Carbon compounds — Eduqas GCSE Combined Science

Carbon compounds form the basis of organic chemistry, a branch of chemistry that studies the structure, properties, and reactions of compounds containing carbon. Carbon is unique because it can form four covalent bonds, allowing it to create a vast array of molecules, from simple hydrocarbons like methane to complex polymers and biological molecules. In WJEC GCSE Combined Science, you will focus on the simplest carbon compounds: hydrocarbons, which are compounds made only of carbon and hydrogen. These include alkanes (saturated hydrocarbons) and alkenes (unsaturated hydrocarbons). Understanding carbon compounds is essential because they are the building blocks of fuels, plastics, and many materials we use daily.

This topic also introduces key concepts such as homologous series, functional groups, and isomerism. You will learn how to name and draw the structures of alkanes and alkenes, and how they react in combustion and addition reactions. The topic connects to real-world applications like fractional distillation of crude oil, cracking to produce smaller molecules, and the environmental impact of burning fossil fuels. Mastering carbon compounds lays the groundwork for more advanced chemistry topics and helps you appreciate the chemistry behind everyday products.

In the WJEC exam, you will be expected to recall the general formulas for alkanes (CnH2n+2) and alkenes (CnH2n), identify and name simple members of each series, and describe their chemical reactions. You should also understand the difference between saturated and unsaturated compounds and how to test for unsaturation using bromine water. This topic is a gateway to understanding organic chemistry and its importance in industry and the environment.

• →Hydrocarbons are compounds containing only carbon and hydrogen. Alkanes are saturated (single bonds only) with general formula CnH2n+2; alkenes are unsaturated (contain a carbon-carbon double bond) with general formula CnH2n.
• →Homologous series: a family of compounds with the same functional group and similar chemical properties, where each member differs by a CH2 unit. Alkanes and alkenes are examples.
• →Functional groups: the part of a molecule that determines its chemical reactivity. For alkenes, the functional group is the C=C double bond, which allows addition reactions.
• →Combustion: alkanes burn in oxygen to produce carbon dioxide and water (complete combustion) or carbon monoxide and water (incomplete combustion). Alkenes also burn with a smoky flame due to incomplete combustion.
• →Addition reactions: alkenes undergo addition reactions at the double bond, e.g., with hydrogen (hydrogenation), water (hydration), or halogens like bromine. The bromine water test (orange to colourless) distinguishes alkenes from alkanes.

• Crude oil is a main source of hydrocarbons and a feedstock for the petrochemical industry
• Separation of crude oil by fractional distillation
• Fractions are largely mixtures of alkanes with the general formula CnH2n+2
• Cracking produces more useful materials
• Crude oil is a finite resource

• 💡Be prepared to explain the environmental and economic implications of using fossil fuels
• 💡Ensure you can describe the process of fractional distillation clearly
• 💡Understand why cracking is necessary for the petrochemical industry
• 💡When naming alkanes and alkenes, always use the correct prefix (meth-, eth-, prop-, but-) and suffix (-ane for alkanes, -ene for alkenes). For example, C3H8 is propane, C3H6 is propene. Don't forget to count the carbon atoms correctly.
• 💡In combustion equations, always balance the equation. For complete combustion of an alkane, the products are CO2 and H2O. For example: C3H8 + 5O2 → 3CO2 + 4H2O. Practice balancing these equations.
• 💡For the bromine water test, state the colour change clearly: orange to colourless for alkenes. Also, explain that the reaction is an addition reaction where bromine adds across the double bond. This shows you understand the chemistry.

• Confusing the process of fractional distillation with cracking
• Failing to identify crude oil as a finite resource
• Incorrectly stating the general formula for alkanes
• Misconception: All hydrocarbons are alkanes. Correction: Hydrocarbons include both alkanes (saturated) and alkenes (unsaturated). Alkenes have a double bond and are more reactive.
• Misconception: Alkanes and alkenes have the same general formula. Correction: Alkanes have formula CnH2n+2, while alkenes have CnH2n. For example, ethane is C2H6, but ethene is C2H4.
• Misconception: The bromine water test turns colourless for both alkanes and alkenes. Correction: Bromine water remains orange with alkanes (no reaction) but turns colourless with alkenes (addition reaction).