BIO146

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Chapter 3
Chapter 3

3.1 - Polar covalent bonds in water molecules result in hydrogen bonding

Water molecules shaped like a wide v

oxygen is more electronegative than hydrogen, so the electrons of the covalent bonds spend more time closer to oxygen than to hydrogen

in water, the oxygen of the molecule has partial negative charges, and the hydrogens have partial positive charges

→ Polar molecule - unequal sharing electrons; its overall charge is unevenly distributed

The properties of water arise from attractions between opposite charged atoms of different water molecules:

the partially positive hydrogen of one molecule is attracted to the partially negative oxygen of a nearby molecule

the two water molecules are held together by a hydrogen bond

the hydrogen bonds are very weak, each only about 1/20 as strong as a covalent bond

3.2 - Four emergent properties of water contribute to Earth’s suitability for life

Cohesion<凝聚>of Water molecules

the hydrogen bonds hold the substance together

at any given moment many of the molecules are linked by multiple hydrogen bonds

contributes to the transport of water and dissolved nutrients against gravity in plants

→ water from the roots reaches the leaves through a network of water-conducting cells

→ as water evaporates from a leaf, hydrogen bonds cause water molecules leaving the veins to tug on molecules farther down, and the upward pull is transmitted through the water-conducting cells all the way to the roots

cohesive behavior

→ either use force or boil the water to break the hydrogen bond

moderate temperature

floating ice

universal solvent (nickname for water) - lots of things can dissolve in water

osmotic pressure - vascular tissue (which can bring water up from the roots to the leaves)

Surface tension

a measure of how difficult it is to stretch or break the surface of a liquid

→ at the air-water interface is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below, but not to the air above

Adhesion<附着力>

the clinging of one substance to another

adhesion of water by hydrogen bonds to the molecules of cell walls helps counter the downward pull of gravity

Moderation of Temperature by Water

→ water moderates air temperature by absorbing heat from air that is warmer and releasing stored heat to air that is cooler

Temperature and Heat

kinetic energy - the energy of motion (anything that moves has kinetic energy)

→ atoms and molecules have kinetic energy because they are always moving(although not necessarily in any direction)

→ the faster a molecule moves, the greater its kinetic energy

Thermal energy - associates with kinetic energy associated with the random movement of atoms or molecules

Temperature

represents the average kinetic energy of the molecules in a body of matter, regardless of volume

the thermal energy of a body of matter reflects the total kinetic energy which depends on the matter’s volume

→ by heating up the water, the average speed of the molecules increases (the thermometer records this as a rising temperature of the liquid), which the total amount of thermal energy also increases

→ when two objects with different temperatures are together, thermal energy always passes from the warmer to the cooler object until they become the same temperature

➡️ Ex. the ice absorbs the thermal energy from the hot water to make it cool down

heat - thermal energy in transfer from one body of matter to another

heat is a measure of the total amount of kinetic energy due to molecular motion(heat is kinetic energy because of that heat moves from high to low)

temperature measures the intensity ...

Units of temperature

calorie(cal) - the amount of heat it takes to raise the temperature of 1g of water by 1 degree celsius

kilocalorie(kcal) - the quantity of heat required to raise the temperature of 1 kg of water by 1 degree celsius

joule(J) - equals 0.239cal; one calorie equals 4.184J

Water’s High Specific Heat

the specific heat of a substance - the amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1 degree celsius

➡️ Ex. the specific heat of water is 1 cal/g°C

because of the high specific heat of water relative to other materials, water changes temperature less than other liquids when it absorbs or loses a given amount of heat

heat must be absorbed in order to break hydrogen bonds; heat is released when hydrogen bonds form

→ because of the high specific heat of water, the water that covers most of Earth keeps temperature fluctuations on land and in water within limits that permit life

1 Cal = 1 kcal = 1000cal

1 cal = 1 cal

Evaporative Cooling

transition from liquid to a gas is called vaporization

→ even at low temperatures, the speediest molecules can escape into the air

→ when a liquid is heated, the average kinetic energy of molecules increases and the liquid evaporates more rapidly

Heat of vaporization - the quantity of heat a liquid must absorb for 1g of it to be converted from the liquid to the gaseous state

➡️ after sweating, the skin temperature is very low

→ water has a high heat of vaporization relative to most other liquids

→ heat burns - caused by the heat energy released (during formation of hydrogen bonds) when steam condenses into liquid on the skin

evaporative cooling

→ occurs because the molecules with the greatest kinetic energy are most like to leave as gas

➡️ Ex. as a liquid evaporates, the surface of the liquid that remains behind cools down

➡️ Ex. high humidity on a hot day increases discomfort because the high concentration of water vapor in the air inhibits the evaporation of sweat from the body. Animals with out sweat glands, such as elephant, may spray water on themselves to cool down

Floating of Ice on Liquid Water

→ water is one of the few substances that are less dense as a solid than as a liquid

> 4°C and above - liquid

> between 4°C and 0°C - starts to freeze

> 0°C and below - molecules become locked into a crystalline lattice, each water molecule hydrogen-bonded to four partners

➡️ if ice sank, then eventually ponds, lakes, and even oceans could freeze solid which will make life impossible on Earth

⁠

Water: The Solvent of Life

solution - a liquid that is a completely homogeneous mixture of two or more substances

solvent - dissolving agent of a solution

solute - the substance that is dissolved

➡️ Ex. water is the solvent and sugar is the solute

aqueous solution - the solute is dissolved in water; water is the solvent

→ water is a very versatile solvent

hydration shell - the sphere of water molecules around each dissolved ion

→ a compound does not need to be ionic to dissolve in water

→ many compounds made up of nonionic polar molecules are also water-soluble (such as sugar)

⁠

Hydrophilic and Hydrophobic Substances

hydrophilic - any substance that has an affinity<亲和力> for water

in some cases, substances can be hydrophilic without actually dissolving

➡️ cotton (hydrophilic substance) does not dissolve in the water because it has giant molecules of cellulose (cotton towel does a good job of drying water on our hands)

hydrophobic - substances that do not have an affinity for water; substances that are nonionic and non-polar (or otherwise cannot form hydrogen bonds) seem to repel water

➡️ oil (high number of relatively non-polar covalent bonds) does not mix with water-based substances

colloid - a stable suspension

Solute Concentration in Aqueous Solutions

molecular mass - the sum of the masses of all the atoms in a molecule

➡️ Ex. CO2 → 12+16*2=44 amu

mole (mol) - represents an exact number of objects: 6.02*10^23; Avogadro’s number

molarity - the number of moles of solute per liter of solution (the unit of concentration most often used by biologists for aqueous solutions)

3.3 - Acidic and basic conditions affect living organisms

The hydrogen atoms leaves its electron behind so that it transfers to a hydrogen ion

➡️ a hydrogen atom participating in a hydrogen bond between two water molecules shifts from one molecule to the other

hydroxide ion(OH-) - the water molecule that lost a proton

hydronium ion (H3O+) - the proton binds to the other water molecule

Acid and Bases

Acid - is a substance that increases the hydrogen ion concentration of a solution

➡️ HCl, HF, HBr, H2SO4, H3PO4, etc.

Base - a substance that reduces the hydrogen ion concentration of a solution

➡️ some bases reduce the hydrogen ion concentration directly by accepting hydrogen ions (NH3→ NH4+)

➡️ other bases reduce the hydrogen ion concentration indirectly by dissociating to form hydroxide ions (NaOH→ Na+ & OH-)

The pH Scale

in any aqueous solution at 25°C, the product of the hydrogen ion and hydroxide ion concentrations is constant at 10^-14

an acid not only adds hydrogen ions to a solution, but also removes hydroxide ions

The pH of a solution is defined as the negative log of the hydrogen ion concentration

➡️ pH =-log[H+] = 10^-x[H+] & pOH = -log[OH-]

→ pH + pOH = 14 → [H+]+[OH-] = 10^-14

➡️ pH 1 = -log[1] = 10^-1 = 0.1M[H+]

2(H2O) → (H3O+) + (OH-)

→ (H3O)+ → (2H+) + (OH-)

Buffers

the internal pH of most living cells is close to 7

buffer - a substance that minimizes changes in the concentrations of hydrogen ion and hydroxide ion in a solution

Acidification: A Threat to Our Oceans

ocean acidification - CO2 dissolves in seawater, it reacts with water to form carbonic acid (H2CO3), which lowers ocean pH

acid precipitation - rain, fog, or snow with a pH lower than 5.2

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