Final exam Flashcards
How strong are hydrogen bonds? How do they compare to covalent bonds?
They are strong intermolecular forces but weak in comparison to covalent bonds
Are hydrogen bonds actually bonds?
No, they are intermolecular forces
How does the distance between donor and acceptor (in a hydrogen bond) affect the energies associated with the force of attraction?
As the distance increases, the force of attraction decreases
Based on Coulomb’s Law, why are ion-dipole forces of attraction stronger than dipole-dipole forces?
The charge is greater in ion-dipole interactions: a true charge interacts with a partial charge, yielding a greater force
When thinking about forces of attraction, does the solvent matter? Why or why not?
Yes, because the solvent can influence the force of attraction
The influence of the solvent on the forces of attraction/repulsion between charges is corrected for by the _______________ ____________.
Dielectric constant
What does water having a dielectric constant of around 80 really mean?
As the dielectric constant increases, the force of attraction decreases between charged species; water will sheild the positive and negative charges from each other; this means that water shields charges from one another 80 times more than does a vacuum
What trends are noticeable regarding dielectric constants?
Polar substances have higher dielectric constants; nonpolar substances have lower dielectric constants
Considering the overall trend regarding dielectric constants, is the force of attraction between potassium ions and chloride ions greater in water or benzene?
Benzene
How does the dielectric constant relate to solublity?
Charged species are less soluble in solvents with low dielectric constants; charged species are more soluble in solvents with high dielectric constants
How many hydrogen bonds can water form with itself in liquid form? Solid form?
4 hydrogen bonds
3 - 4 hydrogen bonds
How would you describe the behavior of pure liquid water?
Rapidly fluctuating hydrogen-bonded network that reorients on the order of picoseconds with approximately 3.4 bonds hydrogen bonds per water molecule and three to five water-member rings
What is different about the water molecules that surround a solute (i.e., waters of hydration)?
Waters around a solute are more constrained; they form a cage-like structure around the solute in an attempt to maximize hydrogen bonding; this means that waters farther away from the solute are freer
What thermodynamic quantitiy determines if a process or reaction is spontaneous?
Gibbs Free Energy
A negative G means a reaction is spontaneous
What do H and S mean in thermodynamics?
H refers to enthalpy change
S refers to entropy
Enthalpy is the driving force of a reaction when its value is _______________; entropy is the driving force of a reaction when its value is ________________.
Negative
Positive
The enthalpy change for placing a nonpolar solute in water is often positive (endothermic) and the entropy change is negative. What can you say about the energetics of placing a nonpolar solute in water? What happens when many nonpolar particles are placed in water?
Nonpolar solutes aggregate with one another in a process called the hydrophobic effect; however, this is not the driving force for aggregation. As solutes are added, the surface area of the aggregation increases, which decreases the number of water molecules in the caged-like network around the aggregation. This means that water molecules are released into the bulk and increase in entropy as they are less constrained. Entropy of water molecules is the driving force for the hydrophobic effect.
What happens when a substance like sodium dodecyl sulfate is placed in water?
A water cage will form around the nonpolar, hydrophobic tail
Water will solvate the polar, hydrophilic tail
When a collection of sodium dodecyl sulfate particles are placed in water, what happens (e.g., polar head, nonpolar tail)?
The particles aggregate into a micelle, a spherical aggregate
What is the driving force for the formation of micelles?
Increased entropy
What structure would form if SDS was placed in a nonpolar solvent?
A reverse micelle
What if you consider a collection of molecules like the one shown below places in aqueous solution?
A bilayer or vesicle/liposome will form
What factors dictate when a micelle or vesicle will form?
The number of nonpolar tails present:
When one hydrophobic tail is present, a micelle will form
When two hydrophobic tails are present, a bilayer or vesicle will form
Calculate the amount of H+ in blood if the pH is 7.4.
[H+] = 10-pH
pH = - log [H+]
[H+] = 10-7.4
= 4.0 x 10-8 H+ ions
A 10-fold change in the concentration of H+ inside the cell results in a change of _________ pH unit.
1
Stronger acids have higher ______ but lower _____.
Ka
pKa
What components make up a buffer?
A weak acid and its conjugate base or a weak base and its conjugate acid
On a titration curve, the pH equals the pKa at ________ of the equivalence volume.
One half
Is there a point in a titration where a buffer exists? If yes, how is it determined?
Yes, the buffering region is the pH range +/- 1 pKa
(e.g., if pKa is 3, the buffering region is pH levels between 2 and 4)
Be very familiar with reading a titration curve.
