test 2: Flashcards
London forces:
London forces occur between all molecular substances and noble gases, they do not occur in ionic substances
London forces: information
-London forces are also called instantaneous, induced dipole-dipole interactions
-they occur between all simple covalent molecules and the separate atoms in noble gases
-in any molecule the electrons are moving constantly and randomly, as this happens the electrons density can fluctuate and parts of the molecule become more or less negative i.e. small temporary or transient dipoles form
-these temporary dipoles can cause dipole to form in neighbouring molecules -> these are called induced dipoles
-the induced dipole is always the opposite sign to the original one
Main factor affecting London forces: size
-the more electrons there are in the molecule the chance that temporary dipoles will form
-this makes the London forces stronger between the molecules and more energy is needed to break them so boiling points will be greater
Trend boiling points: halogens
the increasing boiling points of the halogens down the group 7 series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the London forces between the molecules, this is why I2 is a solid whereas Cl2 is a gas
Trend boiling points: alkanes
-the increasing boiling points of the alkane homologous series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the London forces between molecules
Main factor affecting London forces: shape
the shape of the molecule can also have an effect on the size of the the London forces
-long straight chain alkanes have a large surface area of contact between molecules for London forces to form compared to spherical shaped branched alkanes and so have stronger London forces
Permanent dipole-dipole forces:
-permanent dipole-dipole forces occurs between polar molecules
-it is stronger than London forces and so the compounds have higher boiling points
-polar molecules have a permanent dipole (commonly compounds with Cl-Cl, C-F, C-Br-H-Cl, C=O bonds)
-polar molecules are asymmetrical and have a bond where there is a significant difference in electronegativity between the atoms
-permanent dipole forces occur in addition to London forces
Hydrogen bonding:
it occurs in compounds that have a hydrogen atom attached to one of three most common electronegative atoms of nitrogen, oxygen and fluorine, which must have an available lone pair of electrons, e.g. O-H, N-H, F-H bond
-there is a large electronegativity different between the H and the O,N,F
-always show the lone pair of electrons on the O,F,N and the dipoles and all the -+ charges
-hydrogen bonding occurs in addition to London forces
-the hydrogen bond should have a bond angle of 180º with one of the bond in one of the molecules
-the bond angle is 180º around the H atom because there are two pairs of electrons around the H atom involved in the hydrogen bond, these pairs of electrons repel to a position of minimum repulsion, as far apart as possible
-alcohols, carboxylic acids, proteins, amides all can form hydrogen bonds
Hydrogen bonding: alcohols
alcohols form hydrogen bond this means alcohols have higher boiling points and relatively low volatility compared to alkanes with a similar number of electrons
-hydrogen bonding is stronger than the other two types of intermolecular bonding
Hydrogen bonding: water
-water can form two hydrogen bonds per molecule, because the electronegative oxygen atom has two lone pairs of electrons on it
-it can therefore form stronger hydrogen bonding and needs more energy to break the bonds, leading to a higher boiling point
Hydrogen bonding: ice
in ice the molecules are held further apart by the hydrogen bonds than in liquid water and this explains the lower density of ice
boiling points: graphs
-the anomalously high boiling points of H2, O, NH3 and HF are caused by the hydrogen bonding between these molecules in addition to their London forces
-the additional forces require more energy to break and so have higher boiling points
-the general increase in boiling from HCl to Hl is caused by increasing London forces between molecules due to an increasing number of electrons
Solvent and solubility:
solubility of a solute in a solvent is a complicated balance of energy required to break bonds in the solute and solvent against energy given out making new bonds between the solute and solvent
Ionic substances dissolving in water:
-when an ionic lattice dissolved in water it involved breaking up the bonds in the lattice and forming new bonds between the metal ions and water molecules
-the negative ions are attracted to the +(weird sign) hydrogen on the polar water molecules and the positive ions are attracted to the - oxygen on the polar water molecules
Solubility of simple alcohols:
-the smaller alcohols are soluble in water because they can form hydrogen bonds with water
-the longer the hydrocarbon chain the less soluble the alcohol
Insolubility of compounds in water:
-compounds that cannot form hydrogen bonds with water molecules, e.g. polar molecules such as halogen alkanes or non polar substances like hexane will be insoluble in water
Solubility in non-aqueous solvents:
-compounds which have similar intermolecular forces to those in the solvent will generally dissolve
-non-polar solutes will dissolve in non-polar solvents, e.g. iodine which has only London forces between its molecules will dissolve in a non polar solvent such as hexane which also only has London forces
Propanone:
-propanone is a useful solvent because it has both polar and non polar characteristics
-it can form London forces with some non polar substances, such as octane with its CH3 groups
-its polar C=O bond can also hydrogen bond with water
