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Chemistry 2.5

AS edexcel chemistry - intermolecular bonding

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
Created by: 11043



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