2. Water Flashcards
How many hydrogen bonds are there in ice? in water?
ice: 4
water: 3.4
List the bond types from weakest to strongest (5)
- Van der waal interactions
- Hydrogen bonds - bent
- Hydrogen bonds - linear
- Electrostatic interactions
- Covalent bonds
What are hydrogen bonds (4)
- electrostatic attraction between polarized molecules containing O, N, or F.
- Strongest when linear
- Weak interaction compared to covalent bonding
- 8-21 KJ/mol
Why does water melt so easily at 25 degrees? What type of bond is this related to? (3)
- Increase in entropy is favoured - more disorder
- The temperature times entropy is greater than the change in heat energy which means a negative free energy value which is spontaneous and preferred
- Related to hydrogen bonding
What are electrostatic interactions? (3)
- attraction between oppositely charged ions OR repulsion between like-charges
- D-D, I-D, I-I, induced
- up to 200 KJ/mol
Explain NaCl dissolving in water. What type of bond is this related to? (6)
- Enthalpy is required to break the Na and Cl bonds
- Enthalpy is required to disrupt hydrogen bonds in water
- Enthalpy is released when solvation occurs (new water-ion interactions form)
- Net enthalpy is small but positive
- Solid nacl is highly ordered while nacl solution is disordered which is favored
- Related to electrostatic interactions
What are van der waal interactions? (4)
- Short range, very weak attractions
- Bonds non-polar hydrocarbons
- Stabilizes membranes
- 4 KJ/mol
what happens when a non-polar hydrocarbon is dissolved in water? What type of bond is this related to? (6)
- Hydrocarbon van der waal interactions are broken (+H)
- Water hydrogen bonds are broken (+H)
- New water hydrogen bonds are formed in an organized cage around the hydrocarbon
- Optimizes van der waal interactions between the hydrocarbon and water
- Optimizes the H-bonding among the water molecules (-H)
- Related to VW
OVERALL: LOWERS ENTROPY
What is the hydrophobic effect? (2)
- When the entropy of water is reduced, it disfavors the dissolution of hydrocarbons in water
- Non-polar substances tend to minimize their exposure to water
What are amphipatic molecules?
- Contain both polar and non-polar groups
- Ex: detergents, lipids, proteins, nucleic acids
- Cluster hydrophobic groups away from water to have the lowest free energy
What is a micelle?
When a detergent is amphipathic, the non-polar groups (hydrophobic) are on the inside while the polar groups (hydrophilic) associated itself with water through van der waals interactions
What is the equation for pH? pOH?
pH = -log[H+]
pOH = -log[OH-]
pH + pOH = 14
Which acids have a large Ka? Small Ka? Large pKa? Small pKa?
weak acids: small Ka, large pKa
strong acids: large Ka, small pKa
What is the formula for Ka? pKa?
Ka = [H+][A-]/[HA]
pKa = -log[Ka]
What are titration curves?
indicate the pH values of different mixtures of weak acid and its conjugate base
What is present at point A? B? C?
A: All weak acid (HA) present
B: [HA]=[A-] therefore pH=pKa, effective buffering
C: all conjugate base (A-) present
