Fuels - Topic 8 Flashcards
Hydrocarbons:
compounds that contain carbon and hydrogen only
What is crude oil?
- a complex mixture of hydrocarbons
- containing molecules in which carbon atoms are in chains or rings
- an important source of useful substances (fuels and feedstock for the petrochemical industry)
- a finite resource
Explain the change in boiling points of alkanes:
- boiling points increase as length of chain increases
- as the chain is longer so there are more places for IMFs so more forces to be broken
What does crude oil contain and how can these substances be separated?
- crude oil is a mixture which contains a large number of different liquids which have dif. boiling points
- these types of mixtures can be separated by fractional distillation
The easier something is to boil…
the more flammable it is
As the boiling points of the fractions increases what happens to their properties?
- appearance darkens
- ease of lighting becomes more difficult
- smokiness of the flame increases
- viscosity (stickiness) increases
- thicker liquids and not runny
How is crude oil separated by fractional distillation in the lab?
- if crude oil is heated the liquids with the lowest bp’s will evaporate first and pass up the flask and fractionating column
- this vapour then passes down the side-arm and through the condenser
- this cools down the vapour and so it eventually condenses as a liquid which forms in the conical flask
- if the mixture is heated very gently between a number of temp ranges - different liquids can be collected at dif. temps - these are called fractions
How is crude oil separated by fractional distillation industrially?
- crude oil is heated/boiled and then it enters the fractionating column
- as they rise up the column the temp becomes lower
- when the temps becomes just below the bp of the a substance that substance will melt and a liquid will be collected in a tray at that level in the column
- at different levels a mixture of liquids called fractions are collected - fractions will consist of a mixture of liquids containing similar sized molecules with similar boiling points
- substances with low boiling points will remain gases even at the top of the column as the temp is higher than their boiling points
- these gases are called petroleum gases and will be composes of the shorter chains of alkanes e.g. ethane, methane, propane etc.
- some v. long alkanes are still liquid even at high temps as the both of the column - this fraction is called residue
Uses of (petroleum) gases:
- domestic heating
- cooking
Uses of petrol:
fuel for cars
Uses of kerosene:
fuel for aircraft
Uses of diesel oil:
fuel for some cars and trains
Uses of fuel oil:
- fuel for large ships
- used in some power stations
Uses of bitumen:
- surface roads - tar
- roofs
Examples of some fractions of crude oil:
- (petroleum) gases
- petrol
- kerosene
- diesel oil
- fuel oil
- bitumen
Organic compound:
compounds which contain carbon
Homologous series:
a group of chemical compounds which share certain characteristics
What do compounds in the same homologous series share?
- each compound had the same functional group and therefore the same general formula
- have similar chemical properties
- differ by CH2 units in molecular formulae from neighbouring compounds
- show gradual trend in physical properties (as exemplified by their boiling points)
The complete combustion of hydrocarbon fuels is a reaction in which…
- CO2 and H2O are produced
- energy is given out - exothermic
Which alkanes are used as fuels and why?
the shorter alkanes are used as fuels are they react easily with oxygen
When does complete combustion take place?
when an alkane burns in sufficient oxygen
Balanced equation for complete combustion of ethane:
Balanced equation for complete combustion of hexane:
How can impurities in some hydrocarbon fuels result in the production of sulphur dioxide?
- most fuels inc. coal, carbon and/or hydrogen may also contain some sulphur
- when the fuels are burnt in oxygen this sulphur can react with the oxygen to form sulphur dioxide
How is acid rain caused?
when sulphur dioxide dissolves in rain water
What are the problems with acid (sulphuric acid) rain?
- damages buildings and statues (made of limestone)
- reduces the growth of or kills trees and crops
- cause pH of water tables to be lower which kills fish
What happens when fuels are burned in engines?
- When burned in engines at high temperatures nitrogen and oxygen from the air combine to produce nitrogen monoxide
- When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide
- nitrogen monoxide and nitrogen dioxide are pollutants
Advantages of using hydrogen as a fuel in cars rather than petrol:
- Use of hydrogen – petrol is from crude oil, a finite resource
- Only produces water – no CO2 produced which contributes to global warming
Disadvantages of using hydrogen as a fuel in cars rather than petrol:
- Expensive
- Difficult to transport and store hydrogen - as H2 takes up a lot of space since it is a gas
- Dangerous – hydrogen can be explosive
What are petrol, kerosene, diesel oil and methane?
non-renewable fossil fuels
Where are petrol, kerosene and diesel oil obtained from?
obtained from crude oil
Where is methane obtained from?
found in natural gas
What does the fractional distillation of crude oil provide in general?
fractional distillation of crude oil provides a surplus of the long chain hydrocarbons e.g. decane but a shortage of short chains hydrocarbons e.g. methane
What does cracking involve?
cracking involves the breaking down of larger, saturated hydrocarbon molecules (alkanes) into smaller, more useful ones, some of which are unsaturated (alkenes)
What type of reaction is the cracking of hydrocarbons?
thermal decomposition
General cracking equation:
longer alkane → shorter alkane + short-chain alkene
What is the process of cracking?
- Hydrocarbons can be cracked to produce smaller, more useful molecules
- heat the hydrocarbons to vaporise them
- the vapours are then:
- either passed over hot catalyst
- mixed with steam and heated to a v. high temp so that thermal decomposition reactions can occur
What are the products of cracking?
alkanes + alkenes (unsaturated hydrocarbons)
Why is cracking necessary?
- vital process as it converts long chain molecules which are less useful into short chain molecules which are more useful
- demand for smaller chained alkanes is much greater for longer chained alkanes as they are more useful - demand > supply
- shorter chained hydrocarbons ignite more easily and so are more useful as fuels
When does incomplete combustion take place?
Incomplete combustion happens when the supply of air or oxygen is poor
How much energy is released in incomplete combustion?
Less energy is released than during complete combustion
Products of incomplete combustion:
- carbon monoxide
- carbon (soot)
- water
Incomplete combustion of propanol example:
C3H8 + 3O2 → 2CO + C + 4H2O
What is the carbon released as in incomplete combustion?
carbon is released as fine black particles (soot)
Harmful impacts of soot:
- Soot can cause breathing problems and it blackens buildings
- It may block boilers and other appliances, or cause a fire
Harmful impacts of carbon monoxide:
- Carbon monoxide is a toxic gas
- It is absorbed in the lungs and binds with the haemoglobin in the red blood cells - this reduces the capacity of the blood to carry oxygen
- Carbon monoxide causes drowsiness, and affected people may fall unconscious or even die
Why can the incomplete combustion of hydrocarbons produce carbon and carbon monoxide?
less oxygen so carbon atoms cannot be completely oxidised to produce CO2 instead CO and O are produced
Why can the incomplete combustion of hydrocarbons produce carbon and carbon monoxide?
- If there’s not enough oxygen, some of the fuel doesn’t burn – this is partial
combustion - solid particles of soot (carbons) and unburnt fuel are released
- Carbon monoxide (CO) is also released when there isn’t enough oxygen to produce CO2 instead
How does carbon monoxide behave as a toxic gas?
carbon monoxide is a colourless and odourless gas which, if breathed in, prevents red blood cells carrying oxygen around your body, which leads to death
What problems can be caused by the incomplete combustion producing carbon monoxide and soot in appliances that use carbon compounds as fuels?
- Carbon monoxide causes health problems
- Soot causes global dimming
