Mod 2 Flashcards
What are covalent bonds?
Strong links, very stable, take lots of energy to break
What are non-covalent interactions?
Weak interactions that require little energy to break
What is the dissociation energy of hydrogen bonds in water?
23 kJ/mol
What type of atoms form hydrogen bonds?
Between electronegative O/N and hydrogen bound to an electronegative atom
What percentage of hydrogen bonds are covalent?
10% covalent and 90% electrostatic
What are the dissociation energies of O-H and C-C covalent bonds?
470 kJ/mol for O-H and 348 kJ/mol for C-C
What is the most abundant substance in living systems?
Water, comprising >70% weight in most organisms
What properties make water unique as a solvent?
High melting point, boiling point, and heat of vaporization due to internal cohesion
What is the tetrahedral geometry of water?
Allows water molecules to hydrogen bond with 4 other water molecules
but forms hydrogen bonds with an average of 3.4 other molecules in liquid water
What does the delta negative charge in water signify?
Oxygen attracts electrons more strongly, creating partial charges
How long do hydrogen bonds last in liquid water?
Short-lived, typically within picoseconds
What is the effect of thermal energy on hydrogen bonds in water?
At room temperature, thermal energy is comparable to that required to break hydrogen bonds
What are electrostatic interactions?
Charge-charge interactions, such as those involving ions
Why does salt dissolve in water?
Covalent bonds of water are stronger than ionic bonds in the salt molecule
Define hydrophilic.
Water loving; readily dissolves in water
Define hydrophobic.
Water hating; do not dissolve/mix in water
What are amphipathic compounds?
Compounds that have both hydrophilic and hydrophobic parts
What does Coulomb’s law describe?
Electrostatic interaction between electrically charged particles
According to Coulomb’s law, what happens to the force between charged particles as distance decreases?
The force increases due to an inverse square relationship
What is relative permittivity?
The factor by which the electric field between charges is decreased relative to a vacuum
What is the epsilon value of water?
80
What do non-polar/hydrophobic compounds do in water?
Do not dissolve and interfere with hydrogen bonding of water molecules
Also constrain the orientation of water molecules = decrease in entropy
What is the hydrophobic effect?
Amphipathic compounds arrange to minimize/maximize interaction with water
What are the three types of molecule-ion interactions?
- Charge-charge
- Charge-dipole
- Dipole-dipole
What are Van der Waals interactions?
Nearby atoms influence each other’s electron clouds to create transient dipoles
What is a solvent?
Dissolves the solute, resulting in a solution
What is the definition of a solution?
Homogeneous mixture composed of two or more substances
What is the role of water in cells?
Cells are aqueous environments
What does it mean that water is amphiprotic?
It can act as both an acid (proton donor) and a base (proton acceptor)
What is the equilibrium constant for water’s ionization?
Keq = [H+][OH-] / [H2O]
What is the concentration of water at 25℃?
55.5 M
What does a low pH indicate?
Acidic solution
What does a high pH indicate?
Basic solution
How does the pH scale work?
Logarithmic; an increase of pH by 1 increases it by 10x
What does a large Ka indicate?
Stronger acid with a strong tendency to dissociate
Where do H vs OH migrate to?
H migrates to cathode, OH to anode
What is the Henderson Hasselbalch equation used for?
Titrations involving weak acids and their conjugate bases
What is a buffer solution?
A solution that resists changes in pH
What characterizes molecules with multiple ionizing groups?
Ampholyte; can have more than one pKa
acidic = protonated
What is the first law of thermodynamics?
Energy is not created or destroyed, only transformed
What does the second law of thermodynamics state?
The entropy of the universe is always increasing
What does the third law of thermodynamics state?
A perfectly crystalline solid at absolute zero has an entropy of zero
bc most ordered state
What is entropy a measure of?
How energy is distributed within a system
S (Joules/Kelvin)
What is enthalpy a measure of?
The energy of a system
H
What does a negative ∆G indicate?
Spontaneous process
Can entropically unfavourable processes be spontaneous?
Yes, at lower temperatures
What is an example of a structure that forms spontaneously despite entropic considerations?
Soap micelles
Is water polar or non-polar?
a polar solvent
Why do polar molecules dissolve in water?
They replace water-water interactions with water-solute interactions
Why don’t nonpolar molecules dissolve in water?
They interfere with water-water interactions but can’t form new water-solute interactions.
What are the key weak interactions in aqueous systems?
- Hydrogen bonds, ionic interactions, hydrophobic interactions, van der Waals forces.
Individually weak but collectively significant.
What orientation makes hydrogen bonds strongest?
When bonds are straight (linear), maximizing electrostatic interaction.
Bent bonds are weaker.
What is the bond angle of H₂O and why?
104.5° (less than 109.5° due to repulsion from oxygen’s nonbonding electrons).
How do water molecules interact dynamically?
Exist in flickering clusters of short-lived H-bonded groups.
Each water molecule forms H bonds with ~3.4 others in liquid water, 4 in ice.
Why does ice require heat to melt?
- Heat is needed to break enough H bonds and destabilize the lattice.
- Melting increases entropy, making water molecules more disordered.
How does hydrogen bonding affect boiling points?
- Increases boiling points by strengthening intermolecular forces.
- Seen in alcohols, aldehydes, ketones, N-H containing compounds.
Bond type strength order
…
How does NaCl dissolve
Water dissolves salts by hydrating and stabilizing the ions, weakening electrostatic between them and countering lattice strategy
How does solvent polarity affect ionic interactions?
- Stronger in less polar environments (fewer competing interactions).
- Water weakens ionic interactions due to its high dielectric constant (E = 78.5).
Equation for strength of ionic interactions in a solution
F = Q1Q2 / Er^2
Do polar or nonpolar gases dissolve in water
Nonpolar gases (ex. CO2, O2, and N2) are poorly soluble in water
Polar gases (NH3, NO, and H2S) readily dissolve in water
Why does binding a substrate to an enzyme increase entropy?
- Disrupts ordered water molecules surrounding enzyme and substrate.
- Water molecules become freer, increasing disorder.
Why are nonpolar molecules poorly soluble in water?
- Cannot form energetically favorable interactions with water.
- Water molecules form ordered shells around them, decreasing entropy.
How do weak interactions determine macromolecule structure?
Proteins, DNA, RNA fold to maximize weak interactions.
Examples:
- Antibody-antigen binding
- Enzyme-substrate interactions
- Hormone binding to receptors
What is the role of water in hemoglobin?
- Some water molecules are tightly bound and part of the structure.
- Not osmotically active.
How does water facilitate proton movement in photosynthesis?
Proton hopping occurs through chains of bound water molecules.
Example: cytochrome f in electron transport
What happens to water when solutes are added?
- Lowers water concentration, increasing osmotic pressure.
What do colligative properties depend on?
solute NUMBER, not type
NOT the chemical properties
NOT their mass
How is osmotic pressure calculated?
Van’t Hoff equation:
𝜋=𝑖𝐶𝑅𝑇
i = Van’t Hoff factor (ionic dissociation measure).
C = solute concentration.
What is osmosis?
Movement of water across a semipermeable membrane from high to low water concentration.
What happens to a cell in a hypertonic solution?
Water leaves the cell, causing it to shrink.
What happens in osmotic lysis?
happens in hypotonic solution
Water enters the cell excessively, causing it to burst.
How do cells prevent osmotic lysis?
Plant cells: Cell wall maintains rigidity with turgor pressure.
Freshwater protists: Use a contractile vacuole to pump out water.
Animals: Maintain osmotic balance with blood plasma and interstitial fluid.
