THE IMPORTANCE OF WATER TO LIFE

Hydrogen Bonds Give Water Unique Properties

• Water molecules are polar molecules• Unequal sharing of electrons & V-like shape– They can thus form hydrogen bonds with each other

and with other polar molecules

• Each hydrogen bond is very weak– However, the cumulative effect of enormous

numbers can make them quite strong

• Hydrogen bonding is responsible for many of the physical properties of water

HYDROGEN BONDING: DRAW 2-3 MOLECULES OF WATER & INDICATE THE

HYDROGEN BONDS

HYDROGEN BONDING: DRAW 2-3 MOLECULES OF WATER & INDICATE THE

HYDROGEN BONDS

Formed by the attraction of opposite partial electric charges between two polar molecules

WEAK BONDSNot effective over long distances

COHESIVE PROPERTIES

Heat vs Temperature

• Heat= is a measure of the total amount of kinetic energy due to molecular motion in a body of matter.– Calorie= the amount of heat it takes to raise the

temperature of 1g of water by 10C• Kilocalorie= 1,000 calories

– Joule (J) one joule = 0.239 calories and 1 calorie= 4.184J

• Temperature= the intensity of heat due to the average kinetic energy of the molecules.– We will use Celsius to indicate temperature

THERMAL PROPERTIES

High Specific HeatWater can absorb or release a lot of heat without changing its own temperature by very much. It’s ability to store heat

High Heat of VaporizationWater absorbs a lot of heat, hydrogen

bonds break, then water turns to vapor & then evaporates.

Have to add a lot of heat to change its temp.

Helps keep oceans relatively stable

Makes water a good coolant

Heat needed for the evaporation of water in sweat is taken from the tissues of the skin

WATER AS ICE, FLOATS

Liquid water

Ice

SOLVENT PROPERTIES

Water is a versatile solventsolvent because of its polarity

Most of the important molecules in and out of the cell are polar molecules. These molecules create solutionssolutions that enable for biochemical processes to occur.

Gas Exchange

Protein synthesis & glycolysis

Light independent processes of photosynthesisWater forms a hydration shell hydration shell around each solutesolute ion.

Salt dissolves when all ions have separated

from the crystal

HYDROPHILLIC vs HYDROPHOBIC

Hydrophillic

• Molecules that “love” water– With positive or negative

charges including polar molecules

Hydrophobic

• Molecules that “fear” water– Do not have positive or

negative charges & are nonpolar

EX: all substances that dissolve in water like glucose

EX: all substances that do not dissolve in water like fats & oils

If substances are soluble in water can be freely transported in the blood plasma. Ex: glucose amino acids, & sodium chloride

If they are hydrophobic they are transported inside a lipoprotein complex. EX: fats and cholesterol

Solute Concentration in Aqueous Solutions

• Molecular mass: sum of the masses of all the atoms in a molecule. = number of daltons

• Measuring is done in moles=6.02 x 1023

• Once the molecular mass of a molecule is determined, that number is used with a unit in grams.

How would you make 1 liter of a 1M solution of sucrose (C12H22O11)

• Review Avogadro’s number (a mole)– There are 6.02 x 1023 daltons in 1g.

Carbon has an atomic mass of 12 & there are 12 carbons so 12 x 12 = 144

Hydrogen has an atomic mass of 1& there are 22 hydrogens so 1 x 22 = 22

Oxygen has an atomic mass of 16& there are 11 oxygens so 16x 11 = 176

For a total of ….342 daltons

So, this means you would need 342g of sucrose in a liter of 1M sucrose solution

Solution Grams of Sucrose Total Volume in Flask

0.2 M

0.4M

0.6M

0.8M

1.0M 342g 1,000mL

Calculate the solutions of sucrose (C12H22O11)

273.6g

205.2g

68.4g

136.8g

1,000mL

1,000mL

1,000mL

1,000mL

Water Ionizes

• Covalent bonds within a water molecule sometimes break spontaneously

H2O +OH–

hydroxide ion

H+

hydrogen ion

This process of spontaneous ion formation is called ionization

It is not common because of the strength of covalent bonds

Simplified version

BELOW IS A BEAKER OF DISTILLED WATER

Is the concentration of water in the beaker the same as the total concentration of H+ + OH- ?

What does it mean to be at equilibrium?

WHAT IS THE CHEMICAL FORMULA FOR THIS BEAKER OF DISTILLED WATER? H2O H+ + OH-

Which is in higher concentration in this beaker; H2O or H+ + OH- ?

NO

The reactions is flowing back & forth, but there is no longer any net gain in either the concentration of the products or the reactants.

H2O

H2O +H2 H3O+ + OH-

Simplified version

• A convenient way to express the hydrogen ion concentration of a solution

pH

pH = log [H+]_

The pH scale is logarithmicA difference of one unit represents a ten-fold change in H+ concentration

AcidDissociates in water to increase H+ concentration

BaseCombines with H+ when dissolved in water

Remember:

• The pH scale is logarithmic.– A change in one pH number actually represents a

tenfold change in hydrogen ion concentration

• EX: pH of 3 is actually ten times more acidic than a pH of 4.

CALCULATE THE RATIO OF DISSOCIATED TO INTACT WATER MOLECULES IN A BEAKER OF DISTILLED WATER OF pH at 250 C

If pH =7, then the concentration of hydrogen [H+ ] = 1 x 10-7 , which is the same as…

[H+ ] = 1/10,000,000 , which is the same as

Saying there is 1 dissociated molecule of H2 O for every 10 million intact H2 O molecules in the beaker of distilled water on the table.

IN OTHER WORDS….

If you could reach in & pick out a single molecule from the beaker of water, 9,999,999 times out of 10,000,000 you would pull out a molecule of H2 O & 1 out of 10,000,000 times you would pick out a hydrogen ion.

pH chart (from WS)pH

levelExponetial notation

Decimal notation Fraction notation OH-

1 1 X 10-1 0.1 1/10 1 X 10-13

4 1 X 10-4 0.0001 1/10,000 1 X 10-10

7 1 X 10-7 0.0000001 1/10,000,000 1 X 10-7

10 1 X 10-10 0.0000000001 1/1,000,000,000 1 X 10-4

14 1 X 10-14 0.00000000000001 1/10,000,000,000,000 1 X 10-0

DISCUSSION QUESTIONS

Considering the equation H2O H+ + OH- at equilibrium to answer the following questions

Does the equation tend to one direction of the other? If so, which direction does it tend to go?

This reaction tends towards the left

Which is in higher concentration in the above equation: (H2O) or (H+ + OH- )

H2O

DISCUSSION QUESTIONSHow many times different is a change in one unit of the pH scale?

10X more or less concentrated

What’s the difference in hydrogen concentration between pH5 & pH2?

pH2 has 1,000 times the concentration of H+ than a solution of pH5

What is the difference between a strong acid/base & a weak acid/base?

A strong acid/base dissociates readily because the differences in the electronegativity of the ions are greater; a weak acid/base has atoms that are not as different in their electronegativity so they don’t tend to the dissociated side of the equation as easily.

DISCUSSION QUESTIONSWhich acids/bases are weaker & which are stronger ---

HCl, NH3, H2CO3 & NaOH?

Cl & Na both have very high electronegativity – they are both in column VII of ther periodic table with nearly full valence shells - & so they tend to dominate the electrons of H+ or OH- since the electronegativity of these atoms are very low. This huge difference in electronegativity causes a strong tendency for NaOH and HCl to dissociate.

When the H+ or OH- dissociate from the Na & the Cl they are free to react with other molecules making them strong acids & bases.

N & CO3 don’t have as strong of an electronegative difference with H since they are in columns IV-VI. Because of their lower electronegativity they do not dissociate as readily as HCl & NaOH so they are considered weaker acids & bases.

• Hydrogen ion reservoirs that take up or release H+ as needed

• The key buffer in blood is an acid-base pair (carbonic acid-bicarbonate buffering system)

Buffers

Response to a drop in pH

H2OWater in

blood plasma

CO2

Carbon dioxide

+

H2CO3

Carbonic acid

++

–

HCO3–

Bicarbonateion

H+

Hydrogenion

Response to a rise in pH