12.4 Combustion of alkanes Flashcards
Describe reactivity of alkanes
un-reactive, they don’t react with acids, bases, oxidising agents or reducing agents
However they do burn with halogens under simple conditions
Shorter alkanes burn completely in oxygen to give CO2 and water
Give the reaction for combustion of methane
And ethane
CH4(g) + 2O2(g) —> 2H2O(g) + CO2(g)
C2H6(g) + 3.5O2 —> 3H2O + 2CO2
Describe relationship between length of hydrocarbon, and enthalpy change when they’re burnt
The longer the chain, the more heat given out so there is a more negative enthalpy change
What are fuels
Give some examples of alkane fuels
Substances that release heat energy when they undergo combustion.
They store a large amount of energy for a small amount of weight
- Methane (Natural gas)
- Propane (camping gas)
- Butane (Calor gas)
- Petrol (Mix of hydrocarbons of length C8)
- Paraffin (Mixture of hydrocarbons of length C10 to C18)
Describe incomplete combustion of alkanes
Eg propane
Is incomplete combustion more common in shorter or longer chain hydrocarbons?
. In a limited supply of oxygen, the poisonous gas CO is produced
Eg propane:
C3H8(g) + 3.5O2(g) —> 3CO + 4H2O
With even less oxygen, carbon (soot) is produced eg bunsen burner with closed air hole
Incomplete combustion happens more often with longer chain hydrocarbons which need more oxygen to burn
When hydrocarbon fuels are burned, they produce the harmful pollutant nitrogen oxides, what do these do
. NO, NO2, N2O4
When there is enough energy for nitrogen and oxygen to combine,
In the internal combustion engines of cars, the temperatures are high enough for nitrogen and oxygen in the air to combine to form eg NO
These oxides may react with water vapour to make nitric acid to contribute to acid rain
What is the problem with carbon particles resulting from burning fossil fuels
What is the problem with unburnt hydrocarbons from burning fossil fuels
These particulates cause respiratory illness and make asthma worse.
They also cause smog in the air
These can enter the atmosphere and are significant with greenhouse gases
They contribute to smog which can cause lots of health problems
What does CO2 do to the planet which is produced by burning fossil fuels
. Although it is necessary in the atmosphere, the levels are increasing leading to Global warming and climate change
.CO2 absorbs IR radiation from the sun and then reflects some of it back to earth
Where does sulfur dioxide come from
What is the equation for it forming its product when reacting with more oxygen and water
What problems does this cause
. Sulphur dioxide is produced when burning fossil fuels eg coal or natural gas in industry. As there are sulphur impurities in crude oil
.S02 + 1/202 + H20 —> H2S04(l)
. The H2SO4 is sulphuric acid which is a cause of acid rain
. Acid rain kills plants, destroys buildings, changes PH of ponds
What is flue gas desulphurisation
Why is it called this
This is the process of removing sulphur dioxide
It is called this because the gases given out by power stations are called flue gases
What happens in flue gas desulphurisation
What are the two methods
What are the reactions for them
. In one method, a mix of calcium oxide and water is sprayed into the flue gas in a scrubber
. It reacts with the gas to form calcium sulphite
This can be further oxidised to make calcium sulphate aka gypsum
CaO(s) + 2H2O(l) + S02(g) + 1/202 —->
CaS04.2H20(s)
. Or you can use calcium carbonate aka limestone with no water
CaC03(s) + S02(g) + 1/202–>
CaS04(s) + CO2(g)
What is a catalytic converter
All cars with petrol engines have catalytic converters in their exhaust systems
These reduce the output of CO, nitrogen oxides, and unburnt hydrocarbons from the gas mixture
Describe structure of a catalytic converter
What are the catalysts in it
. Honeycomb made of ceramic material
For a large surface area so a little amount of expensive catalyst goes a long way
Catalysts:
. Coated with rhodium and platinum metals
The exhaust gas pass over catalyst, reacting with each other to form less harmful gases
What are the reactions for
carbon monoxide and nitrogen oxide in catalytic converter
unburnt Hydrocarbons and nitrogen oxide in catalytic converter
So nitrogen oxide NO reacts with the other products to make safer products
- 2CO(g) + 2NO(g) –> 2CO2(g) + N2(g)
- C8H18(g) + 25NO–>
8C02 + 9H2O + 12.5N2
What are some examples of greenhouse gases
How do greenhouse gases cause global warming
Carbon dioxide, methane, water vapour
Short wavelengths of visible light from the sun penetrate the earths atmosphere
Then the earth re-emits back to space this as infrared light
But greenhouse gases trap some of this infrared radiation and keep it in the atmosphere, heating it up
.The earths temperature has been rising and the cause of this is thought to be greenhouse gases
Greenhouse gases have a polar bond!! which is why they absorb the IR
