topic 7 - Organic Chemistry

What is organic chemistry?

The study of compounds containing carbon, including hydrocarbons.

What is crude oil?

A finite resource found in rocks, formed from the remains of ancient biomass.

How was crude oil formed?

The remains of ancient plankton were buried in mud and, over millions of years, converted into crude oil by heat and pressure in the absence of oxygen.

Why is crude oil described as a finite resource?

It takes millions of years to form and is being used much faster than it is replaced.

What is crude oil made of?

A mixture of a very large number of different compounds, most of which are hydrocarbons.

What is a hydrocarbon?

A compound containing hydrogen and carbon atoms only.

What are fossil fuels?

Finite fuels formed from the remains of ancient organisms, such as crude oil, coal and natural gas.

What is crude oil used for?

It is separated into fuels and used as a feedstock to produce chemicals and materials.

What does feedstock mean?

A raw material used to make other substances or products.

Why is crude oil useful as a feedstock?

It contains hydrocarbons that can be separated and used to make fuels, polymers and other useful chemicals.

What is an alkane?

A saturated hydrocarbon containing only single carbon-carbon bonds.

What does saturated mean in organic chemistry?

The carbon atoms are joined by single bonds only, so the molecule contains the maximum possible number of hydrogen atoms.

What is the general formula for alkanes?

CₙH₂ₙ₊₂.

What is the formula of methane?

CH₄.

What is the formula of ethane?

C₂H₆.

What is the formula of propane?

C₃H₈.

What is the formula of butane?

C₄H₁₀.

How do you work out the formula of an alkane with 6 carbon atoms?

Use CₙH₂ₙ₊₂. If n = 6, the formula is C₆H₁₄.

How can you recognise an alkane from its molecular formula?

Check whether it fits the general formula CₙH₂ₙ₊₂.

What is a homologous series?

A family of organic compounds with the same functional group and general formula, similar chemical properties, and a gradual change in physical properties.

What is fractional distillation?

A process used to separate crude oil into fractions according to their different boiling point ranges.

Why can crude oil be separated by fractional distillation?

The hydrocarbons in crude oil have different boiling points.

Describe how fractional distillation works.

Crude oil is heated so that most of it vaporises. The vapours enter a fractionating column, which is hot at the bottom and cooler at the top. The vapours condense at different heights according to their boiling points.

Why is the bottom of a fractionating column hotter than the top?

The temperature gradient allows hydrocarbons with different boiling points to condense at different heights.

Where do hydrocarbons with high boiling points condense in the column?

Lower down, where the temperature is hotter.

Where do hydrocarbons with low boiling points condense in the column?

Higher up, where the temperature is cooler.

What is a fraction of crude oil?

A mixture of hydrocarbons with similar boiling point ranges and similar-sized molecules.

Does fractional distillation create new hydrocarbons?

No. It separates the hydrocarbons already present in crude oil; it is a physical separation process.

What is the difference between fractional distillation and cracking?

Fractional distillation separates hydrocarbons into fractions; cracking breaks large hydrocarbon molecules into smaller molecules.

What are the three important physical properties of hydrocarbons that change with molecular size?

Boiling point, viscosity and flammability.

How does boiling point change as hydrocarbon molecule size increases?

Boiling point generally increases.

How does viscosity change as hydrocarbon molecule size increases?

Viscosity increases; larger hydrocarbons flow less easily.

How does flammability change as hydrocarbon molecule size increases?

Flammability generally decreases; larger hydrocarbons are harder to ignite.

Why are smaller hydrocarbons useful as fuels?

They are more flammable and have lower boiling points, so they vaporise and ignite more easily.

Why are larger hydrocarbons less useful as fuels in some applications?

They are more viscous, less volatile and less flammable, making them harder to ignite and burn efficiently.

What is combustion?

A reaction in which a substance reacts with oxygen and releases energy.

What is complete combustion of a hydrocarbon?

Combustion with enough oxygen to produce carbon dioxide and water.

What are the products of complete combustion of a hydrocarbon?

Carbon dioxide and water.

What is the word equation for complete combustion of a hydrocarbon?

Hydrocarbon + oxygen → carbon dioxide + water.

What is the balanced equation for complete combustion of methane?

CH₄ + 2O₂ → CO₂ + 2H₂O.

What is the balanced equation for complete combustion of propane?

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

What is the balanced equation for complete combustion of butane?

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.

Why does combustion of hydrocarbons release energy?

Energy is released overall when new bonds form in carbon dioxide and water; the energy released exceeds the energy needed to break bonds in the reactants.

What does it mean when carbon and hydrogen are oxidised during combustion?

They react with oxygen to form oxides: carbon forms carbon dioxide and hydrogen forms water during complete combustion.

What is cracking?

The thermal decomposition of large hydrocarbon molecules into smaller, more useful molecules.

Why is cracking needed?

There is greater demand for small hydrocarbons used as fuels than for some of the larger hydrocarbons obtained from crude oil.

What are the products of cracking?

Smaller hydrocarbons, including alkanes and alkenes.

What bonds are broken during cracking?

Carbon-carbon bonds in large hydrocarbon molecules.

What conditions are used for catalytic cracking?

Vaporise the hydrocarbons and heat them, then pass the vapour over a hot catalyst.

What conditions are used for steam cracking?

Mix the hydrocarbons with steam and heat them to a high temperature.

Why is cracking economically useful?

It converts less useful large hydrocarbons into more useful smaller fuels and alkenes that can be used to make polymers and other chemicals.

Why are alkenes useful products of cracking?

They can be used to manufacture polymers and other chemicals.

What is an alkene?

An unsaturated hydrocarbon containing at least one carbon-carbon double bond.

Why are alkenes more reactive than alkanes?

The carbon-carbon double bond allows alkenes to react more readily.

How can you test for an alkene?

Add bromine water to the sample and shake it.

What is the positive result for the bromine water test for an alkene?

Bromine water changes from orange to colourless.

Why does bromine water become colourless when it reacts with an alkene?

Bromine reacts with the carbon-carbon double bond in an addition reaction.

What is the difference between an alkane and an alkene?

Alkanes have only single carbon-carbon bonds and are saturated; alkenes contain a carbon-carbon double bond and are unsaturated.

How can cracking equations be balanced?

Make sure the number of each type of atom is the same on both sides of the equation.

Complete this cracking equation: C₁₀H₂₂ → C₈H₁₈ + ?

C₂H₄. The balanced equation is C₁₀H₂₂ → C₈H₁₈ + C₂H₄.

Why must cracking equations conserve atoms?

Atoms are rearranged but not created or destroyed in a chemical reaction.

Give two uses of hydrocarbons from crude oil.

They are used as fuels and as feedstocks to make chemicals and polymers.

How does modern life depend on crude oil?

Crude oil provides fuels for transport and heating and raw materials for manufacturing many chemicals and materials.

What is one disadvantage of using crude oil as a fuel?

It is finite, and burning it releases carbon dioxide, which contributes to climate change.

What is one environmental impact of extracting or transporting crude oil?

Oil spills can damage habitats and harm wildlife.

Why are smaller hydrocarbons often in greater demand than larger ones?

Smaller hydrocarbons are useful fuels, while alkenes made by cracking can be used to manufacture polymers and other chemicals.

What is the difference between a physical property and a chemical property?

A physical property, such as boiling point or viscosity, can be observed without changing the substance into a different substance. A chemical property describes how a substance reacts.