lecture 1 - polarity

Question 1

how does polarity affect how molecules react with water?

Answer 1

• polarity causes water to interact favouribly w other polar molecules
• also affects how water affects with non-polar molecules

Question 2

H bonds

Answer 2

• 20x weaker than covalent
• 5x stronger than vdw forces
• need 2 linear electronegative atoms and 1 H atom
• alignment and length affect strength (more straight, shorter = stronger)

Question 3

liquid water structure

Answer 3

• h2o can tumble (rotate/translate in 3D space)
• hbonds break and reform rapidly
• creates flickering clusters
• each h2o forms ~3.4 hbonds to its neighbour - maintains some short range order

Question 4

lifetime of an hbond

Answer 4

1-20ps

Question 5

polar solutes and water (salts, sugars)

• are these interactions favourable? why or why not?
• bonds?

Answer 5

• favourable interactions due to dipoles
• typically ~5-6 h2o surround each salt ion
• not worried about orientation as much
• electrostatic interactions compensate for loss of some hbonds in water

Question 6

deltaG = deltaH -T(deltaS)

Answer 6

deltaG less than 0 = favourable reaction

Question 7

non polar solutes in h2o

• what bonds form and what bonds don’t form
• deltaH/S

Answer 7

• no hbonds form between h2o and solute
• only weak vdw interactions
• fewer hbonds for h2o near solute
• detlaH > 0
• deltaS < 0 bc h2o near solute are more odered (less free to tumble)

Question 8

oil and water (aggregation)

• what causes it
• is it favourable and why

Answer 8

• causes aggregation of non-polar solute so there’s fewer h2o molecules in contact with the non-polar solute
• some of ordered h20 near non-polar solute are released back into solvent, forming more hbonds, decreasing number of ordered h2o molecules, increasing entropy (deltaS&raquo_space;>0), deltaH <0, deltaG <0 = favourable

Question 9

hydrophobic effect

• is it a force?
• what defines this effect?
• why is it important?

Answer 9

• not a force
• tendancy of non-polar molecules to aggregate
• important for protein folding (structure/stability), protein-protein/ligand interactins & purification methods.

Question 10

entropic effect

Answer 10

vdw forces btwn h2o and non-polar solutes are weak and similar to vdw interactions bewteen non-polar molecules