Alkanes and fuels Flashcards
Review of alkanes and problems with them as fuels
A substances boiling point is a good indicator of what?
The boiling point of a substance is a good indication of the strength of forces between the particles in it.
What, in general, do high boiling points indicate?
High boiling points mean large intermolecular forces
What are van der Waals forces?
Van der Waals forces are a weak form of intermolecular force caused by electrons creating instantaneous dipole–induced dipole forces.
What are intermolecular forces?
They are forces between different molecules
Are van der Waals forces strong or weak?
They’re the weakest.. boo hoo.
What causes van der Waals forces to increase in alkanes?
van der Waals forces increase with increasing numbers of electrons/increasing chain length.
What will have the highest boiling point, a straight chain alkane or a branched chain alkane?
Straight chain
Why do straight chain alkanes have the biggest boiling point compared to branched chains?
Van der Waals forces are highest when molecules have lots of surface contact. Straight chain molecules can pack closer so have more surface contact.
Describe the trend in the boiling points of the alkanes
the boiling point increases as the length of the carbon chain increases
Why does the boiling point of alkanes increase as the number of carbon atoms in the main chain increase?
The intermolecular (van der Waals) forces increase with increasing numbers of electrons/increasing chain length.
What do all alkanes react with when they burn in air?
Oxygen gas (O2)
What do we mean by complete combustion?
There is enough oxygen gas for all the carbon and hydrogen in the hydrocarbon to be fully oxidised.
OR
Products of burning the alkane are water and carbon dioxide
What is needed for complete combustion to occur?
Complete combustion occurs in a plentiful supply of air/oxygen.
What are the products of complete combustion of a hydrocarbon?
carbon dioxide and water
complete the equation:
alkane + oxygen →
alkane + oxygen → water + carbon dioxide
Why are alkanes used as fuels?
They burn exothermically
What does exothermic mean?
Releases energy to the surroundings
What could be a toxic gas product of incomplete combustion?
carbon monoxide, CO
why is carbon monoxide toxic?
It binds irreversibly to haemoglobin and so stops oxygen being transported around the body.
OR
you suffocate
OR
stops oxygen travelling around the body
What is a solid product of incomplete combustion of alkanes?
soot
or
solid carbon
What is incomplete combustion
not enough oxygen/air for the hydrocarbon to fully oxidise to carbon dioxide and water.
Why is carbon monoxide difficult to detect
it has no colour or odour
Give an equation for the incomplete combustion of propane that has only a solid product and water only
C3H8(g) + 2 O2(g) → 4H2O(l) + 3C(s)
Give an equation for the incomplete combustion of propane that has a toxic gaseous product and water only
C3H8(g) + 7/2 O2(g) → 4H2O(l) + 3CO(g)
Soot can block engine parts.
What problems can this cause?
Give 2 reasons
- Less energy given out by the fuel
- Engine is less powerful
- Engine needs to burn more fuel to get the same energy
- Increased costs due to need to use more fuel
Soot can contribute to global dimming. What is global dimming?
Global dimming is a reduction in sunlight reaching Earth’s surface caused by air pollution/soot.
What is a greenhouse gas?
A gas that contributes to the greenhouse effect by absorbing infrared radiation
Name 2 greenhouse gases
- Carbon dioxide
- Hydrocarbons
- Water vapour
- Methane
- Nitrous oxide (N2O)
What is the greenhouse effect?
The greenhouse effect is the process by which radiation from a planet’s atmosphere warms the planet’s surface to a temperature above what it would be without its atmosphere.
Some hydrocarbon fuels can pass through the engine unburnt. Why are they a problem?
- wastes fuel
- harmful - carcinogenic
- air pollutant - greenhouse gases
How do you remove unburnt hydrocarbons from a cars exhaust?
Use a catalytic convertor
What gas do unburnt hydrocarbons react with in a catalytoc convertor?
NO (nitrogen monoxide)
What are the products of unburnt hydrocarbons reacting with NO in a catalytic convertor?
carbon dioxide
water
nitrogen gas (N2)
What gases are normally found in the atmosphere?
Nitrogen (N2), Oxygen (O2), Carbon Dioxide (CO2), Rare gases (Group 0)
How is SO2 produced when hydrocarbons are burnt?
Some fossil fuels contain sulphur as an impurity which reacts with oxygen
Give an equation for the combustion of the sulphur that is contained in a fuel.
S + O2 → SO2
What kind of oxide is Sulphur Dioxide?
acidic oxide
How does sulphur dioxide cause acid rain?
This is an acidic oxide and can react with water to form acid rain.
Why does sulphur dioxide cause problems when fuel is burnt?
It is toxic
can cause respiratory problems
causes acid rain
How is the effect of sulphur in fuels reduced?
Sulfur is usually removed from the fuel before burning; or SO2 is removed from fumes after burning.
How is SO2 removed from waste gases after it is burnt?
React with calcium oxide (CaO) or calcium carbonate (CaCO3)
Why does sulphur dioxide react with calcium oxide/calcium carbonate?
sulphur dioxide is an acid; the calcium compounds are bases
What kind of reaction is sulphur dioxide reacting with calcium oxide?
Acid-base or neutralisation.
What is the product of calciumoxide and sulphur dioxide reacting? What can it be used for?
The reaction forms gypsum [calcium sulfate(IV), CaSO3.
Gypsum can make plasterboard
How is nitrogen oxide formed in a car engine?
There are sparks and very high temperatures in an engine.
This causes N2(g) from the air to react with O2(g) .
Why are nitrogen oxides removed from exhaust gases?
They can cause acid rain; greenhouse effect
Whats the equation for the reaction in a catalytic convertor?
2CO+2NO → 2CO2 +N2
What elements act as the catalyst in a catalytic convertor?
Platinum - Pt
Rhodium - Rh
Palladium - Pd
Why do catalytic converters have a ceramic honeycomb coated with a thin layer of catalyst?
This is cheaper (thin layer) and gives a large surface area (so faster reaction).
