Freeman Chapter 2 Basic Chem, Water and pH

Chapter 2

_{Water and}

Carbon: The

Chemical Basis

of Life

Key Concepts

• _{Molecules form when atoms bond to each other. Chemical bonds}

are based on electron sharing. The degree of electron sharing

varies from nonpolar covalent bonds, to polar covalent bonds, to ionic bonds.

• Of all small molecules, water is the most important for life.

• _{Chemical reactions tend to be spontaneous if they lead to lower}

potential energy and higher entropy.

The Building Blocks of Chemical Evolution

• _{Each element has a unique atomic number and contains a}

characteristic number of protons, called the atomic number.

• _{The mass number (indicated in superscript) is the number of}

protons + neutrons of the most common isotope.

• _{Elements commonly found in organisms have at least one}

Chemical Bonding

• _{Unfilled electron orbitals allow formation of chemical bonds,}

and atoms are most stable when each electron orbital is filled.

Covalent bond: Each atom’s unpaired electrons are shared by

both nuclei to fill their orbitals

Nonpolar covalent bond: Electrons are evenly shared between

two atoms and the bond is symmetrical.

Covalent Bonds

Covalent bond

Hydrogen atoms each have

Ionic Bonds

A sodium ion being formed

A chloride ion being formed

Table salt is a crystal composed of two ions. Cl–

Na+

Loss of electron Cation formation

Anion formation Gain of electron

Ionic Bonds

A sodium ion being formed

A chloride ion being formed

Loss of electron Cation formation

Anion formation Gain of electron

Ionic Bonds

Table salt is a crystal composed of two ions.

Cl–

Electronegativity of Atoms

• _{An atom in a molecule with a high electronegativity will hold the}

electrons more tightly and have a partial negative charge (δ–_),

whereas the other atom will have a partial positive charge (δ+_).

• _{Cation: An atom that loses an electron and becomes positively}

charged.

• _{Anion: An atom that gains an electron and becomes negatively}

How Many Bonds Can an Atom Have?

• _{The number of unpaired electrons determines the number of}

bonds an atom can make.

• _{Atoms with more than one unpaired electron can form multiple}

Representing Molecules

• _{The shape of a simple molecule is governed by the geometry of}

its bonds.

• _{The molecular formula indicates the numbers and types of atoms}

in a molecule (e.g., H₂O, CH₄).

• _{Structural formulas indicate which atoms are bonded together}

and whether the bonds are single, double, or triple bonds.

Molecular Representations

Methane Ammonia Water Oxygen

Molecular formulas:

Structural formulas:

Quantifying Molecules

• _{The molecular weight of a molecule is the sum of the mass}

numbers of all the atoms in the molecule.

• _{One mole, or 6.022  10}23 _{molecules, has a mass equal to the}

molecular weight expressed in grams.

• _{The concentration of a substance in a solution is typically}

The Properties of Water

• _{Life is based on water because water is a great solvent.}

• The covalent bonds in water are polar because oxygen and

hydrogen differ in their electronegativity: oxygen has a partial negative charge and hydrogen has a partial positive charge.

• _{Ions and polar molecules stay in solution because of their}

Interaction between Hydrogen Bonds and Water

• Hydrogen bonds form between H₂O and other polar molecules or

ions. Ions and polar molecules stay in solution because of their interactions with water’s partial charges.

• Hydrogen bonding makes it possible for almost any charged or polar molecule to dissolve in water and hydrogen bonds are

Water Molecules and Hydrogen Bonding

Water is polar. Hydrogen bonds form between water molecules.

The Structure of Water

• _{The structure of water is unique—its small size, bent shape,}

highly polar covalent bonds, and overall polarity.

• _{Water also has several striking physical properties:}

(1) It expands as it changes from a liquid to a solid.

The Properties of Water

• _{Water is cohesive and has high surface tension}

(1) Binding between like molecules is called cohesion. (2) Binding between unlike molecules is called adhesion.

• _{Water is denser as a liquid than as a solid:}

(1) Ice: Hydrogen bonds connect water molecules in an open crystal pattern.

Structure of Water: Liquid and Ice

In ice, water molecules form a crystal lattice. In liquid water, no

Acid–Base Reactions and pH

• _{In acid–base reactions, a proton donor (acid) transfers a proton}

to a proton acceptor (base).

• _{The pH scale is logarithmic:}

pH = −log [H+_].

pH Scale

Neutral

Oven cleaner

Household bleach Household ammonia

Milk of magnesia

Baking soda Seawater Human blood Pure water Milk Urine Black coffee Tomatoes Wine

Vinegar, soft drinks, beer Lemon juice