Alkanes as Fuels

Alkanes

What is combustion?

A chemical reaction in which a compound reacts with oxygen.

What happens during the complete combustion of an alkane?

The alkane reacts completely with oxygen to produce carbon dioxide and water.

What are the products of complete combustion of an alkane?

Carbon dioxide and water.

What does complete combustion mean?

All of the atoms in the fuel are fully oxidised.

What is the equation for the complete combustion of propane?

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Why are alkanes useful as fuels?

They undergo combustion and release energy.

What are the main products formed when hydrocarbon fuels burn completely?

Carbon dioxide and water.

What environmental problem is caused by carbon dioxide from burning fuels?

Carbon dioxide is a greenhouse gas and increasing its concentration contributes to global warming and climate change.

How can carbon dioxide emissions from fossil fuels be reduced?

By using fuels more efficiently and reducing the amount of fossil fuel burned.

What is incomplete combustion?

A reaction in which some of the atoms in the fuel are not fully oxidised.

Why does incomplete combustion occur?

There is not enough oxygen available or combustion occurs too rapidly for complete oxidation.

What products can be formed during incomplete combustion of an alkane?

Carbon dioxide, carbon monoxide, soot (carbon) and water.

What happens to the hydrogen atoms during incomplete combustion of an alkane?

All of the hydrogen atoms are completely oxidised to form water.

What is soot?

Particles of unburnt carbon produced during incomplete combustion.

What problems can soot cause?

It contributes to air pollution and can cause respiratory problems.

What is an example equation showing carbon formation during incomplete combustion?

C₃H₈ + 2O₂ → 3C + 4H₂O.

What is carbon monoxide?

A toxic gas produced during incomplete combustion.

Why is carbon monoxide dangerous?

It prevents the blood from carrying enough oxygen around the body.

Why is carbon monoxide particularly dangerous?

It is colourless and odourless, so people may breathe it in without knowing.

What is an example equation showing carbon monoxide formation during incomplete combustion?

C₃H₈ + 3½O₂ → 3CO + 4H₂O.

What are unburnt hydrocarbons?

Hydrocarbons that do not burn at all and are released unchanged.

What abbreviation is used for unburnt hydrocarbons?

UHC.

How can unburnt hydrocarbons be produced by engines?

Some hydrocarbon fuel may pass through the engine without being completely burned.

What pollutants can be produced when sulfur-containing fuels are burned?

Oxides of sulfur.

How do sulfur compounds get into fuels?

Some chemicals in crude oil contain sulfur atoms and may remain in fractions after fractional distillation.

What happens to sulfur during combustion?

It reacts with oxygen to form sulfur dioxide.

What is the equation for the formation of sulfur dioxide?

S + O₂ → SO₂.

How can sulfur trioxide be formed?

Sulfur dioxide can react with more oxygen: 2SO₂ + O₂ → 2SO₃.

Why are sulfur dioxide and sulfur trioxide described as acidic oxides?

They react with water to form acids.

What happens when sulfur dioxide reacts with water?

It forms sulfurous acid: SO₂ + H₂O → H₂SO₃.

What happens when sulfur trioxide reacts with water?

It forms sulfuric acid: SO₃ + H₂O → H₂SO₄.

What environmental problem can oxides of sulfur cause?

They contribute to acid rain.

What damage can acid rain cause?

It can damage plants, aquatic ecosystems, buildings and other materials.

What are oxides of nitrogen collectively known as?

NOₓ.

What are two important oxides of nitrogen produced by engines?

Nitrogen monoxide (NO) and nitrogen dioxide (NO₂).

How are oxides of nitrogen formed in vehicle engines?

The high temperatures cause nitrogen and oxygen from the air to react.

What is the equation for the formation of nitrogen monoxide?

N₂ + O₂ → 2NO.

How is nitrogen dioxide formed from nitrogen monoxide?

Nitrogen monoxide reacts with more oxygen: 2NO + O₂ → 2NO₂.

What happens when nitrogen dioxide dissolves in water in the atmosphere?

It forms acids including nitrous acid and nitric acid.

What is the equation for nitrogen dioxide reacting with water?

2NO₂ + H₂O → HNO₂ + HNO₃.

Why are oxides of nitrogen environmental pollutants?

They contribute to acid rain and air pollution.

What is a catalytic converter?

A device fitted to vehicle exhaust systems that uses catalysts to convert harmful pollutants into less harmful substances.

Why are catalytic converters used in vehicles?

To reduce the amount of harmful pollutants released in exhaust gases.

What metals are commonly used as catalysts in catalytic converters?

Platinum, rhodium and palladium.

Why are platinum, rhodium and palladium spread thinly over a honeycomb structure?

To provide a large surface area for reactions while using only a small amount of the expensive metals.

What is a three-way catalytic converter?

A catalytic converter that removes three types of pollutants: carbon monoxide, unburnt hydrocarbons and oxides of nitrogen.

What happens to carbon monoxide in a catalytic converter?

It is oxidised to carbon dioxide.

What is the equation for the oxidation of carbon monoxide in a catalytic converter?

2CO + O₂ → 2CO₂.

What happens to unburnt hydrocarbons in a catalytic converter?

They are oxidised to carbon dioxide and water.

What is an example equation for the oxidation of octane in a catalytic converter?

C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O.

How can a catalytic converter remove both nitrogen monoxide and carbon monoxide?

2NO + 2CO → N₂ + 2CO₂.

What happens to nitrogen monoxide in a catalytic converter?

It is converted into nitrogen gas.

Why are catalytic converters not effective at preventing sulfur pollution?

The catalysts currently used are not very good at removing sulfur compounds.

What is the best way to reduce sulfur-based pollution from fuels?

Remove sulfur compounds from the fuel before it is burned.

What is low-sulfur or ultra-low-sulfur fuel?

Fuel from which much of the sulfur has been removed before combustion.