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How can a gecko cling to any surface and yet still be able to move?. Intermolecular forces Intermolecular forces arise between molecules. They can be thought of as holding individual molecules together. These weak forces must be overcome for substances to boil.
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How can a gecko cling to any surface and yet still be able to move?
Intermolecular forces Intermolecular forces arise between molecules. They can be thought of as holding individual molecules together. These weak forces must be overcome for substances to boil.
There are three types of intermolecular forces Van der Waals forces Permanent dipole-dipole forces Hydrogen bonds
If there were no attractions between molecules it would be impossible to turn a gas into a liquid by cooling. But why are there attractions between uncharged non-polar molecules?
Van der Waals forces These are weak forces that arise from the fluctuating movement of electrons. This movement causes temporary dipoles, which, in turn, induce attraction between molecules (or atoms). These attractions are called Van der Waals forces.
Temporary dipoles occur in one molecule owing to an unequal distribution of the electron cloud around it. This temporary dipole induces a dipole in another molecule and the two attract each other. London forces are also known as induced dipole-dipole forces or Van der Waals forces.
Van der Waals These weak intermolecular forces explain how a gecko (and a spider) can climb a wall!
The strength of Van der Waalsforces increases: The larger the molecule (due to a larger electron cloud) The larger the surface area of the molecule (due to a larger electron cloud exposed).
Van der Waalsforces affect: The boiling point The melting point Viscosity The more Van der Waalsforces exist between molecules, the more ‘sticky’ they become as they attract each other more strongly. Energy is required to overcome these forces of attraction.
Boiling temperatures of noble gases Have a go at explaining this graph
What about the Halogens? At room temperature..... Chlorine is a gas Iodine is a solid Bromine is a liquid
Alkanes Draw 2 isomers of butane Which do you think will have the highest boiling temperature?
Permanent dipole-dipole forces These forces occur between molecules with a permanent dipole. This happens when the electronegativities of the elements are very different, resulting in a polar bond.
The δ+ atom of one molecule attracts the δ- atom of another molecule This results in a weak electrostatic force of attraction. Remember, no overall dipoles are seen in symmetrical molecules, due to the equal pull on the bonding electrons.
The Saturated Hydrocarbons methane CH4 -164 ethane C2H6 -89 propane C3H8 -42 butane C4H10 -0.5pentane C5H12 36 hexane C6H14 69 heptane C7H16 98 octane C8H18 125nonane C9H20 151 decane C10H22 174
Using what you know about inter-molecular forces explain the following graph
Why is the boiling point of water 100ᵒC, when the boiling point of Hydrogen Sulphide is -60ᵒC and that of Hydrogen Selenide is -40ᵒC
Hydrogen bonds These are very strong intermolecular interactions. They are a special case of a permanent dipole-dipole interaction. They are stronger than Van der Waalsforces and permanent dipole-dipole interactions (but still 10X weaker than covalent bonds)
Hydrogen bonds Hydrogen bonds ONLY form when Hydrogen is covalently bonded to one of three very small electronegative atoms..... Oxygen, Nitrogen and Fluorine A lone pair is also required.
Nitrogen, Oxygen and Fluorine are very electronegative and cause the bond with hydrogen to be polar. The bond is so polarised that the hydrogen forms a weak bond with the nitrogen, fluorine or oxygen of another molecule. In addition, N, O and F are all small enough to approach a hydrogen atom – their lone pair on the N, O or F forms a bond with the hydrogen.
δ+ Because Oxygen is so electronegative, Hydrogen is effectively a proton – it has no outer shell. This means it can interact strongly with a lone pair from another oxygen to produce............. δ-
The three atoms associated with a hydrogen bond are ALWAYS in a straight line.
The structure of ice Molecules in ice are held together by hydrogen bonding. The molecules form a giant lattice in which each is bonded to two H atoms by covalent bonds, and two others by hydrogen bonds.
The structure of ice In liquid water hydrogen bonds break and reform easily as the molecules move about. When water freezes the hydrogen bonds hold the water molecules in fixed positions. In order to fit into this structure the molecules are less tightly packed than in liquid water. This means that ice is less dense than water and floats on water. **This is thought to have helped life to continue during the ice ages – ice would form an insulating layer on bodies of water**
Questions: In which of these does hydrogen bonding NOT occur? H2O, NH3, HBr, HF? Explain why hydrogen bonds do not form between methane molecules Draw a dot and cross diagram of water How many lone pairs does it have? How many hydrogen atoms does it have? Explain why water molecules form two hydrogen bonds per molecule, but ammonia molecules only form one. 4) Why do alcohols have a lower volatility compared to alkanes of a similar size?
