Concepts to understand

Water has several unique properties that permit life:

WATER

Polarity makes water a perfect ___ for polar substances.

Ionic solids dissolve in H2O to form ___ ions.

Polarity is also essential for the formation of micelles (soap) and bilayers (__ ___).

Non-polar substances are ___ in water.

Electrostatic interactions

1. Charge - Charge

Electrostatic interactions

2. Hydrogen Bonds

H bonds & properties of water:
-high boiling point
-high melting point
-heat of vaporization
-heat capacity
-surface tension
-viscosity

___ of H bond is important for STRENGTH
--> Strongest when ___

Partial electron pair ___ occurs b/w donor & acceptor

Hydrogen bonds ___ double helix structure in DNA

H bonds stabilize protein ___ structure (i.e. beta sheets).

Important concept: The ___ effect of many weak interactions could be a strong force

Water can form H bonds with??

3. Van der Waals interactions

the interaction between permanent or transient __ of two molecules'.

This technically includes the hydrogen bond interactions. However, H- bond is often separated as a special case.

There are classes of interactions can be distinguished:

1. Between two dipoles (two H20 molecules)
2. Between a dipole and non-polar atoms (Debye force)
3. Between non-polar atoms (___ ___ force)

In 2 and 3, non-polar atoms get polarized by the neighbor

*Van der Waals interactions are forces that can be BOTH ___ and ___

Water auto-ionization

Because of electron-rich O atom, H20 is a ___

It attracts (rather attacks) a positively charged (electron-deficient atom).

A water molecule may attack another __ molecule to cause ionization.
--> The result is hydronium (H3O+) and hydroxide (OH-) ions.

Ion product of water

__ = ion product of water

Kw = 1X10^-14

IMPORTANT:
Kw does __ change when acid or base are added to the solution, assuming [H2O] does not change significantly.

[H+] is an important biochemical parameter

pH = -log [__]

In pure water [H+] = 10^-7 and pH = 7

In blood = about __

IN tumors pH is often <7

*The "right" pH is required for enzymes to function.

pH values depend on the degree of ____ dissolved in water molecules, such as acids and bases.

HCI is an example of a strong acid, it dissociates almost ___

Example:
If 1mM HCI is prepared, pH= -log(0.0001) =3

[H+][OH-] = 10^-14 (Kw)

**If a low concentration of acid added in a lot of water, the diluted water dominates and the pH remains close to __
(cutoff around 1 micro Molar - 10^-6)

Weak acids are only ___ dissociate.

Ka is an acid dissociation constant:

The same convenience logic as in pH, logarithmic values are used:
pKa = -log Ka

Henderson--Hasselbalch Equation

-The relation between pH and pKa

pH = pKa + log ( [proton acceptor] / proton donor])

pKa = pH when [proton acceptor] = [proton donor]

Addition of NaOH to a solution containing a weak acid and its conjugate base does not substantially change pH in the region pH=pKa ±1.

This phenomenon is called ___.

Equilibrium shifts to offset +/-[OH]

pH = pKa at the ___ of the titration curve