Chapter 3, Water

 

 

Figure 3.x1  Water

 

Figure 3.0  Earth

 

The Chemistry of Water

•      Earth is the water planet. 

•      Water exists as a liquid at relatively low temperatures.

•      75% Earth’s surface is covered by liquid water – moderates climate.

•       Life originated in a medium of water, and evolved for 3 billion years before moving to land.

•      2/3 of an organisms body is composed of water.

•      Life on earth is possible because of the unique properties of water.

 

 

Water molecules act like tiny magnets.

•     Water molecules form weak chemical associations with each other called hydrogen bonds.  Hydrogen bond are the underpinning for all the unique properties of water.

 

How hydrogen bonds are formed:

•     The water molecule has distinct + and – charged ends, like two poles of a magnet.  A polar molecule.

•     polar molecules interact to form hydrogen bonds

•     weak electrostatic attractions between opposite partial charges create a force that is 5-10% as strong a covalent bond.

 

Figure 3.1  Hydrogen bonds between water molecules 

 

 

The Properties of Water

•     Water clings to polar molecules

•     Water stores heat

•     Water is an Excellent Solvent

•     Water influences the arrangement of nonpolar molecules

•     Water ionizes

 

Water clings to polar molecules

•     cohesion: water to water

–  responsible for surface tension

•     adhesion: water to other polar molecules

–  capillary action

 

 

Figure 3.3  Walking on water

 

Figure 3.2  Water transport in plants

 

Figure 3.2x  Trees

 

Water stores heat

•     Water moderates temperature because of two properties: high specific heat and high heat of vaporization.

•     Large input of thermal energy is required to break hydrogen bonds.

 

Figure 3.x1  Water

 

 

Specific heat

•     Specific heat is defined as the amount of heat that must be absorbed or lost by 1 gram of substance to change its temperature by 1 degree Celsius.

•     Water has a specific heat of 1 calorie/gram/◦C

–  This is 2X that of most carbon compounds

–  9X higher than iron

–  NH3, which is more polar than water, has a higher specific heat (1.23 calories/ gram/◦C).

 

 

Effects of having a high specific heat

•     heats up more slowly and holds temperature longer

–  allows organisms to regulate body temperature

–  large bodies of water on Earth moderate climate

 

 

Heat of Vaporization

•     Transition of from liquid to a gas require input of energy to break H-bonds, therefore evaporation of water from a surface causes cooling.

–  Temperature regulation by evaporative cooling – sweating

 

Figure 3.4  Evaporative cooling

 

 

Ice is less dense than water

•     Water freezes at lower temperature than predicted because of cohesion from H bonds.  Ice is less dense than water because H-bonds now are rigid, and making spaces in the crystal.

 

Figure 3.6x2  Ice floats and frozen benzene sinks

 

Figure 3.5  The structure of ice (Layer 1)

 

Figure 3.5  The structure of ice (Layer 2)

 

Figure 3.5x1  Ice, water, and steam

 

Water is an Excellent Solvent (for polar molecules!)

•     interacts with polar groups on molecules, pulls them away from the solid, then covers them with a hydrogen shell so they don’t re-associate

–  blood, cytoplasm are complex solutions using water as the solvent

 

Figure 3.7  A crystal of table salt dissolving in water

 

Figure 3.8  A water-soluble protein

 

Water influences the arrangement of nonpolar molecules

•     polar molecules are hydrophilic, whereas non-polar molecules are hydrophobic

•     Carbon compounds largely hydrophobic because covalent bonds in carbon share electron equally, nonpolar.

 

 

Water influences the arrangement of nonpolar molecules, con’t

•     when polar molecules aggregate in water, they push away hydrophobic molecules:  hydrophobic exclusion

•     This cause particular shapes and arrangements to form.

•     Ex. cellular membranes, protein structure 

 

Water ionizes

•     H2O  →  H+  +  OH-

•     At 25 degrees Celsius

–  1 out of every 550 million water molecules dissociates

–  one liter of water contains 10-7 moles of H+ ions, or 10-7 moles/liter.

•   one mole is 6.02 x 1023 molecules, (Avagadro’s number)

•   [  ] means concentration

 

 

Unnumbered Figure (page 47)  Chemical reaction: hydrogen bond shift

 

 

The pH Scale

•     pH = -log [H+]

•     So, pure water, with a [H+] of 10-7 moles/liter, has a pH of 7.

•     The pH scale is logarithmic

–  the lower the pH value, the greater the [H+].

–  Each difference of 1 indicates a 10X change in pH.

 

Figure 3.9  The pH of some aqueous solutions

 

Acids and Bases

•     Acid: a substance that increases the [H+] when in dissociates in water.

•     Base: a substance that increases the [OH-] when in dissociates in water.

 

Figure 3.10  The effects of acid precipitation on a forest

 

Figure 3.10x1  Pulp mill

 

Figure 3.10x2  Acid rain damage to statuary, 1908 & 1968

 

Buffers

•     Essential that pH inside living cells and surrounding fluids is kept to a narrow range, usually ph 7.

•     However, chemical reactions of life constantly produce acids and bases!

•     Buffer: a substance that acts as a reservoir for hydrogen ions.  Donates to solution when they fall, or takes them up from the solution when they are in excess.

 

Examples of Buffers

•     Example: carbonic acid and bicarbonate in human blood, Fig. 2.2

 

•     H2O + CO2 ↔ H2CO3 ↔ HCO3- + H+

 

The End.