Water and the Fitness of the Environment

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Chemical reactions depend on collisions of molecules and therefore on the concentration of solutes in an aqueous solution

Slide 30

Molecular mass is the sum of all masses of all atoms in a molecule

Numbers of molecules are usually measured in moles, where 1 mole (mol) = 6.02 x 1023 molecules

Avogadro’s number and the unit dalton were defined such that 6.02 x 1023 daltons = 1 g

Molarity (M) is the number of moles of solute per liter of solution

Slide 31

Concept 3.3: Acidic and basic conditions affect living organisms

A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other:

The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+)

The molecule with the extra proton is now a hydronium ion (H3O+), though it is often represented as H+

The molecule that lost the proton is now a hydroxide ion (OH–)

Slide 32

Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed

Slide 33

Fig. 3-UN2

Hydronium

ion (H3O+)

Hydroxide

ion (OH–)

2H2O

H

H

H

H

H

H

H

H

O

O

O

O

Slide 34

Though statistically rare, the dissociation of water molecules has a great effect on organisms

Changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell

Slide 35

Effects of Changes in pH

Concentrations of H+ and OH– are equal in pure water

Adding certain solutes, called acids and bases, modifies the concentrations of H+ and OH–

Biologists use something called the pH scale to describe whether a solution is acidic or basic (the opposite of acidic)

Slide 36

Acids and Bases

An acid is any substance that increases the H+ concentration of a solution

A base is any substance that reduces the H+ concentration of a solution

Slide 37

The pH Scale

In any aqueous solution at 25°C the product of H+ and OH– is constant and can be written as [H+][OH–] = 10–14

The pH of a solution is defined by the negative logarith of H+ concentration, written as pH = –log [H+]

For a neutral aqueous solution [H+] is 10–7 = –(–7) = 7

Slide 38