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 Hs.

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 scaleScale conversion 100. C (212. F) = Water boils 37. C (98.6. F) = Human body temp. 23. C (72. F) = Room temp. 0. C (32. F) = Water freeze C. = 5( F – 32) 9 F = 9 5 C +32 K = C + 273

Specific Heat of water Specific heat = Amount of heat that must be absorbed or lost for one gram of a substance to change its 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 liquids 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.

Waters 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 + ions = number of OH - 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 - ] = M 2 For example: In a neutral solution [H + ] = M and [OH - ] = 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 2 CO 3 HCO H + H + donor response to a drop in pH H + acceptor weak acid weak base HCl + NaHCO 3 H 2 CO 3 + NaCl Strong acid weak acid NaOH + H 2 CO 3 NaHCO 3 + H 2 O Strong baseweak base