IB DP Chemistry: SL复习笔记10.2.1 Alkanes - Combustion

Unreactive Alkanes

Strength of C-H bonds

  • Alkanes consist of carbon and hydrogen atoms which are bonded together by single bonds
  • Unless a lot of heat is supplied, it is difficult to break these strong C-C and C-H covalent bonds
  • This decreases the reactivity of alkanes in chemical reactions

Lack of polarity

  • The electronegativities of the carbon and hydrogen atoms in alkanes are almost the same
  • This means that both atoms share the electrons in the covalent bond almost equally

3.2-Hydrocarbons-Pauling-Scale-of-Elements

The Pauling Scale shows that the difference in electronegativity between carbon and hydrogen is only 0.4

  • As a result of this, alkanes are nonpolar molecules and have no partial positive or negative charges (δ+ and δrespectively)
  • Alkanes therefore do not react with polar reagents
    • They have no electron-deficient areas to attract nucleophiles
    • And also lack electron-rich areas to attract electrophiles

3.2-Hydrocarbons-Polarity-Alkanes

Ethane is an example of an alkane that lacks polarity due to almost similar electronegativities of the carbon and hydrogen atoms

  • Due to the unreactivity of alkanes, they only react in combustion reactions and undergo substitution by halogens

Exam Tip

Remember: nucleophiles are negatively charged and are attracted to electron-deficient regions.Electrophiles are positively charged and attracted to electron-rich regions.

Combustion of Alkanes

  • Alkanes are combusted (burnt) on a large scale for their use as fuels
  • They also react in free-radical substitution reactions to form more reactive halogenoalkanes

Complete combustion

  • When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively
    • For example, the complete combustion of octane to carbon dioxide and water

3.2-Hydrocarbons-Complete-Combustion

The complete combustion of alkanes

Incomplete combustion

  • When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
  • Some carbon is only partially oxidised to form carbon monoxide
    • For example, the incomplete combustion of octane to form carbon monoxide

3.2-Hydrocarbons-Incomplete-Combustion_1

The incomplete combustion of alkanes

  • Carbon monoxide is a toxic gas as it will bind to haemoglobin in blood which can then no longer bind oxygen
  • As no oxygen can be transported around the body, victims will feel dizzy, lose consciousness and if not removed from the carbon monoxide, they can die
  • Carbon monoxide is extremely dangerous as it is odourless (it doesn’t smell) and will not be noticed
  • Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present
  • With a reduced supply of oxygen, carbon will be produced in the form of soot:

10.2.1-Incomplete-Combustion-of-Alkanes-2-1

The incomplete combustion of alkanes (2)

Exam Tip

Incomplete combustion of alkanes never produces hydrogen as it is always preferentially oxidised in any available oxygen, rather than carbon

 

 

 

 

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