Transcript Water and pH

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
WEAK BONDS
Not effective over
long distances
Formed by the attraction of opposite partial
electric charges between two polar molecules
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 Heat
Water 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 Vaporization
Water absorbs a lot of heat, hydrogen
bonds break, then water turns to vapor &
then evaporates.
Makes water a
good coolant
Have to add a
lot of heat to
change its temp.
Helps keep
oceans relatively
stable
Heat needed for
the evaporation of
water in sweat is
taken from the
tissues of the skin
WATER AS ICE, FLOATS
Ice
Liquid water
SOLVENT PROPERTIES
Water is a versatile
solvent because of its
polarity
Most of the important molecules in and out of
the cell are polar molecules. These molecules
create solutions that enable for biochemical
processes to occur.
Protein synthesis & glycolysis
Gas Exchange
Salt dissolves when all
ions have separated
from the crystal
Water forms a hydration shell
around each solute ion.
Light independent processes of photosynthesis
HYDROPHILLIC vs HYDROPHOBIC
Hydrophillic
• Molecules that “love” water
– With positive or negative
charges including polar
molecules
EX: all substances that
dissolve in water like glucose
Hydrophobic
• Molecules that “fear” water
– Do not have positive or
negative charges & are
nonpolar
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
Calculate the solutions of sucrose
(C12H22O11)
Solution
0.2 M
Grams of Sucrose
68.4g
Total Volume in Flask
1,000mL
0.4M
136.8g
1,000mL
0.6M
205.2g
1,000mL
0.8M
273.6g
1,000mL
1.0M
342g
1,000mL
Water Ionizes
• Covalent bonds within a water molecule
sometimes break spontaneously
H2O
OH–
hydroxide
ion
+
H+
hydrogen
ion
Simplified
version
This process of spontaneous ion formation is
called ionization
It is not common because of the strength of covalent
bonds
BELOW IS A BEAKER OF DISTILLED WATER
WHAT IS THE
CHEMICAL FORMULA
FOR THIS BEAKER OF
DISTILLED WATER?
H2O +H2
H3O+ + OH-
H2O
H+ + OHSimplified version
Is the concentration of water in the beaker the same as the total
concentration of H+ + OH- ? NO
What does it mean to be at equilibrium?
The reactions is flowing back & forth, but there is no longer any net gain in
either the concentration of the products or the reactants.
Which is in higher concentration in this beaker; H2O or H+ + OH- ?
H2O
pH
• A convenient way to express the hydrogen ion
concentration of a solution
pH = _ log [H+]
The pH scale is logarithmic
A difference of one unit represents a ten-fold change in H+
concentration
Acid
Dissociates in water to increase H+ concentration
Base
Combines 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 Exponetial
level 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
1/1,000,000,000
1 X 10-4
10 1 X 10-10 0.0000000001
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- )
H 2O
DISCUSSION QUESTIONS
How 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 QUESTIONS
Which 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.
Buffers
• 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)
Response to a rise
in pH
–
+
+
H2O
Water in
blood plasma
CO2
Carbon dioxide
H2CO3
Carbonic acid
HCO3–
Bicarbonate
ion
Response to a drop
in pH
+
H+
Hydrogen
ion