C6 - Shapes of Molecules and Intermolecular Forces Flashcards

Question

What are London forces also known as?

Answer 1

Induced dipole-dipole forces

Answer 2

[London forces] are weak intermolecular forces which exist between ALL molecules, polar and non-polar. They act between induced dipoles in different molecules.

Answer 3

Random movement of electrons produces a changing dipole in a molecule. (interactions of e-) At any instant, an 'instantaneous' dipole will exist however its position is continuously changing. The instantaneous dipole induces a dipole on a neighbouring molecule. (The e- of one molecule repel the e- of another molecule so there is a slight positive charge nearer to the electrons and repulsion is reduced, forming another dipole) The 'induced' dipole induces further dipoles on further neighbouring molecules which then attract one another (+ - + - + -) They are the weakest of intermolecular forces. This is due to how the electrons are constantly moving so the interaction is only brief. This then results in the formation of London forces elsewhere.

Answer 4

Van der Waals' forces implies both induced and permanent dipole-dipole interactions whereas London forces is just for induced interactions.

Answer 5

The number of e- The more e- in each molecule: - the larger the instantaneous and induced dipoles - the greater the induced interactions - the stronger the attractive forces between molecules

Answer 6

Interactions between the permanent dipoles in different polar molecules. (the slight charges present in polar molecules only). They are stronger than induced dipole-dipole interactions therefore molecules with induced d.d. forces AND permanent d.d. forces require MORE energy for the interaction to be broken. e.g HCl (requires more energy) compared to simple molecules like F2.

Answer 7

A special type of permanent dipole-dipole interaction found between molecules that contain a hydrogen atom attached to a very electronegative atom with a lone pair of electrons e.g. O, N or F It is represented as a dashed line. They are the strongest type of intermolecular force (not as strong as single covalent bonds)

Answer 8

- Ice is less dense than water - Water has relatively high melting/boiling points - Surface tension

Answer 9

Each water molecule can form 4 hydrogen bonds which hold water molecules apart in an open tetrahedral lattice structure when frozen therefore the water molecules in ice are further apart than in water. The gaps/holes within the structure therefore decrease the density of ice so it is able to float. This enables ice to act as an insulating layer and doesn't destroy marine habitats

Answer 10

Water has many hydrogen bonds as well as London forces. The hydrogen bonds are much stronger than London forces so require much more energy for these forces to be overcome. When ice lattices break, the rigid arrangement of H bonds are broken and when water boils, the bonds are broken completely.

Answer 11

Surface tension is a result of the forces present at the surface of a liquid due to cohesion. The water molecules at the surface of water have no other water molecules above them so only have hydrogen bonds below them which bind the molecules together in cohesion. Cohesion therefore causes the molecules to be pulled down producing a force or ‘tension’ across the surface of the water. This appears as though the water has an elastic ‘skin’ which has been stretched across the water. This is all as a result of the hydrogen bonds formed between water molecules.

Answer 12

DNAs double helix structure is held together by H bonds which enables a single DNA strand to create a perfect copy of itself by replication. Replication depends on the four bases, A, T, G and C. - A & T pair by forming 2 H bonds - C & G pair by forming 3 H bonds The shape and chemical structure of these bases ensure they pair up correctly. A & G are both purines with double ringed structures and T & C are single ringed pyrimidines. H bonding in the double helix can only take place between a purine and pyrimidine. The bases must fit together so that a H atom from one molecule and an electronegative atom (O or N) from the other molecule are aligned correctly.

Answer 13

Small units containing a definite number of atoms with a definite molecular formula e.g. Ne, H2, H2O and CO2. Simple molecular substances are made up of these simple molecules. When solid, they form simple molecular lattices held by weak intermolecular forces and atoms within each molecule are bonded together strongly by covalent bonds. They're all covalently bonded and have no charged particles so aren't conductive.

Answer 14

They have low melting and boiling points. In a simple molecular lattice, weak intermolecular forces can be broken by energy present at low temperatures. Only the weak intermolecular forces break, not the strong covalent bonds.

Answer 15

- Non-polar simple substances are soluble in non-polar substances as intermolecular forces are formed between the molecules and the solvent. The interactions weaken the intermolecular forces in the simple lattice and break the compound. (Similarly, polar substances dissolve in polar solvents) Simple molecules aren't soluble in polar substances as there is little interaction between the molecules in the lattice and the solvent and the permanent d.d. bonds in the polar substance are too strong to be broken E.g. iodine (n.p.) is soluble in cyclohexane (n.p.) but not water

Answer 16

They dissolve in polar solvents as the polar solute and solvent molecules attract each other. E.g. HCl is soluble in water Solubility depends on the strength of the dipole. Some compounds have polar (hydrophilic) and non-polar (hydrophobic) parts to their structure

Answer 17

Instantaneous dipoles followed by induced dipoles