Forces Between Molecules. Flashcards
What causes a dipole moment?
When two atoms with differ electronegativities are bonded together there will be both a negative and a positive side to the molecule. In an electric field this experiences a twisting force and is said to have a dipole moment.
What are van der waals forces?
These are the weak intermolecular forces that exist between all molecules, including both dipole-dipole interactions and dipole-induced dipole interactions.
How will a molecule with a dipole moment orientate itself in an an electric field?
It will rotate so that the negAtive side is towards the positive side and vice versa but will not move towards either plate.
Describe dipole-dipole interactions in the solid state.
The polar molecules interact to form an ordered arrangement where they pack in such a way that the partial negative charges will be adjacent to the partial negative charges.
How may a dipole-dipole moment arise in non-polar molecules?
If two non-polar molecules are very close together the arrangement of electrons is on average symmetrical, however at a given moment the electron distribution around one of the molecules may become unsymmetrical and there may be a dipole in the molecule for an instant.
What will the creation of a temporary dipole on a molecule cause in a sample of those molecules?
The temporary (or induced) dipole can attract the electron cloud of the neighbouring molecule meaning that the molecules will have dipoles which will attract each-other. The induce dipoles may in turn induce dipoles in other molecules.
What can be said to exist between molecules in the liquid or solid state?
Van der waals forces must be present to hold them together.
What determines the strength of van der waals forces?
The greater the number of electrons in the molecule, and the more weekly they are held by the positive nucleus the greater the forces between them.
What does the strength of the van der waals forces determine?
They determine the physical properties of the substance i.e melting and boiling points of substances not the chemical properties.
Why do covalent substances often have low melting and boiling points?
Because there are very weak van der waals forces between the molecules, holding them together.
What allows noble gasses to be liquified?
The presence of van der waals forces.
What is a hydrogen bond?
These for, when a hydrogen atom is bonded to a highly electronegative element. This is different to dipole-dipole interactions because hydrogen has no inner electrons to set up forces of repulsion with the non bonding electrons of other atoms.
This results in hydrogen bonds being much stronger than the usual van der waals forces..
What evidence is there for the existence of hydrogen bonding?
This is given by considering the boiling and melting points of the hydrides of N, O and F. They are much higher than those seen in the other hydrides in the same group, so they must be held together by stronger intermolecular forces.
Describe the structure of ice.
Ice forms when the hydrogen bonds all align in a regularity that extends throughout the whole structure. This structure spaces the molecules out making them further apart than they are in the liquid state. Causing the water to expand by 9% and so ice is less dense than water.
How does hydrogen landing affect the solubility in water?
If a solute has a hydroxyl group it is likely to be soluble in water because it can form a hydrogen bond with water.
