Unit 1 Ch.3 H

2

O & the fitness of the Environment.

Presented by Mrs. Knopke Fullerton Science Dept.

Ch. 3 Notes

 Water contributes to the fitness of the environment to support life.

 Life on earth probably evolved in water.

  Living cells are 70%-95% H 2 O.

Water covers about ¾ of the earth.

 In nature, water naturally exists in all three physical states of matter- solid, liquid, and gas.

I. Water molecules and hydrogen bonding.

 Four valence orbitals of O point to corners of a tetrahedron.

 2 corners are orbitals with unshared pairs of electrons and weak negative charge.

 2 corners are occupied by H atoms which are in polar covalent bonds with O. Oxygen is so electronegative, that shared electrons spend more time around the O causing a weak positive charge near H’s.

I. Cont.

 Positively charged H of one molecule is attracted to the negatively charged O of another water molecule.

 Each water molecule can form a maximum of four hydrogen bond with neighboring water molecules.

 Hydrogen bonding orders water into a higher level of structural organization.

Water has a variety of unusual properties because of attractions between these polar molecules.

II.A. Liquid water cohesive.

 Cohesion: phenomenon of a substances being held together by hydrogen bonds.

 Through hydrogen bonds are transient, enough, water molecules are hydrogen bonded at any given time to give water more structure than other liquids.

 Contributes to upward water transport in plants by holding the water column together.

Cohesion

among water molecules plays a key role in the transport of water against gravity in plants.

Adhesion,

clinging of one substance to another

II.A. Cont.

 Surface tension: Measure of how difficult it is to stretch or break the surface of a liquid.

 Water has a greater surface tension then most liquids.

 Function of the fact that at the air/ H 2 O interface, surface water molecules are hydrogen bonded to each other and to the water molecules below.

 Causes H 2 O to bead (shape with smallest area to volume ratio and allows maximum hydrogen bonding).

 Adhesion: Clinging of water to hydrophilic substances (e.g. glass)

Surface tension,

a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion.

II.B. Water has a high specific heat.

 Kinetic energy: The energy of motion.

 Heat: Total kinetic energy due to molecular motion in body of matter.

 Calorie (cal): Amount of heat it takes to raise temperature of one gram of water by one degree Celsius.

 Kilocalorie: (Kcal or Cal): Amount of heat required to raise the temperature of one kilogram of water by one degree Celsius.

 Temperature: Measure of heat intensity due to average kinetic energy of molecules in a body of matter.

II.B. Cont.

Celsius scale 100 .

C (212 .

F) = Water boils Scale conversion C .

= 5( F – 32) 9 37 .

C (98.6

.

F) = Human body temp.

23 .

C (72 .

F) = Room temp.

F = 9 5 C +32 K = C + 273 0 .

C (32 .

F) = Water freeze

Specific Heat of water

 Specific heat = Amount of heat that must be absorbed or lost for one gram of a substance to change it’s temperature by one degree Celsius.

 Specific heat of Water = One calorie per gram per degree (1cal/g/ C)

How Water Stabilizes Temperature:

 Hydrogen bonding among water molecules (it takes relatively large heat loss or gain for each 1 C change in temperature.

 Hydrogen bonds must absorb heat to break, and they release heat when they form  Much absorbed heat energy is used to disrupt H-bonds before water molecules can move faster (increase temperature)

When water reaches 0 o C, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners.

A large body of water can act as a heat sink:

 Water, which covers ¾ of the planet, keeps temperature fluctuations within a range suitable for life.

 Coastal areas have milder climates than inland  The marine environment has a relatively stable environment

Water has a high heat of vaporization:

 Vaporization = Transformation from liquid to gas.

- molecules with enough kinetic energy to overcome the mutual attraction of molecules in a liquid, can escape into the air.

Heat of Vaporization:

 Quantity of heat a liquid must absorb for 1g of H 2 O to be converted to the gaseous state.

-For water molecules to evaporate, hydrogen bonds must be broken increase heat energy.

 Water has a relatively high heat of vaporization (540 cal/g)

Evaporative cooling:

 Cooling of a liquid’s surface when a liquid evaporates -The surface molecules with the highest kinetic energy are most likely to escape into gaseous form: the average kinetic energy in the remaining surface moleculeis lowered.

Water’s high heat of vaporization:

 Earth: - Solar heat absorbed by tropical seas dissipates when surface water evaporates (evaporative cooling) - As moist tropical moves towards the poles, water vapor releases heat and condenses into rain.

Water expands when it freezes:

 Water contracts as it cools to 4 C  As water cools from 4 C to freezing (O C) it expands and becomes less dense than liquid water (ice floats)  When water begins to freeze, the molecules do not have enough kinetic energy to break H-bonds  As crystalline lattice forms, each water molecule forms a max of 4 H-bonds. Keeping molecules further apart compared to the liquid state. Thus less dense

Expansion of water contributes to the fitness of the environment for life

 Prevents deep bodies of water from freezing solid from the bottom up  Since ice is less dense it forms on the surface first. As it freezes it releases heat to the water below and insulates.

 Makes transition between seasons less abrupt.

 H-bonds form = released heat  H-bonds break = absorbed heat

The surface area of ice insulates liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.

Water a Versatile Solvent

 Solution = A liquid that is homogenous mixture of two or more substances  Solvent = dissolving agent  Solute = substance dissolved in the solution  Aqueous solution = Solution in which water is the solvent

Water is an effective solvent because it so readily forms hydrogen bonds with charged and polar covalent molecules.

Water as a solvent:

 Ionic compounds – Charged regions of polar water molecules have an electrical attraction.

-Water surrounds individual ions, separating and shielding them from one another.

 Polar compounds – Charged regions of polar H 2 O molecules have an affinity for oppositely charged regions of other polar molecules.

 Nonpolar – Not water soluble ex. fats

Water loving or Hating

 Hydrophilic = (hydro = water, philo = loving) Property of having an affinity for water ex. Ionic and polar  Hydrophobic = (hydro = water, phobos = fearing) Property of not having an affinity for water , and thus not being water soluble ex. nonpolar

Aqueous solutions: Acids and Bases

 At Equilibrium – the number of H + number of OH - ions ions =  [H + ] = [OH ]

Acid vs Bases

ACID Substance the increases the relative [H + ] of a solution Also removes OH because it tends to combine with H + to form H 2 O.

For example: (in water) HCl H + + Cl        Base Substance that reduces the relative [H + ] of a solution May alternately increase OH Ex.

A base may reduce [H + ] directly: NH 3 + H + NH 4 + May reduce indirectly: NaOH Na + + OH OH + H + H 2 O

A Solution which:

 [H + ] = [OH ] is a neutral solution  [H + ] > [OH ] is an acidic solution  [H + ] < [OH ] is a basic solution

pH Scale

 [H + ] [OH ] = 10 -14 M 2  For example:  In a neutral solution [H + ] = 10 -7 M and  [OH ] = 10 -7 M  What would make an acid, a base?

pH Scale:

 Scale used to measure the degree of acidity. It ranges from 0 – 14  pH of 7 is neutral  pH < 7 is an acidic solution  pH > 7 is a basic solution  Most biological fluids are within the pH range of a 6-8. There are some exceptions such as stomach acid with the pH = 1.5

 Each pH unit represents a tenfold difference (scale is logrithmic) so a slight change in pH represents a large change in actual [H + ]

Buffers:

 Buffer = Substance that prevents large sudden changes in pH.

-Are combinations of H + donor and H + acceptor forms weak acids or bases -Work accepting H + ions from solution when they are in excess, and donating H + ions to the solution when they have been depleted

Example of Buffers:

 Ex. Bicarbonate Buffer  response to arise in pH  H + H 2 CO 3 HCO 3 + H donor response to a drop in pH H + acceptor + weak acid weak base HCl + NaHCO 3 Strong acid NaOH + H 2 CO 3 Strong base H 2 CO 3 + NaCl weak acid NaHCO 3 + H 2 O weak base