Chapter 2: Aqueous Chemistry Flashcards
Why is water an effective polar solvent?
Tetrahedral shape and the difference in electronegativity between oxygen and hydrogen
What difference in electronegativity makes a bond polar?
> 0.5
Why can water dissolve polar compounds such as glucose?
Because multiple H-bonds can be formed.
Multiple H-bonds make a very stable molecule (ex. DNA).
Name the type of van der Waals interactions occurring in the following molecule
dipole dipole polar due to electrostatice interactions
Name the type of van der Waals interactions occurring in the following molecule
London dispersion forces with non polar molecules
Are noncovalent bonds weaker than covalent bonds?
Yes!
Note: H-bonds are the weak noncovalent bonds most commonly seen in biochemistry. They are strong in numbers.
Name the phenomenon by which nonpolar molecules aggregate to avoid contact with water
Hydrophobic effect
What type of molecules are lipids?
Amphiphilic molecules because the molecules can experience both hydrophobic and hydrophilic effects
Shape and bond types in water
Water is tetrahedral in shape and can form H-bonds (an electrostatic interaction) in water and between water and other molecules such as hydroxyl (OH) and amine (NH) groups.
Why is water an effective polar solvent?
Water is polar.
The H-bond definition and components
H-bonds are partially covalent in nature and are classified as a type of electrostatic interaction. Since they have a longer bond length than covalent O-H bonds they are weaker.
Identify hydrogen bond donor and acceptor groups
Hydrogen bond donors: N-H, O-H, and S-H
Hydrogen bond acceptors: electronegative N, O, or S atoms ** electronegativity is a measure of an atom’s affinity for electrons
Examples of functional groups that can form H-bonds
Hydroxyl and Amine
Identify dipole-dipole and London dispersion forces
Van der Waals interactions (weaker than H-bonds) are electrostatic interactions that occur between particles that are polar but not actually charged.
Dipole-dipole interactions are between two strongly polar groups (ex. two carboxyl groups)
London dispersion forces occur between nonpolar molecules as a result of small fluctuations in their distribution of electrons that create a temporary separation of charge. (ex. methyl groups)
How water dissolves ionic compounds
Water has a relatively high dielectric constant, which is a measure of a solvent’s ability to diminish the electrostatic attractions between dissolved ions. The higher the dielectric constant of the solvent, the less able the ions are to associate with each other.
How the weak interactions can become strong
The cumulative effect of small forces
What is meant by the hydrophobic effect?
Nonpolar molecules aggregate to avoid contact with water.
Effects on entropy when adding nonpolar molecules to water
When a single nonpolar molecule is hydrated by water, the system loses entropy.
When many nonpolar molecules are hydrated, they aggregate and the entropy of the system increases.
What are amphiphilic molecules?
Molecules with both hydrophobic and hydrophilic portions are said to be amphiphilic or amphipathic. (ex. lipids)
How do amphiphilic molecules arrange in water?
They create micelles.
The polar groups of amphiphiles orient themselves toward the solvent molecules and are therefore hydrated, while the nonpolar groups tend to aggregate due to the hydrophobic effect.
Why is the lipid bilayer a barrier to diffusion
The hydrophobic core of a lipid bilayer serves as a barrier to diffusion
When the vesicle forms, it traps a volume of the aqueous solution. Polar solutes in the enclosed compartment tend to remain there because they cannot easily pass through the hydrophobic interior of the bilayer. The energetic cost of transferring a hydrated polar group through the nonpolar lipid tails is too great. (In contrast, small nonpolar molecules such as O2 can pass through the bilayer relatively easily.)
Draw the equation of the ionization of water
The ionization of water produces the hydronium and hydroxide ions
Draw the ionization constant of water (Kw) expression and state the value
Relationship between [H+] and [OH-] and the quantitative expression of pH
[H+] and [OH-] are inversely related
pH = -log[H+]
The definition used by biochemists of acids and bases and their effect on pH
Bronsted Lowry acid is a proton donor (decrease pH)
Bronsted Lowry base is a proton acceptor (increase pH)
Meaning of Ka and pKa and using these to determine the strength of acids
The larger the Ka, the stronger the acid
The smaller the pKa, the stronger the acid
Quantitative expression of the pKa
pKa= -log(Ka)
Write the Henderson-Hasselbalch equation
Predominant chemical species of a weak acid at a given pH
pH > pKa, A- (conj base) predominates
pH < pKa, HA (weak acid) predominates
Definition of a buffer solution
Solution of 50/50 weak acid and the salt of its conjugate base that resists change in pH when small amounts of acid or base are added.
Their buffering capacity is most effective at or near the acid pKa.
How buffer solutions resist changes in pH
Neutralization.
At the start of the titration, all the acid is present in its protonated (HA) form. As base (NaOH) is added, protons begin to dissociate from the acid, producing A-. The continued addition of base eventually causes all the protons to dissociate, leaving all the acid in its conjugate base (A-) form. At the midpoint of the titration, exactly half the protons have dissociated, so [HA] =[A-] and pH 5 pK.
How to interpret the titration curve of acetic acid
The effective buffering capacity of acid is generally taken to be within one pH unit of its pK.
Components of the phosphate buffer system and know how it works.
The phosphate buffer acts under physiological conditions.
pKa = 6.82
The components of the bicarbonate buffer system and how it works.
The bicarbonate buffer system acts in blood
Acid-Base effect on pH
B. Methane because no electronegative atoms and nonpolar
A
B
C
E
