Water And The Intermolecular Forces Flashcards
Water and intermolecular forces
Water molecules have strong intermolecular forces between them.
These forces are affected but the shape of the water molecules and are important for some practical applications
Forces between molecules
To break the covalent bonds in water you need to heat over 3000 degrees Celsius
When ice melts and water vaporises at 100 degrees Celsius, these relatively low temperatures are evident that covalent bonds are not breaking. The bond between the water molecules, the inter molecule forces, are broken. These low melting point shows that the intermolecular forces are much weaker than the covalent bonds within the molecule
Shapes of molecules
The structure and bonding in molecules is described by using either electron dot formulas or valence structures
Electron dot diagrams
Atoms more stable when 8 electron around central atom, the octet rule
In covalent bonds, the electrons shared between two atoms are called the bonding pair of electrons
A single bond is formed when one pair of electrons is shared
A double bond occurs when two pairs of electrons are shared between the two atoms. The pair of valence electrons not involved with bonding are called lone pairs or non bonding pair of electrons
Valence structures
Are used as an alternative to drawing electron dot formulas
A single bond, shown by a line joining the two atoms, has one pair of electrons
A double bond, shown by two short lines, has two pairs of electrons
A triple bond has three short lines or three pairs of electrons
Lone pairs are drawn as two dots, a short line or another line that points away from the element
Predicting the shape of molecules
VSEPR (valence she’ll electron pair resolution) theory is based upon the number of electron pairs surrounding the central atom. The electron pairs arrange themselves as if they repel each other
The three dimensional structure of simple molecules depended upon the number of electron pairs around the central atom
Lone pairs are closer to the single nucleus and so occupy more space
Bonding pairs are shared by two atoms and are attracted by the two nuclei, they occupy less space and cause less repulsion than lone pairs
To work out the shape of a molecule:
- draw the electron dot diagram
- count the lone pairs and bonding pairs. Remember that lone pairs are electron pairs that are not shared, while bonding pairs form the covalent bonds
- treat the electrons in a covalent bond, whether a single, double or triple bond as one set of electrons
Three types of intermolecular forces
All bonding is due to attraction of oppositions (electronmagnetic attraction)
Intermolecular forces occur between the molecules, of the same substance or of different substances
Intramolecular forces are much weaker than the intramolecular forces (covalent, ionic, metallic)
Weakest to strongest forces
Dispersion force, dipole-dipole forces, hydrogen bond
Dipole-dipole forces
An attraction between two dipoles in two individual molecules
Molecules line up so that the slightly positive end of the molecule attracts the slightly negative end of another molecule
One end of the molecule is slightly negative while the other need is slightly positive
The presence of dipoles creates an attraction between neighbouring molecules
Hydrogen bonds
Boiling and melting points give an estimation of the strength of the intermolecular forces between molecules
When nitrogen, oxygen or fluorine is attracted to a hydrogen atom, has an unusually high boiling point
When very high electronegative elements bond to hydrogen - hydrogen bonding
Strongest of the weakest intermolecular force
Shown as dotted line in between molecules
Dispersion
Polar molecules can form dipole-dipole or hydrogen bonds between the molecules. Non polar molecules cannot, so there must be some other force than enables non-polar molecules such as dry ice, liquid nitrogen and oils and fats, to form liquids and solids.
