Hydrogen Bonds
What arrangement creates the strongest hydrogen bonds? What about weaker hydrogen bonds?
Hydrogen bonds are the strongest when the donor and acceptor atom are 180 degrees apart, the nonlinear hydrogen bonds are going to be weaker
What is polarity? And what determines polarity of a molecule?
Polarity is an unequal sharing of electrons, it is determined by the difference in electronegativity of the atoms in the molecule.
What types of molecules dissolve readily in water? What types of molecules do not dissolve readily in water?
Hydrophilic (polar) dissolves readily, while hydrophobic (non-polar) does not.
If I have one solution with a pH of 5 and another with a pH of 7, what is the difference in H+ concentration between the solutions?
There would be 100x more H+ in the solution with a pH of 5 compared to a pH of 7!
Acids ______ a proton while bases _______ a proton
donate, accept
What two groups are present in a hydrogen bond?
The weakly acidic donor group and weakly basic acceptor group! (Remember: BAAD, bases accept acids donate)
What is required for the presence of a dipole?
Each atom of the bond has an unequal sharing of electrons causing a partially positive or partially negative charge, this will create a dipole
What makes water a great solvent?
It contains both a partial positive and partial negative charge due to its dipole. This allows for interactions with ions and polar molecules allowing them to easily dissolve in water.
How do we calculate pKa?
It is the -log of Ka, usually Ka will be given to you!
Do strong acids/bases dissociate completely or partially in water?
Strong acids and bases dissociate completely in water
Is the weakly acidic donor or weakly basic acceptor in a hydrogen bond more electronegative?
The weakly basic donor is more electronegative and thus carries a partially negative charge meaning it can accept a partially positive-charged atom
What type of bond is the strongest? What about the weakest?
Polar covalent bonds are the strongest, and nonpolar covalent bonds are also very strong. The weakest is van der waals which consists of hydrogen bonds, dipole-dipole interactions and london dispersion forces. However, hydrogen bonds are very strong when they are high in number.
What is a sphere of hydration?
This is when the polar nature of water allows for electrostatic interactions with ions. Water molecules will form shells around the ions and the ions will dissolve.
At what point on a titration curve does pH = pKa
the inflection point!
What is an inflection point on a titration curve?
this is the point in an acid base titration at which enough base has been added to neutralize 50% of the acid so [acid]=[base]
Are hydrogen bonds present within a water molecule?
No! They are not present within a singular water molecule, these are called polar covalent bonds. However, hydrogen bonds can exist between multiple water molecules due to the presence of a dipole
What are some key things to remember about non-covalent bonds?
non-covalent bonds are reversible, non-covalent interactions are specific, 3D structure of most macromolecules are determined as a result of the collective influence of the many weak interactions, size, shape and type of interaction all must be correct for binding and proper folding, and non-covalent bonds allow for dynamic biological processes
What is the hydrophobic effect? How does the nonpolar solute respond to being in water?
This is the tendency of water to minimize its contacts with hydrophobic molecules. The water molecules will form a cage around the solute (sphere of hydration), the nonpolar solute will cluster together in an aggregation which allows for greater disorder of the water molecules (universe favors disorder).
Also remember it is weakly temperature dependent, at lower temps water is less mobile so this may not occur
What causes a titration curve to shift up or down?
if the pKa value is large the curve will move up, if it is small it will move down.
low pKa = more acidic
so pH and pKa are directly related
What is a buffer?
this is a solution that can resist changes in pH, it requires that there is a weak acid and its conjugate base present
Hydrogen bonding is important for establishing 3-D structures. What were the 2 types of hydrogen bonding discussed in lecture that contribute to macromolecule formation?
There is interstrand hydrogen bonding and intrastrand hydrogen bonding. Interstrand holds more than 1 strand together (this is seen in DNA double helix and in some proteins). Intrastrand hydrogen bonds are formed within a singular strand (these are seen in some proteins as well).
What is proton jumping?
Remember, H+ does not actually exist in water, rather H3O+ does! So 2H2O -> H3O+ + OH-
proton jumping allows for this because it accounts for increased mobilities of H+ and OH- as compared to other ions, this is why acid-base reactions occur fast in aqueous solutions
Is the dissolving of a salt in water entropically or enthalpically driven?
It is entropically driven, we go from having a uniformed compound (salt) to free ions causing disorder to increase and entropy to be high. In this reaction enthalpy is also a small positive number because it requires energy to separate the ions from each other. So this overall reaction is favorable and entropically driven.
What types of acids do we need to use in the Henderson-Hasselbach Equation?
Weak acids only!
If I have a diprotic acid, how many species will I have? How many dissociation events will I have?
I will have 3 species and 2 dissociation events