Consequences of Intermolecular Interactions

  • Created by: E.H13
  • Created on: 28-05-20 10:13

Real Gases

Ideal gases obey pV=nRT, where R = 8.314 JK-1mol-1. It can never liquify, no matter the temperatue. This assumes;

  • atoms/molecules have no size
  • no interactions between the atoms/molecules 

Van der Waals equation of state; in the form (peff)(Veff) = nRT

(p + a/Vm2)(Vm-b) = RT           Where Vm = V/n. (molar volume - it's easier to write on a computor).

a - accounts for interactions; attractive long range interactions decrease the frequency and force of collisions, decreasing pressure

b - accounts for size; reduction of available volume means less volume to move in, which means more frequent collisions, which increases pressure

When p = 0, liquid and gas coexist, and at high T or high Vm, tends toward ideal gas.

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  • Different phases adopted by different elements and compounds
  • Structures of solids and liquids
  • Transport properties
  • Suromolecular chemistry
  • Properties of electrolyte solutions
  • Liquid crystals - due to anisotropic polarisability
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Anomalous Properties of Water

Despite it being abundant, it is considered anomalous as it behaves differently from simple liquids due to H-bonding. Relatively high boiling and melting points, but also has a high liquid range (100K). 

H-bonding is condensed water is cooperative; the strength of H-bonding increases with increasing no. of molecules as this increases the amount of polarisation. Shown in the dipole moments (1.85D in isolated H2O to 2.4-2.6 in liquid H2O at 0oC). 

Ice; based on tetrahedral coordination. Each molecule has 4 H-bonds. Multiple different solid ice phases.

Liquid; more dense than ice.  Maximum density at 4oC.  Fluctuating network of H-bonds. Higher densities favoured by increasing VdWs, but H-bonding favours lower co-ordination and lower density. This means on melting, even though hydrogen bonding is weaker, Van der Waals is stronger. Less compressible than ice. High dielectric constant as it is a good polar solvent. 

According to Clayeyron eqn; every 133 atm of applied pressre decreases the melting temperature by 1 K. 

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The Hydrophobic Effect

Hydrophobic effect; the low solubility of hydrocarbons and other non-polar molecular in water, and their increased tendancy to aggregate.

Enhanced effective attractions can occur between hydrocarbons when in water. Can be explained as A-B interactions are weaker than A-A and B-B- interactions, so UAB < UAA, UBB. Sadly this doesn't explain why the solubility of oil in water is a minimum at T = 25oC, as we'd expect solubility to increase as T increases.

Actual explanation; enthalpy isn't particularly unfavourable, as the non-polar molecules induce cage-like ordering of water molecules, strengthening the H-bonding. BUT this means entropy is significantly decreased. Below 25oC, the entropy term dominates, meaning solubility decreases as T rises. Above this point, the 'water cages' start to break down, meaning dH and dS increase so solubility increases.

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Clathrates, Micelles and more

Clathrate; a single hydrocarbon/non-polar molecule, surrounded by a polyhedral cage of water molecules. At high p, low T these can precipitate out as solids. 

Micelles; examples of colloids. Hydrophilic heads (polar/charged) form a close packed shell, with hydrophobic tails (hydrocarbon chains) aggregating together, forming the centre of the micelle. 

Used to solubilise hydrocarbons in aq solutions. Works as surfactants. 

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Proteins are polypeptides; chains of amino acids joined by peptide links (-CO-NH-).

Protein folding; folding of polypeptide chains due to intermolecular forces. 

Primary structure; sequence of amino acids

Secondary structure; alpha-helices and beta-sheets, due to H-bonding between peptide groups.

Tertiary structure; folding due to -S-S- links, ionic interactions and H-bonding between side chains. (Hydrophobic effect might be important here)

Quaternary structure; aggregation of more than one polypeptide chain, due to similar interactions to tertiary structure

Protein aggregation can be beneficial (e.g. haemoglobin), but also harmful (protein misfolding diseases). 

Hydrophobic effect - partically drives protein folding. This reduces the unfavourable decrease in entropy of the surrounding water molecules. 

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