EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.7.3 Simple Molecular Structures

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.7.3 Simple Molecular Structures

Simple Molecular Structures

 

  • Simple molecular structures have covalent bonds joining the atoms together, but intermolecular forces that act between neighbouring molecules
  • They have low melting and boiling points as there are only weak intermolecular forces acting between the molecules
  • These forces are very weak when compared to the covalent bonds and so most small molecules are either gases or liquids at room temperature
    • Often the liquids are volatile

     

  • As the molecules increase in size the intermolecular forces also increase as there are more electrons available
  • This causes the melting and boiling points to increase

Intermolecular Forces Vs Covalent Bonds, downloadable IGCSE & GCSE Chemistry revision notes

Covalent bonds are strong but intermolecular forces are weak

 

Exam Tip

The atoms within covalent molecules are held together by covalent bonds while the molecules in a covalent substance are attracted to each other by intermolecular forces.

Melting & Boiling Point Patterns

 

Melting and Boiling Point of Simple Compounds in Relation to Molecular Mass

  • As the relative molecular mass of a substance increases, the melting and boiling point will increase as well
  • An increase in the relative molecular mass of a substance means that there are more electrons in the structure, so there are more intermolecular forces of attraction that need to be overcome when a substance changes state
  • So larger amounts of heat energy are needed to overcome these forces, causing the compound to have a higher melting and boiling point
  • The family of organic molecules called alkanes show a clear increase in boiling point as the size of the molecule increases

 

10.1.1-Alkanes-Boiling-Point-Graph

 

Graph showing the increase in boiling point as the molecular size increases

 

 

Conductivity & Covalent Compounds

 

  • They are poor conductors of electricity as there are no free ions or electrons to carry the charge
  • Most covalent compounds do not conduct at all in the solid state and are thus insulators
  • Common insulators include the plastic coating around household electrical wiring, rubber and wood

 

Electrical-wire-insulator

 

The plastic coating around electrical wires is made from covalent substances that do not allow a flow of charge

 

 

Exam Tip

When a covalent molecule melts or boils the covalent bonds do not break, only the intermolecular forces. If you think about it, when you boil a kettle full of water you are not generating large volumes of hydrogen and oxygen gas in your kitchen – this might give you an interesting unwanted chemical reaction ! Boom !

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