of cracking?
Catalytic
Thermal
What are fuels?
Compounds that release heat energy when burnt
What are alkanes used as?
Fuels
Describe the two types of combustion
Complete Combustion
➤ Excess oxygen
➤ Products are CO2 and H2O
Incomplete Combustion
➤ Limited amount of oxygen
➤ Products are CO and H2O and sometimes C ( soot )
Explain why alkanes are used as fuels
They readily burn in the precense of oxygen. Their combustion is highly exothermic.
What can soot cause?
Global dimming - reflection of the sun’s light
Internal combustion engines can produce a number of pollutants, give exmaples.
NOx CO, C, unburned hydrocarbons, SOx
Explain how the sulfur in petroleum fractions can lead to pollution
Petroleum fractions contain sulfur.
When the sulfur burns, it produces SO2.
SO2 will dissolve in atmospheric water and can produce acid rain.
What are the 3 steps of free radical substitution? What do they consist of?
Initiation
Propagation
Termination
Describe the Initiation Step of FRS
In the precense of UV light, the halogen is broken down.
This is an example of homolytic fission - each atom gets one electron from the covalent bond.
Describe the Propagation step of FRS
A hydrogen is replaced and the Cl free radical is reformed as a catalyst.
Describe the Termination step of FRS
Two radicals join to end the chain reaction and form a stable product.
When writing out the termination step, what’s important to note?
You should write teh step using the structural formulae
Why is the free radical mechanism described as a chain reaction prior the the termination step?
The Cl free radical is regenerated - it can hence react with several more alkane molecules.
What is a free radical?
A reactive species with possess an unpaired electron
How do we classify halogenoalkanes?
According to how many carbon atoms are attached to the C-H group.
Primary = 1
Secondary = 2
Tertiary = 3
What is a nucleophile?
An electron pair donator
What is substitution?
Swapping a halogen atom for another atom/groups of atoms
Describe the process of a Nucleophilic Substitution reaction
- The nucleophiles attack the positive carbon atom
- The carbon atom has a delta positive charge due to the electronegativity difference between the halogens and carbon
- The nucleophile will replace the halogen, to form a new compound + halogen free radical
ON GOOGLE DOC
What does the rate of a substitution reaction depend on?
The strength of the C-X bond.
A weaker bond will be easier to break, hence the reaction will be faster. As you go down group 7, the C-X become weaker.
GOOGLE DOC
State the reagents / conditions of nucleophilic substitution with hydroxide ions.
Reagents:
Potassium or Sodium Hydroxide
Conditions:
In aqueous solution
Heat under reflux
State the reagents / conditions of nucleophilic substitution with cyanide ions.
Reagents:
KCN dissolved in ethanol/water mixture
Conditions:
Heating under reflux
State the reagents / conditions for the nucleophilic substitution with ammonia.
Reagents:
NH3 dissolved in ethanol
Conditions:
Heating under pressure;
Haloalkane in ethanol
excess ammonia
Why is the naturally occuring ozone layer beneficial?
It filters out much of the sun’s UV radiation
Describe how chlorine radicals have led to the depletion of the ozone layer. Give equations.
Man-made CFCs have caused a hole to form in the ozone.
Chlorine radicals are formed in the upper atmoshpere when UV causes C-Cl bonds in these CFCs to break.
Chlorine free radicals catalyse the decomposition of the ozone layer.
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Why is the C-F bond not affected by UV?
It is stronger than the C-Cl bond
What have scientists done to combat ozone depletion?
They’ve banned the use of CFCs and developped alternative chlorine-free compounds.
For example, HFCs are now used for refrigerators and air conditioners. They don’t contain the C-Cl bonds.
How can SO2 be removed from waste gases produced from furances?
By flue gas desulfurisation.
The gases pass through a scrubber containing basic calcium oxide, which reacts with the acidic sulfur dioxide in a neutralisation reaction.
SO2 + CaO ➤ CaSO3
During flue gas desulfurisation, CaSO3 ( calcium sulfite ) is a product. What can it be used for?
It can be used to make calcium sulfate, for plaster board.
How are NOx compounds produced inside car engines?
High temperatures and spark in the engine provide sufficient energy to break strong N2 bond.
Describe 5 pollutants (excluding SOx) and their environmental consequences
NOx
➤ formed in car engines due to high temperatures
➤ NO is toxic, can form NO2 which is acidic and can form acid rain
CO
➤toxic
CO2
➤ contributes to global warming
Unburnt hydrocarbons
➤ contributes towards formation of smog
Soot
➤ global dimming and respiratory problems
What are catalytic converters?
A device in cars that removes CO, NOx and unburned hydrocarbns from exhaust gases, turning them into ‘‘harmless’’ CO2, N2 and H2O.
What do catalytic converters contain?
A ceramic honeycomb coated with a thin layer of a catalyst metals, platinum, palladium, rhodium, to give them a large surface area.
What are the 3 main greenhouse gases?
Carbon Dioxide
Methane
Water Vapour
Describe the mechanism of the greenhouse effect (4)
➤ UV wavelength radiation passes through the atmosphere and heats up Earth’s surface
➤ The Earth radiates infrafred long wavelength radiation
➤ The C=O bonds in CO2 absorb IR radiation, so it can’t escape
➤ Energy is transferred to other molecules in the atmosphere via collisions hence the atmosphere is warmed
Why have CO2 levels risen significantly in the last years?
due to increasing burning of fossil fuels
Write the overall equation for the reaction of fluorine with methane to form trifluoromethane (CHF3): MS [1]
CH4 + 3F2 →CHF3 + 3HF
Outline the essential features of the fractional distillation of crude oil that enable the crude oil to be separated into fractions: MS [4]
-Fractions are made of hydrocarbons that have different boiling points
-Boiling point depends on size/length
-The tower is coolest at the top
-Higher boiling point longer chained hydrocarbons will condense at the bottom
Give the conditions for thermal cracking.
High pressure → 7000kPa
High temperature → 700 oC
