5.2 Hydrocarbons
Hydrocarbons are organic carbon compounds that play a vital role in our daily lives as primary sources of fuel and raw chemical materials.
Learning Objectives
- Define hydrocarbons as organic carbon compounds composed only of carbon and hydrogen elements.
- Explain the formation of petroleum, natural gas, and coal from natural resources.
- Describe the fractional distillation of petroleum based on differences in boiling points.
- Differentiate between saturated and unsaturated hydrocarbons.
- Identify the general formulas and members of the alkane and alkene homologous series.
- Identify alternative and renewable energy sources in daily life.
Comparison between Alkane and Alkene Series
Clarify the structural differences, saturation status, general formula, and carbon bond types between alkanes and alkenes.
Alkane
Saturated hydrocarbons with only single covalent bonds between carbon atoms.
General formula: \(\text{C}_n\text{H}_{2n+2}\), where \(n = 1, 2, 3, \dots\)
Starts at \(n = 1\): Methane \(\text{CH}_4\).
Alkene
Unsaturated hydrocarbons containing at least one double covalent bond between carbon atoms.
General formula: \(\text{C}_n\text{H}_{2n}\), where \(n = 2, 3, \dots\)
Starts at \(n = 2\): Ethene \(\text{C}_2\text{H}_4\).
Short Explanation
Definition
Hydrocarbon compounds are organic carbon compounds made up of only carbon and hydrogen elements.
Hydrocarbon Compounds from Natural Sources
Petroleum and natural gas are formed from the remains of dead marine life buried in the seabed. Over millions of years, these remains are buried deeper under thick layers of rock and mud. The combined effects of pressure, heat absorbed from the surroundings, and decomposition caused by bacteria change the buried remains into petroleum and natural gas.
Coal is formed from the remains of dead plants that were naturally buried underground millions of years ago. Over millions of years, the remains become buried deeper under thick layers of rocks. Pressure, heat, and bacterial decomposition change the buried plant fossils into coal.
Fractional Distillation of Petroleum
Petroleum is a mixture of hydrocarbons. It must be separated using fractional distillation because different petroleum fractions have different boiling points.
| Fraction | Temperature range |
|---|---|
| Fraction 1 | Room temperature to \(80^\circ\text{C}\) (approx. \(30^\circ\text{C} - 80^\circ\text{C}\)) |
| Fraction 2 | \(80^\circ\text{C} - 150^\circ\text{C}\) |
| Fraction 3 | \(150^\circ\text{C} - 230^\circ\text{C}\) |
| Fraction 4 | \(230^\circ\text{C} - 250^\circ\text{C}\) |
Saturated and Unsaturated Hydrocarbons
Saturated hydrocarbons contain only single covalent bonds between carbon atoms, for example alkanes such as propane \(\text{C}_3\text{H}_8\).
Unsaturated hydrocarbons contain at least one double covalent bond or triple covalent bond between carbon atoms, for example alkenes such as propene \(\text{C}_3\text{H}_6\).
Homologous Series
A homologous series is made up of a specific group of organic compounds that have similar chemical properties.
Alkane
Saturated hydrocarbons with single covalent bonds.
General formula: \(\text{C}_n\text{H}_{2n+2}\), where \(n = 1, 2, 3, \dots\)
First six members: Methane \(\text{CH}_4\), Ethane \(\text{C}_2\text{H}_6\), Propane \(\text{C}_3\text{H}_8\), Butane \(\text{C}_4\text{H}_{10}\), Pentane \(\text{C}_5\text{H}_{12}\), Hexane \(\text{C}_6\text{H}_{14}\).
Alkene
Unsaturated hydrocarbons with at least one double covalent bond.
General formula: \(\text{C}_n\text{H}_{2n}\), where \(n = 2, 3, \dots\)
First five members: Ethene \(\text{C}_2\text{H}_4\), Propene \(\text{C}_3\text{H}_6\), Butene \(\text{C}_4\text{H}_8\), Pentene \(\text{C}_5\text{H}_{10}\), Hexene \(\text{C}_6\text{H}_{12}\).
Alternative Energy and Renewable Energy Sources
Fossil fuels such as petroleum, coal, and natural gas are non-renewable and rapidly depleting. Alternative energy sources do not deplete easily; they include nuclear energy and renewable energy sources.
Examples of renewable energy sources include solar energy, wind energy, hydroelectric energy, biomass energy, geothermal energy, tidal energy, and wave energy. Rubbish disposal sites and organic food waste decay release carbon dioxide and methane gas, which can be harnessed to generate electrical energy.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the main characteristic that allows petroleum fractions to be separated during fractional distillation?
Why is ethene considered an unsaturated hydrocarbon, while ethane is a saturated hydrocarbon?
What flammable gas is released from the decay of organic food waste that can be used to generate electricity?
Common Mistakes
- Writing the general formula of alkenes as \(\text{C}_n\text{H}_{2n+2}\) instead of \(\text{C}_n\text{H}_{2n}\).
- Assuming that the alkene series starts with a 1-carbon molecule (“methene”), when in fact the simplest alkene is ethene (\(n = 2\)) because a double bond requires at least two carbon atoms.
- Confusing the origin of fossil fuels: thinking coal comes from marine life and petroleum from land plants, whereas marine life forms petroleum/natural gas and land plants form coal.
Concept Misunderstandings
Hydrocarbons contain oxygen.
Hydrocarbons contain only carbon and hydrogen. Compounds containing oxygen, such as alcohols, are not hydrocarbons.
Crude oil is a pure compound.
Crude oil is a mixture of various hydrocarbons that can be separated physically by fractional distillation.
Nuclear energy is a renewable energy source.
Nuclear energy is an alternative energy source that does not deplete easily, but it is distinguished from renewable energy sources such as solar, wind, and biomass.
Summary
- Hydrocarbons are organic compounds composed entirely of hydrogen and carbon.
- Natural fossil fuels—petroleum, natural gas, and coal—were created by heat, pressure, and bacterial action on dead organisms over millions of years.
- Fractional distillation separates petroleum into fractions according to boiling points.
- Saturated hydrocarbons (alkanes, \(\text{C}_n\text{H}_{2n+2}\)) contain single carbon-carbon bonds, while unsaturated hydrocarbons (alkenes, \(\text{C}_n\text{H}_{2n}\)) contain carbon-carbon double bonds.
- Depleting fossil fuels drives the transition to alternative and renewable energy sources like solar, biomass, and methane gas.
Short Activity
A. Objective Quiz
1 Which of the following is an unsaturated hydrocarbon?
2 What physical property allows crude oil to be separated into distinct fractions during fractional distillation?
B. Fill in the Blanks
3 The general formula for the alkane homologous series is .
4 Plant fossils buried deep under thick layers of rock over millions of years are converted into by pressure, heat, and bacterial decomposition.
C. Drag and Drop: Match the Term to Its Description
Drag each term to the correct description. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match the Term to Its Description
Write the correct term next to each description.
| Description | Term |
|---|---|
| 1. Saturated hydrocarbon with single covalent bonds between carbon atoms. | |
| 2. Unsaturated hydrocarbon containing at least one carbon-carbon double covalent bond. | |
| 3. Gas produced from food waste decay that can be used to generate electrical energy. | |
| 4. Liquid mixture of hydrocarbons formed from ancient dead marine life. |
Answer choices: Alkane • Alkene • Methane • Petroleum
