Water Flashcards
what does the electronegativity of the O atom in water do?
induces a net dipole moment and allows water to serve as both a hydrogen bond donor and acceptor
what are the common non-covalent attractive forces in order of weakest to strongest?
van der Waals hydrophobic interactions dipole-dipole hydrogen bonding ionic interactions ((not totally sure about order))
what are hydrogen bonds?
strong dipole-dipole interactions that arises between proton donor and proton acceptor. involves two electronegative atoms. bonds are strongest when molecules are oriented to maximize interactions (co-linear)
why is H bonding important in water?
water can serve as both the H acceptor and donor and can make up to 4 bonds per water molecule. this gives it a high bp, mp, heat capacity, enthalpy of vaporization, and surface tension.
the H bonds are short lived, so water can organize with itself or solute nearly instantaneously
what is the most common form of ice? why?
hexagonal ice. it forms a regular lattice and has low entropy, but contains more H bonds per water molecule so it’s favorable. ice has lower density than liquid water and floats, which is not typical.
what does the high enthalpy of vaporization of water mean for us?
it takes a lot of energy to cause a phase change (boil) and break the H-bonds. This relates to our regulation of body temperature (sweating) requiring energy
What is water a good solvent for? what is it not a good solvent for?
good for charged and polar substances. like amino acids, peptides, small alcohols, carbohydrates
poor for non polar substances. like non polar gases, aromatic groups, aliphatic chains
why is water good at dissolving salts?
high dielectric constant reduces attraction between oppositely charged ions in salt.
strong electrostatic interactions between the solvated ions and water lower the energy of the system
entropy increases as ordered crystal lattice is dissolved
what is the hydrophobic effect?
the association or folding of non polar molecules in aqueous solution. it is one of the main factors behind protein folding, protein-protein association, and formation of lipid micelles. it does NOT arise due to direct attractive force between two non polar molecules
how can the hydrophobic effect be explained with entropy?
bulk water has little order and high entropy. water near a hydrophobic solute becomes highly ordered and entropy is lowered. this is unfavorable and so hydrophobic solutes have low solubility. when multiple hydrophobic solutes gather together, they lower the amount of water molecules interacting and increase entropy as water is not ordered
water can autoionize. what does this mean and what are the products?
the O-H bonds are polar and can dissociate heterolytically to produce H+ and OH-. this is a rapid, reversible process that has equilibrium lying to towards H2O. extent of dissociation depends on temp (high temp favors ionization)
protons do not actually exist in solution. what happens to the proton when water is ionized to form H+ and OH-?
the proton is immediately hydrated to form hydronium ions (H3O+). the covalent and hydrogen bonds are interchangeable and allow for rapid mobility of protons in water via “proton hopping”
what is pH?
the negative log(base 10) of the hydrogen ion concentration. pH and pOH add to 14. in neutral solution [H] = [OH] and pH is 7. (pH can be negative)
pH = -log[H]
remember lewis vs Arrhenius vs bronsted-lowry? which one do we care about in biochem?
lewis: electron pair acceptor or donor
Arrhenius: dissolves H2P to increase [H] or [OH]
bronzed-lowry: proton acceptor or donor
In biochem we use Arrhenius
can you calculate Ka and pH if given one? There’s a normal way (quadratic equation) and a simplified way. when can you use simplified way?
see slides 32-34
simplification can be used if amount of dissociated species is much less than amount of undissociated species. basically if x «_space;total acid then you can change products from 1-x to just 1
