Hydrocarbons - GCSE Chemistry

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Introduction

  • In organic chemistry, the study of hydrocarbons is important for understanding all carbon compounds.
  • They show how carbon and hydrogen combine to form the simplest organic structures, which later give rise to complex substances such as alcohols, acids, fuels, medicines, and plastics.
  • In this blog, we’ll study the two main classes of hydrocarbons — alkanes and alkenes — along with their important reactions and properties.

Uses of Hydrocarbons

  • Fuels — in the form of petrol, diesel, and LPG for transport and cooking.
  • Used in manufacturing medicines and cosmetic products.
  • Provide energy through combustion in industries and power plants.
  • Act as lubricants in engines and machinery.

What are Hydrocarbons?

  • Hydrocarbons are organic substances composed only of carbon (C) and hydrogen (H) atoms.
  • Based on the type of bonding between carbon atoms, hydrocarbons are classified into two major categories:

Alkanes –

  • Alkanes are saturated hydrocarbons where each carbon forms four single covalent bonds.

Hydrocarbons diagram showing two carbon atoms bonded together

  • They are quite stable and unreactive, mainly reacting through combustion.

Example:

Hydrocarbons showing the structures of methane, ethane, propane and butane

Alkanes –

  • Alkenes are unsaturated hydrocarbons with one or more double bonds.

Hydrocarbons diagram showing a carbon–carbon bond

  • This double bond is their functional group and the reason for their higher reactivity.

Example:

Structural formulae of ethene and propene alkenes in hydrocarbons

Why are Alkanes Saturated Hydrocarbons?

  • Alkanes are called Saturated Hydrocarbons because each carbon atom forms only single bonds (C–C) and is bonded to the maximum number of hydrogen atoms possible.
  • Since their carbon atoms are fully “saturated” with hydrogen, no more atoms can join unless a bond is broken.
  • This is why alkanes are not very reactive and do not react with bromine water or undergo addition reactions.

Why are Alkenes Unsaturated Hydrocarbons?

  • Alkenes are called unsaturated hydrocarbons because they contain at least one double bond (C=C) between carbon atoms.
  • The double bond means that more atoms can join the molecule without breaking existing single bonds.
  • This makes alkenes more reactive than alkanes.
  • They react with bromine water, which turns from orange to colorless, and can undergo addition reactions.

How do Alkenes React with Bromine?

  • The reaction in which an alkene reacts with bromine is called an addition reaction.
  • The double bond in the carbon molecule breaks, and each carbon atom bonds to one bromine atom, forming a saturated compound.
  • This happens because alkenes are more reactive due to their double bond, which easily opens to add new atoms.

Example:

Addition reaction of ethene with bromine forming 1,2-dibromoethane

How does Bromine Water Test distinguish Alkanes and Alkenes?

  • Bromine water test is a simple way to distinguish between alkanes and alkenes.
  • Bromine water is orange-brown in color.

Working/Procedure:

  • Take two test tubes — one with an alkane and one with an alkene.
  • Add a few drops of bromine water to each test tube.
  • Gently shake or stir both tubes.

Result:

For Alkanes:

  • Do not react with bromine water because they are saturated hydrocarbons.
  • The orange color stays the same.

For Alkenes:

  • Undergo an addition reaction with bromine water because they are unsaturated hydrocarbons.
  • The orange color turns colorless.

Bromine water test showing colour change with alkenes and no change with alkanes

How Do Hydrocarbons Undergo Combustion?

  • Both alkanes and alkenes burn in oxygen in a reaction called combustion.
  • This reaction releases energy as heat and light, which is why hydrocarbons are widely used as fuels.

1. Complete Combustion:

  • Hydrocarbons burn completely when there is enough oxygen.
  • In this process, the carbon and hydrogen atoms are oxidised, forming carbon dioxide (CO₂) and water (H₂O).

Example:

Combustion of methane hydrocarbon showing reaction with oxygen to form carbon dioxide and water

2. Incomplete Combustion:

  • Hydrocarbons burn incompletely when there is not enough oxygen.
  • In this process, carbon monoxide (CO) is formed instead of carbon dioxide (CO₂).

Example:

Incomplete combustion of ethane hydrocarbon producing carbon monoxide and water

Frequently Asked Questions

Solution:

  • Hydrocarbons are compounds made only of carbon and hydrogen atoms.
  • They are the basic fuels like methane and petrol.
  • Example: CH₄ is the simplest hydrocarbon.

Solution:

  • Alkanes contain only single bonds between carbon atoms.
  • This means no more atoms can be added, so they are called saturated.
  • Example: Ethane (C₂H₆).

Solution:

  • Alkenes contain a C=C double bond, which can open and add more atoms.
  • Because they are not fully bonded with hydrogen, they are unsaturated.
  • Example: Ethene (C₂H₄).

Solution:

The double bond in alkenes breaks open during the reaction. Bromine atoms add across the double bond, forming a colourless product.

Solution:

  • Alkenes decolourise brown bromine water because they react with it.
  • Alkanes do not react, so the brown colour stays the same.

Solution:

Hydrocarbons burn in oxygen to form carbon dioxide and water. This reaction releases heat energy, which is why fuels are useful.

Solution:

Alkanes burn completely when oxygen is enough. This produces a blue, clean flame with no smoke.

Solution:

Alkenes burn less completely because of their double bond. This forms carbon particles, which give a yellow, smoky flame.

Solution:

Alkanes follow the formula CₙH₂ₙ₊₂. This fits all single-bonded hydrocarbons like methane and ethane.

Solution:

Alkenes follow the formula CₙH₂ₙ. This matches hydrocarbons with one double bond such as ethene and propene.

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