Hydrocarbon property trends with molecular size

Hydrocarbons are substances made only of hydrogen and carbon. As hydrocarbon molecules become larger, their physical properties change in predictable ways. These trends can be used to compare smaller and larger hydrocarbons.

Boiling point and viscosity

Larger hydrocarbon molecules have higher boiling points. The attractions between larger molecules are stronger, so more heating is needed to separate the molecules and make the liquid boil. Smaller hydrocarbons therefore boil at lower temperatures and are generally more volatile: they evaporate more readily.

Larger hydrocarbon liquids also have higher viscosity. A viscous liquid is thicker and flows less easily. The stronger attractions between larger molecules make it harder for them to move past one another, so larger hydrocarbon liquids flow less readily than smaller ones.

Flammability and fuel use

Flammability decreases as hydrocarbon molecules get larger. Smaller hydrocarbons generally ignite more easily because they evaporate more readily, producing vapour that can mix with oxygen and burn. Larger hydrocarbons can still burn, but they are generally harder to ignite.

These trends help determine which hydrocarbons are suitable as fuels. Some uses require a fuel that evaporates and ignites easily. Others use less volatile hydrocarbons whose thicker liquids flow differently.

Complete combustion of hydrocarbons

A hydrocarbon is a substance made only of carbon and hydrogen. When it burns completely in oxygen, it releases energy, which is why hydrocarbons are used as fuels.

In complete combustion, there is enough oxygen for the carbon in the hydrocarbon to form carbon dioxide and the hydrogen to form water. Both the carbon and the hydrogen are oxidised: they combine with oxygen. The products are therefore always carbon dioxide ($$\ce{CO2}$$) and water ($$\ce{H2O}$$).

Balancing complete combustion equations

A balanced equation has the same number of atoms of each element on both sides. To balance the complete combustion of a hydrocarbon:

1. Write the hydrocarbon and oxygen as reactants, and carbon dioxide and water as products.
2. Balance carbon by placing a number in front of $$\ce{CO2}$$.
3. Balance hydrogen by placing a number in front of $$\ce{H2O}$$.
4. Count the oxygen atoms in the products, then balance oxygen by placing a number in front of $$\ce{O2}$$.
5. Check the number of carbon, hydrogen and oxygen atoms on both sides.

Change the numbers in front of formulae, not the subscripts within them. Changing a subscript would change the substance.

For example, propane has the formula $$\ce{C3H8}$$. Its three carbon atoms require three $$\ce{CO2}$$ molecules, and its eight hydrogen atoms require four $$\ce{H2O}$$ molecules. These products contain ten oxygen atoms in total, so five $$\ce{O2}$$ molecules are needed:

Both sides contain three carbon atoms, eight hydrogen atoms and ten oxygen atoms.

If balancing oxygen requires a fraction in front of $$\ce{O2}$$, multiply every coefficient in the equation by the same number to obtain whole-number coefficients.