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Intermolecular forces for OCR GCSE Chemistry

Intermolecular forces

This page covers the following topics:

1. Introduction to intermolecular forces

Small molecules are held together by strong covalent bonds and impacted by weak intermolecular interactions. For such molecular compounds to melt or boil, only the intermolecular forces have to be broken, not the covalent bonds within molecules. Thus, the energy required for the molecules to escape a lattice in solids or to escape a liquid is low. This results in low temperatures needed for a material to melt or boil, that is low melting and boiling points. Due to different melting and boiling points, fluorine and chlorine at room temperature are gases, bromine - liquid and iodine - solid.

Small molecules do not conduct electricity as they do not have charged particles. Compounds made of small molecules usually have a low viscosity (resistance to liquid flow) due to weak intermolecular forces.

Introduction to intermolecular forces

1

The molecules of several compounds are presented in the image. Explain why the boiling point of these compounds significantly increases going towards the right despite very similar molecular masses.

Small molecules are held together by strong covalent bonds and impacted by weak intermolecular interactions. For such molecular compounds to boil, only the intermolecular forces have to be broken, not the covalent bonds within molecules. Thus, the energy required for the molecules to escape a liquid and consequently the boiling point is determined by the intermolecular forces.

intermolecular forces

The molecules of several compounds are presented in the image. Explain why the boiling point of these compounds significantly increases going towards the right despite very similar molecular masses.

2

What type of bonds hold together small molecules like COโ‚‚?

Small molecules are held together by strong covalent bonds.

covalent

What type of bonds hold together small molecules like COโ‚‚?

3

A dot and cross diagram for compound X is presented in the image. Explain why compound X exists naturally in a gas state.

Small molecules of compound X are held together by strong covalent bonds, but weak intermolecular interactions. For compound X to boil, the covalent bonds within molecules do not have broken, just the intermolecular forces. Thus, the energy required for the molecules of compound X to escape a liquid is low. This results in a low temperature needed for the compound to boil, that is low boiling point. Therefore, compound X exists as a gass.

weak intermolecular interactions โ†’ low energy required โ†’ low temperature required โ†’ low boiling point โ†’ gas

A dot and cross diagram for compound X is presented in the image. Explain why compound X exists naturally in a gas state.

4

Which of the following compounds has the lowest viscosity? Provide a reasoning to your answer.

โ‹… CHโ‚ƒCHโ‚‚OH
โ‹… CHโ‚ƒCHโ‚‚CHโ‚‚OH
โ‹… CHโ‚ƒCHโ‚‚CHโ‚‚CHโ‚‚OH

Compounds made of small molecules usually have a low viscosity (resistance to liquid flow) due to weak intermolecular forces.

CHโ‚ƒCHโ‚‚OH, smallest molecule

Which of the following compounds has the lowest viscosity? Provide a reasoning to your answer. 

โ‹… CHโ‚ƒCHโ‚‚OH
โ‹… CHโ‚ƒCHโ‚‚CHโ‚‚OH
โ‹… CHโ‚ƒCHโ‚‚CHโ‚‚CHโ‚‚OH

5

Why does ice melt at a relatively low temperature of 0ยฐC?

Water has small molecules that are held together by strong covalent bonds and impacted by weak intermolecular interactions. For water to melt, only the intermolecular forces have to be broken, not the covalent bonds within molecules. Thus, the energy required for water molecules to escape a lattice in ice is low. This results in low temperature needed for ice to melt, that is low melting point. Therefore, ice melts at a relatively low temperature of 0ยฐC.

small molecules โ†’ weak intermolecular forces โ†’ low energy needed โ†’ low temperature needed โ†’ low melting point

Why does ice melt at a relatively low temperature of 0ยฐC?

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