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Chemistry 2.5
AS edexcel chemistry - intermolecular bonding
| Question | Answer |
|---|---|
| What is intramolecular bonding? | Strong ionic, covalent and metallic bonds between atoms in a lattice or a molecule |
| What is intermolecular bonding + different types? | Weak electrostatic forces of attraction between neighbouring molecules, affects the physical properties of the substance - permanent dipole forces, instantaneous dipole-induced dipole forces (london forces), hydrogen bonds |
| What are permanent dipole forces? | Intermolecular forces that exist between polar covalent molecules - the delta plus end of one molecule attracts the delta minus end of another molecule |
| What are london forces? | intermolecular forces-electrons in covalent molecules oscillate in bonds/orbitals. at any point in time, electrons will be closer to one end of the molecule than the other, instantaneous dipole, which induces an instantaneous dipole in adjacent molecule |
| What molecules do london forces, permanent dipole forces and hydrogen bonds occur between? | London - all covalent molecules, polar and non-polar. Hydrogen - between a hydrogen atom and an electronegative atom, fluorine, nitrogen or oxygen. Permanent dipole forces - only polar molecules |
| Which is stronger - london forces or permanent dipole forces? | London forces - affect boiling/melting points, more energy is required to separate the molecules |
| What is the strength of a london force dependent on? | Number of electrons and size of electron cloud - larger means electrons are held less strongly, larger distance over which they can move, greater instantaneous dipoles, larger london forces. Number of points of contact bewteen adjacent molecules |
| Why do hydrogen atoms for strong intermolecular bonds? | Hydrogen atoms have no shielding electrons so are exposed to shifts in electron density |
| Why don't hydrogen bonds form with chlorine? | It is too large to get close enough to the hydrogen atom to form an intermolecular bond. |
| What happens to the boiling point of the alkanes as the number of carbon atoms increases? | Boiling point increases-more points of contact + more electrons, greater instantaneous dipoles + London forces, stronger bonding requires > energy to overcome. Polymeric alkanes- jumbled up chains, don't fit tightly, large distances, few points of contact |
| What happens when alkanes are branched? | Branching of the carbon chain lowers the boiling point because molecules are bulkier when branched - can't fit together tightly, larger distance between moelcules, fewer poitns of contact bewteen adjacent molecules, weaker london forces |
| What is the pattern of the melting points of the alkanes? | Alkanes with an even number of carbon atoms have higher melting points - fit together more tightly, stronger london forces |
| Why do alcohols have low volatilities and high boiling points compared to alkanes with a similar number of electrons? | Alcohols have a hydroxyl group so undergo intermolecular hydrogen bonding as well as london forces so it requires more energy to separate the molecules |
| What is the trend of the boiling point of hydrogen halides? | Boiling point increases down group - more electrons, greater instantaneous dipoles, stronger london forces, outweighs decrease in strength of permanent dipole forces. HF has highest becuase it has hydrogen bonding as well |
| What is solubility? | The mass of solute that dissolves in 100g of solvent at a particular temperature. Like dissolves in like |
| What are the conditions for a solute to dissolve? | Enthalpy of hydration (energy released when bonds are made between solute and solvent) > lattice enthalpy (energy required to break apart lattice in solute). like dissolves like |
| Why do polar and non-polar solutes dissolve? | Polar solutes dissolve in polar solvents - allows hydrogen bonding, more london forces. Non-polar solutes dissolve in non-polar solvents with similar intermolecular bonding - allows more london forces to form |
| What is seen in a separating funnel? | An organic layer, in which non-polar solutes dissolve, and an aqueous layer, in which polar solutes dissolve |
| Why can't non-polar solutes dissolve in water? | They can't react or form bonds with water, so the enthalpy of hydration would be smaller than the lattice enthalpy, not energetically favoruable |
| What happens when ionic solids dissociate in water? | Hydration energy is released when delta minus oxygen atoms in water molecules surround cations, and delta plus hydrogen atoms in water molecules surround anions |
| When temperature increases, what happens to solubility? | Increases - there is more energy to break apart the bonds of the lattice of the solute |
| What happens as the number of carbon atoms in an alcohol icnreases? | Decreases in solubility in water - a larger proportion of the molecule is a non-polar carbon chain that can't form H bonds with water |
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