Ch.7 - Shapes of Molecules and Intermolecular Forces Flashcards
What are intermolecular forces? What affect do these forces have on the chemical properties of molecules?
Intermolecular forces are forces between molecules. These include van der Waals’ forces, dipole-dipole interactions and hydrogen bonding. The presence of these attractive forces means that the molecules are more difficult to separate than would otherwise be the case, for this reason melting and boiling point are affected by the strength of these forces.
What are van der Waals’ forces?
Van der Waals’ forces are weak, temporary, attractive forces caused by the movement of electrons within a molecule. The electrons move randomly, at a particular point when they are nearer to one atom than the other, there is a temporary polarity (temporary dipole) in the molecule.
A temporary dipole in one molecule will induce a similar dipole in a neighbouring molecule. These combined will increase boiling points.
The greater the number of electrons in a molecule, the greater are the number of temporary dipoles, increasing attraction.
What is a dipole?
A dipole is a bond or molecule whose ends have opposite charges.
What are dipole-dipole interactions?
Dipole-dipole interactions are permanent and occur between polar molecules. The negative end of a dipole in a polar molecule is attracted to the positive end.
What is hydrogen bonding?
Hydrogen bonding is a special type of dipole-dipole interaction, which occurs when hydrogen is bonded to small, highly electronegative atoms such as O, N or F.
Rate the intermolecular forces in strength.
Hydrogen bonding, the dipole-dipole interaction then van der Waals’ forces.
Why is a meniscus formed with water in glass?
Dipole-dipole interactions between water molecules and the molecules in the wall of glass attract each other, causing the water to stick to the glass in a concave bowl shape.
What is the electron repulsion theory?
The electron repulsion theory may be used to explain the shape of molecules consisting of atoms bonded to a central atom. It states:
The electron pairs in the valence shell of the central atom repel each other and end up as far apart as is geometrically possible.
Lone pairs have a greater repelling effect than bonding pairs.
Match all the molecular shapes to their corresponding bond pairs, lone pairs and bond angles.
What is intramolecular bonding?
Intramolecular bonding is the bonding within the molecule that holds the atoms together. Covalent and ionic bonds are intramolecular forces.
How is the polarity of a molecule containing polar covalent bonds determined?
The polarity of a molecule containing polar covalent bonds depends on its shape. If it has a symmetrical shape , the molecule is non-polar. If it has an assymmetrical shape, its polar.
